a r^s.5/ fin (DrtumWa (Hntoetsfitp mtfjeCttpafltengiirk College of ^(jps'ictans anb Hifcrarp mrgeons ^S^ w@ A TEXT-BOOK OF HISTOLOGY By FREDERICK R. BAILEY, A.M., M.D. Adjunct Professor of Normal Histology, College of Physicians and Surgeons- Medical Department, Columbia University, New York City SECOND AND REVISED EDITION PROFUSELY ILLUSTRATED NEW YORK WILLIAM WOOD &? COMPANY MDCCCCV1I 'Copyright, igo6. By WILLIAM WOOD AND COMPANY PREFACE TO THE SECOND EDITION. There has been no change in the general plan and scope of the Text-Book as outlined in the Preface to the First Edition. Some errors have been corrected, some drawings improved, some new drawings substituted and added. The Chapter on the Nervous System, for the elaborateness of which the author was inclined to make some apologies, has proved a most valuable feature of the book. More changes have been made in this chapter than in any other, some necessitated by the advances which have been made in Neuro-histology in the past two years, others to further facilitate the teaching of the subject. Several new diagrams have also been added to this chapter. For these the author is again deeply in- debted to Dr. O. S. Strong and Mr. A. M. Miller. Digitized by the Internet Archive in 2010 with funding from Columbia University Libraries http://www.archive.org/details/textbookofhistolOObail PREFACE TO THE FIRST EDITION. The primary aim of the writer in the preparation of these pages has been to give to the student of medicine a text-book of histology for use in connection with practical laboratory instruction, and espe- cially to furnish to the instructor of histology a satisfactory manual for classroom teaching. With these objects in view, the text has been made as concise as possible consistent with clearness, and the writer has attempted to make the more essential elements stand out somewhat from the necessarily accompanying details. It has been impossible to accomplish this without some sacrifice of uniformity of treatment and of logical sequence. This is espe- cially noticeable in the chapter on the nervous system, which has been made much fuller aud more "practical" than is usual. The author's reason for the method of treatment there adopted and for the considerable amount of anatomy which this chapter contains being the apparent success the method has met with in the teaching of this always difficult subject to students. The chapter on general technic is intended to furnish the student with only the more essential laboratory methods. For special and more elaborate methods the student is referred to the special works on technic mentioned at the close of the chapter. The special technic given in connection with the different tissues and organs is in most cases such as can be conveniently used for the preparation of class sections. The original illustrations are from drawings by Mr. A. M. Miller, to whom the writer is greatly indebted for his careful and accurate work. The uselessness of redrawing perfectly satisfactory illustra- tions has led the writer to borrow freely from various sources, each cut being duly accredited to the work from which it has been taken. iii iv PREFACE. For all of these the author wishes to express his appreciation and obligation. He is also deeply indebted to Dr. O. S. Strong for his careful review and criticism of the chapter on the nervous system and for his supervision of the drawing of Figs. 263 and 264 ; to Dr. G. C. Freeborn, his predecessor as Instructor of Histology at the College of Physicians and Surgeons, for many valuable suggestions ; and to Dr. T. Mitchell Prudden for his careful and critical review of the author's copy. CONTENTS. PART I. — HISTOLOGICAL TECHNIC. CHAPTER I. General Technic. General Considerations, Examination of Fresh Tissues, Dissociation of Tissue Elements, Teasing, Maceration, . Fixation, . . Hardening, Preserving, . Decalcifying, Embedding, Celloidin Embedding, Paraffin Embedding, Section Cutting, Celloidin Sections, Paraffin Sections, Staining, Nuclear Dyes, Plasma Dyes, Staining Sections, Staining in Bulk, Mounting, Staining; and Mounting Paraffin Sections lection, . 13 14 18 10 20 CHAPTER II. Special Staining .Methods. Silver 1\ uate Method of Staining the Intercellular Substance, Chlon'd of Gold for Demonstrating Connective-tissue Cells. Weigert's Elastic Tissue Stain, ..... Goigi's Chrome-silver for Staining Secretory Tubules, 2 S VI CONTEXTS. Mallory's Hasmotoxylin Stain for Connective Tissue, Osmic Acid Stain for Fat. . Jenner's Blood Stain ...... PAGE 26 26 26 CHAPTER III. Special Neurological Staining Methods. Weigert's Method of Staining Medullated Ner\ Weigert-Pal Method, Golgi Methods of Staining Nerve Tissue, Slow Method, Rapid Method, Mixed Method, Formalin-bichromate Method, Bichloride Method, Golgi-Cox Method, Cajal's Method, Nissl's Method, .. General References on Technic, e Fibres, 27 2S 29 29 29 3° 3° 30 3° 3 1 32 33 PART II. — THE CELL. CHAPTER I. The Cell, General Structure, Structure of a Typical Cell, The Cell Body, The Cell Membrane, The Nucleus, The Centrosome, Vital Properties of Cells, Metabolism, . Function, Irritability, . Motion, Amoeboid, Protoplasmic, Ciliary, . Reproduction, Direct Cell-division, Indirect ('ell-division, . Fertilization of the Ovum, Technic, .... References for further study, 37 37 37 38 39 39 40 41 41 41 41 42 42 42 42 43 43 43 46 50 51 CONTENTS. VI 1 PART III. — THE TISSUES. CHAPTER I. Histogenesis — Classification . Tissues Derived from Ectoderm, Tissues Derived from Entoderm, Tissues Derived from Mesoderm, PAGE 55 55 55 56 CHAPTER II. Epithelium (Including Mesothelium and Endothelium), . . 57 Histogenesis, ........ 57 General Characteristics, 57 Classification, 58 Simple Epithelium, 5S Simple Squamous, 5S Simple Columnar, . 58 Pseudostratified, 60 Stratified Epithelium, . 60 Stratified Squamous, 60 Stratified Columnar, 61 Transitional, 62 Modified Forms of Epithelium, 63 Ciliated Epithelium, 63 Pigmented Epithelium, . 64 Glandular Epithelium, . 64 Neuro-epithelium, 64 Mesothelium and Endothelium, 64 Technic, .... 65 CHAPTER III. The Connective Tissues, Histogenesis, General Characteristics, Classification, Fibrillar Connective Tissue, Areolar Connective Tissue, Formed Connective Tissue, Development, .... Elastic Tissue, ..... Technic for Fibrillar and Elastic Tissue, Embryonal and Mucous Tissue, Technic, ..... Recticular Tissue, .... 67 67 67 67 6S 7i 7 1 72 I 2 74 7 5 77 77 COXTEXTS. Lymphatic Tissue, ..... Technic for Reticular and Lymphatic Tissue, Fat Tissue, . Technic, Cartilage, Hyaline, Elastic, Fibrous, Technic, Bone Tissue, Technic, Neuroglia, . The Blood, Red Blood Cells, . White Blood Cells, Blood Platelets, . Development, Technic, CHAPTER IV. CHAPTER V. Muscle Tissue, Involuntary Smooth Muscle, Voluntary Striated Muscle, Involuntary Striated Muscle, Development of Muscle Tissue, Technic, .... CHAPTER VI. Nerve Tissue, The Neurone, General Structure, The Cell Body, . The Nucleus, The Cytoplasm, Neurofibrils, Perifibrillar Substance, Chromophilic Bodies, The Dendrites, The Axone, Non-Medullated Axone (Non-Medullated Nerve Fibres), Medullated Axones (Medullated Nerve Fibres), Theories as to Physiology of the Neurone, Neuroglia, ........ Technic, ......... General References, ....... CONTENTS. IX PART IV. — THE ORGANS. CHAPTER I. PAGE The Circulatory System, . . . . . . . .121 The Blood-vessel System, 121 General Structure, 121 Capillaries, .... 121 Arteries, .... 122 Veins, ..... 1 26 Technic, 12S The Heart, .... 129 Technic, 131 Development of the Blood-vessel Sy stem, 132 The Lymph-vessel System, . 132 Lymph Capillaries. *33 Lymph Spaces, 133 Serous Membranes, 133 Technic, .... 133 The Carotid Gland, J 34 The Coccygeal Gland, . 134 Technic, .... J 34 General References on Circulatory System, 134 CHAPTER II. Lymphatic Organs, . . . . . . . . . . 135 The Lymph Nodes, 135 Technic, 138 Hsemolymph Nodes, 139 Technic, 141 The Thymus, 142 Technic, 143 The Tonsils, i44 The Palatine Tonsils, 144 The Lingual Tonsils, M5 The Pharyngeal Tonsils. 146 Technic, 146 The Spleen, 146 Technic, 150 General References, r S ' CHAPTER III. The Skeletal System, ... ... 152 The Bones, ..... 152 Bone Marro'w, 156 Red Marrow, 156 YelloAv Marrow, 158 CONTENTS. Technic, ..... Development of Bone, Intramembranous Development, Intracartilaginous Development, Subperiosteal, Growth of Bone, Technic, The Cartilages, Articulations, Technic, General References, PAGE 160 160 161 163 166 167 168 16S 169 170 170 CHAPTER IV. The Muscular System. A Voluntary Muscle, Tendons, Tendon Sheaths and Bursas, Growth of Muscle, Technic, 171 172 172 173 173 CHAPTER V. Glands and the General Structure of Mucous Membranes . 174 Glands — General Structure and Classification, . . . .174 General Structure of Mucous Membranes, . . . . .178 CHAPTER VI The Digestive System, ........ 179 Anatomical Divisions, ........ 179 The Headgut, ......... 180 The Mouth, 180 The Mucous Membrane of the Mouth, . .•■•'. 180 Glands of the Oral Mucosa, .... 181 Technic, ........ 183 The Tongue, . . . . . . 183 Technic, ........ 186 The Teeth, ........ 187 Development of the Teeth, .... 191 Technic, ........ i93 The Pharynx, ........ 194 Technic, ........ 194 The Foregut, ......... 195 The (Esophagus, ........ J 95 Technic, ........ 196 General Structure of the Walls of the Gastro-intestinal Canal, 196 The Stomach, ........ 198 Technic, ........ 203 CONTENTS. XI The Midgut, The Small Intestine, The Endgut, The Large Intestine, The Rectum, Blood-vessels of the Stomach and Intestines. Lymphatics of the Stomach and Intestine Secretion and the Absorption of Fat, Technic, ..... The Larger Glands of the Digestive System, The Salivary Glands, The Parotid, . The Sublingual, The Submaxillary, Technic, The Pancreas, Technic, The Liver, Excretory Ducts of the Liver, The Gall-bladder, . Technic, Development of the Digestive System, General References, PAGE 204 20-). 2IO 2IO 214 2I 5 217 218 220 221 221 222 225 231 231 237 237 238 238 240 CHAPTER VII. The Respiratory System, . 241 The Nares, ..... • 241 The Larynx, ..... • 243 The Trachea, ..... • 243 Technic, ..... . 246 The Bronchi, ..... . 246 The Lungs, ..... . 248 Development of the Respiratory System, • 254 Technic, ..... • 255 The Thyroid, ..... • 255 The Parathyroids, .... • 256 Technic, ..... • 257 General References, .... • 257 CHAPTER VIII. The Urinary System, The Kidney, The Kidney-Pelvis and Ureter, The Urinary Bladder, . The Adrenal, Technic, General References, 258 258 268 269 270 273 273 Xll CONTENTS. CHAPTER IX. The Reproductive System, Male Organs, The Testis, The Seminal Ducts, The Epididymis, The Vas Deferens, The Seminal Vesicles and Ejaculatory Ducts, . Rudimentary Structures Connected with the Development Genital System, The Spermatozoon, Development of the Spermatozoon, Technic, The Prostate Gland, Cowper's Glands, Technic, The Penis, The Urethra, Technic, Female Organs, . The Ovary, . Rudimentary Structures Genital System, The Oviduct, Technic, The Uterus, The Mucosa of the Resting Uterus, The Mucosa of the Menstruating Uterus, . The Mucosa of the Pregnant Uterus, The Placenta, ..... The Vagina, ...... Development of the Urinary and Reproductive Systems, Technic, ........ General References, ...... Connected with the Development PAGE 274 274 274 280 280 281 282 of the of the CHAPTER X. The Skin and its Appendages, The Skin, Technic, The Nails, . Technic, The Hair, Technic, Development of Skin, Nails, rmd II The Mammary Gland, Technic, General Reference; CONTENTS. XI 11 CHAPTER XI. The Nervous System, Histological Development, Membranes of the Brain and Cord Technic, The Ganglia, Technic, The Peripheral Nerves, Technic, The Spinal Cord, . Technic, General Structure and Practical Study, Origin of the Fibre Tracts of the Cord, The Spinal Ganglion Cell and the Origin of the Posterior Col umns, Afferent Nerve Terminations Ceils Situated in Other Parts of the Central Nervous System which Contribute Axones to the White Matter of the Cord Root Cells — Motor Cells of the Anterior Horn, Column Cells, ...... Cells of Golgi, Type II., . Technic and Practical Study, . . ... Fibre Tracts of the Cord, ..... Ascending Fibre Tracts, ..... Descending Fibre Tracts, .... Fundamental Columns, ..... Technic, ....... The Medulla Oblongata (including the Pons Varolii), General Structure, ...... Technic, ........ Practical Study, ....... Transverse Section through Pyramidal Decussation Transverse Section through Sensory Decussation, Transverse Section through Lower Part of Olivary Nu cleus, ...... Transverse Section through Middle of Olivary Nucleus, Transverse Section through Exit of Cranial Nerve VIII Transverse Section through Exits of Cranial Nerves VI. and VII., Transverse Section through Exit of Cranial Nerve V. The Midbrain, General Structure, Practical Study, Transverse Section through Exit of Cranial Nerve IV. Transverse Section through Exit of Cranial Nerve III. The Cerebral Peduncles, .... The Cerebellum, ...... General Histology of the Cerebellar Cortex, PAGS 336 338 339 339 342 342 344 344 344 345 35° 35o 35 1 356 356 35 7 360 360 362 364 366 368 37 1 37 1 37i 374 3 74 374 376 3 So 382 38S 3S8 392 395 395 395 395 398 400 401 401 CONTENTS. PAGE The Cerebrum, ........ . 406 General Histology of the Cerebral Cortex, . 406 Technic, ........ . 411 The Pituitary Body, ....... . 412 The Pineal Body, ....... • 415 Technic, ........ • 415 General References, ....... • 415 CHAPTER XII. The Organs of Special Sense, The Organ of Vision, The Eyeball, The Optic Nerve, . The Relations of Optic Nerve The Lacrymal Apparatus, The Eyelid, . Development of the Eye, Technic, The Organ of Hearing, The External Ear, The Middle Ear, The Internal Ear, Development of the Ear Technic, The Organ of Smell, Technic, The Organ of Taste, Technic, General References, to Retina and Brain 416 416 416 428 428 433 434 435 43 6 437 437 43^ -139 -148 449 45° + 52 452 453 453 Index, 455 PART I. HISTOLOGICAL TECHNIC. CHAPTER I. GENERAL TECHNIC. Certain body fluids, blood, urine, etc., may be examined by simply placing them on a slide under a cover-glass. A few tissues, e.g., thin membranes, such as the omentum and the mesentery, may be examined fresh in some inert medium, as normal salt solution (0.75 per cent aqueous solution sodium chlorid). For such exam- ination the tissue is immersed in the salt solution on a slide and covered with a cover-glass. Most tissues and organs, however, re- quire much more elaborate preparation to render them suitable for microscopic examination. Tissues too dense and thick to be readily seen through with the microscope must be so treated as to make them transparent. This is accomplished either by pulling the tissue apart into fine shreds, teasing, or by cutting it into thin slices; sec- tion cutting. Some tissues admit of teasing in a fresh condition ; others can be satisfactorily teased only after they have been subjected to the action of a chemical which breaks down the substance holding the tissue elements together, maceration. Fresh tissue can rarely be cut into sections sufficiently thin for microscopic examination. It must first be killed in such a manner as to preserve as nearly as pos- sible the living tissue relations, fixation. If too soft for section cutting it must next be put through a process known as hardening. If, how- ever, as in the case of bone, the tissue is too hard, it must be soft- ened by dissolving out the mineral salts, decalcification. If very thin sections are to be cut, it is further necessary to impregnate the tissue with some fluid substance which will harden in the tissue and give to the mass a firm, even consistency. This is known as embedding. Furthermore, most tissue elements have refractive indices which are so similar that their differentiation is often extremely difficult. To overcome this difficulty, recourse is had to staining the tissue with dyes which have an affinity for certain only of the tissue elements, or 4 HISTOLOGICAL TECH NIC. which stain different elements with different degrees of intensity. This is known as differential or selective staining. The final step in the process is the mounting of the specimen, after which it is ready for microscopic study. Only the more common procedures used in the preparation of tissues for microscopic study are described in this section. At the end of each section are given the technical methods most satisfactory for the demonstration of the tissues described in that section. For other methods the student is referred to special works upon micro- scopic technic. I. Dissociation of Tissue Elements. This is accomplished by (i) teasing, or (2) maceration, or both. (1) Teasing. — This consists in pulling apart fresh or preserved tissues by means of teasing needles. Instructive specimens of such tissues as muscle and nerve may be obtained in this way. (2) Maceration.— This is the subjecting of a tissue to the action of some chemical which breaks down the substance uniting the tissue elements, thus allowing them either to fall apart or to be more easily dissociated by teasing. The most commonly used macerating fluids are : (a) Ranvier s Alcohol (33 per cent, made by adding 35 c.c. of 96-per-cent alcohol to 65 c.c. of water). — Bits of fresh tissue are placed in this fluid for from twenty-four to forty-eight hours. The cells may then be easily separated by shaking or by teasing. Ran- vier's alcohol is an especially satisfactory macerating fluid for epi- thelia. •* (b) Formalin, in very dilute solutions (0.2 to 0.4 per cent). — Tissues should remain in the formalin solution from twenty-four to forty-eight hours. This also is especially useful for dissociating epi- thelial cells. (c) Sodium or Potassium Hydrate (30- to 35-per-cent aqueous solution). — From twenty minutes to an hour is usually sufficient to cause the tissue elements to fall apart or to be readily pulled apart with the teasing needles. If it is at any time desirable to stop the action of the caustic alkali, this may be accomplished by neutralizing with glacial acetic acid or by replacing the alkali with a 60-per-cent aqueous solution of potassium acetate. The specimens may then be preserved in the potassium-acetate solution, in glycerin, or in 50-per- GENERAL TECH NIC. 5 cent alcohol. This dissociating fluid is largely used for muscle cells and fibres. II. Fixation. Fixation is the first step in the preparation of sections of tissues for microscopic study. Its object is to preserve as nearly as possible the structures as they exist in the living tissues. This is accom- plished by means of chemicals in solution, the solution being known as a fixing agent or fixative. Pieces of tissue are immersed in the fixative and allowed to remain there until fixation is complete. The length of time required depends upon the character of the tissue and upon the fixative used. The pieces of tissue should be small, and it is important that large quantities of the fixative be used. Furthermore, it is sometimes necessary to change the fluid after a short time, in order to keep it up to the proper strength. Whole organs and even bodies may be fixed in toto by injecting the fixative through an artery and allowing it to escape through the veins. After the injection, the whole specimen should be placed in a large quantity of the same fixative. This method is used only in cases where it is necessary to preserve the topographic relations of various parts of an organ or a body. Satisfactory fixation is largely dependent upon the freshness of the tissue when placed in the fixa- tive. The following are the fixatives in most common use : (i) Strong Alcohol (96-per-cent). — This is a rapid fixative and should be used on small pieces of tissue. The time required is from six to twenty-four hours, though tissues may remain longer without injury. The alcohol should be changed after two or three hours. This fixative should not be used where fine histological detail is desired, since it causes some shrinkage. One advantage in its use is the fact that tissues are hardened and ready for embedding at the end of fixation. (2) Dilute Alcohol (30-per-cent to 80-per-cent). — This, as a rule, gives unsatisfactory results, causing much shrinkage of the tissue elements. (3) Formalin (2-per-cent to 10-per-cent aqueous solution). — For- malin is rapid in its action and probably has better penetrating qual- ities than any other fixative. For general purposes a 4-per-cent solution (1 part commercial formalin to 9 parts water) should be used, in which fixation is accomplished in from six to twenty-four hours. 6 HISTOLOGICAL TECHNIC. After a few hours the fluid should be changed. The results after formalin are not always among the best, owing to the fact that it has little hardening power, and the subsequent action of alcohol is likely to cause some distortion of the tissues. It acts better when com- bined with other fixatives than when used alone. (See Orth's Fluid.) (4) Muller's Fluid. Potassium bichromate 2.5 gm. Sodium sulphate 1.0 gm. Water 100. o c.c. This fluid gives very good results, but is extremely slow in its action, requiring from a week to several months. Fairly large pieces of tis- sue may be fixed, but in all cases large quantities of the fixative should be used and frequently renewed. (5) Formalin- Mullcr s Fluid {Ortli s Fluid). Midler's fluid (double strength]. Formalin, S-per-cent > Equal parts This is one of the best general fixatives. The action is similar to that of Miiller's fluid but much more rapid, fixation being accom- plished in from twenty-four to forty-eight hours, though specimens may remain in the fluid several days without disadvantage. Fairly large pieces of tissue may be fixed with good results. The fixative should be changed after a few hours. Fixation with Orth's fluid gives an excellent basis for a haematoxylin-eosin stain (see (1), p. 17). The fixative should always be freshly prepared. A convenient way is to keep the 8-per-cent formalin solution and the double-strength Miiller's fluid in stock. Orth's fluid is then prepared by simply tak- ing equal parts of each. (6) Osmic Acid. — This, in a i-per-cent aqueous solution, is a quick fixative of poor penetrating power. Very small pieces of tissue must therefore be used. They should remain in the fluid from twelve to twenty-four hours. Osmic acid stains fat and myelin black and is consequently useful in demonstrating their presence in tissues. Fixation should take place in the dark. (7) Flemming 1 s Fluid. Chromic acid, i-per-cent aqueous solution 25 c.c. Osmic acid, i-per-cent aqueous solution 10 c.c. Glacial acetic acid, i-per-cent aqueous solution 10 c.c. Water 55 c.c. GENERAL TECH NIC. 7 Flemming's fluid is one of the best fixatives for nuclear struct- ures, and is of especial value in demonstrating mitotic figures. Very small pieces of tissue should be placed in the fixative, where they remain for from twenty-four hours to three days. The solution should be freshly made as required, or a stock solution without the osmic acid may be kept and the latter added at the time of using. (8) Mercuric Chlorid. — This may be used either in saturated aqueous solution or in saturated solution in 075-per-cent salt solu- tion. Fixation is complete in from twelve to twenty-four hours, and is usually very satisfactory. A saturated solution of mercuric chlorid in 5-per-cent aqueous solution of glacial acetic acid also gives good results. (c) Zenker s Fluid. Potassium bichromate 2.5 gm. Sodium sulphate 10 gm. Mercuric chlorid 5-° S m - Glacial acetic acid 5.0 c.c. Water 100.0 c.c. This fluid should be freshly made, or the salts may be kept in solu- tion and the acetic acid added at time of using. Zenker's fluid is a good general fixative, but usually causes some shrinkage of the tissue elements. Fixation requires from six to twenty-four hours. The most serious drawback to Zenker's fluid is the fact that the mercuric chlorid sometimes produces dark irregular precipitates in the tissues. This may be remedied, however, by the use of iodine and iodide of potassium in the hardening process (see Hardening, p. 8). (10) Picric acid is an excellent fixative for cytoplasm. It may be used in : (a) Saturated aqueous solution ; (b) saturated solution of picric acid in i-per-cent aqueous solution of acetic acid; (e) saturated solution of picric acid in 2-per-cent aqueous solution of sulphuric acid. III. Hardening. Most fixatives are also hardening agents if their action is pro- longed. This is, however, often detrimental. It is, therefore, cus- tomary, after fixation is complete, to carry the specimens, with or without washing, through successively stronger grades of alcohol for the purpose of hardening the tissues. For general histological pur- 8 HISTOLOGICAL TECHNIC. poses the specimens may be transferred directly to 70-per-cent or 80-per-cent alcohol, which should be changed once or twice. In the case of delicate tissues the first grade of alcohol should be 40 per cent or 50 per cent, the second 70 per cent, and the third 80 per cent. The specimens should remain in each grade from twelve to twenty-four hours. Washing the tissues after fixation is not a matter of indifference. In some cases water should be used, while in other cases water is liable to undo the action of the fixative, in which cases alcohol must be used for washing. After fixation in alcohol no washing, of course, is necessary. Specimens fixed in strong alcohol are embedded immediately (see Embedding, p. 10), or preserved (see Preserving, p. 8). After fixa- tion in dilute alcohol the specimens are passed through the graded alcohols up to 80-per-cent. After fixation in plain formalin the specimens are passed directly through the graded alcohols without washing in water. Specimens fixed in any solution containing picric acid should not be washed in water, but passed directly through the alcohols ; and it is usually necessary to change each grade in order to wash out the picric acid. Specimens fixed in osmic acid or any solution containing osmic acid should be washed in running water before being passed through the graded alcohols. After solutions containing potassium bichro- mate the specimens should be washed in water sufficiently to remove the excess of bichromate, though too prolonged washing seems to be detrimental. A precipitate forms in the alcohols, but this appar- ently does no harm. After mercuric chlorid or Zenker fixation the washing may be done either in water or in alcohol. To avoid precipitates in the tis- sues add a small quantity of an iodine solution (equal parts tincture iodine and 10-per-cent aqueous solution potassium iodid) to any of the grades of alcohol. As the alcohol becomes clear more of the solution is added until the alcohol remains slightly tinged. IV. Preserving. Hardened tissues are usually preserved in 80-per-cent alcohol. Formalin in aqueous solutions of 1 per cent to 10 per cent is also used as a preservative. In either case, when it is necessary to pre- serve the specimens for a considerable length of time (several months GENERAL TECHNIC. 9 or longer), the tissues are likely to lose their staining qualities to a certain extent. Preserving the specimens in equal parts of strong alcohol, glycerin, and distilled water is successful as a partial remedy for this. V. Decalcifying. Tissues containing lime salts, like bones and teeth, must have the lime salts dissolved out before sections can be cut. This process is known as decalcification. Tissues to be decalcified must be first fixed and hardened. Fix- ation in Orth's fluid and hardening in graded alcohols give good results. After hardening, the tissue is washed in water and placed in one of the following decalcifying fluids. The quantity of fluid should always be large and the fluid frequently changed. The com- pletion of decalcification can be determined by passing a needle through the specimen or by cutting it with a scalpel. The time required varies with the size and hardness of the specimen and the decalcifying fluid used. (1) Hydrochloric Acid. — This may be used in aqueous solutions of from 0.5 per cent to 5 per cent. A very satisfactory decalcifying mixture is that known as Ebner's hydrochloric-salt solution. It con- sists of : Sodium chlorid, saturated aqueous solution 1 part. Water 2 parts. Hydrochloric acid, sufficient to make a from 2-per-cent to 5-per- cent solution. The addition of the salt prevents swelling of the tissue. This fluid is slow in acting and should be changed every day. When decalcifi- cation is complete, the specimen is washed in sufficient changes of water to remove all trace of acid. This may be quickly accomplished by the addition of a little ammonium hydrate to the water. The specimen is then carried through graded alcohols. (2) Nitric Acid. — This is less used than the preceding. The strength should be from i-per-cent to 10-per-cent aqueous solution. Weak solutions (1 per cent to 2 per cent) will decalcify small fcetal bones in from three to twelve days. For larger bones stronger solu- tions and longer time are required. (3) Small bones may be satisfactorily decalcified in Zenker s fluid (see fixatives, page 7), or in the following: io HISTOLOGICAL TECHNIC. Picric acid i part. Chromic acid i part. Glacial acetic acid 5 parts. VI. Embedding. Most hardened tissues are still not firm enough to be cut into the thin sections suitable for microscopic study. In order to support the tissue elements and render them more firm for section cutting, recourse is had to embedding. This consists in impregnating the tissues with some substance which is liquid when the tissues are placed in it, but which can be made to solidify throughout the tis- sues. In this way the tissue elements are held firmly in place. The embedding substances most used are celloidin and paraffin. Celloidin Embedding. (1 ) Alcohol-Ether Celloidin. — Two solutions should be made. Solution No. 2. TJiick celloidin — a 5-per-cent solution of cel- loidin in equal parts 96-per-cent alcohol and ether. Solution No. I . Thin celloidin — made by diluting solution No. 2 with an equal volume of equal parts of alcohol and ether. The hardened tissues are placed successively in : Strong alcohol (96-per-cent) twelve to twenty-four hours, to dehy- drate. Equal parts alcohol and ether, twelve to twenty-four hours. Thin celloidin, twenty-four hours to several days. Thick celloidin, twenty-four hours or longer. . The exact time tissues should remain in the different grades of celloidin depends upon the character of the tissue, the size of the specimen, and the thinness of section desired. Many tissues may be advantageously left for weeks in thin celloidin. The specimen must now be " blocked " and the celloidin hardened. By blocking is meant fastening the specimen impregnated with celloidin to a block of wood or other suitable material which may be clamped in the microtome (see Section Cutting, p. 13). The speci- men may be taken from the thick celloidin, considerable of the latter adhering to the specimen, quickly pressed upon a block of wood or vulcanized fibre, allowed to harden five to ten minutes in air, and then immersed in 80-per-cent alcohol. The alcohol gives an even harden- ing of the celloidin, attaching the specimen firmly to the block. GEXERAL TECHXIC. n Another method, and one by which very even-shaped blocks may be obtained, is to place the specimen from the thick celloidin into a little paper box (made by folding paper over a wooden block), slightly larger than the specimen, and covering with thick celloidin. The celloidin should dry slowly under a bell -jar for from two to twelve hours, according to the amount of celloidin, after which it should be immersed in 8o-per-cent alcohol and the paper pulled off. Such a block may be cut into any desired shape. It is attached to the wooden or vulcanized block by dipping for a moment in thick cel- loidin, and then pressing firmly down upon the block. After five to ten minutes' drying in air it is transferred to 8o-per-cent alcohol. Old, hard, celloidin-embedded specimens are sometimes difficult to attach to blocks. This usually may be accomplished by first thor- oughly drying the specimen and then dipping it in equal parts alco- hol and ether for a few minutes. This softens the surface of the cel- loidin, after which the specimen is dipped in thick celloidin and blocked. Embedded or blocked specimens can be kept in 80-per- cent alcohol. After several months, however, the celloidin is likely to become too soft for good section cutting. In that case the speci- mens can be readily re-embedded by dissolving out the old celloidin with alcohol and ether and putting them again through the regular embedding process. (2) Clove-oil Celloidin.- — A more rapid impregnation of the tissue may be obtained by means of what is known as clove-oil cel- loidin. Celloidin 30 gm. Clove oil 100 c.c. Ether. . . 400 c.c. Alcohol, absolute 20 c.c. The celloidin is first placed in a jar and the clove oil and ether added. From two to four days are required for solution of the cel- loidin. During this time the jar should be shaken several times. After the celloidin is dissolved the absolute alcohol is added and the solution is ready for use. The specimen must be thoroughly dehydrated, placed in alcohol and ether or pure ether for a few hours, and then transferred to the clove-oil celloidin. From six to twelve hours is sufficient to impreg- nate small pieces of tissue. The tissue is now taken from the cel- loidin, placed directly upon a wooden or vulcanized block, and im- 12 HISTOLOGICAL TECHXIC. mersed in chloroform. The celloidin hardens in about an hour, and is then ready for sectioning. The specimen is very firm, and very thin sections can be cut. A disadvantage in clove-oil celloidin is that neither the blocks nor the sections can be kept permanently in alcohol, as can those embedded in alcohol-ether celloidin. They may, however, be kept for several weeks in pure chloroform. Paraffin Embedding. For paraffin embedding a thermostat or paraffin oven is necessary in order that a constant temperature may be maintained. The tem- perature should be about 5 6° C. Pure paraffin, the melting point of which is from 50 to 55 C, is used. In very warm weather it may be necessary to add to this a little paraffin the melting point of which is 62 C. The hardened tissue is first put in 96-per-cent alcohol for from twelve to twenty-four hours, and then completely dehydrated by put- ting in absolute alcohol for the same length of time, or less for small specimens. It is then transferred to some solvent of paraffin. Some of the solvents used are xylol, oil of cedarwood, chloroform, and to- luol. Of these the best are perhaps xylol and oil of cedarwood. The tissue should remain in either of these for several hours, or until the tissue becomes more or less transparent. It is then placed in melted paraffin, in the paraffin oven, for from one to three hours, according to the size of the specimen and its density. This allows the tissues to become impregnated with the melted paraffin. It is best to change the paraffin once or twice. In case of very delicate tissues it is well to transfer them from the absolute alcohol to a mixture of equal parts absolute alcohol and xylol for a short time before putting them into the pure xylol. In the same way a mixture of equal parts xylol and paraffin may be used before putting the tissues into pure paraffin. For hardening the paraffin in and around the tissue a very con- venient apparatus consists of a plate of glass and several L-shaped pieces of iron or lead. Two of these are placed on the glass plate in such a manner as to enclose a space of the desired size. Into this are placed the specimen and sufficient melted paraffin to cover it. Both glass and irons should be smeared with glycerin to prevent the GENERAL TECHNIC 13 paraffin from adhering, and should be as cold as possible, so that the paraffin may harden quickly. The same paper boxes described under celloidin embedding may also be used for paraffin. Another good method for small pieces of tissue is to place the specimen in paraffin in an ordinary watch-glass which has been coated with glycerin. Both paper-box and watch-glass specimens are immersed in cold water as soon as the surface of the paraffin has become hard. After the paraffin has hardened any excess may be cut away with a knife. Paraffin-embedded specimens may be kept indefinitely in air. For section- cutting, the block of paraffin is attached to a block of wood or of vulcanite or to the metallic block-holder of the micro- tome. This is done by heating one surface of the paraffin until it becomes soft and then pressing this side down firmly upon the block. VII. Section Cutting. The older method of making free-hand sections with a razor has been almost completely superseded by the use of a cutting instru- ment known as the microtome. This consists essentially of a clamp for holding the specimen and a microtome knife or razor. The two are so arranged that when knife and specimen meet, a section of any desired thickness may be cut. The technic of section cutting differs according to whether the specimen is embedded in celloidin or in paraffin. In cutting celloidin sections the knife is so adjusted that it passes obliquely through the specimen, as much as possible of the cutting edge being used. The knife is kept flooded with 80-per-cent alco- hol and the specimens are removed by means of a camel's-hair brush to a dish of 80-per-cent alcohol where they may be kept for some time if desired. When colloidin sections tear or when very thin sec- tions are desired, it is often of advantage to paint the surface of the block after cutting each section with a coat of very thin celloidin. Celloidin sections are usually not thinner than 10 //., although under favorable conditions sections 5 p. 1 or even 3 ,) Rapid Method. — Small pieces of tissue, 2 to 4 mm. thick, are placed in the following solution for from two to six days, the time depending upon the age and character of the tissue, the tem- perature at which fixation is carried on, and the elements which it is desired to impregnate : 30 HISTOLOGICAL TECH NIC. Osmic acid, i-per-cent aqueous solution i part. Potassium bichromate, 3.5-per-cent aqueous solution ... 4 parts. As a rule, the longer the hardening the fewer are the elements stained, but these few are clearer. The tissue is next transferred to silver nitrate as in the slow method. Pieces of tissue should be tried each day until a satisfactory result is obtained. The pieces may be kept in silver nitrate some time, but not in alcohol, and are better cut without embedding, the pieces being simply washed in 95 -per- cent alcohol several hours, then gummed to the block with celloidin, cut in 95-per-cent alcohol, and mounted as in the slow method. (e) Mixed Method. — Specimens are placed in the bichromate solution for about four days, then from one to three days in the osmium-bichromate mixture (see Rapid Method), after which they are transferred to the silver solution (see Slow Method). (a) Formalin-Bichromate Metliod. — Tissues are placed for from two to six days in the following solution : Formalin 10 to 20 parts. Potassium bichromate, 3-per-cent aqueous solu- tion 90 to 80 parts. Subsequent treatment with silver is the same as in the previously described method. The results resemble those of the slow method. The specimens may be kept in strong alcohol. The method is satis- factory only for the adult cerebrum and cerebellum. (2) Golgi Bichlorid Method. — Material, which need not be cut into small pieces, remains for several months in the potas- sium-bichromate solution (see Slow Silver Method), after which it is transferred to a o. 25-per-cent to i-per-cent aqueous solution of mer- curic chlorid for from four to twelve months or longer, the solution being changed as often as discolored. The degree of impregna- tion must be determined by frequently testing the material, but is usually indicated by the appearance of small white spots on the surface of the tissue. A modification of the bichlorid method, known as the Cox-Golgi Method, often gives good results. The following fixing solution is used : Potassium bichromate, 5-per-cent aqueous solution. ... 20 parts. Mercuric chlorid, 5-per-cent aqueous solution 20 parts. Distilled water 4° parts. Aiter mixing the above, add SPECIAL NEUROLOGICAL STAINING METHODS. 31 Potassium chromate, 5-per-cent aqueous solution. .... 16 parts. Tissues remain in this fluid for from two to five months. In the Golgi silver methods the result of the treatment first with bichromate and then with silver nitrate, is that a precipitate is formed in the tissue, a chromate or some other silver salt, which in favorable cases is largely confined to certain of the nerve cells and their processes. It must be remembered that only a few of the cells and processes are stained, these often only partially, and that other irregular precipitations are usually present. In the mercury methods, the bichromate of potassium and the bichlorid of mercury may be used combined in the same solution. There are other mod- ifications of the Golgi methods, in which similar precipitates of other metallic salts are secured. Golgi specimens should be dehydrated and embedded as rapidly as possible. This is especially true of specimens treated by the rapid and the mixed methods. Those treated by the slow silver method and by the bichlorid method are more permanent, and more time may be taken with their dehydration. Sections should be cut thick (75 to 100 //) and mounted in xylol-balsam. After the rapid method, it is safer to mount without a cover-glass ; after the slow method, specimens may be mounted with or without a cover. The balsam should be hard, and melted at the time of using (see Mounting, page 20). Cajal's Methods for Staining the Neurofibrils in the Nerve-Cells. In these methods, besides the neurofibrils, the cell processes and especially the axis cylinders are often beautifully displayed, the stain giving a picture in this respect much more general than that of the Golgi methods, but much more specific, and sharper than that of the ordinary stains. The methods consist mainly of two steps : (1) The staining of the tissue in a solution of silver nitrate; (2) the further reduction of the silver stain with a weak photographic developer. Three methods, or variations, are here given : (1) Pieces about 0.5 cm. thick are placed in a liberal quantity of from i-per-cent or 1. 5-per-cent (new-born or embryonic mammalian material) to 5-per-cent (adult material) solution of silver nitrate and kept at a temperature of 32 to 40 C. for 2 to 5 days. When 2,2 HISTOLOGICAL TLCHNIC. ■ properly stained (shown by a yellowish or light brown coloiation of freshly cut surfaces) the pieces are very briefly rinsed in distilled water and placed in : pyrogallol (or hydroquinone) I gram, distilled water ioo c.c, formalin 5-10 c.c, for twenty-four hours or more. They are then washed a few minutes in water and transferred to 95- per-cent alcohol, which is changed when discolored, and where they may often be kept for some time without injury. They may then be embedded in celloidinor paraffin and sections cut, usually 15-25 p. in thickness. Different depths of the blocks of tissue usually vary in stain, the most favorable being intermediate between the surface and centre of the block. Celloidin sections usually keep well in 95 -per- cent alcohol. They may be cleared in carbol-xylol, rinsed in xylol, and mounted in xylol-balsam or xylol-damar in the usual way. In delicate objects (study of pathological changes in neurofibrils) it may be best to abbreviate the dehydration, and block and cut without infiltration with celloidin. (2) Pieces are first placed in 95-per-cent alcohol or in absolute alcohol (32 ° to 40 C.) for twenty-four hours. For neurofibrils it is better to add from 0.25 c.c. to 1 c.c. of ammonium hydrate to each 100 c.c. of the alcohol. They are then treated with silver nitrate i-per-cent or i^-per-cent, as in (1). This method gives better pict- ures of the cell processes and axis cylinders and a better fixation of the cells. (3) Pieces are first placed in distilled water 100 c.c, formalin 20 c.c, ammonia 1 c.c, for twenty-four hours at 32°-40° C. , washed in water twelve to twenty-four hours, and then treated with silver nitrate i-per-cent or 1.5-per-cent, as in (1). This method gives pictures of the terminations of nerve fibres on the periphery of nerve- cells and their dendrites (end-feet or end-buttons of Auerbach). In general it is best to avoid, in the above methods, any excessive exposure to the light while the pieces are in the silver bath (espe- cially when the pieces are very small), though they may be brought into the light for examination and while being transferred to the reducing fluid. Nissl's Method. This method is useful for studying the internal structure of the nerve cell. It depends upon a rapid fixation of the tissue, its subse- quent staining with an aniline dye, and final decolorization in alcohol. SPECIAL NEUROLOGICAL STAINING METHODS. 33 The aniline dye most commonly used is methylene blue. There are many variations and modifications of Nissl's method. The fol- lowing is simple and gives uniformly good results : Specimens are first fixed in mercuric-chlorid solution (page 7), in formalin (10-per-cent aqueous solution), or in absolute alcohol, and embedded in celloidin. Thin sections are stained in a i-per-cent aqueous solution of pure methylene blue (Grubler). The sections are gently warmed in the solution until steam begins to be given off. They are then washed in water and differentiated in strong alcohol. The degree of decolorization which gives the best results can be learned only by practice. Several alcohols must be used, and the last alcohol must be perfectly free from methylene blue. The sections are cleared in equal parts xylol and cajeput oil and mounted in xylol-balsam. A contrast stain may be obtained by having a little eosin or erythrosin in the last alcohol. General References for Further Study of Technic. Lee: The Microtomist's Vade-mecum. Mallory and Wright: Pathological Technique. Freeborn : Histological Technic. Reference Handbook of Medical Sciences, vol. iv. PART II. THE CELL. CHAPTER I. THE CELL. In the simplest forms of animal life the entire body consists of a little albuminous structure, the essential peculiarity of which is that it possesses properties which we recognize as characteristic of living organisms. This albuminous material basis of life is known as, pro- toplasm, while the structure itself is known as a cell. Within the cell membrane metaplasm i granules ( karyosome or 1 net-knob f hyaloplasm spongioplasm linin network nucleoplasm ... attraction-sphere --- centrosome ;.-> plastids ----- chromatin network — nuclear membrane ""•- nucleolus — vacuole FlG. i.— Diagram of a Typical Cell. cell is usually found a specially formed part, the nucleus. Peripher- ally some cells are limited by a distinct cell wall ox cell membrane. An actively multiplying cell contains a minute structure associated with the reproductive function and known as the centrosome. A typical cell thus consists of the following structures (Fig. i) : (i) The cell body; (2) the cell membrane; (3) the nucleus ; (4) the centrosome. Of these the cell body is the only one present in all cells. Most animal cells have no cell membrane. A few cells con- tain, in their fully developed condition, no nuclei. In many mature cells it is impossible to distinguish a centrosome. All plants and animals consist of cells and their derivatives, and if an attempt be made to resolve any of the more complex living" 3§ THE CELL. structures into its component elements, it is found that the smallest possible subdivision still compatible with life is the cell. The cell may therefore be considered as the histological clement or unit of structure. i. The Cell Body (protoplasm — cytoplasm). — This is a semi-fluid substance of complex chemical composition, belonging to the general class of albumins. It contains a peculiar nitrogenous ^roteid, pi as tin. Many theories have been advanced as to the ultimate structure of protoplasm (Fig. 2). The earliest theory, that protoplasm is homo- a geneous, having no definite structure, has been quite gen- erally discarded. Other investigators con- sider protoplasm as made up of (1) a fibrillar element, either in the form of a network of anasto- mosing fibrils (cytoreticulum) /' or of a feltwork of independent fibrils (filar mass or miton), and (2) a fluid or semi-fluid sub- stance which fills in the meshes of the reticulum or separates the fibrils (interfilar mass or para- myon) (Fig. 2, a). Altmann's granule theory considers protoplasm as com- posed of fine granules embedded in a gelatinous intergranular substance. Altmann believed these granules the ultimate vi- tal elements, and for this reason gave them the name of bioblasts (Fig. 2, b). According to Butchli, protoplasm is a foam or emulsion, the appearance of a reticulum being due to the fact that each little foam space forms a complete cavity filled with fluid, the cut walls of these spaces giving a reticular appearance on section (Fig. 2, c). Protoplasm varies somewhat both as to structure and as to chem- ical composition in different cells. It is thus probably best consid- ered as the material basis of cell activity rather than as a substance having fixed and definite chemical and morphological characteristics. The formed element of protoplasm, whether reticular or fibrillar in structure, is known as spongioplasm, the homogeneous element as hyaloplasm (Fig. 1). Peculiar bodies known as plastids (Fig. 1) are Fig. 2.— Diagram Illustrating- Theories of Proto- plasmic Structure, a, Fibrillar theory ; b, granule theory ; c; " foam " theory. (The gen- eral structure of cell body and nucleus corre- sponds.) THE CELL. 39 of frequent occurrence in vegetable cells, and are also found in some animal cells. They are apparently to be regarded as a differentiation of the cytoplasm, but possess a remarkable degree of independence, being capable of subdivision and in some cases of existence outside of the cell. Fat droplets, pigment granules, excretory substances, etc., may be present in cell protoplasm. These bodies represent for the most part either food elements in process of being built up into the proto- plasm of the cell or waste products of cellular activity. To such protoplasmic "inclusions" the terms deiLtoplasm, paraplasm, meta- plasia, have been applied (Fig. i). When the protoplasm of a cell can be differentiated into a cen- tral granular area and a peripheral clear area, the former is known as cndoplasm, the latter as exoplasm. When the exoplasm forms a dis- tinct limiting layer, but blends imperceptibly with the rest of the protoplasm, it is known as the crusta. 2. The Cell Membrane (Fig. i). — This is present in but few ani- mal cells, and is a modification of the peripheral part of the protoplasm. When a membrane surrounds the cell, it is known as the pellicula ; when cells lie upon the surface, and only the free surface of the cells is covered by a membrane, it is known as the culicula. 3. The Nucleus (Fig. 1). — This is a vesicular "body embedded in the cytoplasm. Its size and shape usually correspond somewhat to the size and shape of the cell. Considered by earlier cytologists an unessential part of the cell, the nucleus is now known to be most intimately associated with cellular activities. It is not only essen- tial to the carrying on of the ordinary metabolic processes of the cell, but is an active agent in the phenomena of mitosis, which in most cases determine cell reproduction. As a rule each cell contains a single nucleus. Some cells con- tain more than one nucleus, as, e.g., such multinuclear cells as are found in marrow and in developing bone. Some cells, such as the human red blood cell and the respiratory epithelium, are, in their mature condition, non -nucleated. All non-nucleated cells, however, contained nuclei in the earlier- stages of their development. Non- nucleated cells, while capable of performing certain functions, are wholly incapable of proliferation. The non-nucleated condition must therefore be regarded as not only a condition of maturity, but of actual senility, at least so far as reproductive powers are concerned. 4 o THE CELL. Chemically the nucleus is extremely complex, being composed of the proteids nuclein, paranuclein, linin, nuclear fluid, and lantanin. Morphologically also the nucleus is complex, much of the struct- ural differentiation being determined by the staining reactions of the different elements when treated with certain aniline dyes. The nuclear structures and their relations to the chemical constituents of the nucleus are as follows r 1 (a) The nuclear membrane {amphipyreniii). This forms a limit- ing membrane separating the nucleus from the cell protoplasm. It is wanting in some nuclei. (b) The intranuclear network, or nucleoreticidum, consists of a chromatic element {iiuclein or chromatin) and of an achromatic ele- ment {linin). At nodal points of the network there are often con- siderable accumulations of chromatin. These nodal points, at first thought to be nucleoli, are now known as false nucleoli, or karyosomes. Instead of a distinct network there may be disconnected threads or simply granules of chromatin. Chromatin is the most characteristic of the chemical constituents of the nucleus and the only one which contains phosphoric acid. Within the linin, fine granules occur (lan- tanin). These are differentiated from chromatin by the fact that they are most susceptible to acid dyes, while chromatin takes basic dyes. {c) The n ucleol us or plasmqsome [paranuclein ,py renin) is a small spherical body within the nucleus. It stains intensely with basic dyes. Its function is unknown. (d) Nucleoplasm {karyoplasm, nuclear fluid, nuclear sap). This is the fluid or semi-fluid material which fills in the meshes of the nucleoreticulum. While the nucleus is a perfectly distinct structure and is usually separated by a membrane from the rest of the cell, a marked simi- larity exists between the structure of nucleoplasm and cytoplasm. This similarity is emphasized by the absence in some resting cells of any nuclear membrane, by the apparent direct continuity in some cases of nucleoreticulum and cytoreticulum, and by the continuity of nucleoplasm and cytoplasm in all cells during cell division. 4. The centrosome (Fig. 1) is a small spherical body found some- times in the nucleus, or more commonly in the cytoplasm near the nucleus. Surrounding the centrosome there is usually an area of fine radiating fibrils, the centrosphcre {attraction sphere, protoplasmic ra- 1 Bracketed words refer to chemical substance of which structure is composed. THE CELL. 41 diation, arcJwplasvi) . The main significance of the centre-some is in connection with cell division, under which head it will be further considered (page 44). Vital Properties of Cells. It has already been noted that the essential peculiarity of the cell is that it possesses certain properties which are characteristic of life. By this is meant that a cell is able : 1 . To nourish itself and to grow — metabolism. —2— -To-do ■ work- — -function. 3. To respond to stimula- tion — irritability. 4. To move — motion. 5. To produce other cells — reproduction. In single-cell animals such as the amoeba, all of these functions are performed by the single cell. In all higher multicellular animals, there is a morphological differentiation of cells corresponding to a physiological division of function. 1. Metabolism. — This term is used to designate those cellular activities which have to do with the nutrition of the cell. A cell is able (1) to take up from without substances suitable for its nutrition and to transform these into its own peculiar structure, and (2) to dis- pose of the waste products of intracellular activities. The former is known as constructive metabolism or anabolism, the latter as destruc- tive metabolism or katabolism. 2. Function. — This is the special work which it is the part of the cell to perform. It varies greatly for different cells. Some cells, as, e.g., the surface cells of the skin, appear to act mainly as protec- tion for more delicate underlying structures. Other cells — gland cells — in addition to maintaining their own nutrition, produce specific substances (secretions'), which are of great importance to the body as a whole. Still other cells, e.g., nerve cells and muscle cells, have the power to store up their food substances in such a way as to make them available in the form of energy. This appears to be accom- plished by the building up within the cell of highly complex and, con- sequently, unstable molecular combinations. By reduction of these un- stable combinations, molecules of greater stability and less complexity are formed. This results in the transformation of potential into kinet- ic energy, and the expenditure of this energy is expressed in function. 3. Irritability is that property which enables a cell to respond to external stimuli. Cells vary in respect to their irritability, the most markedly irritable cells in higher animals being those of the neuro- 42 THE CELL. muscular mechanism. Stimulation may be mechanical, electrical, thermal, chemical, etc. The response of the cell to certain forms of chemical stimulation is known as chemotaxis. Some substances attract cells (positive chemotaxis) ; others repel cells (negative chemo- taxis). Stimuli other than chemical possess similar properties, as : ) Fig. 3. — Amoeboid Movement. Successive changes in shape and position of fresh-water amoeba. indicated by the terms thermotaxis, galvanotaxis, etc. Some cells are so specialized as to react only to certain kinds of stimulation, e.g., the retinal cells only to light stimuli. 4. Motion. — This is dependent wholly upon the protoplasm of the cell, and is exhibited in several somewhat different forms. (a) Amoeboid movement. This consists in the pushing outward by the cell of processes (pseudopodia). These may be retracted or may draw the cell after them. In this way the cell may change both its shape and position (Fig. 3). (b) Protoplasmic movement. This occurs wholly within the limits of the cell, changing neither its shape nor position. It occurs in both plant and animal cells, and consists of a sort of circulation or "streaming" of the protoplasm. It is evidenced by the move- ment of minute granules present in the protoplasm, by changes in the position of the nucleus, etc. ( These daughter stars are at first connected by the fibrils of the achro- matic spindle. In this stage may also occur beginning division of the cell body. (d) Telophase (Fig. 5, H). — This is marked by division of the cell protoplasm and consists of a cycle of changes, by means of which each group of daughter chromosomes is transformed into the chromatin network of a resting nucleus. These changes are the same as those described in the prophase, but occur in the reverse order, the chromo- somes uniting to form the spireme, and the spireme becoming trans- formed into the nuclear network. The result is the formation of two daughter cells. The nuclear membrane reappears, as does also the nucleolus. Each daughter cell is thus provided with a rest- ing nucleus. It is through the above-described process of cell-division that the vast number of cells which make up the adult body are developed from one original cell — the ovum. Such powers of evolution are not, however, inherent in the ovum itself, but are acquired only after its union with germinal elements from the male. This union of male and female germinal elements is known as fertilization of the ovum. Fertilization of the Ovum. Prior to and in preparation for fertilization, both male and female cells must pass through certain changes. These are known as matu- ration of the spermatozoon on the male side and of the ovum on the female. The spermatozoon (Fig. 6) is developed from a cell of the seminifer- ous tubule of the testis. The nucleus of this cell so divides its chro- mosomes that each spermatozoon contains just one-half the number of chromosomes characteristic of cells of the species. These are con- tained in the head of the spermatozoon, which thus represents the nucleus of the male sexual cell, the middle piece being the ccntro- some, the tail piece the remains of the protoplasm. The nucleus of the ovum or germinal vesicle also passes through a series of changes by which it loses one-half its chromosomes. The germinal vesicle or nucleus of the ovum first undergoes mi- totic division with the usual- longitudinal cleavage of its chromosomes THE CELL. 47 and the formation of two daughter nuclei. One of these and its cen- trosome are extruded from the cell as the first polar body. The re- maining nucleus and centrosome again divide mitotically only in this second division, instead of the usual longitudinal cleavage of chro- mosomes, by which each daughter nucleus is provided with the same number of chromosomes as the mother nucleus : the chromosomes simply separate, one-half going to each daughter nucleus. One of the daughter nuclei and its cen- trosome is now extruded as the second polar body. The polar bodies ultimately disappear, as does also the centrosome remaining within the egg. This leaves in the now matured ovum a single nucleus, which is known as the female pronucleus, and which contains one-half the number of chromosomes characteristic of cells of the species. During this process in some animals — in others after its completion — the spermatozoon enters the ovum, losing its now useless tailpiece. The head of the spermatozoon becomes the male pronucleus, while the middle piece becomes a centrosome. The chromatin of the male next becomes ar- ranged as chromosomes. Male and female pro- nuclei now lose their limiting membranes and approach each other, their chromosomes intermin- gling. As each pronucleus contained onc-Jialf the number, the monaster thus formed contains the full number of chromosomes characteristic of the species. Meanwhile the male centrosome, formed from the body of the spermatozoon, divides into two daugliter ccntrosomes. These with their radiating fibrils have the same arrangement relative to the monaster of mingled male and female chromosomes, already described under mitosis. By longitudinal cleavage of these chromosomes, as in ordinary mitosis, two sets of daughter chromosomes are formed. Each set passes along the filaments of the achromatic spindle to its centrosome. Thus is formed the diastcr. By continuation of the mitotic process two new nuclei are formed, each nucleus containing the number of chromo- somes characteristic of the species, and each being made up equally of male and female chromosome elements. Thus occurs the first Fig. 6. — Human Spermatozoa. (Af- ter Re t zi u s . ) /, Head seen on flat ; 2, head seen on edge ; k, head ; m, body ; f, tail ; e, end piece. 48 THE CELL. division of the fertilized ovum into two daughter cells. By similar mitotic processes these two cells become four, the four cells become eight, etc. This is known as segmentation of the ovum. The earlier generations of these cells are morphologically alike and membrane of ovum nucleus of ovum entering sper- matozoon. protoplasm of ovum with deutoplasm granules -- female pronucleus male pronucleus female pronu- cleus head of sper- matozoonwith centrosome centrosome chromosome of female pronucleus chromosome of male pronucleus centrosome chromosome from female pronucleus chromosome from male pronucleus —•centrosome Fig. 7.— Diagram of Fertilization of the Ovum. (The somatic number of chromosomes being four.) (From Bohm and von Davidoff, after Boveri.) 1, Ovum surrounded by spermatoza, only one of which is in the act of penetration. Tow- ard the latter the protoplasm of the ovum sends out a process; 2, Head of spermato- zoon has entered ovum, its body becoming the male centrosome, its tail having disap- peared ; 3, The head of spermatozoon has become the male pronucleus. Male and female pronuclei approach each other. Between them is the (male) centrosome ; 4, The spiremes of male and female pronuclei have each formed two chromosomes. The centrosome has divided ; 5, Male and female chromosomes have mingled and by longitudinal cleavage (see Mitosis, p. 39) have become eight. These become arranged in the equatorial plane of the ovum. Mantle fibres extend from centrosomes to chromosomes; 6, Division of the ovum ; two daughter cells, each containing a daughter nucleus. Each daughter nucleus contains four chromosomes, two derived from each pronucleus. THE CELL. 49 are known as blastomcrcs. Soon, however, these cells become spread out and at the same time separated into two primary germ layers. The outer of these is known as the ectoderm or epiblast, the inner as SEGMENTA- TION CAVITY. Fig. 8.— Segmentation of the Ovum. (From Gerrish, after van Beneden.) a, Two-cell stage resulting from first division of fertilized ovum ; b, four-cell stage; c, if, e, later stages. A, Differentiation into inner and outer cells; B, Formation of segmental tion cavity ; C, Embryonic vesicle, showing two primary germ layers. Outer cells, ec- toderm ; inner celts, entoderm. 4 5° THE CELL. the entoderm or Jiypoblast. Between these two layers and derived from them a third layer is formed, the mesoderm or mesoblast. These three layers constitute the blastoderm. TECHNIC. i. Fresh cells may be studied by gently scraping the surface of the tongue, transferring the mucus thus obtained to a glass slide and covering with a cover- glass. 2. Red blood cells from the frog are prepared as follows : After killing the frog the heart is opened and the blood allowed to drop into a tube containing Hayem's fluid (sodium chlorid i gm., sodium sulphate 5 gm., mercuric chlorid 0.5 gm., dis- tilled water 100 c.c). After shaking, the cells are allowed to settle for from twelve to twenty-four hours. The fixative is then replaced by water, the tube again shaken, the cells allowed to settle, and the water is replaced with 80-per-cent alco- a Fig. 9. — The Two Primary Germ Layers ; from transverse section through primitive groove of a chick of 27 hrs. incubation, a, Ectoderm (outer germ layer); b, entoderm (inner germ layer); c, mesoderm (middle germ layer); d. anlage of notochord. hol tinged with iodin. After from twelve to twenty-four hours the alcohol is de- canted and the tube partly filled with alum-carmine solution (page 16). About twenty -four hours usually suffices for staining the nuclei. The alum-carmine is then poured off and the cells well shaken in water. After settling, the water is replaced by glycerin, to which a small amount of picric acid has been added. In this the cells may be permanently preserved. The nuclei are stained red by the carmine, the cytoplasm yellow by the picric acid. 3. Surface cells from the mucous membrane of the bladder. The bladder is removed from a recently killed animal, pinned out mucous membrane side up on a piece of cork and floated, specimen side down, on equal parts Midler's fluid and Ranvier's alcohol (technic 4, p. 6, and a, p. 4) for from twenty-four to forty-eight hours. The specimen is then washed in water and the cells removed by gently scraping the surface. These may then be stained and preserved in the same manner as the preceding. Cells from the different layers should be studied ; also the appearance of the large surface cells seen on flat and on edge, showing pitting of under surface by cells beneath. 4. Amoeboid movement may be studied by watching fresh-water amoebae or white blood cells. A drop of water containing amoebas is placed on a slide, covered and a brush moistened with oil is passed around the cover to prevent evaporation. The activity of the amoebae may be increased by slightly raising the temperature. An apparatus known as the warm stage is convenient for demonstrating amoeboid movement. A drop of blood, human, or, better, from one of the cold-blooded THE CELL. 5r animals, may be used for the study of amoeboid movement in the white blood cells. It should be placed on a slide, covered, and immediately examined on the warm stage. 5. Ciliary movement is conveniently studied by removing a small piece of the gill of an oyster or mussel, teasing it gently in a drop of normal salt solution and covering. The cilia being very long, their motion may be easily studied, especially after it has become slow from loss of vitality. 6. Mitosis. The salamander tadpole and the newt are classical subjects for the study of cell-division. The female salamander is usually full of embryo tad- poles in January and February. The embryos are removed and fixed in Flem- ming's fluid (technic 7, p. 7), after which they may be preserved in equal parts of alcohol, glycerin, and water. Mitotic figures may be found in almost any of the tissues. Pieces of epidermis from the end of the tail, the parietal peritoneum, and bits of the gills are especially satisfactory. If the newt's tail is used, it should be fixed in the same manner, embedded in paraffin and cut into thin sections. These are stained with Heidenhain's haematoxylin, technic 3, p. 15. Certain vegetable tissues, such as the end roots of a young, rapidly growing onion, or magnolia buds are excellent for the study of mitosis. The technic is the same as for animal tissues. General References for Further Study of the Cell. Wilson : The Cell in Development and Inheritance. McMurrich : The Development of the Human Body. Minot: Human Embryology. A Laboratory Text-book of Embryology, Hertwig: Die Zelle und die Gewebe. PART III. THE TISSUES. CHAPTER I. HISTOGENESIS— CLASSIFICATION. Ectoderm, mesoderm, and entoderm (see page 50) are known as the primary layers of the blastoderm. They differ from one another not only in position, but also in the structural characteristics of their cells. The separation of the blastomeres into these three layers represents the first morphological differentiation of the cells of the developing embryo. By further and constantly increasing differentiation are developed from these three primary layers all tis- sues and organs, each layer giving rise to its own special group of tissues. The tissue derivations from the primary layers of the blasto- derm are as follows : Ectoderm. — (1) Epithelium of skin and its appendages — hair, nails, sweat, sebaceous and mammary glands, including smooth mus- cle of sweat glands. (2) Epithelium of mouth and anus, of glands opening into mouth and enamel of teeth. (3) Epithelium of nose and of glands and cavities connected with nose. (4) Epithelium of external auditory canal and of membranous labyrinth. (5) Epithelium of anterior surface of cornea, of conjunctiva, and of crystalline lens. (6) Epithelium of male urethra, except prostatic portion. (7) Epithelium of pineal bodies and of pituitary body. (8) Entire nervous system, including retina. Entoderm. — (1) Epithelium of digestive tract excepting mouth and anus, and of glands connected with digestive tract. (2) Epithelium of respiratory tract and of its glands. (3) Epithelium of bladder, ureters, female urethra, and of prostatic portion of male urethra. (4) Epithelium of tympanum and of Eustachian tube. (5) Epithelium of thyroid and of Hassall's corpuscles of thymus. 55 5 6 THE TISSUES. Mesoderm. — The cells of the mesoderm soon differentiate to form three sub-groups — the mesothelium, the mesenchyme, and the mes- amoeboid cells. Mesothelium. — The mesothelial cells form tissues resembling epi- thelium. They line the serous membranes — pleura, pericardium, and peritoneum ; form the epithelium of the genito-urinary system except that of ureters, bladder, and urethra ; and give rise to striated and heart muscle. MesencJiyme. — From the mesenchyme cells are derived all con- nective tissues; the lymphatic organs, including the spleen; cells classed as " endothelial " cells, which line the vascular and lymphatic systems ; smooth muscle and bone marrow. Mesamaboid Cells. — From these are derived the embryonic red and the white blood cells. In all but the lowest forms of animal life the body consists of an orderly arrangement of many kinds of cells. From the cells is de- veloped a substance which lies outside the cells and is known as in- tercellular substance. The association of a particular type of cell with a particular type of intercellular substance is known as a tissue. The character of a tissue depends upon the character of its cells, of its intercellular substance, and their relations to each other. Further differentiation of cells and intercellular substance within a particular tissue gives rise to various sub-groups of the tissue. The association of tissues to form a definite structure for the performance of a par- ticular function is known as an organ. A scientific classification of the tissues is at present impossible. The foregoing list of tissue derivations shows how unsatisfactory is any attempt at classification on the basis of histogenesis, many tissues which are morphologically similar being derived from two or even all three of the blastodermic layers. The following is the usual classification of adult tissues: (i) Epithelial trssues; (2) connective tissues ; (3) blood; (4) muscle tis- sue ; (5) nerve tissue. Of these, epithelium and connective tissue may be regarded as the more elementary tissues, being common to both plants and animals. Blood is sometimes classified among the connective tissues. Muscle and nerve are the most highly specialized tissues and are peculiar to animals. CHAPTER II. EPITHELIUM (INCLUDING MESOTHELIUM AND ENDOTHELIUM). Histogenesis. — Epithelium is derived from all three of the pri- mary blastodermic layers. It is at first a thin membrane-like struc- ture composed of a single layer of cells. This condition may persist or new cells may develop between the older cells and the underlying connective tissue, thus forming epithelium several layers of cells in thickness. General CJiaracteristics. — Epithelium consists almost wholly of cells. The intercellular substance is merely sufficient to attach the cells to one another and is, consequently, known as cement sub- stance. In some instances the protoplasm of adjacent epithelial cells is seen to be even more closely associated, the intervening cement substance being bridged over by delicate processes of protoplasm which pass from one cell to another and are known as " iiitercellular brid ges " . (see Fig. 14, p. 61). It seems probable that the minute spaces between the processes serve as channels for the passage of food (lymph) to the cells. The surface cells of epithelium are united by continuous cement substance in which there are apparently no spaces. In this way escape of lymph is prevented. Epithelial cells vary in size and shape, the element of pressure being a frequent determining factor as regards shape. Their proto- plasm may be clear, finely or coarsely granular, or pigmented. Each cell usually contains a single well-defined nucleus. Two or more nuclei are sometimes present. Some epithelial cells are, when fully matured, non-nucleated, e.g., respiratory epithelium of lung. When epithelium rests upon connective tissue, it is usually sepa- rated from the latter by a thin, apparently homogeneous membrane known as the basal membrane or viembrana propria. Authorities differ as to whether this membrane is of connective-tissue or of epi- thelial origin. Surface epithelial cells frequently have thickened free borders or 57 5 S THE TISSUES. cuticulce, which unite to form a continuous membrane, the cuticular membrane. Striations extend from the cytoplasm into the cuticulae. A still greater specialization of the surface of the cell is seen in the ciliated cell. In these cells fine hair-like projections — cilia — extend from the surface of the cell. Some epithelial cells show important changes dependent upon their functional activities. An example of this is seen in the mucous cell in which there is a transformation of the greater part of the cyto- plasm into, or its replacement by, mucus. Epithelia are devoid, as a rule, of both blood and lymph vessels. An exception to this is the stria vascularis of the cochlea. Nerves, on the other hand, are abundant. Classification. — Epithelia may be classified according to shape and arrangement of cells as follows : (i) Simple Epithelium. — (a) Squamous; (/;) columnar. (2) Stratified Epithelium. — (a) Squamous; (a) columnar. (3) Mesothelium and Endothelium. Specializations of the above-mentioned types are known as : (a) Ciliated epithelium; (/>) pigmented epithelium: (c) glandular epi- thelium ; (<7) neuro-epithelium. 1. Simple Epithelium. In simple epithelium the cells are arranged in a single layer. (a) Simple squamous epithelium consists of flat scale-like cells which are united by an extremely small amount of intercellular substance. The edges of the cells are smooth or serrated. Seen on flat, they present the appearance of a mosaic. Seen on edge, the cells appear fusiform, being thickest at the centre, where the nucleus is situated, and thinning out toward the periphery. Simple squamous epithelium has but a limited distribution in man. It occurs in the lungs as non-nucleated respiratory epithelium, in Bowman's capsule of the renal corpuscle, in the descending arm of Henle's loop of the uriniferous tubule, in the retina in the form of pigmented cells, and on the posterior surface of the anterior lens capsule. (In Simple columnar epithelium consists of a single layer of elon- gated cells. The bases of the cells are usually separated from the EPITHELIUM. 59 underlying connective tissue by a basement membrane. The nu- cleus is, as a rule, in the deeper part of the cell, near the basement membrane. Many of these cells have prominent thickened free FlG. io. — From Section of Cat's Lung, stained with silver nitrate, showing- outlines of the Sim- ple Squamous Epithelium Lining- the Air Vesicle, a. Two epithelial cells; b, the wavy stained intercellular substance; c, foetal cells ; d, connective tissue. borders or cuticulae. This form of epithelium is often ciliated. When the height of the cell about equals its other dimensions, the epithelium is called citboidal. Simple columnar epithelium lines the gastro-intestinal canal, the uriniferous tubule (excepting the de- FlG. ii.— Simple Columnar Epithelium from the Human Small Intestine, j, Mucous (goblet) cell ; f>, basement membrane ; c, thickened free border (cuticula); (2) Formed Connective Tissue. — In this the fibres all run in approximately the same direction, and are united by a small amount of ground substance (Fig. 27). There are but few cells and these are flattened out by pressure and lie between the fibres, their long axes corresponding to the direction of the fibres. This arrangement of tissue elements forms a firm, dense tissue, FIG. 27.— Longitudinal Section of Tendon from Frog's Gastrocnemius. X 250. The nuclei of the flattened cells are seen lying in rows be- tween the connective-tissue fibres. 72 THE TISSUES. such as is found in tendons and ligaments. Formed connective tissue also occurs as anastomosing networks of fibres, as, e.g., in the omentum (page 64). It should be noted that between dense 'formed" connective tissue on the one hand and loose "areolar" ssue on the other, all gradations exist. Regarding the development of the connective-tissue fibrils, there are two theories : (1) According to one, they are developed directly from the protoplasm of the connective-tissue cells. The cells in- crease in length, and fine granules appear, which arrange themselves in rows in the cytoplasm ; these granules unite to form fibrils. Such cells are known as fibroblasts, and their fibrils are the forerun- ners of the intercellular fibrils of connective tissue. A modification of this theory derives the fibrils from the peripheral portion of the cell — the exoplasm. (2) According to the other theory the fibrils are developed from the matrix, minute granules first becoming arranged in rows- and later uniting to form fibrils. Regarded as opposing theories, there is in reality but little an- tagonism between them. There is no doubt as to the intercellular matrix being a product of the cell. Whichever theory, therefore, is accepted, the entire intercellular substance, fibres and ground sub- stance are ultimate derivatives of the cell. Recent studies, espe- cially those of Mall, are confirmatory of the second of the theories given above. He maintains that the connective-tissue fibrils, both white and elastic, are derivatives of an active intercellular matrix, which latter is a direct product of the cell. Two similar theories exist as to the development of elastic fibres, a cellular theory and an extracellular theory. According to some advocates of the cellular theory, the elastic fibres are derived from the exoplasm ; according to others, from the cytoplasm immediately surrounding the nucleus. Recent researches favor the extracellular theory. Mall describes extremely minute fibrils in the ground sub- stance, which later develop into elastic fibres. 2. Elastic Tissue. Elastic fibres occurring in fibrous connective tissue have been described. When the elastic fibres are greater in amount than the white fibres, the tissue is known as clastic tissue. Almost pure elastic tissue is found in the ligamentum nuchas of quadrupeds. Here the fibres are coarse and held together by a small amount of cement substance. A few white fibres and connective-tissue cells are also present (Figs. 28 and 29). Elastic tissue may be arranged as thin membranes, as, e.g., in the walls of blood-vessels. These membranes are usually de- THE CONNECTIVE TISSUES. 73 scribed as composed of a dense mass of flat, ribbon-like elastic fibres, which interlace in such a manner as to leave openings in the Fig. 28. — Coarse Elastic Fibres from Ligamentum Nuchae. X 500. Teased specimen. (Tech- nic 10. p. 75.) membrane. Hence the term " fenestrated membrane. " They have been recently described as consisting of a central layer composed of elastin, staining with magenta, and on either side a thin, transparent A-T" 6 ; ~H~ - 3, i ; FlG. 29.— Cross Section of Coarse Elastic Fibres from Ligamentum Nuchas. X 500. (Tech- nic 10, p. 75). <7, Elastic fibres ; />, white fibrous tissue and cement substance. The nu- clei are the nuclei of fixed connective-tissue cells. sheath unstained by magenta. This is seen to correspond to Mall's description of the structure of the elastic fibre. Only the middle of these lavers is fenestrated. 74 THE TISSUES. TECHNIC. i. Areolar Tissue, to show White and Elastic Fibres. — Remove a bit of the subcutaneous tissue, as free from fat as possible, from a recently killed animal. Place it upon a mounting slide and with teasing needles quickly spread it out in a thin layer. During this manipulation the specimen should be kept moist by breathing on it. Put a drop of sodium chlorid solution upon the specimen and cover. As the specimen is unstained, a small diaphragm should be used for the micro- scopic examination. The white fibres are straight or wavy, are crossed in all directions, and are longitudinally striated. The elastic fibres have been stretched and show as sharp lines with curled ends where the fibres are broken. Place a drop of hydric acetate, i-per-cent aqueous solution, at one side of the cover and a bit of filter paper at the other side. The filter paper absorbs the salt solution, which is replaced by the hydric acetate. The latter causes the white fibres to swell and become indistinct while the elastic fibres show more plainly. 2. Areolar Tissue, to show Cells and Elastic Fibres. — Prepare second speci- men of areolar tissue in the same manner as the preceding. Instead of mounting in salt solution, allow it to become perfectly dry, then stain in the following solu- tion : Gentian violet saturated aqueous solution 40 c.c. Water 60 c.c. Wash thoroughly, dry, and mount in balsam. The nuclei of the fixed connective-tissue cells are stained violet. Their deli- cate cell bodies show as an irregular haze around the nuclei. Both nuclei and cell bodies appear cut in all directions by the stretched elastic fibres. Wandering cells (leucocytes) may usually be seen. Plasma cells are frequently not demonstrable, and mast cells are only occasionally present. The elastic fibres are stained violet. The white fibres are almost unstained. 3. Formed Connective Tissue. — Fibrous tissue arranged in the form of a net- work may be seen in the specimen of omentum (technic 7, p. 66). 4. Densely formed connective tissue may be studied in tendon. Cut through the skin of the tail of a recently killed mouse about half an inch from the tip and break the tail at this point. By pulling on the end of the tail this portion may now be separated from the rest of the tail, carrying with it long delicate tendon fibrils, which have been pulled out of their sheaths. This should be immediately ex- amined in salt solution, using the high power and a small diaphragm. The fibrils are seen arranged in parallel bundles. 5. Place a drop of hydric acetate (2-per-cent aqueous solution) at one side of the cover-glass, absorbing the salt solution from the opposite side by means of filter paper. The fibres swell and become almost invisible, while rows of connec- tive-tissue cells (tendon cells) can now be seen. The cells may be stained by allow- ing a drop of hematoxylin or of carmine solution to run under the cover. After the cells are sufficiently stained, the excess of stain is removed by washing and the specimen mounted in glycerin. 6. Fix a small piece of any good-sized tendon in formalin-Miiller's fluid (page (>). After a week, harden in alcohol, embed in celloidin, and make longitudinal THE CONNECTIVE TISSUES. 75 and transverse sections. Stain strongly with hematoxylin, followed by picro-acid fuchsin (page 16). Mount in balsam. 7. Pigmented connective-tissue cells are most conveniently obtained from the chorioid coat of the eye. Fix an eye in formalin-Midler's fluid (see page 5), cut in half, remove chorioid and retina and pick of the dark shreds which cling to the outer surface of the chorioid and inner surface of the sclera. These may be trans- ferred directly to glycerin, in which they are mounted, or the bits of tissue may be first stained with hematoxylin (page 14). In addition to the pigmented cells should be noted the ordinary fixed connective-tissue cells which lie among them. Only the nuclei of these cells can be seen. S. Connective-Tissue Cells to show Anastomosing Processes. — Stain a cornea with gold chlorid (see page 25). Sections are made tangential to the convex sur- face and are mounted in glycerin. 9. Connective-tissue cell spaces (lacunae) and their anastomosing canaliculi may be demonstrated by staining a cornea with silver nitrate (see page 25). The silver stains the ground substance of the cornea, leaving the lacunae and canaliculi unstained. The relation which this picture bears to the preceding should be borne in mind (see Figs. 24 and 25). 10. Coarse elastic fibres may be obtained from the ligamentum nuchas, which consists almost wholly of elastic tissue. A piece of the ligament is fixed in satu- rated aqueous solution of picric acid and hardened in alcohol. A bit of this tissue is teased apart on a glass slide in a drop of pure glycerin, in which it is also mounted. Before putting into glycerin, the specimen may be stained with picro- acid-fuchsin. This intensifies the yellow of the elastic fibres and brings out in red the fibrillar connective tissue. Pieces of the ligament fixed and hardened in the same manner may be embedded in celloidin and cut into longitudinal and trans- verse sections. These stained with picro-acid-fuchsin show well the relation of the coarse elastic fibres (yellow) to the more delicate fibrous tissues (red). 3. Embryonal and Mucous Tissue. Embryonal and mucous tissue are essentially developmental forms and represent early differentiations from the general parent type. They consist according to their age of oval, fusiform, or irregular branching and anostomosing cells, lying in a matrix, which is just beginning to show evidences of a fibrillar structure. By some his- tologists the term "embryonic " connective tissue is limited to the stage of fusiform cells with slightly fibrillar matrix (Fig. 30), the term "mucous" tissue being applied to an embryonic form of con- nective tissue in which irregular branching and anastomosing cells lie in a slightly fibrillated matrix which gives the chemical reaction for mucin (Fig. 31). Embryonal tissue is not found in the adult, while mucous tissue has only a very restricted distribution. Much variation exists as to the shape and size of the cells in em- bryonal and mucous tissue. This is clue to the fact that these cells represent transition stages in the development of the adult connec- 7 6 THE TISSUES. tive-tissue cell. Thus in embryonic connective tiss ue, while most of the cells are fusiform, one finds spherical and oval cells and some Fig. 30.— Embryonal Connective Tissue from Axilla of Five-inch Fcetal Pig. X 600. (Technic 1, p. 77-) Various shaped connective-tissue cells are seen lying in a slightly fibrillated matrix. few cells which are triangular or stellate. The same holds true of mucous tissue, where, while most of the cells are of the triangular or stellate variety, round, oval and fusiform cells are also present. . Fig. 31.— Mucous Connective Tissue from Umbilical Cord of Eight-inch Foetal Pig. X 6oo (Technic 2, p. 77.) THE CONNECTIVE TISSUES. 77 TECHNIC. i. Embryonal Tissue. — Bits of the subcutaneous tissue from the axilla or groin of a five-inch foetal pig are fixed in Zenker's fluid (technic 9, p. 7), hardened in alcohol and stained for twelve hours in alum-carmine (technic b, p. 16). They are then transferred to eosin-glycerin, in which they are teased and mounted. Note the intercellular substance, that it is composed of delicate single fibrils inter- lacing in all directions with no arrangement into bundles, as in adult tissue, and that there is as yet no differentiation into two kinds of fibres. 2. Mucous Tissue. — The umbilical cord of a four or five months human foe- tus, or of a nine-inch foetal pig is fixed in formalin-Midler's fluid (page 6), hard- ened in alcohol, and transverse sections stained with haematoxylin-eosin (technic 1, p. 17) and mounted in eosin-glycerin. Note the central blood-vessels with their thick walls and the surface epithelium. The mucous tissue is best studied near the surface just beneath the epithelium. 4. Reticular Tissue. Reticular connective tissue is a form of fibrillar connective tissue. It consists of small bundles of extremely delicate white fibrillae. These interlace in all directions and form a network enclosing spaces of various sizes and shapes (Fig. 32). The cells are flat and wrap FIG. 32.— Reticular Connective Tissue from Human Lymph Node. X 600. (Technic, p. 79.) The nuclei belong to flat connective-tissue cells which lie upon the fibres of the reticulum, their cell bodies being invisible. themselv es around the bundles of fibrils. This led to the belief that reticular connective tissue was composed wholly of anastomosing cells. ' Later, when the underlying fibrillar basis was understood, the overlying cells were referred to as " epithelioid " cells, the designation being based upon morphological characteristics. With the recogni- ' In some lower animals and in the embryos of some higher animals, such wholly cellular reticular tissues are found. 7§ THE TISSUES. tion of the impossibility of differentiating on a morphological basis between certain forms of epithelial and of connective-tissue cells, these cells were classified where their histogenesis properly places them, as connective-tissue cells. Reticular connective tissue differs in chemical composition from both fibrous and elastic tissue. It does not yield gelatin in boiling and Fig. 33. — Diffuse and Dense (Circumscribed) Lymphatic Tissue from a Human Lymph Node. CTechnic, p. 79.) a, Reticular connective tissue, in the meshes of which are suspended by leucocytes, and c, lymphocytes. The reticular connective tissue is present also in the dense lymphatic tissue, but is not visible on account of the closely packed cells. is not digested by pancreatin, while white fibres yield gelatin and are slowly digested by pancreatin. Reticular connective tissue forms the framework of adenoid tissue and of bone marrow. Fibrils giving THE CONNECTIVE TISSUES. 79 the chemical reaction of reticular tissue are associated with the fibrous and elastic-tissue framework of the lung, liver, kidney, and other organs. 5. Lymphatic Tissue. Lymphatic tissue consists of reticular connective tissue and a spe- cial type of connective-tissue cells, lymphoid cells, filling the meshes of the reticulum. Lymphoid cells are small spherical cells. Each cell has a single nucleus which almost fills the cell. In lymphatic tissue the cell is a much more important factor in determining the character of the tissue than in most forms of connective tissue. Lymphatic tissue may be diffuse or circumscribed. In diffuse lymphatic tissue (Fig. 33) the cells are not closely packed and there is no distinct demarcation between the lymphatic and the surround- ing tissues. An example of diffuse lymphatic tissue is seen in the stroma of the mucous membrane of the gastro-intestinal canal. In circumscribed lymphatic tissue (Fig. 34) the cells are very closely packed, often completely obscuring the reticulum. There is also a quite distinct demarcation between the lymphatic and the surround- ing tissues. Such a circumscribed mass of lymphatic tissue is known as a lymph nodule. TECHNIC. Fix a lymph node in formalin-Midler's fluid (technic 5, p. 6), and stain very thin sections with hematoxylin and picro-acid-fuchsin (technic 3, p. 17). In the lymph sinuses of the medulla the reticulum can usually be plainly seen. This specimen serves also for the demonstration of diffuse and compact lymphatic tis- sue, the former in the lymph sinuses of the medulla, the latter in the nodules of the cortex and in the medullary cords. 6. Fat Tissue. Adipose tissue ox fat tissue is a form of connective tissue in which most of the cells have become changed into fat cells. Fat tissue is peculiar among the connective tissues in that the cells and not the intercellular substance make up the bulk of and determine the char- acter of the tissue. The adult fat cell is surrounded by a distinct cell membrane, and almost the entire cell is occupied by a single spherical droplet of fat (Figs. 36 and 37). The nucleus, flattened and surrounded by a small amount of cytoplasm, is usually found pressed against the cell wall (Fig. ^7). This appearance of a dis- tinct cell membrane enclosing the spherical fat droplet, with the nucleus and cytoplasm pressed into a crescent-shaped mass at one So THE TISSUES. side, has given rise to the term "signet-ring cell." Fat cells which occur singly, or in small groups, or in the developing fat of young Fig. 35.— Fat Tissue from Human Subcutaneous Tissue (Child) to show Lobulation. X 25c (Technic 1, p. 83.) FlG. 36.— Young Fat from Human Subcutaneous Tissue (Child). X 175. (Technic 1, p. 83.) a, Interlobular connective tissue ; b, fixed connective-tissue cell ; c, fat cells ; d, artery ; e, nucleus of fat cell and remains of cytoplasm ("signet ring "). THE CONNECTIVE TISSUES. 81 animals, are spherical (Fig. 36). In large masses of adult fat, the closely packed cells are subjected to pressure and are polyhedral (Fig. 37). Fat cells are usually arranged in groups or lobules, each lobule being separated from its neighbors by fibrillar connective tissue (Fig. 35). Adipose tissue is usually associated with loose fibrous tissue. The appearance which adult fat presents can be understood only by reference to its histogenesis. Eat cells are developed directly from embryonic connective-tissue cells. In the human embryo they are first distinguishable as fat cells about the thirteenth week. The — b FIG. 37.— Adult Fat Tissue from Human Subcutaneous Tissue. X 175. (Technic 1, p. S3.) a, Fat cells ; b, interlobular connective tissue ; c, nucleus of fat cell and remains of cyto- plasm (" signet ring ") ; d, artery. connective-tissue cells which are to become fat cells gather ; groups in the meshes of the capillary network which marks the end- ing of a small artery. Each group is destined to become an adult fat lobule (Fig. 38). Fat first appears as minute droplets in the cytoplasm of the em- bryonic connective-tissue cell (Fig. 39). These small droplets in- crease in number and finally coalesce to form a single larger droplet. This increases in size and ultimately almost wholly replaces the cyto- plasm. In this way the nucleus and remaining cytoplasm are pressed to one side and come to occupy the inconspicuous position which they have in adult fat. 6 82 THE TISSUES. The blood supply of fat is rich and the adult lobule maintains its embryonic vascular relations, in that the vascular supply of each lobule is complete and independent. One artery runs to each lobule, Fig. 38.— Developing Fat Tissue from Subcutaneous Tissue of Five-inch Fcetal Pig-. X 75. (Technic 2, p. 83.) £, Arteriole breaking up into capillary network ; b, embryonal con- nective tissue ; c, embryonal fat lobules developing around blood-vessels. •?#..' ••?•!• '•#• & PlG. 39.— Developing Fat Tissue from Subcutaneous Tissue of Five-inch Foetal Pig. (Technic 2, p. 83.) a, Arteriole breaking up into capillary network ; b, embryonal connective tissue, embryonal cells from which fat cells are developing ; c, capillaries. Fat droplets stained black. At the right are five individual cells showing stages of development from an embryonal cell to an adult fat cell. THE CONNECTIVE TISSUES. S3 where it breaks up into an intralobular capillary network, which in turn gives rise to the intralobular veins, usually two in number. Fat is thus seen to be a connective tissue in which some of the cells have undergone specialization. There still remain, however, embryonal connective-tissue cells which are not destined to become fat cells, but which develop into cells and fibres of ordinary fibrous con- nective tissue. A few of these remain among the fat cells to become the delicate intralobular connective tissue seen in adult fat. The majority are, however, pushed to one side by the developing lobules, where they form the interlobular septa. TECHNIC. 1. Fat Tissue. — Human subcutaneous fat as fresh as possible is fixed in forma- lin-Miiller*s fluid (technic 5, p. 6), hardened in alcohol and embedded in celloidin. Sections are stained with hematoxylin and picro-acid-fuchsin (technic 3, p. iS). The alcohol and ether of the celloidin remove the fat from the fat cells, leaving only the cell membranes. The fat gives the celloidin a milky appearance. Such celloidin does not cut well. The celloidin should, therefore, be changed until it ceases to turn white. The sections are cleared in oil of origanum or carbol xylol, and mounted in balsam. The fibrillar tissue is stained red by the fuchsin. and the protoplasm of the fat cell yellow by the picric acid. 2. Developing Fat Tissue. — Remove bits of tissue from the axilla or groin of a five-inch foetal pig, or other foetus of about the same development. Fix twenty- four hours in a i-per-cent aqueous solution of osmic acid (technic 6, p. 26), wash thoroughly and mount in glycerin. A part of the tissue mounted should be thor- oughly teased, the rest gently pulled apart. The teased portion will show the fat cells in various stages of development. The unteased part will usually show brownish blood-vessels and the grouping of fat cells around them, to form embry- onic fat lobules. Note the developing connective tissue between the groups of fat cells. It is from this that the areolar tissue, which envelops and separates the lobules of adult fat, is developed. 7. Cartilage. Cartilage is a form of connective tissue in which the ground sub- stance is firm and dense and determines the physical character of the tissue. Cartilage cells are differentiated connective- tissue cells^ While varying greatly in shape they are most frequently spherical or oval. Each cell lies in a cell space or lacuna, which it completely fills. The intercellular substance immediately surrounding a lacuna is frequently arranged concentrically, forming a sort of capsule. Fine canaliculi connecting the lacunae are present in some of the lower animals and have been described in human cartilage. They 8 4 THE TISSUES. can be demonstrated, however, in human cartilage, only by special methods, and probably represent artefacts. Cartilage contains no blood-vessels, and in human cartilage no lymph channels have been positively demonstrated. s~\ i ■;- r -, Hi H '.'■' MP *.) (' - '^i4) ( m *';£$'-, flll^ *■."':. fill '■-'■'^■^ Wk y 11 111 Fig. 40. — Hyaline Cartilage from Head of Frog's Femur. X 350. (Technic 1, p. of cartilage cells in apparently homogeneous matrix. i.) Groups Cartilage is subdivided according to the character of its intercel- lular substance into three varieties : (i) Hyaline, (2) elastic, (3) fibrous. 1. Hyaline Cartilage (Fig. 40). — The cells occur singly or in An entire group of cells frequent- ly lies in one lucuna surrounded by a single capsule. Such a groups of two or multiples of two. 1 - ■ - A.M.M* Fig. 41.— Elastic Cartilage from Dog's Ear. X 350. (Technic 2, p. 86.) Groups of car- tilage cells in fibro-elastic matrix. group of cells has developed within its capsule from a single parent cell. In other cases del- icate hyaline partitions separate the cells of a group. The cells are spherical or oval, with flatten- ing of adjacent sides. The nu- cleus is centrally placed, and has a distinct intranuclear net- work and membrane. The cyto- plasm is finely granular, and may contain droplets of fat, of glyco- THE CONNECTIVE TISSUES. *S gen, or of both. Toward the perichondrium the arrangement of the cells in groups is less distinct. Here the cells are fusiform and parallel to the surface. The intercellular matrix, wh en subjected to the usual technic, appears homogeneous. By the use of special methods, such, e.g., as artificial digestion, this apparently structureless matrix has been shown to be made up of bundles of fibres, quite similar to those found in fibrous connective tissue. Hyaline cartilage forms the articular cartilages of joints, the costal cartilages, and the cartilages of the nose, trachea, and bronchi. In the embryo a young type of hyaline cartilage, known as embryonal cartilage, forms the matrix in which most of the bones are developed. 2. Elastic cartilage (Fig. 41) resembles hyaline, but differs from the latter in that its hyaline jnatrix contains a large number of elastic fib res. These vary in size, many being extremely fine. The elastic fibres branch and run in all directions, forming a dense network of interlacing and anastomosing fibres. Elastic cartilage occurs in the external ear, the Eustachian tube, the epiglottis, and in some of the laryngeal cartilages. 3. Fibrous cartilage (Fig. 42) is composed mainly of fibrillar connective tissue. The fibres may have a parallel arrangement, or Fig. 42.— Fibrous Cartilage from Dog's Intervertebral Disc. X 350. (Techuic 3, p. 86.) Groups of cartilage cells in matrix of fibrillar connective tissue. may run in all directions. Cells are few, and are usually arranged in rows of from two to six, lying in elongated cell spaces between the fibre bundles. 86 THE TISSUES. Fibrous cartilage occurs in the inferior maxillary and sterno- clavicular articulations, in the symphysis pubis, and in the interver- tebral discs. Cartilage, except where it forms articular surfaces, is covered by a membrane, the perichondrium. This is composed of fibrillar con- nective tissue, and blends without distinct demarcation with the superficial layers of the cartilage. Like the other connective tissues, cartilage develops from mesen- chyme. It is at first wholly cellular. Each cell forms a capsule around itself, and by blending of these capsules are formed the first elements of the intercellular matrix. This increases in quantity and assumes the structural characteristics of one of the forms of carti- lage. The white fibres of fibro-cartilage and the yellow fibres of elastic cartilage develop in the same manner as in fibrillar and elastic tissue. TECHNIC. (i) Hyaline Cartilage. — Remove a frog's femur and immediately immerse the head in saturated aqueous solution of picric acid. Cut sections tangential to the rounded head, keeping knife and bone wet with the picric-acid solution. As bone must be cut, a special razor kept for the purpose should be used. Cut sections as thin as possible. The first sections consist wholly of cartilage. As bone is reached, the cartilage is confined to a ring around the bone. Mount in the picric- acid solution, cementing the cover-glass immediately. (2) Elastic Cartilage. —Remove a piece of cartilage from the ear and fix in formalin-Muller's fluid (technic 5, p. 6). Stain sections strongly with haematoxylin, followed by picro-acid-fuchsin (technic 3, p. i8j. Clear in carbol-xylol and mount in balsam. The capsules around the cartilage cells are thick and, as they usually retain some haematoxylin, can be readily seen. Note also the flattened cartilage cells near the surface, and the perichondrium. (3) Fibro-cartilage. — Fix pieces of an intervertebral disc in formalin-Muller's fluid. Sections are stained either with haematoxylin-eosin or with haematoxylin- picro-acid-fuchsin and mounted in balsam. 8. Bone Tissue. Bone is a form of connective tissue in which the matrix is ren- dered hard by the deposition in it of inorganic matter, chiefly the phosphate and the carbonate of calcium. These salts are not merely deposited in the matrix, but are intimately associated and combined with its histological structure. The intimacy of this association of the organic and inorganic constituents of bone is shown by the fact that, though the salts compose two-thirds of bone by weight, it THE CONNECTIVE TISSUES. 87 Fig. 43. — Bone Tissue showing Lacuna? and Canali- culi. X 700. (Technic 1, p. 87.) is impossible to distinguish them by the highest magnification. Furthermore, if either the lime salts are dissolved out by means of acids (decalcification) or the organic matter removed by heating (calcination), the histological structure of the bone still remains. Like the other connec- tive tissues, bone consists morphologically of cells and intercellular substance. Bone cells or bone cor- puscles lie in distinct cell spaces or lacuna. From the lacunae pass off in all direc- tions minute canals — canaliculi — which anastomose with canaliculi of neighboring lacunas (Fig. 43). At the surface of bone these canaliculi open into the periosteal lymphatics. A complete system of canals is thus formed, which traverse the bone and serve for the passage of nutritive fluids. The bone cells themselves (Fig. 44) are flat, ovoid, nucleated cells, with numerous fine processes, which extend in all directions into the canaliculi. In young developing bones the processes of adjacent cells anastomose. In adult bone the processes extend but a short distance into the canaliculi, and probably do not anas- tomose. The basement substance or matrix has a wing the outline of the fib strU cture, closely resembling that of lacuna to be seen. 'J ° fibrillar connective tissue, and it is in this fibrillar matrix that the lime salts are deposited. The fibrils are held together by cement substance into bundles. In most bone the bundles are fine and arranged in layers or lamella. Less commonly the fibre bundles are coarser and have an irregular arrangement. Fig. 44.— Bone Cell and La- cuna. (After Joseph.) At a the cell body has shrunken, TECHNIC. (1) For the study of the minute structure of bone a section of undecalcified or hard bone is required. Part of the shaft of one of the long bones is soaked for several days in water and all the soft parts are removed. It is then placed in equal 88 . THE TISSUES. parts alcohol and ether to remove all traces of fat and thoroughly dried (the handle of a tooth or nail brush frequently furnishes good material and is already dried). Thin longitudinal and transverse sections are now cut out with a bone saw. One surface is next ground smooth, first on a glass plate, using emery and water, then on a hone. The specimen is now fastened polished side down on a block of wood or glass by means of sealing wax, and the other side polished smooth in the same manner as the first, the bone being ground as thin as possible. The sealing wax is removed by soaking in alcohol and the specimen looked at with the low power. If not thin enough, it is gently rubbed on a fine hone. It is then soaked in equal parts alcohol and ether, dried thoroughly and mounted in hard balsam. This is accomplished by placing a small bit of hard balsam on a slide, melting, pushing a bit of the bone into the hot balsam, covering and cooling as quickly as possible. The object of the hard balsam and quick cooling is to prevent the balsam running into the lacuna; and canaliculi and obscuring them by its transparency. The air imprisoned in the lacunae and canaliculi causes them to appear black when viewed by reflected light. (2) The structure of the bone cell is best studied in sections of decalcified bone which has first been carefully fixed. (See technic 1, p. 160.) 9. Neuroglia. This peculiar form of connective tissue is confined entirely to the central nervous system and is most conveniently studied in connec- tion with nervous tissue (see page 115). CHAPTER IV. THE BLOOD. o o Blood is best considered as a tissue, the intercellular substance of which is fluid. This fluidity of the intercellular substance allows the formed elements or cells to move about freely, so that there is not the same definite and fixed relation between cells and intercellu- lar substance as in other tissues. The formed elements of the blood are: (i) Red blood cells (red blood corpuscles, erythrocytes) ; (2) white blood cells (colorless cor- puscles — leucocytes); (3) blood platelets (thrombocytes). 1. Red blood cells (erythrocytes) (Fig. 45, /, 2, J) are in man non-nucleated circular discs. Their average diameter is about 7.5 ,«, their thickness 2 p. at the thin centre. They are biconcave, with rounded edges. Seen on the flat, the difference in thickness between centre and periphery is evidenced by the difference in refraction (Fig. 45, /). Seen on edge, the shape resembles that of a dumbbell (Fig. 45, 2). Singly or in small numbers, red blood cells have a pale straw color. Redness of the cells is ap- parent only when they are seen in large numbers. If fresh blood be allowed to stand for a moment, the red cells are seen to adhere to one another by their flat sur- faces, forming rows or rouleaux (Fig. 45, J)- Subjected to the usual technic, the red blood cell appears homogeneous. By the use of special methods, this apparently homogeneous substance can be separated into {a) a color-bearing proteid — hamoglobin, and (b) a stroma, the latter representing the protoplasm of the cell. It is the haemoglobin S9 Fig. 45. — Cells from Human Blood. X 600. (Technic 2, p. 94.) /, Red blood cell seen on flat ; 2, red blood cell seen on edge ; j, red blood cells forming rouleaux ; 4, 4, small and large lympho- cytes ; j, mononuclear leuco- cyte ; 6, transitional leucocyte ; 7, polymorphonuclear leucocyte, containing neutrophile gran- ules ; S, polynuclear leucocyte, containing eosinophile gran- ules; ., except in the pregnant uterus, where the cells frequently attain a much greater size. At the centre of the cell, which 95 96 THE TISSUES. is its thickest portion, is a long rod-shaped nucleus surrounded by an area of finely granular cytoplasm. The rest of the cytoplasm shows delicate longitudinal striations, which probably represent a longitu- dinal arrangement of the spongioplasm. The cells are united by a small amount of cement substance. Intercellular "bridges " similar to those connecting epithelial cells have been described (Fig- 47). Smooth muscle cells may be arranged in layers of considerable thickness, the cells having a definite direction, as in the so-called "musculature" of the intestine (Fig. 48). In such masses of smooth muscle the cells are separated into groups or bundles by connective tissue. Smooth muscle cells may be arranged in a sort of network, the cells crossing and interlacing in all directions, as in : ■ rjnu Fig. 48. — Smooth Muscle from Longitudinal Section of Cat's Small Intestine, showing' Por- tions of Inner Circular and Outer Longitudinal Muscle Coats with Intervening Connec- tive Tissue. X 350. (Technic 3, p. 104.) a, Transversely cut cells of inner circular layer ; in comparatively few has the plane of section passed through the nucleus; b, longitudi- nally cut cells of outer longitudinal layer. In many of the cells the plane of section has not passed through the nucleus; c, intermuscular septum (connective tissue); d, small artery. the wall of the frog's bladder. Again, they may be scattered in small groups or singly among connective-tissue elements, as in the villi of the small intestine. 2. Voluntary Striated Muscle. — This consists of cylindrical fibres from 30 to 120 //. in length and from 10 to 60 p. in di- ameter. Each muscle fibre consists of (a) a delicate sheath, the sarco- Icmuia, enclosing (p) the muscle substance proper, in which lie (c) the muscle nuclei. The sarcolemma is a clear, apparently structureless, membrane, MUSCLE TISSUE. 97 which adheres so closely to the underlying muscle substance as to be indistinguishable in most preparations. In teased specimens it may frequently be seen at the torn ends of the fibres (Fig. 49). The muscle substance consists of fibrillce and sarcoplasm, and shows two sets of striations (Fig. 50), longitudinal striations and cross stria- tions. The longitudinal striations are due to parallel running ultimate fibrillae, of which the muscle fibre is composed. These fibrillae are united by a minute amount of interfibrillar cement substance. The transverse striations appear in the unstained fibre examined by re- flected light as alternate light and dark bands (Figs. 50 and 5 1). The light band is composed of a singly refracting (isotropic) substance, the dark band of a doubly refracting (anisotro- pic) substance. Through the middle of the light band runs a fine dark (anisotropic) line {Krause s line), while an even finer light (iso- tropic) line {Henscri 's line) runs through the middle of the dark band. As both dark and light substances run through the entire thick- ness of the fibre, they in reality constitute discs of muscle substance (Fig. 51). By means of certain chemicals these discs may be separated, the separation taking place along the lines of Krause. Each " muscle disc " thus consists of that portion of a fibre included between two adjacent lines of Krause and is composed of a central dark disc, and on either side one-half of each adjacent light disc. A muscle fibre is thus seen to be divisible longitudinally into ultimate fibrillce, transversely into muscle discs. What is known as the sarcous element of Bow- man is that portion of a single fibrilla which is included in a single disc, i.e., between two adjacent lines of Krause (Fig. 51). The sarcoplasm is not evenly distributed 7 77K FIG. 49.— Semidiagramma- tic Drawing of Parts of two Muscle Fibres which have been broken, show- ing the relations be- tween Muscle Substance Proper and Sarcolemma. (Ranvier.) m, a, Retract- ed ends of muscle sub- stance, between which is seen the sarcolemma with several adherent muscle nuclei ; B, thin layer of muscle sub- stance which has adhered to the sarcolemma ; ft, muscle nucleus ; s, sar- colemma; p, space be- tween sarcolemma and muscle substance. 9 S THE. TISSUES. throughout the fibre. On cross section irregular trabeculge of sar-- coplasm are seen extending in from the sarcolemma (Fig. 52). These separate the fibrillae into bundles, the muscle columns of Kbllikcr. A transverse section of one of these columns presents the appearance of a network of sarcoplasm and of interfibrillar cement substance enclosing the fibrillae. This appearance is known as Cohnheini s field (Figs. 51 and 52). The contractile clement of the fibre, the fibrilla, is anisotropic, d . Fig. 50. Fig. si. Fig. 50. — Portion of Striated Voluntary Muscle Fibre. X 350. (Technic 4, p. 104.) The fibre is seen to be marked transversely by alternate light and dark bands. Through the centre of the light band is a delicate dark line (Krause's line) ; through the centre of the dark band a fine light line indicates Henson's line. The black line outlining the fibre repre- sents the sarcolemma. a, Fibrillae ; b, muscle nucleus ; c, Krause's line ; d, Henson's line. Fig. 51. — Diagram of Structure of a Muscle Column of Kolliker. The appearance presented by the cross-cut muscle column = Cohnheim's field, a, Muscle fibrillse ; b, sarcous element ; c, Krause's line, d, Hensen's line ; e, Cohnheim's field ; f, muscle disc. the sarcoplasm isotropic; the former, therefore, appears dark, the latter light by transmitted light. Upon this is based Rollet's theory of the structure of the striated muscle fibre (Fig. 53). According to this theory, each fibrilla consists of a number of rod-shaped segments joined end to end. Each segment consists of a thicker central por- tion, which tapers almost to a point where it joins the next adjacent segment. The point of union is marked by a minute globular swell- MUSCLE TISSUE. 59 ing. Between the fibrillae is the semi-fluid sarcoplasm. In the for- mation of a fibre similar parts of each fibril segment lie in the same transverse plane. The thicker portions lying side by side form the dark disc in which there is comparatively little sarcoplasm. The attenuated portions, with their relatively large amount of sarcoplasm, form the light disc. The row of globular swellings forms the line of Krause. Two varieties of striated voluntary muscle fibres are distinguished, white fibres and red fibres. The difference between the two is due to the amount of sarcoplasm — the red fibres being rich in sarcoplasm, the white fibres poor. Red fibres fl contract less rapidly than white, but are less easily fatigued. In man white fibres are in the large majority, red fibres never occurring Fig. 52. d c Fig. 53 . FIG. 52. — Semidiagrammatic Drawing of Transverse Section of a Voluntary Muscle Fibre, showing Sarcolemma; sarcoplasm separating fibrils into bundles, each bundle constitut- ing a muscle column of Kolliker and the appearance of its cross cut end being Cohnheim's field, a, Sarcoplasm ; b, Cohnheim's fields ; c, sarcolemma. FIG. 53. — Diagram representing Rollet's Theory of the Structure of a Voluntary Muscle Fibre, a, Dark disc ; b, light disc ; c, sarcoplasm ; d, fibrilla ; e, Krause's line. alone, but mingled with white fibres in some of the more active muscles, such as those of respiration and mastication. In some of the lower animals are found muscles made up w'holly of red fibres. Muscle fibres ending within the substance of a muscle have pointed extremities. Where muscle fibres join tendon, the fibre ends in a rounded or blunt extremity, the sarcolemma being continu- ous with the tendon fibres (Figs. 54 and 55). IOO THE TISSUES. m. &s Muscle fibres are usually unbranched. In some muscles — e.g.. those of the tongue and of the eye — anastomosing branches occur. When muscle fibres end in mucous membranes — e.g. , the muscle fibres of the tongue, — their terminations are often branched. Muscle fibres are multinuclear, some of the larger fibres containing a hundred or more nuclei. In the white fibres the nuclei are situated at the periphery just beneath the sarco- lemma. In red fibres they are centrally placed. 3. Involuntary Striated Muscle (Heart Muscle). — This occupies an intermediate posi- tion, both morphologically and embryologically, relative to smooth muscle and to striated vol- untary muscle. Like the former, it is com- posed of cells. Like the latter, it is both longitudinally and transversely striated. Heart- muscle cells are short, thick cylinders. These are joined end to end to form long fibres. By means of lateral branches the cells of one fibre anastomose with cells of adjacent fibres. Each heart-muscle cell usually contains one nucleus ; some cells contain several nuclei. While there is no distinct sarcolemma, the sar- coplasm is more abundant at the surface of the cell, thus giving much the appearance of an enclosing membrane. The amount of sarco- plasm throughout the cell is large. Around the nucleus is an area of sarcoplasm free from fibrillae. This area often extends some distance toward the ends of the cell. The striations of heart muscle are less dis- tinct than are. those of voluntary muscle. Ac- cording to McCallum, they represent very sim- ilar structures. The longitudinal striations indicate fibrillce united by cement substance. From the central mass of sarcoplasm which surrounds the nucleus, strands radiate toward the periphery. These strands, anastomosing, FlG. 54. — Semidiagram- matic Illustration of Endings of Muscle Fi- bres within a Muscle and in Tendon. (Gage.) a, Tapering end of fibre terminating within the muscle; the lower end of the central fibre shows the same method of termination ; c, c, each fibre terminates above in pointed intra- muscular ending, below in blunt ending con- nected with tendon. MUSCLE TISSUE. IOI separate the fibrillar into columns, the muscle columns of Kbllikcr. In cross section these present the appearance described under vol- untary muscle as Cohnheim s fields. The disposition of the sarco- plasm, extending outward from the region of the nucleus like the spokes of a wheel, gives to the cross section a characteristic radiate appearance (Fig. 57). The transverse markings represent, as in vol- untary muscle, alternate light and dark discs. Through the middle of the light disc can be seen the membrane of Kransc. McCallum describes Krause's membrane as belonging not only to the fibrillar element, but also to the sarcoplasm. The latter he describes as further subdivided by membranes, which are transversely continuous Fig. 55. Fig. 56. Fig. 55. — Two Muscle Fibres from Upper End of Human Sartorius, to show connection of muscle and tendon, x 350. (Gage.) m, Muscle fibres; t, tendon fibres. Fig. 56. — Muscle Cells from the Human Heart (techhic 6, p. 104), showing lateral branches and lines of union between cells. X 500. with Krause's membranes, into minute discs. The centre of the cell around the nucleus is wholly composed of these little discs of sarcoplasm. McCallum describes two appearances which the lines of union between the muscle cells present. In one each fibrilla shows a thickening at the cement line, from which one or more delicate fila- ments cross the cement to unite with similar filaments from an oppo- 102 THE TISSUES. site fibrilla. In the other form of union the cement substance is crossed by intercellular bridges similar to those described under epithelium. Recent investigations tend to prove that what have been described as heart-muscle cells are not separate units, but that heart muscle is a syncytial tissue, each cell representing only a growth segment of the whole muscle fibre. The occurrence of non-nucleated seg- FIG. 57.— Section of Heart Muscle. X 350. (Technic7, p. 104.) a, Cells cut longitudinally; 6, cells cut transversely (only three nuclei have been included in the plane of section) ; c, cells cut obliquely ; d, connective-tissue septum. ments and the fact that the longitudinal fibrillae are described by some observers as passing uninterruptedly through the " intercellu- lar " cement substance favor this view. On the other hand, the ease with which heart muscle may be separated into cells, especially in young animals and in the lower vertebrates, and the definite staining reaction which the intercellular substance gives when subjected to the action of silver nitrate are in favor of a cellular structure. Development of Muscle Tissue. In the higher animals muscle tissue, with the single exception of the sweat-gland muscles (page 55), is derived wholly from mesoderm. Smooth muscle is developed from the mesenchyme, while heart mus- cle and voluntary muscle are derived from the mesothelium. The smooth muscle cell shows the least differentiation. In be- coming a smooth muscle cell the embryonal cell changes its shape, MUSCLE TISSUE. 103 becoming greatly elongated, while at the same time its spongioplasm is arranged as longitudinally disposed contractile fibrils. A voluntary muscle fibre is a highly differentiated multinuclear cell or syncytium. Each fibre is developed from a single cell {myo- blast) of one of the embryonic muscle segments or myotomes. These cells, which are at first spherical, become elongated and spindle- shaped. The nucleus is at this stage centrally placed, and the spongioplasm occurs in the form of a reticulum. Regular arrange- ment of the spongioplasm first appears around the periphery, while the central portion of the cell is still occupied by reticular spongio- plasm and the nucleus. The fibrils extend toward the centre until they fill the entire cell, which has now become a muscle fibre. Dur- ing this process of fibrillation the nucleus has been undergoing mi- totic division. In the white fibres these nuclei migrate to the surface and come to lie just beneath the sarcolemma. The cement substance which unites the fibrils, as well as the larger masses of sarcoplasm, represents the remains of still undifferentiated protoplasm (hyalo- plasm). McCallum describes the development of heart muscle in the pig as follows : In embryos 10 mm. long the heart muscle consists of closely packed spindle-shaped cells, each containing an oval nucleus. The spongioplasm is arranged in the form of a network, no fibrils being present. In embryos 25 mm. long the shape of the cell re- mains unchanged, but on cross section there can be seen around the periphery a row of newly formed fibril bundles which have developed from the spongioplasm. From the periphery fibril bundles spread toward the centre. In embryos 70 mm. long the heart-muscle cell has assumed its adult shape and structure. Attention has already been called (page 42) to the spongio- plasm as the contractile element of protoplasm. It is to be noted that in the development of muscle no new element appears, the con- tractile fibrillce representing nothing more than a specialization of the already contractile spongioplasm. TECHNIC. (1) Isolated Smooth Muscle Cells. — Place small pieces of the muscular coat of the intestine in 0.1-per-cent aqueous solution of potassium bichromate, or in 30-per- cent alcohol for forty-eight hours. Small bits of the tissue are teased thoroughly and mounted in glycerin. Nuclei may be demonstrated by first washing the tissue and then staining for twelve hours in alum-carmine (page 16). This is poured off, 104 THE TISSUES. the tissue again washed in water and preserved in eosin-glycerin, which gives a pink color to the cytoplasm. (2) Potassium hydrate in 40-per-cent aqueous solution is also recommended as a dissociater of smooth muscle cells. Pieces of the muscular coat of the intestine are placed in this solution for five minutes, then transferred to a saturated aqueous solution of potassium acetate containing i-per-cent hydric acetate for ten minutes. Replace the acetate solution by water, shake thoroughly, allow to settle, pour off water, and add alum-carmine solution (page 16). After twelve hours' staining, wash and transfer to eosin-glycerin. (3) Sections of Smooth Muscle. — Fix small pieces of intestine in formalin- Miiller's (technic 5, p. 6) or in Zenker's fluid (technic 9, p. 7). Thin transverse or longitudinal sections are stained with haemotoxylin-eosin (technic 1, p. 17), and mounted in balsam. As the two muscular coats of the intestine run at right angles to each other, both longitudinally and transversely cut muscle may be studied in the same section. (4) Striated Voluntary Muscle Fibres. — One of the long muscles removed from a recently killed animal is kept in a condition of forced extension while a i-per- cent aqueous solution of osmic acid is injected into its substance at various points by means of a hypodermic syringe. Fixation is accomplished in from three to five minutes. The parts browned by the osmic acid are then cut out and placed in pure glycerin, in which they are teased and mounted. (5) Sections of Striated Voluntary Muscle. — Fix a portion of a tongue in forma- lin-Muller's fluid or in Zenker's fluid (page 7). Thin sections are stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 18) and mounted in balsam. As the muscle fibres of the tongue run in all directions, fibres cut transversely, longitudi- nally, and obliquely may be studied in the same section. The sarcolemma, the pointed endings of the fibres, and the relation of the fibres to the connective tissue can also be seen. (6) Isolated heart-muscle cells may be obtained in the same manner as smooth muscle cells (see technic 1, p. 103). (7) Sections of Heart Muscle. — These are prepared according to technic 3, (above). By including the heart wall and a papillary muscle in the same section, both longitudinally and transversely cut cells are secured. The stain may be either haematoxylin-eosin (technic 1, p. 17), or haematoxylin-picro-acid-fuchsin (technic 3, p. r8). CHAPTER VI. NERVE TISSUE. The Neurone. In most of the cells thus far described the protoplasm has been confined to the immediate vicinity of the nucleus. In the smooth muscle cell was seen an extension of protoplasm to a considerable distance from the nuclear region, while in the connective-tissue cells of the cornea the protoplasmic extensions took the form of distinct processes. Processes, often extending long distances from the cell body proper, constitute one of the most striking features of nerve- cell structure. Some of these processes are known as nerve fibres; and nerve tissue was long described as consisting of two ele- ments, nerve cells and nerve fibres. With the establishment of the unity of the nerve cell and the nerve fibre, the nerve cell with its processes was recognized as the single structural unit of nerve tissue. This unit of structure is known as a neurone. The neurone may thus be defined as a nerve cell with all of its processes. In the embryo the neurone is developed from one of the ectoder- mic cells which constitute the wall of the primitive neural canal. This embryonic nerve cell, or neuroblast, is entirely devoid of proc- esses. Soon, however, from one end of the cell a process begins to grow out. This process is known as the axone (axis-cylinder process, neuraxone, neurite). Other processes appear, also as outgrowths of the cell body ; these are known as protoplasmic processes or dendrites. Each adult neurone thus consists of a cell body, and passing off from this cell body two kinds of processes, the axis-cylinder process and the dendritic processes (Fig. 58). I. The Cell Body. Like most other cells, the nerve cell body consists of a mass of protoplasm surrounding a nucleus (Fig. 59). Nerve cell bodies vary in size from very small cell bodies, such as those found in the granule layers of the cerebellum and of the olfac- tory lobe, to the large bodies of the Purkinje cells of the cerebellum and of the motor cells of the ventral horns of the cord, which are *°5 io6 THE TISSUES. among the largest in the body. There is as much variation in shape as in size, and some of the shapes are characteristic of the regions in which the cells are situated. Thus the bodies of the cells of the spinal ganglia are spheroidal; of most of the cells of the cortex cerebri, pyramidal ; of the cells of Purkinje, pyrif orm ; of the cells of the ventral horns of the cord, irreg- ularly stellate. According to the num- FIG. 59. Fig. 58. — Scheme of Lower Motor Neurone. The cell body, protoplasmic processes, axone, collaterals, and terminal arborizations in muscle are all seen to be parts of a single cell and together constitute the neurone. (Barker), c, Cytoplasm of cell body containing chromophilic bodies, neurofibrils, and perifibrillar substance ; n, nucleus ; «', nucleolus ; d, dendrites ; a h, axone hill free from chromophilic bodies; ax, axone ; sf, side fibril (collateral) ; m, medullary sheath ; n R, node of Ranvier where side branch is given off ; s/, neurilemma and incisures of Schmidt ; m', striated muscle fibre ; tel, motor end plate. FlG. 59. — Large Motor Ganglion Cell from Ventral Horn of Spinal Cord of Ox, showing Chromophilic Bodies. (From Barker, after von Lenhossek.) a, Pigment; b, axone; c, axone hill ; d, dendrites. ber of processes given off, nerve cells are often referred to as ttnipolar, bipolar, or multipolar. The nucleus of the nerve cell (Fig. 59) differs in no essential from the typical nuclear structure. It consists of (1) a nuclear mem- brane, (2) a chromatic nuclear network, (3) an achromatic nucleo- plasm, and (4) a nucleolus. NERVE TISSUE. 107 The cytoplasm of the nerve cell consists of two distinct ele- ments : (1) Neurofibrils, and (2) perifibrillar substance. In most nerve cells a third element is present, (3) chromophilic bodies. (1) The neurofibrils are extremely delicate fibrils which are con- tinuous throughout the cell body and all of its processes. Within the body of the cell they cross and interlace and probably anastomose (Fig. 60). (2) The perifibrillar substance (Fig. 60) is a fluid or semifluid substance which both in the cell body and in the processes sur- Ik Fig. 60.— Ganglion Cells, Stained by Bethe's Method, showing Neurofibrils. A, Anterior horn cell (human) ; B, cell from facial nucleus of rabbit ; C, dendrite of human anterior horn cell showing arrangement of neurofibrils. (Bethe.) rounds and separates the neurofibrils. It is believed by some to be like the fibrils, continuous throughout cell body and processes, by others to be interrupted at certain points in the axone (see page 1 13). ioS THE TISSUES. (3) The ■ chromophilic bodies (Fig. 59) are granules or groups of granules which occur in the cytoplasm of all of the larger and of some of the smaller nerve cells. They are best demonstrated by means of a special technic known as the method of Nissl (page 32). When subjected to this technic, nerve cells present two very different types of reaction. In certain cells, only the nuclei stain. Such cells are found in the granule layers of the cerebellum, olfactory lobe, and retina. They are known as earyochromes, and apparently consist wholly of neurofibrils and peri- fibrillar substance. Other cells react, both as to their nuclei and as to their cell bodies, to the Nissl stain. These cells are known as somcitocliromes. Tak- ing as an example of this latter type of cell one of the motor cells of the ventral horn of the cord and subjecting it to the Nissl technic, we note that the cyto- plasm is composed of two distinct ele- ments : (a) a clear, unstained ground substance, and, scattered through this, (b) deep blue- staining masses, the chromo- philic bodies (Fig. 59). These bodies are granular in character and differ in shape, size, and arrangement. They may be large or small, regular or irregU- fig. 61. -Pyramidal Cell from Human lar in shape, may be arranged in rows or Cerebral Cortex. (Golgi bichlorid ... . . method, see 2 , p 97 .) Goigi cell m an irregular manner, may be close type i. a , ceii body; d, main or together, almost filling the cell body, or apical dendrite showing gemmules ; c lateral dendrites showing gem- quite separated from one another. Pre- oS?l5T ae with . co ; ,a ; e , rals - senting these variations in different. Only part of axone is included in t> drawing. types of cells, the appearance which the chromophilic bodies present in a particular type of cell remains, constant, and has thus been used by Nissl as a basis of classification. It is important to note in studying the nerve cell by this method 1 For this classification, the significance of which is somewhat doubtful, the reader is referred to Barker, "The Nervous System and Its Constituent Neurones," p. 121. NERVE TISSUE. 109 that somatochrome cells of the same type frequently show marked variations in staining intensity. This appears to depend upon the size and closeness of arrangement of the chromophilic bodies, and this again seems dependent upon changes in the cytoplasm connected with functional activity. In cells stained by Nissl's method the cytoplasm between the chromophilic bodies remains unstained and apparently structureless, and it is this part of the cytoplasm that corresponds to the neuro- fibrils and perifibrillar substance. -The relation which the appearance of the Nissl-stained cell bears to the structure of the living protoplasm is still undetermined. Ac- cording to some investigators the Nissl bodies exist as such in the living cell. Others believe that they are not present in the living Fig. 62.— Golgi Cell Type II. from Cerebral Cortex of Cat. (Kolliker.) .r, Coarse proto- plasmic processes with gemmules easily distinguishable irom the more delicate, smoother axone, a. The latter is seen breaking up into a rich plexus of terminal fibres near its cell of origin, practically the entire neurone being included in the drawing. cell, but represent precipitates due either to post-mortem changes or to the action of fixatives. The significance of the Nissl picture from the standpoint of pathology lies in the fact that when subjected to a given technic, a particular type of nerve cell always presents the same appearance, and that this appearance furnishes a norm for com- no THE TISSUES. parison with cells showing pathological changes, and which have been subjected to the same technic. Many nerve cells contain more or less brownish or yellowish pig- ment (Fig. 59). This pigment is not present in the cells of the new- born, but appears in increasing amounts with age. Its significance is not known. II. The Protoplasmic Processes or Dendrites.— These have a structure similar to that of the cell body, consisting of neurofibril perifibrillar substance, and, insomatochrome cells, chromophilic bodies (Figs. 59 and 60). Dendrites branch dichotomously, become rapidly smaller, and usually end at no great distance from the cell body (Figs. 61 and 62). III. The Axone. — This differs from the cell body and dendrites in that it contains no chromophilic bodies (Fig. 59), consisting wholly of neurofibrils and perifibrillar substance. Not only is it entirely achromatic itself, but it always takes origin from an area of the cell body, the axo ne hill or implantation cone (Fig. 59), which is free from chromophilic bodies. It is as a rule single, and while usually aris- ing from the body of the cell may be given off from one of the larger protoplasmic trunks. Some few cells have more than one axone, and nerve cells without axones have been described. In Golgi prepara- tions the axone is distinguished by its straighter course, more uni- form diameter, and smoother outline (Fig. 61). It sends off few branches (collaterals) , and these approximately at right angles. Both axone and collaterals usually end in terminal arborizations. In most cells the axone extends a long distance from the cell body. Such cells are known as Golgi cell type 1. (Fig. 61). In others the axone branches rapidly and ends in the gray matter in the vicinity of its cell of origin — Golgi cell type II. (Fig. 62). As they leave the cell body the neurofibrils of the axone converge to a very narrow portion of the axone, where the perifibrillar sub- stance is much reduced in amount, or according to some, entirely interrupted. Beyond this the fibrils become more separated and the perifibrillar substance more abundant. Some axones pass from their cells of origin to their terminations as "naked" axones, i.e., uncovered by any sheath. Other axones are enclosed by a thin membrane, the neurilemma or sheath of Schwann. Still others are surrounded by a sheath of considerable thickness known as the medullary sheatJi. NERVE TISSUE. 1 1 1 Depending upon the presence or absence of a medullary sheath, axones may thus be divided into two main groups — medullated axones and non-medullated axones. r. Nox-medullated axoxes (non-medullated nerve fibres) (Fig. 63). These are subdivided into non-medullated axones without a neurilemma and non-medullated axones with a neurilemma. (a) Non-medullated axones without a neurilemma are merely naked axones. Present in large numbers in the embryo, they are in the n d — B A Fig. 63. Fig. 64. Fig. 63.— Xon-medullated Xerve Fibres with Xeurilemma, only the nuclei of which can be seen. X 300. Fig. 64. — A, Fresh Medullated Xerve Fibre from Sciatic Xerve of Guinea-pig 1 (X 700), show- ing relative size of axone and medullary sheath. B, Medullated Xerve Fibre from Hu- man Cauda Equina (X 700) (technic 4, p. 117), showing shrunken axtne. a, Axone ; d, medullary sheath ; c, node of Ranvier ; J, neurilemma ; //, incisures of Schmidt; /, nu- cleus of neurilemma. adult confined to the gray matter and to the beginnings and endings of sheathed axones, all of the latter being uncovered for a short dis- tance after leaving the nerve cell body, and also just before reaching their terminations. I 12 THE TISSUES. h— Mi (p\ Non-medullated axones with a neurilemma — fibres oj Remak. In these the axone is surrounded by a delicate homogeneous, nucleated sheath, the neurilemma or sheath of Schwann (Fig. 63). These axones are described by some writers as having no true neurilemma, but merely a '"%& mT~ b discontinuous covering of flat connective-tissue 1 cells, which wrap around the axone and corre- spond to the endoneurium of the nerve trunk a b c (see page 343). \\J 2. Medullated axones (med- ullated nerve fibres). — These, like the non-medullated, are subdivided according to the presence or ab- sence of a neurilemma into med- ullated axones with a neurilemma and medullated axones without a neurilemma. (a) Medullated axones with a neurilemma constitute the bulk of the fibres of the cerebro-spinal nerves. Each fibre consists of (1) an axone, (2) a medullary sheath, and (3) a neurilemma. (1) The axone is composed of neurofibrils continuous with those of the cell body, and like them lying in a perifibrillar substance or neuroplasm (Fig. 65). In the fresh condition the axone is broad, and shows faint longitudinal stri- ations corresponding to the neuro- fibrils, or appears homogeneous (Fig. 64, A). Fixatives usually cause the axone to shrink down to a thin axial thread, whence its older name of axis-cylinder (Fig. 64, B). A delicate membrane has been described by some as enveloping the axone. It is known as the axo lemma ox periaxial sheath (Fig. 65). Fig. 65. Fig. 66. Fig. 65.— Diagram of Structure of a Med- ullated Nerve Fibre, showing two differ- ent views as to relations of neurilemma and axolemma and their behavior at the nodes of Ranvier. (Szymonowicz.) a, Neurofibrils; b< cement substance; c, axone ; d, incisure of Schmidt ; e, nucleus of neurilemma ; /, medullary sheath ; jr, sheath of Schwann ; //, axone ; z', axolem- ma ; j, sheath of Schwann; k, node of Ranvier. FlG. 66.— Piece of Medullated Nerve Fibre from Human Radial Nerve. X 400. Os- mic-acid fixation and stain. (Szymono- wicz.) a, Medullary sheath ; b, axone ; c, sheath of Henle ; d, nuclei of Henle's sheath ; e. nucleus of neurilemma. NERVE TISSUE. 113 (2) The medullary sheath (Figs. 64 and 65) is a thick sheath composed of a semifluid substance resembling fat and known as mye- lin. In the fresh state the myelin has a glistening homogeneous appearance. It is not continuous, but is divided at intervals of from So to 600//- by constrictions, the nodes or constrictions of Ranvier. That portion of a fibre included between two nodes is known as an intemode (Fig. 65). The length of the internode is usually propor- tionate to the size of the fibre, the smaller fibres having the shorter internodes. In fresh specimens the medullary sheath of an inter- node is continuous (Fig. 64, A), but in fixed specimens it appears broken up into irregular segments, ScJimidt-Lantermann segments, by clefts which pass from the neurilemma to the axolemma or axone, and are known as the clefts or incisures of Schmidt- L ant ermann (Fig. 64, B). On boiling medullated nerve fibres in alcohol and ether a fine network is brought out in the medullary sheath, the neurokeratin netzvork. Owing to the resistance of neurokeratin to the action of trypsin, it has been considered as possibly similar in composition to horn. (3) The neurilemma or sheath of Sclnvann (Figs. 64, B, and 65) is a delicate structureless membrane which encloses the myelin. At the nodes of Ranvier the neurilemma dips into the constriction and comes in contact with the axone or axolemma. Against the inner surface of the neurilemma, usually about midway between two nodes, is an oval-shaped nucleus, the nucleus of the neurilemma (Figs. 64, B, and 66). Each nucleus is surrounded by an area of granular proto- plasm, and makes a little depression in the myelin and a slight bulg- ing of the neurilemma (Fig. 64, B). In addition to the above-described sheaths, most medullated fibres of peripheral nerves have, outside the neurilemma, a nucleated sheath ■of connective-tissue origin, known as the sheath of Henle (Fig. 66). Two views as to the relation of the axolemma to the neurilemma are illustrated in Fig. 65. According to one the neurilemma is con- tinuous, merely dipping into the nodes of Ranvier, where it touches the axolemma or the axone. According to the second both neurilem- ma and axolemma are interrupted at the node, but unite with each other there co enclose completely the medullary substance of the internode. Recent experiments of Bethe and others tend to prove an inter- ruption of the perifibrillar substance at the node of Ranvier. They consider the axone at the node as probably crossed by a sieve-like 114 THE TISSUES. plate, through the holes of which the fibrils pass, but which com- pletely interrupts the perifibrillar substance. Medullated nerve fibres vary greatly in size. The finer fibres have a diameter of from 2 to 4 //, those of medium size from 4 to \o ',)-, the largest from 10 to 20 p-. They have few branches, and these are always given off at the nodes of Ranvier. (b) Medullated axones without a neurilemma are the medullated nerve fibres which form the white matter of the central nervous sys- tem. Their structure is similar to the above-described structure of a medullated nerve fibre with a neurilemma, except for the absence of the latter sheath. As to the physiological significance of the structural elements of the neurone, we have little absolute knowledge but certain fairly well- grounded theories. That portion of the neurone which surrounds the nucleus — the cell body — is, as already stated, the genetic or birth centre of the neu- rone, the nucleus as in other cells being probably concerned in the general cell metabolism. From the behavior of the processes when cut off from the cell body it is evident that the latter is the trophic or nutritive centre of the neurone. It seems probable that from the standpoint of neurone activity, the cell body usually acts as the func- tional centre of the neurone, the processes acting mainly as channels through which impulses are received and distributed. Certain facts, such for example as the entire absence of chromophilic bodies in many nerve cells, which nevertheless undoubtedly functionate ; the ab- sence of these bodies in all axones ; the diminution of the chromatic substance during functional activity ; its much greater diminution if activity be carried to the point of exhaustion; these together with its behavior under certain pathological conditions all favor the theory that the stainable substance of Nissl is not the active nerve element of the cell, but is rather of the nature of a nutritive element. There thus remain to be considered as possible factors in the transmission of the nervous impulse the neurofibrils and the peri- fibrillar substance. While a few investigators are inclined to mag- nify the importance of the latter, the majority agree in considering the neurofibrils as the actual nervous mechanism oj the neurone. The already referred to observations of Bethe regarding the interruption of the perifibrillar substance at the constricted portion of the axone and at the nodes of Ranvier, thus making the neurofibrils the only continuous structure, are obviously in favor of this view. The neurofibrils are probably a differentiation of the spongioplasm, while the perifibrillar substance and chromophilic bodies are specializations of the hyaloplasm. NERVE TISSUE. 115 As to the manner in which neurones are connected, there are two main theories, the contact theory and the continuity theory. According to the contact theory each neurone is a distinct and separate entity. Association between neurones is by contact or con- tiguity of the terminals of the axone of one neurone with the cell body or dendrites of another neurone, and never by continuity of their protoplasm. This theory, which is known as the " neurone theory " and which received general acceptance as a result of the work of Gol- gi, His, Forel, Cajal, and others, has been recently called in question if not actually disproved by the discovery of the continuity of the neurofibrils. Based upon this theory is the so-called "retraction theory," which held that a neurone being associated with other neu- rones only by contact was able to retract its terminals, thus breaking the association and throwing itself, as it were, out of circuit. According to the more recent continuity theory, while the peri- fibrillar substance is interrupted as above described, the neurofibrils are continuous. According to this theory the neurofibrils, which form a plexus or network within the cell body and dendrites, are connected with a pericellular network — the Golgi net — -which closely invests the cell body and its dendrites. Externally the Golgi net is further connected with the neurofibrils of the axones and collate- rals of other nerve cells. This connection is either direct, or, as some believe, through another general (diffuse) extracellular network. The neurofibrils are thus, according to this theory, continuous and form two or possibly three continuous networks : (a) an intracellular network, (b) a pericellular network (Golgi), and (c) a more diffuse extracellular network, lying between the cells. Neuroglia. This is a peculiar form of connective tissue found only in the central nervous system. Unlike the other connective tis- sues, neuroglia is of ectodermic origin, being developed from the ectodermic cells which line the embryonic neural canal. These cells, at first morphologically identical, soon differentiate into neuro- blasts or future neurones, and spongioblasts or future neuroglia cells. In the adult two main types of neuroglia cells are found — spider cells and mossy cells (Fig. 67). Spider cells consist of a cen- tral portion containing the nucleus and of delicate, radiating, straight, unbranched processes. Mossy cells also have a central nucleated por- tion and processes ; the latter are, however, rough, thick, and branch- ing. As in the nerve cell, the processes of neuroglia cells do not anastomose, but form a network of interlacing fibrils for the support n6 THE TISSUES. of the nervous tissue proper. Spider cells occur chiefly in the white matter, mossy cells in the gray matter in connection with blood- vessels. While these represent the two most common types of neu- roglia cells, many other forms occur, which are probably transitional between the two types described. According to YVeigert, what are in Golgi preparations apparently processes of the cells, are entirely separate neuroglia fibres, the neu- FlG. 67. — A, Neuroglia Cell — Spider Type— Human Cerebrum. B, Neuroglia Cell — Mossy- Type— Human Cerebrum. roglia cells having no processes. Weigert would thus make the structure of neuroglia analogous to that of fibrous connective tissue, i.e., composed of cells and a fibrillar intercellular substance. Other investigators using the special Weigert neuroglia stain claim that this stain fails to act upon the non-fibrillar elements of the cytoplasm body, and that the apparently separate fibrils are really a part of the protoplasm of the neuroglia cell. TECHNIC. (i) Pieces of the cerebral cortex are stained by one of the Golgi methods. If the rapid or mixed silver method is used, sections must be mounted in hard bal- sam without a cover ; if the slow silver or the bichloride method is used, the sections may be covered. Sections are cut from 75 to 100 fi in thickness, cleared in carbol- xylol or oil of origanum and mounted in balsam. This section shows only the ex- ternal morphology of the neurone. It is also to be used for studying the different varieties of neuroglia cells as demonstrated by Golgi's method (see page 29). NERVE TISSUE. 117 (2) Thin transverse slices from one of the enlargements of the spinal cord are fixed in absolute alcohol. Thin sections (5 to io,«) are stained by Nissl's method (page 32) and mounted in balsam. This section is for the purpose of studying the internal structure of the nerve cell and processes as demonstrated by the method of Nissl. (3) Medullated Nerve Fibres (fresh). — Place a small piece of one of the sciatic or lumbar nerves of a recently killed frog in a drop of salt solution and tease longi- tudinally. Cover and examine as quickly as possible. Note the diameter of the axone and of the medullary sheath and the appearance of the nodes of Ranvier. An occasional neurilemma nucleus can be distinguished. (4) Medullated nerve fibres — fibres from the Cauda equina (this material has the advantage of being comparatively free from fibrous connective tissue) are fixed in formalin-MuTler's fluid (technic 5, p. 6), and hardened in alcohol. Small strands are stained twenty minutes in strong picro-acid-fuchsin solution (technic 2, p. 18), washed thoroughly in strong alcohol, cleared in oil of origanum, thoroughly teased longitudinally and mounted in balsam. General References for Further Study of Tissues. Hertwig : Die Zelle und die Gewebe. Kolliker : Handbuch der Gewebelehre. Ranvier: Traite Technique d'Histologie. Cabot: A Guide to the Clinical Examination of the Blood for Diagnostic Pur- poses. Ewing: Clinical Pathology of the Blood. Wood : Laboratory Guide to Clinical Pathology. Prenant, Bourn et Maillard : Traite d'Histologie. Barker: The Nervous System. Van Gehuchten : Le Systeme nerveux de l'homme. Bethe : Allgemeine Anatomie und Physiologie des Nervensystem. PART IV. THE ORGANS. CHAPTER I. THE CIRCULATORY SYSTEM. The circulatory apparatus consists of two systems of tubular structures, the blood-vessel system and the lymph-vessel system, which serve respectively for the transmission of blood and lymph. THE BLOOD-VESSEL SYSTEM. This consists of (a) a central propelling organ, the heart ; {b) a series of efferent tubules — the arteries — which by branching con- stantly increase in number and decrease in calibre, and which serve to carry the blood from the heart to the tissues; (e) minute anasto- mosing tubules — the capillaries — into which the arteries empty and through the walls of which the interchange of elements between the blood and the other tissues takes place ; (d) a system of converging tubules — the veins — which receive the blood from the capillaries, decrease in number and increase in size as they approach the heart, and serve for the return of the blood to that organ. The entire system — heart, arteries, veins, capillaries — has a com- mon and continuous lining, which consists of a single layer of endo- thelial cells. Of the capillaries this single layer of cells forms the only wall. In the heart, arteries, and veins, the endothelium serves simply as the lining for walls of muscle and connective tissue. Capillaries. It is convenient to describe these first on account of their sim- plicity of structure. A capillary is a small vessel from 7 to i6,a in diameter. Its wall consists of a single layer of endothelial cells. 7 i/ The cells are somewhat elongated in the long axis of the vessel. Their edges are serrated and are united by a small amount of inter- 121 122 THE ORGANS. cellular substance. Capillaries branch without diminution in calibre, and these branches anastomose to form capillary networks, the meshes FIG. 68. — Vein and Capillaries. Silver-nitrate and hematoxylin stain (technic 7, p. 66) , to show outlines of endothelial cells and their nuclei. fed FIG. 69.— Diagram of Capillaries, Small Artery, and Vein, showing their structure and rela- tions, a, Capillaries ; b, nuclei of capillary endothelium ; c, precapillary arteries ; d, arte- riole ; e, small vein ; f\ small artery. of which differ in size and shape in different tissues and organs (Figs. 68, 69, 70). Arteries. The wall of an artery consists of three coats : (1) An inner coat, the intima. (2) A middle coat, the media. THE CIRCULATORY SYSTEM. 123 (3) An outer coat, the adventitia. The intima consists of a single layer of endothelial cells, con- tinuous with and similar to that forming the walls of the capillaries, or, in arteries of considerable size, of this layer plus more or less connective tissue. The middle coat consists mainly of smooth muscle, the outer of connective tissue. The structure of these three coats varies according to the size of the artery, and while the transition between them is never abrupt, it is convenient, for purposes of description, to distinguish (a) small arteries, (b) medium-sized arteries, and (e) large arteries. Small Arteries. — Passing from a capillary to an artery, the first change is the addition of a thin sheath of connective tissue around the outside of the endothelial tube. A little farther back isolated smooth muscle cells, circularly arranged, begin to ap- pear between the endothelium and the connective tissue. Such an artery is known as a precapillary artery. The next transition is the FlG. 70. — Capillary Network from Human Pia Mater, showing also an arteriole in "optical section " and a small vein. X 350. (Technic 1, p. 128.) «, Vein ; b, arteriole ; c, large cap- illar}- ; d, small capillaries. completion of the muscular coat, the muscle cells now forming a con- tinuous layer. Such an artery, consisting of three distinct coats, the middle coat composed of a single continuous layer of smooth muscle cells, is known as an arteriole (Fig. 69, d ; Fig. 70, /;). Medium- Sized Arteries. — This group comprises all the named arteries of the body with the exception of the aorta and the pulmo- 124 THE ORGANS. nary. Their walls are formed of the same three coats found in the arteriole, but the structure of these coats is more elaborate. i. The intima consists of three layers (Fig. 71). (a) An inner endothelial layer already described. (&) A middle layer, the intermediary layer of the intima. This is composed of delicatejvvhite and elastic fibrils and connective-tis- sue cells. (c) An outer layer, the elastic layer of the intima, or membrana elastica interna — a thin fenestrated membrane of elastic tissue! This membrane is intimately connected with the media and marks the .-a 71. — From Cross-section through Walls of Medium-sized Artery and its Accompanying Vein. X 75. (Technic 3, p. 129.) A, Tntima of artery; a, its endothelial layer ; d, its in- termediary layer; c, its elastic layer ; B, media of artery ; C, adventitia, the upper part belonging to the artery, the lower to the vein ; within the adventitia are seen the vasa vasorum ; D, media of vein ; E, intima of vein ; t, its intermediary layer ; /, its endothe- lial layer. boundary between the latter and the intima. In the smallest of the medium-sized arteries the intermediary layer is often wanting, the endothelial cells resting directly upon the elastic membrane. Owing to the extensive amount of elastic tissue in their walls, there is a post-mortem contraction of arteries which results in the intima being thrown up into folds. For this reason the elastic membrane pre- sents, in transverse sections of an artery, the appearance of a wavy band (Fig. 71). THE CIRCULATORY SYSTEM. 125 2. The media is a thick coat of circularly disposed smooth jnus- clejcells (Fig. 71). Its thickness depends largely upon the size of the vessel, though varying somewhat for different vessels of the same size. A small amount of fibrillar connective tissue supports the muscl e cells. Elastic tissue is present in the media, the amount being usually proportionate to the size of the vessel. 1 In the smaller of the medium-sized arteries, the elastic tissue is disposed as delicate fibrils among the muscle cells. In larger arteries many coarse fibres are intermingled with the fine fibrils. When much elastic tissue is present the muscle cells are separated into more or less well-defined groups. In such large arteries as the subclavian and the carotid, elastic tissue occurs not only as fibrils but also as circularly disposed plates or fenestrated membranes. 3. The adventitia (Fig. 71) is composed of loose fibrous connec- tiyjtiissue with some elastic fibres. Occasionally there are^caftered smooth muscle cells. Both smooth muscle cells and elastic fibres are arranged longitudinally. The adventitia does not form a definitely outlined coat like the media or intima, but blends externally with the tissues surrounding the artery and serves to attach the artery to these tissues. In some of the larger arteries the elastic tissue of the' ad- ventitia forms an especially well-defined layer at the outer margin of the media. This is known as the membrana elastica externa. In general it may be said that the thickness oftKe adventitialmd the amount of elastic tissue present are directly proportionate to the size of the artery. Large arterie s like the aorta (Fig. 72) have the same three coats as small and medium-sized arteries. The layers are not, however, so distinct. This is due mainly to the excessive amount of elastic tis- sue in the media (Fig. 73), which makes indistinct the boundaries between intima and media, and between media and adventitia. The walls of the aorta are thin in proportion to the size of the vessel, in- creased strength being obtained by the decided increase in the amount of elastic tissue. Of the intima, the endothelial cells are 1 This proportion does not obtain for all vessels. Thus in the radial, femoral, and coeliac arteries there is comparatively little elastic tissue, while in the common iliac, carotid, and axillary the elastic tissue is in excess of the muscular. The disposition of elastic tissue in the walls of arteries which supply the brain is somewhat peculiar. The inner elastic membrane is especially well denned. There are but few elastic elements in the media, and the longitudinally disposed fibres of the adventitia are almost entirely wanting. 126 THE ORGANS. short and polygonal ; the intermediary layer similar to that of a me- dium-sized artery; the elastic layer less distinct and often broken up into several thin layers. The media consists mainly of elastic tissue Fig. 72.— From Transverse Section of Dog's Aorta. X 60. (Technie 4, p. 129.) a, Intima ; b y media ; c, adventitia ; d, vasa vasorum ; e, elastic tissue ; f, endothelium. arranged in circular plates or fenestrated membranes. Between the elastic-tissue plates are groups of smooth muscle cells and some fibrillated connective tissue. The adventitia resembles that of the medium-sized artery. There is no external elastic membrane. Veins. The walls of veins resemble those of arteries. There are the same three coats, intima, media, and adventitia, and the same ele- ments enter into the structure of each coat (Fig. 71). Venous walls are not, however, so thick as those of arteries of the same calibre, and the coats are not so distinctly differentiated from one another. The transition from capillary through the precapillary vein to the small THE CIRCULATORY SYSTEM. 127 vein is similar to that described under arteries (page 123). Unlike the artery, the thickness of the wall of a vein and its structure are not directly proportionate to the size of the vessel, but depend also upon other factors such as the position of the vein and the support given to its walls by surrounding structures. Of the intima the endothelial layer and the intermediary layer are similar to those of the artery. The elastic layer is not always present, is never so distinct, and is not wavy as in the artery (Fig. 71). The result is a lack of demarcation between intima and media, the connective tissue of the intermediary layer of the intima merging with the mixed muscle and connective tissue of the media. Project- ing at intervals from the inner surface of the wall of most veins are mm Fig. 73. — From Transverse Section of Dog's Aorta, to show Elastic Tissue. X 60. (Technic 7, p. 129. Elastic tissue stained black, a. Intima; b, media; ) an outer composed of mixed fibrous and clastic tissue and smooth muscle cells. Externally the endocardium is closely attached to the myocardium. Strong fibrous rings (annuli fibrosi), composed of mixed fibrous THE CIRCULATORY SYSTEM. I 3 l and elastic tissue, surround the openings between auricles and ven- tricles. Similar but more delicate rings encircle the openings from the heart into the blood-vessels. The heart valves are attached at their bases to the annuli fibrosi. They are folds of the endocardium, and like the latter consist of fibrous and elastic tissue continuous with that of the rings and cov- ered by a layer of endothelium. The epicardium is the visceral layer of the pericardium. It is a serous membrane like the endocardium, which it resembles in struc- ture. It consists of a layer of mixed fibrous and elastic tissue cov- ered over by a single layer of mesothelial cells. Beneath the epicar- dium there is usually more or less fat. Blood-vessels. — Blood for the nutrition of the heart is supplied through the coronary arteries. The larger branches run in the con- nective tissue which separates the bundles of muscle fibres. From these, smaller branches pass in among the individual fibres, where they break up into a rich capillary network with elongated meshes. From the myocardium, capillaries penetrate the connective tissue of the epicardium and endocardium. The auriculo- ventricular valves are supplied with blood-vessels, while in the semilunar valves blood- vessels are wanting. Lymphatics. — Lymph channels traverse the epicardium and endo- cardium and enter the valves. Within the myocardium minute lymph vessels have been demonstrated between the muscle fibres and accompanying the blood-vessels. Nerves. — These are derived from both cerebro-spinal and sym- pathetic systems, and consist of both medullated and non-medullated fibres. Sympathetic ganglion cells are distributed in groups through- out the myocardium. Among these cells the nerve fibres form plex- uses from which both motor and sensory terminals are given off to the muscle. (For nerve endings in heart muscle see page 354.) TECHNIC. (1) The Heart. — Cut pieces through the entire thickness of the wall of one of the ventricles, care being taken not to touch either the serous surface or the lining endothelium. Fix in formalin-Miiller's fluid (technic 5. p. 6). Cut transverse and longitudinal sections; stain with hasmatoxylin-eosin (technic 1. p. 17) and mount in balsam. (2) Treat the entire heart of a small animal {e.g., guinea-pig or frog) in the same manner as the preceding, making transverse sections through both ventricles. (3) An entire heart, human or animal, may be fixed in the distended condition 132 THE ORGANS. by tilling with formalin-Muller's fluid under low pressure and then tying off the vessels. The entire heart thus distended is placed in a large quantity of the same fixative. Development of the Circulatory System. The blood-vessels and the heart begin their development sepa- rately and afterward become united. Both are derived from meso- derm. The earliest vessels to be formed are the capillaries. These make their appearance in the mesodermic tissue near the periphery of the area vasculosa which surrounds the developing embryo. Here groups of cells known as "blood islands" differentiate from the rest of the mesodermic cells. Within these islands channels appear which are lined with flat cells derived from cells of the islands. These represent the earliest capillaries. In post-embryonic life new capillaries develop by outgrowths from already existing capil- laries. These capillary "buds," at first solid, push their way through the intervening tissue and unite with similar buds from other capillaries. Through this solid structure a lumen is hollowed out by extension of the lumina of the older capillaries. Arteries and veins are developed from the capillaries by a further differen- tiation of the surrounding mesodermic cells to form the muscular and connective-tissue coats outside the first-formed capillary tube. The heart and the roots of the large vessels which spring from it, while also of mesoblastic origin, have an entirely different early development. The heart first appears as an endothelial tube, which develops like the capillaries by differentiation of mesodermic cells. Other mesodermic cells next form an entirely separate muscular tube around the endothelial tube. This is the primitive myocardium. These two tubes are at first united only in places by bands of con- nective tissue. Later they approach each other so that the inner tube, the endocardium, becomes a lining for the outer tube, the myo- cardium. The epicardium, as the visceral layer of the pericardium, has a separate origin, being constricted off from that portion of the mesoderm which lines the primary body cavity. The Lymph- Vessel System. The larger lymph vessels are similar in structure to veins. Their walls are, however, thinner than those of veins of the same calibre and they contain more valves. They are capable of great distention, and when empty collapse so that their thin walls are in apposition. THE CIRCULATORY SYSTEM. 133 The largest of the lymph vessels, the thoracic duct, has three well- defined coats : an intima consisting of the usual lining endothelium resting upon a subendothelial layer of delicate fibro-elastic tissue, the outermost elastic fibres having a longitudinal arrangement ; a fairly thick media of circularly disposed smooth muscle cells ; and an ad- ventitial which is strengthened by bundles of longitudinal smooth muscle. LympJi capillaries resemble blood capillaries in that their walls are composed of a single layer of endothelial cells. The cells are rather larger and more irregular than in blood capillaries, the capil- laries themselves are larger, and, instead of being of uniform diameter throughout, vary greatly in calibre within short distances. In cer- tain tissues dense networks of these lymph capillaries are found. Cleft-like lymph spaces — perivascular lymph spaces — partially sur- round the walls of the smaller blood-vessels. Lymph spaces without endothelial or other apparent lining also occur. Examples of these are the pericellular lymph spaces found in various tissues and the canaliculi of the cornea and of bone (pages 68 and 87). Similar in character to lymph spaces are the body cavities, peri- toneal, pleural, and pericardial, with their linings of serous membranes. These cavities first appear in the embryo as a cleft in the mesoderm — the coelom, body cavity, or plcuroperitoneal cleft. This cleft is lined with mesothelium beneath which the stroma is formed. These membranes not only line the cavities, but are reflected over most of the viscera of the abdomen and. thorax. They consist of a stroma of mixed fibrous and elastic tissue, covered on its inner side by a layer of mesothelium, the two being separated by a homogeneous basement membrane. The stroma contains numerous lymphatics. These communicate with the free surfaces by means of openings — stomata — surrounded by cuboidal cells, whose shape and granular protoplasm distinguish them from the neighboring flat mesothelium. TECHNIC. (1) Remove a portion of the central tendon of a rabbit's diaphragm. Rub the pleural surface gently with the finger or with a brush to remove the mesothe- lium. Rinse in distilled water and treat with silver nitrate as in technic 7, p. 66. Mount in glycerin. If the silver impregnation is successful, the networks of coarser and finer lymphatics can be seen as well as the outlines of the endothelium of their walls. If care has been taken not to touch the peritoneal surface, the peritoneal mesothelium and the stomata are frequently seen. 134 THE ORGANS. (2) The Thoracic Duct. — Remove a portion of the thoracic duct, fix in for- malin-Miiller's fluid (technic 5, p. 6^. and stain sections with hasmatoxylin-eosin (technic 1, p. 17). The Carotid Gland. This is a small ductless gland which lies at the bifurcation of the carotid artery. It is composed of a vascular connective tissue sup- porting spheroidal groups of polyhedral epithelial cells which are closely associated with tufts of capillaries. Some of the gland cells take a brownish stain with chromic acid similar to the medullary cells of the adrenal. The Coccygeal Gland. This is also a ductless gland similar in structure to the preceding, but with much more irregularly arranged groups of cells. TECHNIC. Technic same as for Thyroid Gland, page 257. General References for Further Study of the Circulatory System. Kolliker: Handbuch der Gewebelehre des Menschen, vol. iii. Stohr: Text-book of Histology. Schafer : Histology and Microscopic Anatomy, in Quain's Elements of Anat- omy, tenth edition. CHAPTER II. LYMPHATIC ORGANS. The Lymph Nodes. Lymph nodes are small bodies, usually oval or bean-shaped, which are distributed along the course of the lymph vessels. In some regions they are arranged in series forming " chains " of lymph nodes as, e.g., the axillary and inguinal. Each lymph node is surrounded by a capsule of connective tissue which sends trabecules or septa into the organ. The capsule and \ ,jt"' h g f FIG. 74. — Section through Entire Human Lymph Node, including: Hilum. X 15. (Technic 1, p. 138.) Dark zone, cortex ; light central area, medulla, a, Lymph nodule of cortex ; />, germinal centres ; c, trabecular containing blood-vessels; d, capsule ; e, hilum ; f\ lymph sinus of medulla ; g y lymph cords of medulla ; //, lymph sinuses of medulla and cortex. .septa constitute the councctivc-tissuc framework of the node, and serve as a support for the lymphatic tissue (Fig. 74). The capsule is composed of fibrous connective tissue arranged in two layers. In the outer the fibres are loosely arranged and serve, 135 1 36 THE ORGANS. like the fibres of the arterial adventitia, to attach the node to the surrounding tissues. The inner layer of the capsule consists of a more dense connective tissue and contains some smooth muscle cells. At one point, known as the hilum (Fig. 74), there is a depression where the connective tissue of the capsule extends deep into the sub- stance of the node. This serves as the point of entrance for the main arteries and nerves, and of exit for the veins and efferent lymph vessels. The connective-tissue septa, which extend from the capsule into the interior of the node, divide it into irregular intercommunicating compartments. In the peripheral portion of the node these compart- ments are somewhat spheroidal or pear-shaped. Toward the centre of the node the septa branch and anastomose freely, with the result that the compartments are here narrower, more irregular, and less well defined. This arrangement of the connective tissue allows the division of the node into two parts, an outer peripheral part or cortex and a central portion, the mediUla (Fig. 74). Within the compartments formed by the capsule and the septa is the lymphatic tissue (for structure see page 79). In the cortex where the compartments are large and spheroidal or pear-shaped, the lymphatic tissue is of the compact variety, and is arranged in masses which correspond in shape to the compartments. These are known as lymph nodules (Fig. 74). In the centre of each nodule is usually an area in which the cells are larger, are not so closely packed, and show marked mitosis. As it is here that active proliferation of lym- phoid cells takes place, this area is known as the germinal centre (Figs. 74 and 75). Immediately surrounding the germinal centre is a zone in which the lymphoid cells are more closely packed than else- where in the nodule (Fig. 75). This is apparently clue to the active production of new cells at the germinal centre and the consequent pushing outward of the surrounding cells. In stained sections the centre of the nodule is thus lightly stained, while immediately sur- rounding" this light area is the darkest portion of the nodule (Fig. 75). From the inner sides of the nodules strands of lymphoid tissue extend into the medulla. These are known as lymph cords, and anas- tomose freely in the small irregular compartments of the medulla. In both cortex and medulla the lymphoid tissue is always separated from the capsule or from the septa by a distinct space — the ly mph sinus — which is bridged over by reticular tissue containing compara- LYMPHATIC ORGANS. *37 tively few lymphoid cells (Fig. 75). These sinuses form a contin- uous system of anastomosing channels throughout the node. The reticular connective tissue (page 77), which forms a part of the lymphatic tissue proper, is closely attached to the fibrous connec- tive-tissue framework of the organ. In the lymph nodules, and wher- ever the lymphoid cells are densely packed, the underlying reticular network is almost completely obscured. Crossing the sinuses, espe- FlG. 75. — Section through Cortex and Portion of Medulla of Human Lymph Node. (Technic 2, p. 13S). a, Capsule ; b, lj'mph sinus ; c, trabecula ; * -* \ ""f . *v» ***•:■•.! 1 -* * ' ' ! f *-* i - *■ ■ » * ,1 M * * m\ -3tf 1 i * ■■ ; %A •* * * -1 ■-flj. >. •.t • ■•'■'W - if ' 4 *■„•■■! B*£ , ■..% iS'^ *?«* ;| v* 1***1 r»v v*ssi ^f '-WU^ * »»*, -^ . ,W. f*-;..***"^ FIG. 105.— Cross-section through Root of Human Canine Tooth (X 25) (Sabotta), showing re- lations of pulp cavity, dentine, andcementum. P, Pulp cavity ; D, dentine ; C, cementum ; K, Tomes' granular layer. and passes into the dentinal pulp, and one or more outer fibre-like processes which enter the dentine, where they form the dentinal fibres. Dentine (Figs. 106 and 107, D) resembles bone. It is peculiar in that it contains canaliculi, dental canals (Figs. 106 and 107, Dk), but no lacunae or bone cells. The latter are represented by the odontoblasts of the pulp, which, as already noted, lie at the inner side of the dentine, into the canaliculi of which they' send the dentinal fibres. Dentine is non-vascular. The dental canals begin at the THE DIGESTIVE SYSTEM. 189 dental pulp, where they have a calibre of 2 to 3 //. They pass out- ward, taking a somewhat curved course, to the limit of the dentine. In their passage through the dentine the main canals give off side branches, which anastomose with similar branches from other canals. This anastomosis takes place not only between branches of adjacent canals, but also between branches of canals some distance apart. The main canals terminate either in blind extremities, or form loops by anastomosing with neighboring tubules. A few tubules run slightly beyond the limits of the dentine into the enamel. They do not pass into the cementum. The dentine immediately around a dental canal is more dense and hard than elsewhere and forms a sort of sheath for the canal — Neumann s dental sheath. Between the dental canals is a calcified ground substance, in which are connec- tive-tissue fibres running in the long axis of the tooth. Spaces which probably represent incomplete calcification of the dentine occur in the peripheral portion of the dentine of the crown. KB Dk, Fig. 106.— From Longitudinal Section through Root of Human Molar Tooth (X 200) (Sabotta), showing junction of dentine and cementum. C, Cementum; £), dentine; A', Tomes' granular layer ; Dk, dental canals ; KH, lacunar of cementum. These are known as interglobular spaces (Fig. 1 07, Jg). They are filled with a substance resembling" uncalcified dentine. In the outer part of the dentine of the root are similar spaces 190 THE ORGANS. which are smaller and more closely placed. These form the so-called Tomes' granular layer (Fig. 106, K). The enamel is the hardest substance in the body. It contains little more than a trace of organic substance (3 to 5 per cent). It consists of long six-sided prisms — enamel fibres or enamel prisms (Fig. 107, S) — which take a slightly wavy course through the entire thickness of the enamel. The prisms are attached to one another by a small amount of cement substance. In the human adult the prisms $ft Dk Fig. 107. — From Longitudinal Section of Crown of Human Premolar (X 200) (Sabotta), show- ing' junction of enamel and dentine, i 1 . Enamel ; D, dentine; Sp, enamel prisms; Dk, dental canals ; Jg, interglobular spaces. A few dentinal fibres are seen passing beyond the limits of the dentine into the enamel. The oblique dark bands in the enamel are the lines of Retzius. are homogeneous ; in the embryo they show a longitudinal fibrillation. Rather indistinct parallel lines (the lines of Retzius) cross the enamel prisms. They probably represent the deposition in layers of the lime salts. The enamel is covered by an apparently structureless mem. brane, the eutienla dcntis. The cement um (Fig. 106, C) covers the dentine of the root in a manner similar to that in which the enamel covers the dentine of the crown (Fig. 104, /and J). Cementum is bone tissue. It contains lacuna and bone cells, but no distinct lamellation and no Haversian systems or blood-vessels, excepting in the large teeth of the larger THE DIGESTIVE SYSTEM. I9 r mammalia, where they may be present. Many uncalcified Sharpey's fibres penetrate the cementum. The union between the root of the tooth and the alveolar peri- osteum is accomplished by a reflection of the latter over the root, where it forms the dental periosteum, or peridental membrane (Fig. 104, 4). At the neck of the tooth this membrane blends with the submucosa of the gum. The peridental membrane is formed of fibrillar connective tissue free from elastic fibres. These fibres are directly continuous with Sharpey's fibres of the cementum. Blood-vessels of teeth are confined entirely to the pulp cavity. One or two small arteries reach the pulp cavity from the underlying connective tissue, through the foramen in the apex of the root. These break up into a capillary network in the dental pulp. Lymphatics have as yet not been demonstrated in the dental pulp. Medullated nerve fibres accompany the blood-vessels through the apical canal. In the pulp they break up into a number of non- medullated branches, which form a plexus along the outer edge of the pulp, beneath the odontoblasts. From this plexus branches are given off which pass in between the odontoblasts, some terminating there, while others end between the odontoblasts and the dentine. Development. — The enamel of the teeth is of ectodermic origin, the remainder of mesodermic. The earliest indication of tooth formation occurs about the seventh week of intra-uterine life. It consists in a dipping down of the epithelium covering the edge of the jaw into the underlying connective tissue, where it forms the dental ridge, or common dental germ. At intervals along the outer side of this dental ridge, the cells of the ridge undergo proliferation and form thickenings, ten in number, each one corresponding to the position of a future milk tooth. These are known as special dental germs, and remain for some time connected with one another and with the surface epithelium by means of the rest of the dental ridge. Into the under side of each special dental germ an invagination of the underlying connective tissue occurs. This forms the dental papilla (Fig. 108), over which the tissue of the special dental germ forms a sort of a cap, the latter being known from its subsequent function as the enamel organ. The next step is the almost complete separation of the special dental germs and ridge from the surface epithelium (Fig. 108), and the formation around each special dental germ of a vascular membrane, the dental sac. The attenuated strand 192 THE ORGANS. of epithelial cells, which still maintains a connection between the dental germs and the epithelium of the gums, is known as the neck of the enamel organ, and it is from this that an extension soon occurs to the inner side of the dental germs of the milk teeth, to form the dental germs of the permanent teeth (Fig. 108). Into the latter, connective-tissue papillae extend as in the case of the milk teeth. There are thus present as early as the fifth month of fcetal existence the germs of all milk and of some permanent teeth. The enamel organ at this stage consists of three layers : (1) The outer enamel cells, somewhat flattened ; (2) the inner enamel cells, / r f*\ — b e * -.d ■ sy S my FlG. 108.— Developing Tooth from Three-and-one-half-months' Human Embryo. X 63. (Szy- monowicz.) a, Epithelium of gums ; b, neck of enamel organ ; c, dental germ of permanent tooth ; d, bone of lower jaw ; e, dental papilla ; /, inner enamel cells ; g; enamel pulp ; h, outer enamel cells. high columnar epithelium; (3) a layer of enamel pulp, situated between the other layers, and consisting of stellate anastomosing cells with considerable intercellular substance (Figs. 108 and 109). The first of the dental tissue to become hard is the dentine. The surface cells of the papilla differentiate to form a layer of colum- THE DIGESTIVE SYSTEM. 193 - ##\£%® : &02- n nar cells, odontoblasts. Between these and the inner enamel cells a membrane-like structure, the mcmbrana prczformativa, is formed. This becomes converted into den- tine by the deposition of lime salts, the process being similar to the formation of bone by the osteoblasts. Processes of the odontoblasts remain in the developing dentine as the dental fibres, lying in channels, the dental canals. Additional dentine continues to be laid down in layers, each new layer internal to the pre- ceding. In this way the dental papilla is reduced in size to form the pulp cavity. Small spots re- main, in which there is little or no calcification. These are the so- called interglobular spaces. The emamel is formed by the enamel organ. A membrane, the euticular membrane, is first laid down between the inner enamel cells and the dentine. Each of the inner enamel cells now sends out a process, Tomes' process, from its inner end. The processes are separated by a considerable amount of cement, and are the beginnings of the enamel prisms. Calcification now takes place both in the prisms and in the cement substance, the latter at the same time becoming reduced in amount. Further growth in thickness of enamel occurs by lengthening of the enamel prisms. During the formation of the enamel, the enamel pulp and the external enamel cells disappear. The cementum is developed by ossification of that part of the dental sac which covers the root. FIG. 109.— From Cross-section through a De- veloping Tooth. X 720. (Bohm and von Davidoff.) Note close relationship be- tween odontoblasts and tissue of dental pulp, a, Dental pulp ; b, odontoblasts ; c, dentine ; d, inner enamel cells ; e, enamel pulp. TECHNIC. (1) Teeth are extremely difficult organs from which to obtain satisfactory ma- terial for study. Sections of hard (undecalcified) and of decalcified teeth may be prepared in the same manner as sections of bone — technics 1 and 2. p. 160. The decalcified tooth should include if possible the alveolar margin of the jaw, so that 13 194 THE ORGANS. in longitudinal sections the mode of implantation and the relation of the tooth to the surrounding structures can be seen. (2) For the study of developing teeth, embryo pigs, sheep, cats, dogs, etc., are suitable. For the early stages foetal pigs should be five to six inches long ; for the intermediate, ten to twelve inches. The later stages are best obtained from a small new-born animal, e.g., kitten or small pup. The jaw — preferably the lower— or pieces of the jaw are fixed in formal in -Midler's fluid (technic 5, p. 6), hardened in alcohol, and decalcified (page 9). Subsequent treatment is the same as for de- veloping bone (technic 1, p. 16S). The Pharynx. The wall of the pharynx consists of three coats — mucous, mus- cular, and fibrous. 1. The mucous membrane has a surface epithelium and an un- derlying stroma. The epithelium is stratified squamous except in the region of the posterior nares, where it is stratified columnar ciliated, contin- uous with the similar epithelium of the nasal mucosa. The stroma, or tunica propria, consists of mixed fibrous and elas- tic tissue infiltrated with lymphoid cells. In certain regions these cells form distinct lymph nodules (see pharyngeal tonsils, page 146). Beneath the stratified squamous epithelium the stroma is thrown up into numerous low papillcs. These are absent in regions covered by ciliated cells. Bounding the stroma externally is a strongly de- veloped layer of longitudinal elastic fibres, the elastic limiting layer, which separates the stroma from the muscular coat and sends stout bands in between the muscle bundles of the latter. 2. The muscular coat lies beneath the elastic layer and is formed of very irregularly arranged muscle fibres belonging to the constrictor muscles of the pharynx. 3. The fibrous coat consists of a dense network of mixed fibrous and elastic tissue. It has no distinct external limit, and binds the pharynx to the surrounding structures. The distribution of blood-vessels, lymphatics, and nerves is simi- lar to that in the oral mucosa. Small, branched, tubular, mucous glands are present in the stroma, and extend down into the intermuscular connective tissue. They are most numerous near the opening of the PCustachian tube. TECHNIC. For the study of the structure of the walls of the pharynx, material should be prepared as in technic 2, p. 183. THE DIGESTIVE SYSTEM. 195 THE FOREGUT. The (Esophagus. The walls of the oesophagus are continuous with those of the pharynx and closely resemble the latter in structure. They consist of four layers, which from within outward are mucous, submucous, muscular, and fibrous (Fig. no). FIG. no.— Transverse Section through Wall of Dog's (Esophagus. X 18. (Bohm and von Da- vidoff.) a, Epithelium; b, stroma; c, muscularis mucosae; d, submucosa ; e, circular muscle layer ; f, longitudinal muscle layer ; £-, fibrous layer. 1. The mucous membrane resembles that of the pharynx except that beneath the stroma is a well-developed muscularis mucosa com- posed of smooth muscle cells arranged longitudinally. 2. The submucosa is composed of loosely arranged fibrous and elastic tissue. It contains mucous glands, the larger blood-vessels, lymphatics, and nerves. 3. The muscular coat. In the upper portion of the oesophagus this coat is composed of striated muscle fibres; in the middle portion of mixed striated and smooth muscle. In the lower portion there are 196 THE ORGANS. two distinct layers of smooth muscle, an inner circular and an outer longitudinal. The latter is not continuous. 4. The fibrous coat consists of bundles of white fibrous tissue with many elastic fibres. It serves to connect the oesophagus with the surrounding structures. Two kinds of glands occur in the oesophagus. (1) Mucous Glands. — These are of the same structure as those of the tongue, but much smaller. They lie in the submucosa and are distributed throughout the entire oesophagus, though most numerous in its upper third. The ducts pass obliquely downward on their way to the surface. Just before entering the muscularis mucosae the duct widens out to form a sort of ampiilla. Beyond this it again becomes narrow and enters the epithelium in the depression between two adjacent papillae. A small lymph nodule is usually attached to the duct as it passes through the tunica propria. (2) Simple Branched Tubular Glands.— These resemble the glands of the cardiac end of the stomach, but branch much more profusely. Some contain both chief and acid cells, others only chief cells (see stomach, page 199). They lie in the tunica propria, and are for the most part confined to a narrow zone at the lower end of the oesophagus and to the level of the fifth tracheal ring. Scattered groups also occur in other regions. TECHNIC. Remove a portion of the wall of the oesophagus, wash carefully in normal salt solution, and pin out, mucous-membrane side up, on a piece of cork. Fix in formalin-Midler's fluid and harden in alcohol (technic 5, p. 6). Transverse or longitudinal sections should be cut through the entire thickness of the wall. If the details of the muscular coat are to be studied, sections from at least three dif- ferent levels should be taken : one near the upper end, one at about the middle, and the other in the lower third. Stain with haematoxylin-eosin or hsematoxylin-picro- acid-fuchsin (technic 1 or 3, p. 17) and mount in balsam. General Structure of the Walls of the G astro- Intestinal Canal. The walls of the stomach and intestines are made up of four coats (Fig. iii). These from the lumen outward are mucous, submucous, muscular, and serous. 1. The mucous membrane (Fie. in) consists of surface epithe- THE DIGESTIVE SYSTEM. 197 lium, glands, stroma, and muscularis mucosas. The surface epithe- lium is simple columnar and rests upon a distinct basement membrane. The arrangement of the glands and the nature of the gland cells differ in different parts of the tract. The stroma is a richly cellulai con- nective tissue, which in some places is so infiltrated with lymphoid PlG. hi. — Diagram of Strvicture of Wall of Gastro-intestinal Canal. A, Mucous membrane ; a, glands ; b, epithelium ; c, goblet cells ; d, stroma ; e, inner circular, /, outer longitudi- nal layers of g, muscularis mucosae. B, Submucosa. C, Muscular coat ; //, its inner cir- cular layer:/, its outer longitudinal layer; i, intermuscular connective-tissue septum. D, serous coat ; k, its connective-tissue layer ; /, its endothelial layer. cells as to constitute diffuse lymphatic tissue. In other places it contains circumscribed masses of lymphatic tissue, lymph nodules. The amount of stroma depends upon the closeness with which the glands are packed. The muscularis mucosa; consists of smooth muscle cells, which have a generally longitudinal arrangement. Where, however, the muscularis mucosae is thick there are frequently 198 THE ORGANS. two distinct layers — an inner circular and an outer longitudinal. Folds of considerable extent occur in the mucous membrane. Those of the stomach are known as rugce, and are not constant, depend- ing upon the degree of distention of the organ. Those of the small intestine are much more definite, and are known as valvules connivaitcs. 2. The submucosa (Fig. in) is a loose connective-tissue struct- ure. It contains the larger blood-vessels, lymphatics, and nerves. 3. The muscular coat (Fig. 1 1 1) consists of two layers of smooth muscle, which in the intestine are sharply differentiated into an inner circular and an outer longitudinal. In the stomach the direction of the layers of the muscular coat is less definite. A narrow layer of connective tissue separates the two layers of muscle. From this, septa extend into the muscle tissue, separating it into bundles. 4. The serous coat (Fig. 1 1 1) is the visceral layer of the peri- toneum. It consists of a thin layer of connective tissue covered by a single layer of mesothelium. Along the attachment of the mesen- tery the serous coat is wanting. The subdivisions of the gastro-intestinal canal differ from one another mainly in regard to the structure of their mucous mem- branes, and especially in regard to the structure of the glands of the mucous membrane and submucosa. The Stomach. 1. The mucous membrane of the stomach is folded into ridges or rugce, the height and number of which depend, as already noted, upon the degree of distention of the organ. The rugae are most promi- nent in the collapsed organ, almost absent when the organ is fully distended. In addition to the rugae the entire mucous membrane is studded with minute depressions barely visible to the naked eye, the so-called gastric pits or crypts (Fig. 1 12, Mg). These mark the openings of the gastric glands. In the fundus they are compara- tively shallow, extending through about one-fifth the thickness of the mucosa; in the pylorus the crypts are much deeper, extending through half or more of the thickness of the mucous membrane (compare Figs. 1 12 and 1 16). The Epithelium. — At the junction of oesophagus (Fig. 113) and THE DIGESTIVE SYSTEM. 199 stomach the stratified squamous epithelium of the former ends rather abruptly, being replaced by the simple columnar epithelium, which covers the entire surface of the gastric mucosa and extends down into the crypts (Fig. 112). The cells are of the high, clear, mucous type (Fig. 114, M and M'). The end of the cell tow- ard the lumen is clear, usually consists mostly of mucus, and consequently stains lightly. The basal end of the cell contains the spheroidal, oval, or sometimes flattened nucleus, is granular, and takes a darker stain. The cells rest upon a distinct basement membrane. The Gastric Glands. — Extending from the bottoms of the crypts, their epithelium continuous with that of the crypts themselves, are the gastric glands. These are of two kinds, peptic or fundus glands, distributed throughout the greater part of the gastric mucosa, and pyloric glands, confined to the immediate region of the pylorus. The peptic glands (Fig. 112) are simple, sometimes branched, tubular glands, of which from three to seven open into each gastric crypt. They ex- tend through the entire thickness of the stroma, to the muscularis mucosae. Each gland consists of (1) a mouth opening into the crypt ; (2) a constricted portion, the neck ; (3) the body or main portion of the tubule ; and (4) a slightly dilated and bent blind extremity, the fundus (Fig. 112). The mouth marks the transition from the higher epithelium of the crypt to the low cuboidal of the neck (Fig. 114, //). In the body and fundus of the gland two types of cells are found : (a) chief cells (cen- tral, peptic, or adelomorphous), and (/>) parietal cells (acid, oxyntic, or delomorphous). The chief cells (Fig. 114, a) are the more numerous. They are i Fig. 112.— Vertical Section through the Mucous Membrane of the Fun- dus of the Stomach. X 85. (Kol- liker.) Mg, Gastric crypts; //, neck ; k, body ; g; fundus of peptic glands ; //, chief cells ; b, parietal cells ; m, muscularis mucosae. 200 THE ORGANS. of the low columnar type, often pyramidal with apices directed toward the lumen. Their protoplasm is granular and clear, taking a light stain. Their bases rest either on the basement membrane or against the parietal cells. The parietal cells (Fig. 114, b) are oval or polygonal in shape, and lie against the basement membrane. The nucleus is spherical, somewhat larger than that of the chief cell, and is usually situated at the centre of the cell. The protoplasm is finely granular and stains intensely with the aniline dyes. In stained specimens the two kinds of cells are thus in marked contrast. Although lying against Fig. 113.— Section through Junction of (Esophagus and Stomach of Man. X izi. (Schafer.) Oe, (Esophagus : M, stomach ; cd, cardiac glands ; wd, dilated ducts of cardiac glands ; S, stroma; E, stratified squamous epithelium of oesophagus; mm, muscularis mucosas ; cd, irregularly cut tubules of cardiac glands ; dd, cardiac glands in lower end of the oesoph- agus ; u, limit of stratified oesophageal epithelium. the basement membrane and frequently pushing it out so as to form little protuberances beyond the even line of the gland tubule, the parietal cells always maintain a connection with the lumen. This is accomplished by means of little clefts between the chief cells (inter- THE DIGESTIVE SYSTEM. 2or cellular secretory tubules), which extend down to the parietal cells. By means of the method of Golgi may be demonstrated not only the intercellular secretory tubules, but also the fact that upon reaching the cells these are continuous with a network of minute spaces within the cell — the intracellular secretory tubules (Fig. 115). Parietal cells are not distributed uniformly through- out the gland, but are most numer- ous in the body, where they fre- quently almost obscure the chief cells. In the fundus of the gland parietal cells are less numerous. For this reason and because of the wider lumen of the fundus, trans- verse and longitudinal sections of this part of the tubule are most satisfactory for the study of the re- lations of the two kinds of cells (Figs. 1 12 and 1 14). Lying near the basement mem- brane among the bases of the col- umnar epithelial cells are small spherical or irregular cells with dark nuclei. These are young epithelial cells which from their func- tion are known as " replacing cells " (see page 60). The pyloric glands (Figs. 116 and 117) are simple branched tubular glands, several of which open into each of the deep pyloric crypts. The glands, though short, are quite tortuous, so that in sec- tions the tubules are seen cut mainly transversely or obliquely. In most of the pyloric glands but one type of cell is found. These resemble the chief cells of the fundus, but present a more uniform appearance, probably due to the absence of parietal cells. As in the fundus, " replacing cells " lie between the bases of the columnar epi- thelial cells. Parietal cells are not always entirely absent, but occur here and there in the pyloric tubules, especially near the fundus. The transition from fundus to pylorus is not abrupt, but is marked by a "transitional border zone," in which fundus and pyloric glands are intermingled. FIG. 114. — Cross-sections at Various Levels of Peptic Glands of Stomach. X 400. (Kolliker.) M, Section through gastric pit near surface ; M\ section through gas- tric pit near bottom ; //, mouth of gland ; k, neck ; g; body near fundus ; a, chief cells ; /», parietal cells. 202 THE ORGANS, In the transition zone between oesophagus and stomach are found glands which resemble the peptic glands, but contain no parietal cells. The stroma (Figs. 112 and 116) or tunica propria, in which the glands are embedded, consists of mixed fibrillar and reticular connective tissue infiltrated with lym- phoid cells. In the fundus of the stom- ach the giands are so closely packed that the stroma is reduced to thin strands, which pass up between the glands and also separate them from the muscularis mucosae. In the pylorus the glands are more widely separated and the stroma is correspondingly greater in amount. In both fundus and pylorus thicker strands of stroma surround a number of gland tubules, thus separating them into more or less well-defined groups. In FIG. 115.— Longitudinal Section of •■ ■,.,. , ,-, ,. rr ■, ■, ,. ,. Fundus of Gland from Pyloric End addition tO the diffuse lymphatic tlSSUe of Dog's stomach. ^(Goigi ^method. f t h e stroma, closely packed aggrega- tions of lymphoid cells are found in the shape of distinct nodules, known as " solitary follicles" These occur throughout the entire gastric mucosa, but are most numerous in the pylorus. The nodules are usually egg-shaped, their apices lying just beneath the epithelium, their bases resting upon the muscularis mucosas. Less commonly they lie partly in the submucosa. Over the nodules the epithelium is more or less infiltrated with migratory leucocytes. Most of the nodules contain germinal centres, around which the lymphoid cells are more closely packed than elsewhere (see page 136). The muscularis mucosae (Figs. 112 and 116, m) may consist of a single layer of smooth muscle with cells arranged longitudinally or obliquely, or there may be two distinct layers, an inner circular and an outer longitudinal. From the muscularis mucosae single cells and groups of cells extend into the stroma between the gland tubules. 2. The submucosa consists of connective tissue, loosely arranged, and contains larere blood-vessels. See 4, p. 24.) a, Lumen of gland ; b, intracellular canals in parietal cells; c, cut-off portion of parietal cell ; d, chief cells ; e, intercellular canals leading from lumen of gland to ca- nals in parietal cells. THE DIGESTIVE SYSTEM. 203 3. The muscular coat is usually described as consisting of three layers, an inner oblique, a middle circular, and an outer longitudinal. This division of the muscular coat into layers having definite ■^■r '...., :-.. '■"-._"••"-■ ; ; ;--.-'r . ";'-'_ - ; Fig. 116. Fig. 117. FIG. 116.— Vertical Section through Mucous Membrane of Pyloric End of Stomach. X 85. (Kolliker.) Aig, Gastric crypt ; b, blood-vessel in stroma ; d, longitudinal section of body of gland ; m, muscularis mucosas. Fig. 117.— Pyloric Gland from Vertical Section through Wall of Dog's Stomach. (Ebstein.) m, Gastric pit in which are seen some transversely cut cells ; n, neck of gland ; f, fundus cut transversely. directions can be made out only in the pylorus, the muscle bundles of the fundus running in various directions. 4. The serous coat consists of a layer of loosely arranged connec- tive tissue covered by a single layer of mesothelium. TECHNIC. (1) Remove a human stomach (not more than two or three hours after death) or that of a recently killed dog. Open along the lesser curvature, and carefully re- move the excess of mucus by washing with normal saline. Cut pieces through 204 THE ORGANS. the entire thickness of the wall, one from the fundus and one from the pylorus ; pin out, mucous membrane side up, on pieces of cork, fix in formalin-Midler's fluid (technic 5, p. 6) or in Zenker's fluid (technic 9, p. 7), and harden in alcohol. Sec- tions are cut as thin as possible, care being taken that the plane is such that the glands are cut longitudinally, stained with haematoxylin-eosin (technic 1, p. 17), and mounted in balsam. (2) Instead of removing pieces of stomach and pinning them out on cork, as suggested in the preceding technic, the entire stomach may be filled with the fixa- tive, the ends being tied, and then placed in a large quantity of the fixing fluid. After fixation, pieces are removed and hardened in graded alcohols. If this method is used, great care must be taken not to overdistend the organ, only very moderate distention being desirable. Further treatment is the same as in the pre- ceding technic (1). (3) For comparison of restiDg with active gastric cells, preparations should be made from the stomach of an animal that has been for from twenty-four to forty- eight hours without food, and from a stomach during active digestion. Fix in Zenker's fluid as in technic (1), above. Examine unstained sections and sections stained with hcematoxylin-eosin. (4) Sections through the junction of oesophagus and stomach and through the junction of stomach and duodenum furnish instructive pictures. They should be prepared as in technic (1). (5) For the study of the distribution of the blood-vessels sections of an injected stomach should be made. This is best accomplished by selecting a small animal, such as a rat or guinea-pig, and injecting in toto through the ascending aorta, or by injecting only the hind part of the animal through the abdominal aorta. Tech- nic, p. 21. III. THE MIDGUT. The Small Intestine. On passing from stomach to small intestine the rugae of the former disappear, but are replaced by much more definite foldings of the mucosa, the valvules conniventes (Fig. 119). These folds involve the entire thickness of the mucous membrane and part of the sub- mucosa. They are in general parallel to one another, and pass in a circular or oblique manner, partly around the lumen of the gut. The entire surface of the intestine, including the valvulae, is studded with minute projections just visible to the naked eye, and known as villi (Figs. 119 and 120). These involve only the epithelium and stroma, although they also contain some muscular elements derived from the muscularis mucosas. The villi differ in shape in the different parts of the small intestine, being leaf-shaped in the duodenum, rounded in the jejunum, club-shaped in the ileum. The valvulae conniventes and the villi are characteristic of the small intestine. It is impor- tant to note that while the crypts of the stomach are depressions in THE DIGESTIVE SYSTEM. 205 the mucous membrane, the intestinal villi are definite projections a dove its general surface (Fig. 118). The wall of the intestine consists of the same four coats described as constituting the wall of the stomach, mucosa, submucosa, muscu- laris, and serosa. 1. The mucosa, as in the stomach, is composed of a lining epithe- Hum, stroma, glands, and muscularis mucosa;. Of these the epithe- FlG. 11S.— Section through Junction of Pylorus and Duodenum. (Klein.) v. Villi of duode- num ; d, stomach, showing gastric crypts; 6, apex of a solitary lymph nodule ; c, crypt of Lieberkiihn ; s, secreting tubules of Brunner's glands ; g, pyloric glands ; t, tubules of Brunner's glands in submucosa of stomach ; m, muscularis mucosae. Hum, the stroma, and cells from the muscularis mucosae are concerned in the formation of the villi. The villus consists of a central core — a fold of the stroma — of mixed fibrous and reticular tissue infiltrated with lymphoid cells, and of a covering epithelium. The epithelium is of the simple columnar type. The cells are high and have thickened striated free borders (Figs. 121 and 122). These contiguous thickened free borders unite to form a distinct membrane, the cutieular membrane (Fig. 122, e). Scattered among the columnar cells are numerous mucous or goblet cells (Figs. 121 and 122, b). The goblet cells are derived from the columnar cells, and vary in appearance according to the amount of secretion which 2C>6 THE ORGANS. they contain. A cell at the beginning of secretion contains only a small amount of mucus near its free border. As secretion in- creases the mucus gradually replaces the cytoplasm until the lat- FlG. 119. — Vertical Longitudinal Section of Human Jejunum (X 16) (Stohr), including two val- vulas conniventes. a, Villi, in many of which the stroma has shrunken away from the epithelium leaving a clear space, X X. Lying free in the lumen of the gut are seen sec- tions of villi cut in various directions. 5, Epithelium ; c, stroma ; d, crypts of Lieberkiihn ; X, solitary lymph nodule with germinal centre ; e. tissue of submucosa forming centre of one of the valvulas conniventes ; f, submucosa ; g; inner circular layer of muscle ; /i, outer longitudinal layer of muscle ; z', Auerbach's plexus ;/, serous coat. ter is represented only by a crescentic mass containing a flat- tened nucleus and pressed against the basement membrane. The cell now discharges its mucus upon the free surface. The goblet cells possess no thickened border, appearing, when seen from the surface, as openings surrounded on all sides by the cuticulae of the adjacent columnar cells. Small spherical cells with deeply staining nuclei are found in varying numbers among the epithelial cells. These are so-called wandering cells, migratory leucocytes, from the underlying stroma (Figs. 12 r, Ji, and 122, /). Other cells with dark- staining nuclei, " replacing cells," are found between the bases of the columnar cells (pages 60 and 201). In addition to the connective-tissue and lymphoid cells, which THE DIGESTIVE SYSTEM. 207 constitute the main bulk of the villus core (Figs. 121 and 122), iso- lated smooth muscle cells derived from the muscularis mucosae occur, running in the long axis of the villus. A single lymph or chyle vessel (Fig. 121,/; 122, cJi) with distinct endothelial walls traverses the centre of each villus, ending at its tip in a slightly dilated blind extremity. As it is usually seen collapsed, it appears as two closely approximated rows of flat cells with bulging nuclei. The capillaries of the villus lie for the most part away from the chyle vessel, just beneath the basement membrane (Fig. 121, c ; 122, g). From the depths of the depressions between the villi, simple tubular glands — glands or crypts of Lieberkuhn (Figs. 120 and 123) — extend down through the stroma as far as the muscularis mu- cosas. These crypts are lined with an epithelium similar to and con- tinuous with that covering the villi. The cells are, however, lower, i c d FlG. 120.— Vertical Section through Mucous Membrane of Human Jejunum, x So. (Stohr.) a and b, Artifacts due to shrinkage ; c, intestinal crypts (Lieberkuhn) ; J, oblique and transverse sections of crypts ; e, stroma ;/, epithelium ; g, tangentially cut villi ; /;, mus- cularis mucosa; ; ;', submucosa. and there are fewer goblet cells. In addition to these cells there are also found in the depths of the crypts of Lieberkuhn peculiar coarsely granular cells, the cells of Paneth (Fig. 123, k). They are found in 208 THE ORGANS. man and in rodents, but do not occur in the carnivora. They prob- ably produce a specific secretion, the nature of which is unknown. The stroma, besides forming the centres of the villi, fills in the spaces between the crypts of Lieberkiihn and between the latter and Fig. 122. Fig. 121. — Longitudinal Section of Villus from Small Intestine of Dog. (Piersol.) a, Colum- nar epithelium ; b, goblet cells ; //, leucocytes ; c, basement membrane ; d, core of villus ; e, blood vessels ; /, lacteal. Fig. 122. — Cross-section of a Villus of Human Small Intestine. X 530. (Kolliker.) The stroma of the villus has shrunken away from the epithelium, b, Goblet cell ; c, cuticula showing striations ; e, columnar epithelial cell ; g-m, basement membrane with nuclei ; /, leucocyte in epithelium ; /', leucocyte just beneath epithelium ; m, large leucocyte in stroma ; c/i, central chyle vessel ; g; blood-vessel. the muscularis mucosae. In places the lymphoid cells are closely packed to form distinct nodules or "solitary follicles," such as are found in the stomach (see page 202). Peyer's Patches (agminated follicles) (Fig. 124). — These are groups of lymph nodules found mainly in the ileum, especially near its junction with the jejunum. They always occur on the side of the gut opposite to the attachment of the mesentery. Each patch consists of from ten to seventy nodules, so arranged that the entire patch has a generally oval shape, its long diameter lying lengthwise of the intestine. The nodules of which a patch is composed lie side THE DIGESTIVE SYSTEM. 209 by side. Their apices are directed toward the lumen and project almost through the mucosa, being uncovered by villi, a single layer of columnar epithelium alone separating their surfaces from the lumen of the gut. The bases of the nodules are not confined to the stroma, but usually spread out in the submucosa. The relation of the patch to the stroma and submucosa can be best appreciated by following the course of the muscularis mucosas. This is seen to stop abruptly at the circumference of the patch, appearing throughout the patch as isolated groups of smooth muscle cells. The nodules rarely remain distinct, but are confluent with the exception of their apices and bases. It should be noted that both solitary nodules and Peyer's patches are structures of the mucosa, and that their presence in the submu- cosa is secondary. The muscularis mucosae (Figs. 1 20 and 125) consists of an inner circular and an outer longitudinal layer of smooth muscle. 2. The submucosa (Figs. 119, 120, 125) consists, as in the stomach, of loosely arranged connective tissue and contains the larger blood-vessels. It is free from glands except in the duo- denum, where it contains the glands of Brunner (Fig. 125). These are branched tubular glands lined with a granular columnar epithelium similar to that of the pyloric glands. The ducts are also lined with simple columnar epithelium. They paSS through the muscularis mil- Fig. 123. -Longitudinal Section of . ... . Fundus of Crypt of Lieberkuhn. X cosae and empty either into a crypt of 530 . (Koiiiker.) a.Gobietceiishow- Lieberktihn or on the surface between mg mitosis ; «, epithelial ceil ;£. ceil of Paneth ; /, leucocvte in epithe- the villi. Brunner's glands frequently Hum; m, mitosis in epithelial ceil, occur in the pylorus, and it is not un- f«™ n * n s the ">'P l is * een the 1 J stroma of the mucous membrane. common for the pyloric glands to ex- tend downward somewhat into the duodenum. Meissner s plexus. of nerve fibres, mingled with groups of sympathetic ganglion cells, lies in the submucosa (see page 218). 3. The muscular coat (Figs. 119 and 125) consists of two well- 14 2-# *» 2IO THE ORGANS. defined layers of smooth muscle, an inner circular and an outer longi- tudinal. Connective-tissue septa divide the muscle cells into groups or bundles, while between the two layers of muscle is a connective- FlG. 124.— Transverse Section of Cat's Small Intestine through a Peyer's Patch. (Stohr.) a, Villi ; b, crypts ; c, longitudinal muscle layer ; d, circular muscle layer ; e, lymph nodules ; f, muscularis mucosae ; g; submucosa. tissue septum which varies greatly in thickness at different places and contains a plexus of nerve fibres and sympathetic ganglion cells known as the plexus of Auerbach (see page 217). 4. The serous coat consists as in the stomach of loose connective tissue covered by a single layer of mesothelium. IV. THE ENDGUT. The Large Intestine. The wall of the large intestine consists of the same four coats which have been described as constituting the walls of the stomach and small intestine, mucous, submucous, muscular, and serous. I. The mucous membrane has a comparatively smooth surface, there being neither crypts as in the stomach nor villi as in the small intestine (Fig. 126). The glands are of the simple tubular variety, THE DIGESTIVE SYSTEM. 211 are considerably longer than those of the small intestine, are almost straight, and extend through the entire thickness of the stroma. Owing to the closeness with which the gland tubules are packed, the amount of stroma is usually small. The surface cells (Fig. 127, e) are very high and narrow, with small, deeply placed nuclei, and are not usually intermingled with goblet cells. Passing from the sur- face down into the glands, the cells become somewhat lower and goblet cells become numerous (Fig. 127, B and C). Both super- ficial and deep cells rest upon a basement membrane similar to that in the small intestine. The stroma also, though less-'ih amount, is similar in structure to the stroma of the small intestine. The MUSCULARIS MUCOS.E (Fig. 126) consists of an inner circular and an outer longitud- inal layer of smooth muscle. 2. The submucosa (Fig. 126) consists of loosely arranged connective tissue. It contains large blood-vessels and the nerve plexus of Meissner (see page 218). Solitary lymph follicles occur throughout the mucous membrane of the large intes- tine. While properly consid- ered as structures of the stroma from which they originate, the follicles lie mainly in the sub- mucosa. (For details of struct- ure see page 136.) 3. Of the muscularis (Fig. 126) the inner circular layer only is complete, the muscle tissue of the external longitud- inal coat being arranged mainly as three strong, flat, longitudinal bands, the lineze coli. Between these bands the longitudinal mus- cular coat is either very thin or entirely absent. In the connective tissue, lying to the outer side of the circular muscle coat, is the nerve plexus of Auerbach. (For details see page 217.) "V.t. Fig. 125.— From Vertical Longitudinal Section of Cat's Duodenum to show Brunner's Glands. (Larrabee.) a, Villus; t>, epithe- lium ; c, stroma ; d, crypts ; e, muscularis mucosas ; /, Brunner's glands ; ^, submu- cosa ; //, circular muscle laver. 212 THE ORGANS. 4. The serous coat consists, as in the stomach and small intestine, oi loose connective tissue covered by a single layer of mesothelium. The Vermiform Appendix. The vermiform appendix is a diverticulum from the large intes- tine. Its walls are continuous with those of the latter, and closely mm afe k-1^ e» Fg3 Fig. 126. Fig. 127. Fig. 126.— From Vertical Longitudinal Section of Cat's Large Intestine. (Larrabee.') a Epithelium ; l\ stroma ; c, fundus of gland ; d, muscularis mucosae ; e, submucosa \f, cir- cular muscle layer ; g; longitudinal muscle layer ; //, serous coat ; 2, Auerbaoh's plexus. FIG. 127-From Vertical Longitudinal Section of the Mucous Membrane of the Human Large Intestine. (Technic 1, p. 220.) a, .Mucous (goblet)cells ; b, fundus of a gland cut obliquely; c, muscularis mucosas; d, lumen of a gland cut longitudinally; e, stroma between the glands ; /, leucocytes in the epithelium ; g; stroma between fundi of glands and muscu- laris mucosae. resemble them in general structure. There are the same four coats, mucous, submucous, muscular, and serous. THE DIGESTIVE SYS I EM. 213 1. The mucous membrane (Fig. 128) consists of epithelium, glands, stroma, and muscularis mucosae. The epithelium resembles that of the large intestine. The glands vary in number, but are usually much less closely packed than in the large intestine. They are most numerous in the appendices of infants and children. The gland tubules (Fig. 128,7) are usually rudimentary, but * n most cases Fig. 128.— Transverse Section of Human Vermiform Appendix. (Techmc 2, p. 220.) a, Meso- appendix ; 6, serous membrane (serosa); c, outer longitudinal muscle layer; d, inner circular muscle layer ; e, submucosa ; /, groups of fat cells in submucosa ; g, blood-vessels in submucosa ; //, lymph nodules; /, stroma;/, glands opening into lumen and cut in various planes. have the same structure as the intestinal glands, and are evidently functional as they contain mucous cells in all stages of secretion. In consequence of the wider separation of the tubules the stroma is more abundant than in the large intestine, but has the same structure. The muscularis mucosa (Fig. 128, d) is usually fairly distinct as a thin circularly disposed band of smooth muscle cells just beneath the stroma. In some cases the mucosa as such is practically absent, 214 THE ORGANS. being replaced by fibrous tissue. This condition is especially com- mon after middle age, and may or may not be associated with oblit- eration of the lumen. 2. The submucosa (Fig. I28,r)is similar to that of the intestine. 3. The muscular coat varies greatly, both as to thickness and as to the amount of admixture of fibrous tissue. The inner circular layer (Fig. 128, ^/)is usually thick and well developed. The outer longitudinal layer (Fig. 128, c) differs from that of the large in- testine in having no arrangement into lineae, the muscle tissue forming a continuous layer. Less commonly a more or less marked tendency to an arrangement of the cells of the longitudinal coat into bundles, between which the outer coat is thin or wanting, is observed. 4. The serosa has the usual structure of peritoneum. The lymph nodules (Fig. 128, Ji) constitute the most con- spicuous feature of the appendix. They lie mainly in the submucosa. In children and young adults the nodules are oval or spherical; in later life somewhat flattened. The nodules may be entirely distinct, or may be arranged as in a Peyer's patch with distinct apices and bases, but with their central portions confluent. The muscularis mu- cosae either passes through the superficial portions of the nodules, or, where they are separated from the lumen, passes over them. The distribution of blood-vessels, lymphatics, and nerves is similar to that in the large intestine. The Rectum. 1. The mucous membrane of the rectum has a structure similar to that of the large intestine. The glands are longer and the mucosa consequently is somewhat thicker. In the lower part of the rectum definite longitudinal foldings of the mucosa occur, the so-called columnar rectales. A change in the character of the mucous mem- brane begins at the upper end of the columnse rectales. Here the simple columnar epithelium of the gut passes over into a stratified squamous epithelium, beneath which is a papillated stroma. The glands continue for a short distance beyond the change in the epi- thelium, but soon completely disappear. At the anus there is a transition from mucous membrane to skin similar to that described as occurring at the margin of the lips (page 180). 2. The submucosa is similar in structure to that of the large intestine. THE DIGESTIVE SYSTEM. 215 The muscularis of the rectum differs from that of the large in- testine in that the longitudinal layer is continuous and thick. The serous coat is absent in the lower part of the rectum, being replaced by a fibrous connective-tissue layer, which connects the rec- tum with the surrounding structures. Blood-Vessels of the Stomach and Intestines. The arteries reach the gastro-intestinal canal through the mesen- tery and pass through the muscular coats to the submucosa, where they form an extensive plexus of large vessels (Heller's plexus) (Fig. 129, c). Within the muscular coats the main arteries give off small ABC FlG. 129.— Scheme of Blood-vessels and Lymphatics of Stomach. X 70. (Szymonowicz, after Mall.) a, Mucous membrane ; fi, muscularis mucosae ; c, submucosa ; d, inner circular muscle layer; e, outer longitudinal muscle layer; A, blood-vessels; B, structure ot coats ; C, lymphatics. branches to the muscle tissue. From the plexus of the submucosa two main sets of vessels arise, one passing outward to supply the muscular coats, the other inward to supply the mucous membrane (Fig. 129). Of the former the larger vessels pass directly to the intermuscular septum, where they form a plexus from which branches 2l6 THE ORGANS. are given off to the two muscular tunics. A few small branches from the larger recurrent vessels also supply the inner muscular layer. Of the branches of the submucosa plexus which pass to the mucous membrane, the shorter supply the muscularis mucosae, while W&&MM& Fig. 130.— Scheme of Blood-vessels and Lymphatics of Human Small Intestine. (From Bohm and von Davidoff, after Mall.) a, Central lacteal of villus ; b, lacteal ; c, stroma ; d, mus- cularis mucosas ; e, submucosa ;f, plexus of lymph vessels ; £■, circular mtiscle layer ; //, plexus of lymph vessels ; z', longitudinal muscle layer ; /, serous coat ; k, vein ; /, artery ; «z, base of villus ; 72, crypt ; , parietal cell ; c, lumen ; , stroma ; c, chief cell ; d, parietal cell. differentiation of the ordinary columnar epithelial cell. The chief cells, "peptic cells," of the stomach glands are large and clear dur- ing fasting, become granular and cloudy with the onset of digestion, THE DIGESTIVE SYSTEM. 219 and smaller with loss of granules during the digestive process. As activity of the chief cells (Fig. 132) is coincident with an increase in the pepsin found in the gastric mucosa, it is probable that these cells £)>, :&--■ r> m .— --~$ > 'Ms Tig. 133.— Fat Absorption. Longitudinal section of villus of cat's small intestine, three hours after feeding 1 . X 350. Osmic acid, a, Pat droplets in epithelial cells ; b, fat droplets in leucocytes in stroma ; c, fat droplets in leucocytes within lacteal ; if, fat droplets free in lacteal ; e, capillary containing blood cells ; f, central lacteal of villus. produce pepsin, and that the granules represent some stage in the elaboration of the ferment. As their name of " acid cells " would indicate, the parietal cells were considered the source of the hydrochloric acid of the stomach. While doubt still exists as to the function of these cells, recent investigations make it probable that it is not the secretion of hydrochloric acid. The cells of Bruiv ner's glands undergo changes during digestion, which are quite 220 THE ORGANS. similar to those described as occurring in the chief cells of the stomach glands, and are probably also concerned in the production of pepsin. The only function of the intestinal crypts which has yet been determined is the secretion of mucus. The possibility that certain cells of the crypts of the small intestine produce a specific secretion has been mentioned (page 208). (2) Absorption of Fat. — While various other products of diges- tion are absorbed by the intestine, the absorption of fat is the one most easily observed. After feeding fat, fatty acids, or soaps, fat globules are found to have penetrated the intestinal mucosa, and may be seen in (a) the epithelial cells, (b) the leucocytes, and (c) the lac- teals of the villi (Fig. 133). Fat globules are never seen in the thickened free borders of the cells. Hence it seems probable that the fat before passing through this part of the cell becomes split up into glycerin and fatty acids which are united again to form fat within the protoplasm of the cell. Leucocytes containing fat glob- ules are seen throughout the stroma. Within the lacteals are found fat-containing leucocytes and free fat droplets of various size. It would thus seem probable that the process of fat absorption consisted in: (1) The passage of glycerin and fatty acids through the cell borders; (2) their reunion in the cell to form fat; (3) the trans- ference of these fat globules to leucocytes ; which (4) carry them to the lacteals. In the lacteals the fat is probably set free by disinte- gration of the leucocytes. TECHNIC. (1) The technic for the small and large intestines and rectum is the same as for the stomach. Accurate fixation of the villi is difficult, there being usually some shrinkage of the connective tissue of the core away from the epithelium. A longitudinal section should be made through the junction of small and large intestine, showing the transition from the villus-covered surface of the former to the comparatively smooth surface of the latter. To show Brunner's glands a section of the duodenum is required. To show the varying shapes of the villi in the different regions, sections should also be made of the jejunum and ileum. Solitary follicles may usually be seen in any of the above sections. A small Peyer's patch, together with the entire thickness of the intestinal wall, should be removed, treated as above, stained with ha;matoxylin-eosin (technic 1, ]). 17)7 or with lutmatoxylin-picro-acid-fuchsin (technic 3, p. 18), and mounted in balsam. (2) A vermiform appendix, as fresli as possible, should be cut transversely into small pieces, fixed in formalin-Muller's fluid (technic 5, p. 6), and hardened in THE DIGESTIVE SYSTEM. 221 alcohol. Thin transverse sections are made through the entire wall, stained with haematoxylin-eosin or hasmatoxylin-picro-acid-fuchsin, and mounted in balsam. (3) Fat Absorption. — For the purpose of studying the process by which fat passes from the lumen of the gut into the chyle vessels, an animal should be killed at the height of fat absorption. A frog fed with fat bacon and killed two days later, a dog fed with fat meat, or a cat with cream and killed after from four to eight hours, furnishes good material. Usually if the preparation is to be successful, the lumen of the intestine will be found to contain emulsified fat and the lacteals of the mesentery are seen distended with chyle. Extremely thin slices of the mucous membrane of the small intestine are fixed in i-per-cent osmic acid or in osmium bichromate solution (5-per-cent aqueous solution potassium bichromate and 2-per- cent aqueous solution osmic acid — equal parts) for twelve to twenty-four hours, after which they are passed rather quickly through graded alcohols. Sections should be thin and mounted, either unstained or after a slight eosin stain, in glycerin. (4) The blood-vessels of the stomach are best studied in injected specimens. (See page 21.) The Larger Glands of the Digestive System. The smaller tubular glands which form a part of the mucous membrane and submucosa of the alimentary tract have been already described. Certain larger glandular structures, the development of which is similar to that of the smaller tubules but which come to lie wholly without the alimentary tract, connected with it only by their main excretory ducts, and which are yet functionally an important part of the digestive system, remain to be considered. These are (a) The parotid. 1. The salivary glands -' (I?) The sublingual. 2. The pancreas. S. The liver. (c) The submaxillary. 1. The Salivary Glands. " The salivary glands are all compound tubular glands. In man the parotid is serous; the sublingual and submaxillar}', mixed serous and mucous (page 181). Only the general structure of these glands is here described, the minute structure of mucous and serous glands having been described on page 1 Si. Each gland consists of gland tissue proper and of a supporting connective-tissue framework. The framework consists of a connec- tive-tissue capsule which encloses the gland, but blends externally 222 THE ORGANS. with and attaches the gland to the surrounding structures. From the capsule trabecules pass into the gland, subdividing it into lobes and lobules. The gland tissue proper consists of systems of excretory ducts opening into secretory tubules, all being lined with one or more layers of epithelial cells. Each gland has one main excretory duct. This divides into branches — interlobar ducts — which run to the lobes in the connective tissue which separates them. The interlobar ducts give rise to branches which, as they pass to the lobules in the inter- A B C Fig. 134.— Diagrams to illustrate the Structure of the Salivary Glands. (Stohr.) A, Parotid ; B, sublingual ; C, submaxillary, a, Excretory duct ; b, secreting tubule ; c, intermediate tubule ;rf, terminal tubule. lobular connective tissue, are known as interlobular ducts. From the latter, branches enter the lobules — intralobular ducts — and split up into terminal secreting tubules which constitute the bulk of the lobule. From the interlobular connective tissue delicate extensions pass into the lobules, separating the gland tubules. The glandular tissue is known as the parencJiyni a of t he gland in contradistinction to the connective or interstitial tissue. The parotid gland in man, clog, cat, and rabbit is a purely serous gland. Its duct system is complex. The main excretory duct (Stenoni) is lined by two layers of columnar epithelium resting upon a distinct basement membrane. The main duct divides into numerous branches, which in turn give rise to so-called secreting or salivary tubules. These* are continuous with long narrow inter- THE DIGESTIVE SYSTEM. 223 mediate tubules, from each of which are given off a number of short terminal tubules (Fig. 134, A). The two-layered epithelium of the main duct becomes reduced in the smaller ducts to a single layer of columnar cells. The salivary tubules are lined with high columnar epithelium, the bases of the cells showing distinct longitudinal stri- ations. In the intermediate tubule the epithelium is flat, sometimes spindle-shaped. The terminal tubules are lined with serous cells (page 181). The sublingual gland is a mixed gland in man, dog, cat, and rabbit. The duct system is less complex than in the parotid. The main duct (Bartholin!) sends off branches which are continuous with tubules, showing a few secretory mucous cells. These open directly into the terminal tubules (Fig. 134, E), The excretory duct is like that of the parotid gland, lined with a two-layered columnar epithe- lium resting upon a basement membrane. In the smaller ducts the epithelium is reduced to a single layer of columnar cells. There are no intermediate tubules. The terminal tubules are lined with both serous and mucous cells (page 181). The crescents of Gianuzzi (page 182) are numerous and large. The connective tissue of the gland contains many lymphoid cells. The submaxillary gland is also a mixed gland in man, dog, cat, and rabbit. In complexity of its duct system it stands between the parotid and the sublingual (Fig. 134). The main duct (Wharton's) has not only a two-layered epithelial lining resting upon a basement membrane, but is distinguished by a richly cellular stroma and a thin layer of longitudinally disposed smooth muscle. Branches of the main duct open into long secreting tubules which communicate with the terminal tubules by means of short narrow intermediate tubules (Fig. 1 34, C). The secretory tubules are lined as in the parotid with columnar cells whose bases are longitudinally striated. These cells usually contain more or less yellow pigment. The intermediate tubules have a low cuboidal or flat epithelium. Most of the end tubules contain serous cells only (page 181). The crescents of the mucous tubules (page 182) are less numerous and smaller than those in the sublingual, consisting as a rule of only from one to three cells (Fig- 135)- Blood-vessels. — The larger arteries run in the connective-tissue septa with the ducts, giving off branches which accompany the divisions of the ducts to the lobules, where they break up into capil- 224 THE ORGANS. lary networks among the tubules. These give rise to veins which accompany the arteries. The lymphatics begin as minute capillaries in the connective tis- sue separating the terminal tubules. These empty into larger lymph vessels which accompany the arteries in the septa. The nerves of the salivary glands are derived from both cerebro- spinal and sympathetic systems, consisting of both medullated and non-medullated fibres. The medullated fibres are afferent, probably FIG. 135.— Section of Human Submaxillary Gland. X 252. (Stohr.) a, Mucous tubule ; b, se- rous tubule ; c, intermediate tubule ; d, " secretory " tubule ; e, demilune ; f, lumen ; g, interstitial connective tissue. the dendrites of cells located in the geniculate ganglion. Small bundles of these fibres accompany the ducts. Single fibres leave the bundles, lose their medullary sheaths, and form a non-medullated subepithelial plexus, from which delicate fibrils pass to end freely among the epithelial cells. Efferent impulses reach the gland through the sympathetic. The fibres are axones of cells situated in small peripheral ganglia; the cells sending axones to the submaxillary lying upon the main excretory duct and some of its larger branches; those sending axones to the sublingual being situated in a small ganglion — the sublingual — lying in the triangular area bounded by the chorda tympani, the lingual nerve, and Wharton's duct ; those supplying the parotid probably being in the otic ganglion. Axones from these cells enter the glands with the excretory duct and follow THE DIGESTIVE SYSTEM. 225 its branchings to the terminal tubules, where they form plexuses be- neath the epithelium. From these, terminals pass to the secreting cells. It is probable that the salivary glands also receive sympa- thetic fibres from cells of the superior cervical ganglia. TECHNIC. (1) The salivary glands should be fixed in Flemming's fluid (technic 7, p. 7), or in formalin-Muller's fluid (technic 5, p. 6). Sections are cut as thin as possible, stained with hsmatoxylin-eosin (technic 1, p. 17), and mounted in balsam. (2) For the study of the secretory activities of the gland cells, glands from a fasting animal should first be examined and then compared with those of a gland the secretion of which has been stimulated by the subcutaneous injection of pilo- carpine. Fix in Flemming's or in Zenker's fluid (technic 9, p. 7). Examine some sections unstained and mounted in glycerin, others stained with haematoxylin-eosin and mounted in balsam. (3) The finer intercellular and intracellular secretory tubules are demon- strated by Golgi's method. Small pieces of absolutely fresh gland are placed for three days in osmium-bichromate solution (3-per-cent potassium bichromate solu- tion, 4 volumes; i-per-cent osmic acid, 1 volume), and then transferred without washing to a 0.75-per-cent aqueous solution of silver nitrate. Here they remain for from two to four days, the solution being frequently changed. The processes of dehydrating and embedding should be rapidly done, and sections mounted in glycerin, or, after clearing in xylol, in hard balsam. Pancreas. The pancreas is a compound tubular gland. While in general similar to the salivary glands, it has a somewhat more complicated structure. A connective-tissue capsule surrounds the gland and gives off trabeculae, which pass into the organ and divide it into lobules. In some of the lower animals, as for example the cat, these lobules are well defined, being completely separated from one another by connective tissue. In this respect they resemble the lobules of the pig's liver. A number of these primary lobules are grouped together and surrounded by connective tissue, which is considerably broader and looser in structure' than that separating the primary lobules. These constitute a lobule group or secondary lobule. In the human pancreas the division into lobules and lobule groups is much less distinct, although it can usually be made out. This is clue to the incompleteness of the connective-tissue septa, the human pancreas in this respect resembling the human liver. Rarely the human paricreas is distinctly lobulated. 226 THE ORGANS. The gland has a main excretory duct, the pancreatic duct or duct of Wirsung. In many cases there is also a secondary excretory duct, the accessory pancreatic duct or duct of Santorini. Both open into the duodenum. The main duct extends almost the entire length of the gland, giving off short lateral branches, one of which enters the centre of each lobule group. Here it splits up into branches which pass to the primary lobules. From these intralobular ducts, are given off long, narrow, intermediate tubules, which in turn give rise to the terminal secreting tubules (Fig. 136). The excretory ducts are lined with a simple high columnar epithelium which rests upon a basement membrane. Outside of this is a connective-tissue coat, the thick- ness of which is directly proportionate to the size of the duct. In the pancreatic duct goblet cells are present, and the ac- companying connective tissue contains small mucous glands. As the ducts decrease in size, the epithelium becomes lower until the intermediate tubule is reached where it becomes flat. The terminal tubules themselves are most of them very short, frequently almost spherical. This and the fact that several terminal tubules are given off from the end of each intermediate tubule have led to the description of these tubules as alveoli, and of the pancreas as a tubulo-alveolar gland, although there is no dilatation of the lumen. The terminal tubules are lined with an irregularly conical epithelium resting upon a basement membrane (Figs. 137 and 138). The appearance of these cells depends upon their functional con- dition. Each cell consists of a central zone bordering the lumen, which contains numerous granules known as zymogen granules, and of a peripheral zone next to the basement membrane, which is homo- geneous and contains the nucleus (Fig. 138). The relative size of these zones depends upon whether the cell is in the active or resting state (compare Fig. 139, A and />). During rest (fasting) the two Fig. 136. — Diagram to illus- trate Structure of Pancreas. (Stohr.) a, Excretory duct ; b, intermediate tubule ; c, c, terminal tubules. THE DIGESTIVE SYSTEM. 227 zones are of about equal size. During the early stages of activity (intestinal digestion) the granules largely disappear and the clear >dIllPBft& FIG. 137.— Section of Human Pancreas; X 112. (Kolliker.) av, Alveoli ; a, interlobular duct surrounded by interlobular connective tissue ; L, islands of Langerhans ; v, small vein zone occupies almost the entire cell. During the height of digestion the granules are increased in number, while after prolonged secre- tion they are again almost absent. The cell now returns to the rest- ing state in which the two zones are about equal. The increase and disappearance of the granules are marked by the appearance of the fluid secretion of the gland in _ ^ __ fc3 ■y^- x *?.&& •**.■.& ■ FlG. 141.— Island of Langerhans and few surrounding Pancreatic Tubules. (Bohm and von Davidoff.) a, Capillary ; b, tubule. size, some, especially where the cells are closely packed, being small, others being large and vesicular. Some of the islands are quite sharply outlined by delicate fibrils of connective tissue (Fig. 141). Others blend with the surrounding tissues. The origin, structure, and function of these islands have been subjects of much controversy. For some time they were considered of lymphoid origin. They are now believed to be epithelial cells having a developmental history similar to the cells lining the secret- ing tubules. Each cell-island consists of, in addition to the cells, a tuft or glomerulus of broad tortuous anastomosing capillaries, which arise from the network of capillaries which surround the secreting 230 THE ORGANS. tubules. The close relation of cells and capillaries and the absence of any ducts have led to the hypothesis that these cells furnish a secretion — internal secretion — which passes directly into the blood- vessels. In a recent publication Opie reviews previous work upon the his- tology of the pancreas and adds the results of his own careful re- searches. He concludes that the cell-islands of Langerhans are Fig. 142.— From Section of Pancreas, the blood-vessels of which had been injected (Kuhneand Lea), showing island of Langerhans with injected blood-vessels, surrounded by sections of tubules. Zymogen granules are distinct in inner ends of cells. definite structures "formed in embryological life," that "they possess an anatomical identity as definite as the glomeruli of the kidney or the Malpighian body of the spleen, and that they subserve some special function." He calls attention to the similarity which Schafer noted between these cell- islands and such small ductless structures as the carotid and coccygeal glands and the parathyroid bodies. From his study of the pancreas in diabetes, Opie concludes that the islands of Langerhans are concerned in carbohydrate me- tabolism. Blood-vessels. — The arteries enter the pancreas with the main duct and break up into smaller arteries which accompany the smaller ducts. These end in a capillary network among the secreting tubules. From this, venous radicles arise which converge to form larger veins. These pass out of the gland in company with the arteries. Lymphatics. — Of the lymphatics little is known. Nerves. — The nerves are almost wholly from the sympathetic system, and are non-medullated. Some of them arc axones of cells in sympathetic ganglia, outside the pancreas; others, of cells situ- ated in small ganglia within the substance of the gland. They pass THE DIGESTIVE SYSTEM. 231 to plexuses among the secreting tubules, to which and to the walls of the vessels they send delicate terminal fibrils. TECHNIC. (1) The general technic for the pancreas is the same as for the salivary glands (page 225). (2) Zymogen granules may be demonstrated by fixation in formalin-Muller's fluid (technic 5, p. 6), and staining with picro-acid-fuchsin (technic 2, p. 18), or with Heidenhain's iron haematoxylin (technic 3, p. 15). (3) The arrangement of the blood-vessels in the islands of Langerhans may be studied in specimens in which the vascular system has been injected (page 21). The Liver. The liver is a compound tubular gland, the secreting tubules of which anastomose. There are thus, strictly speaking, no " terminal tubules " in the liver, the lumina and walls of neighboring tubules anastomosing without any distinct line of demarcation. b\d Fig. 143.— Section of Lobule of Pig's Liver X 60 (technic 1, p. 234), showing lobule completely surrounded by connective tissue, it, Portal vein ; b, bile duct ; c, hepatic artery ; d, por- tal canal ; e, capillaries ; /", central vein ; g; cords of liver cells ; //, hepatic vein. The liver is surrounded by a connective-tissue capsule, the cap- sule of Glissoii. At the hilum this capsule extends deep into the 232 THE ORGANS. substance of the liver, giving off broad connective-tissue septa, which divide the organ into lobes. From the capsule and from these interlobar septa, trabecular pass into the lobes, subdividing them into ' ,' , ■ ''•:*-ir- Vf c -*»'»\ ,*=-«■> » •>'•>-,■>■»* ? - i " »" *»'* » r y \ --^„-.'A H •': .■ -- .V"*. ».. *■ ■-.-',•-■ -v y *"A ■ Til*-' 9 , ' y^" ^''•'i'i'V '* rs '^'»"j'V? M* s^^'m^-%^ <&*£ ■„¥*■ t " i E> V H 7-,-/ **' <.;•+* FIG. 144.— Section of Human Liver. X 80. (Hendrickson.) P, Portal vein; B, hepatic ar- tery ; B, bile duct. P, H, B constitute the portal canal and lie in the connective tissue between the lobules. lobules. In some animals, as for example the pig, each lobule is completely invested by connective tissue (Fig. 143). In man, only islands of connective tissue are found, usually at points where three or more lobules meet (Fig. 144). The lobules are cylindrical or irregularly polyhedral in shape, about 1 mm. in breadth and 2 mm. in length. Excepting just beneath the capsule, where they are fre- quently arranged with their apices toward the surface, the liver lob- ules have an irregular arrangement. The lobule (Fig. 143) which may be considered the anatomic unit of structure of the liver, consists of secreting tubules arranged in a definite manner relatively to the blood-vessels. The blood-ves- sels of the liver must therefore be first considered. THE DIGESTIVE SYSTEM. -55 The blood supply of the liver is peculiar in that in addition to the ordinary arterial supply and venous return, which all organs pos- sess, the liver receives venous blood in large quantities through the portal vein. There are thus two afferent vessels, the hepatic artery and the portal vein, the former carrying arterial blood, the latter venous blood from the intestine. Both vessels enter the liver at the hilum and divide into large interlobar branches, which follow the connective-tissue septa between the lobes. From these are given off interlobular branches, which run in the smaller connective-tissue septa between the lobules. From the interlobular branches of the portal vein arise veins which are still interlobular and encircle the lobules. These send off short branches which pass to the surface of the lobule, where they break up into a rich intralobular capillary net- work. These intralobular capillaries all converge toward the centre of the lobule, where they empty into the central vein (Fig. 143). The central veins are the smallest radicles of the hepatic veins, which are the efferent ves- sels of the liver. Each central vein be- gins at the apex of the lobule as a small vessel little larger than a capillary. As it passes through the centre of the long axis of the lobule the central vein con- stantly receives capillaries from all sides, and, increasing in size, leaves the lobule at its base. Here it unites with the central veins of other lobules to form the sublobular vein which is a branch of the hepatic. The hepatic artery accompanies the portal vein, following the branchings of the latter through the interlobar and in- terlobular connective tissue, where its finer twigs break up into capillary net- works. Some of these capillaries empty into the smaller branches of the portal vein ; others enter the lobules and anastomose with the intralobular portal capillaries. The main excretory duct — hepatic duct — leaves the liver at the hilum near the entrance of the portal vein and hepatic artery. Fig. 145. — Portal Canal. X 315. (Klein and Smith.) ,?, Hepatic artery; I', portal vein; d, bile duct. 234 THE ORGANS. Within the liver the duct divides and subdivides, giving off inter- lobar, and these in turn interlobular brandies. These ramify in the connective tissue, where they always accompany the branches of the portal vein and hepatic artery. These three structures — the hepatic artery, the portal vein, and the bile duct, which always occur together in the connective tissue which marks the point of separation of three or more lobules — together constitute the portal canal (Fig. 145). From the interlobular ducts short branches pass to the surfaces of .M'-:y &■ ■;? ; .*■'■'■' - ^ . , . ^ & V • & r :w ay.- '<-. - '.'. % FIG. 146. — Part of Lobule of Human Liver, showing capillaries and anastomosing' cords of liver cells. X 350. a, Liver cells ; b. capillaries. the lobules. From these are given off extremely narrow tubules, which enter the lobule as intralobular secreting tubules. The walls of the ducts consist of a single layer of epithelial cells resting upon a basement membrane and surrounded by connective tissue (Fig. 145). The height of the epithelium and the amount of connective tissue are directly proportionate to the size of the duct. In the largest ducts there are usually a few scattered smooth muscle cells. The walls of the secreting tubules are formed by the liver cells. The liver cells (Fig. 146) are irregularly polyhedral in shape. THE DIGESTIVE SYSTEM. 235 They have a granular protoplasm which frequently contains glycogen, pigment granules, and droplets of fat and bile. Each cell contains ■one or more spJicrical nuclei. Like other gland cells, the granularity Pig. 147.— Part of Lobule of Human Liver, Golgi Method (technic 3, p. 238), to show relations of bile duct to intralobular secretory tubules and of the latter to the liver cells, a, Bile duct ; d, cords of liver cells ; c, blood capillaries ; d, central vein ; e, secretory tubules. of the protoplasm depends upon its functional condition. Within the cells are minute irregular canals, some of which can be injected through the blood-vessels, while others are apparently continuous with the secreting tubules (Fig. 148, A and B). The capillaries of the portal vein, as they anastomose and con- A B FlG. 14S. — A, Cell from human liver showing intracellular canals (Browicz) ; c. intracellular canal ; ;/, nucleus. />', From section of rabbit's liver injected through portal vein, show- ing intracellular canals (continuous with intercellular blood capillaries).. (Schafer.) verge from the periphery to the centre of the lobule, form long- meshed capillary networks. In the meshes of this network lie the anastomosing secreting tubules. On account of the shape of the 236 THE ORGANS. capillary network, the liver cells, which form the walls of these tubules, are arranged in anastomosing rows or cords, known as hepatic cords or cords of liver cells (Fig. 146). The secreting tubules (Fig. 147) are extremely minute channels, the walls of which are the liver cells. A secretory tubule always runs between two contiguous liver cells, in each of which a groove is formed. The blood capillaries, on the other hand, are found at the corners where Fig. 149. — Liver Lobule, to show Connective-tissue Framework. (Mall.)' three or more liver cells come in contact. It thus results that bile tubules and blood capillaries rarely lie in contact, but are regularly separated by part of a liver cell. Exceptions to this rule sometimes occur. While most of the secretory tubules anastomose, some of them end blindly either between the liver cells or, in some instances,, after extending a short distance within the cell protoplasm (Fig. 148, A). At the surface of the lobule there is a modification of some of the liver cells to a low cuboidal type, and these become con- tinuous with the lining cells of the smallest bile ducts, the secretory tubule being continuous with the duct lumen. THE DIGESTIVE SYSTEM. 237 Special methods of technic have demonstrated a connective-tissue framework within the lobule. This consists of a reticulum of ex- tremely delicate fibrils which envelop the capillary blood-vessels, and of a smaller number of coarser fibres which radiate from the region of the central vein — radiate fibres (Fig. 149). Special technical methods also show the presence of stellate cells — cells of Kupffer — within the lobule. These are interpreted by Kupffer as belonging to the endothelium of the intralobular capil- laries. Blood-vessels. — These have been already described. Lymph vessels form a network in the liver capsule. These com- municate with deep lymphatics in the substance of the organ. The latter accompany the portal vein and follow the ramifications of its capillaries within the lobule as far as the central vein. The nerves of the liver are mainly non-medullated axones of sympathetic neurones. The nerves accompany the blood-vessels and bile ducts, around which they form plexuses. These plexuses give off fibrils which end on the blood-vessels, bile ducts, and liver cells. Three main ducts, all parts of a single excretory duct system, are concerned in the transportation of the bile to the intestine, the he- patic, the cystic, and the common. Their walls consist of a mucous membrane, a submucosa, and a layer of smootJi muscle. The mucosa is composed of a simple columnar epithelium resting upon a base- ment membrane and a stroma which contains smooth muscle cells and small mucous glands. The submucosa is a narrow layer of con- nective tissue. Hendrickson describes the muscular coat as consist- ing of three layers, an inner circular, a middle longitudinal, and an external oblique. At the entrance of the common bile duct into the intestine, and at the junction of the duct of Wirsung with the com- mon duct, there are thickenings of the circular fibres to form sphinc- ters. In the cystic duct occur folds of the mucosa — the Heisterian ■valve — into which the muscularis extends. The Gall-Bladder. The wall of the gall-bladder consists of three coats — mucous, muscular, and serous. The mucous membrane is thrown up into small folds or ruges, which anastomose and give the mucous surface a reticular appear- 238 THE ORGANS. ance. The epithelium is of the simple columnar variety with nuclei situated at the basal ends of the cells. A few mucous glands are usually found in the stroma. The muscular coat consists of bundles of smooth muscle cells which are disposed in a very irregular manner, and are separated by considerable fibrous tissue. A richly vascular layer just beneath the stroma is almost free from muscle and corresponds to a submucosa. It frequently contains small lymph nodules. The serous coat is a reflection of the peritoneum. TECHNIC. (i) Before taking up the study of the human liver, the liver from one of the lower animals in which each lobule is completely surrounded by connective tissue should be studied. Fix small pieces of a pig's liver in formalin-Muller's fluid (technic 5, p. 6.) Cut sections near and parallel to the surface. Stain with hae- matoxylin-picro-acid-fuchsin (technic 3, p. 18) and mount in balsam. In the pig's liver the lobules are completely outlined by connective tissue and the yellow picric- acid-stained lobules are in sharp contrast with the red fuchsin-stained connective tissue. (2) For the study of the human liver treat small pieces of perfectly fresh tissue in the same manner as the preceding, but stain with haematoxylin-eosin (technic 1, p. 17). (3) The secretory tubules and smaller bile ducts may be demonstrated by technic 4, p. 25. A light eosin stain brings out the liver cells. (4) For the study of the blood-vessels of the liver, inject the vessels through the inferior vena cava or portal vein. If the vena cava is used, it is convenient to inject from the heart directly through the right auricle into the vena cava. Sec- tions should be rather thick and may be stained with eosin, or even lightly with haematoxylin-eosin (technic 1, p. 17), and mounted in balsam. (5) For demonstrating the intralobular connective tissue Oppel recommends fixing fresh tissue in alcohol, placing for twenty-four hours in a 0.5-per-cent aqueous solution of yellow chromate of potassium, washing in very dilute silver nitrate solution (a few drops of 0.75-per-cent solution to 50 c.c. of water) and then trans- ferring to 0.75-per-cent silver nitrate solution, where it remains for twenty-four hours. Embed quickly in celloidin. The best tissue is usually found near the surfaces of the blocks. A similar result is obtained by fixing fresh tissue in 0.5- per-cent chromic-acid solution for three days, then transferring to 0.5-per-cent sil- ver nitrate solution for two days. Development of the Digestive System. In the development of the digestive system all the layers of the blastoderm are involved. Mesoderm and entoderm are, however, the layers most concerned, as the ectoderm is used only in the formation of the oral and anal orifices. The primitive alimentary canal is formed by two folds which grow out from the ventral surface of the THE DIGESTIVE SYSTEM. 2 39 embryo and unite to form a canal, in a manner quite similar to the formation of the neural canal (page 336). In this way the primitive gut is lined with cells which previously formed the ventral surface of the embryo, i.e., entoderm. A portion of the mesoderm accom- panies the entoderm in the formation of the folds. This is known as the visceral layer of the mesoderm. The primary gut is thus a closed sac or tube. It is connected with the umbilical vesicle, but has no connection with the exterior. These connections are formed later by oral and anal invaginations of ectoderm which extend in- ward and open up into the ends of the hitherto imperforate gut. The ends of the alimentary tract, including the oral cavity and all of the glands and other structures connected with it, are of ectodermic origin. The epithelial lining of the gut and the parenchyma of all glands connected with it are derived from entoderm. The muscle, the connective tissue, and the mesothelium of the serosa are de- veloped from mesoderm. The mesodermic elements show little variation throughout the gut, the peculiarities of the several anatomical divisions of the latter being dependent mainly on special differentiation of the entoderm (epithelium). Beneath the entodermic cells is a narrow layer of loosely arranged tissue which later separates into stroma, muscularis mucosae, and submucosa. Outside of this a broader mesodermic band of firmer structure represents the future muscularis. The stomach first appears as a spindle-shaped dilatation about the end of the first month. Its entodermic cells, which had consisted of a single layer, increase in number and arrange themselves in short cylindrical groups. These are the first traces of tubular glands. They increase in length and extend downward into the mesodermic tissue. For a time the cells lining the peptic glands are all appar- ently alike, but at about the fourth month the differentiation into chief cells and parietal cells takes place. In the intestines a proliferation of the epithelium and of the underlying stroma results in the formation of villi. These appear about the tenth week, in both small and large intestines. In the former they increase in size, while in the latter they atrophy and ultimately disappear. The simple tubular glands of the intestines develop in a manner similar to those of the stomach. The mesothelium of the serosa is derived from the mesodermic cells of the primitive body cavity. 240 THE ORGANS. The development of the larger glands, connected with the diges- tive tract, takes place in a manner similar to the formation of the simple tubular glands. All originate in extensions downward of entodermic cords into the underlying mesodermic tissue. From the lower ends of these cords, branches extend in all directions to form the complex systems of tubules found in the compound glands. The salivary glands being developed from the oral cavity, origi- nate in similar invaginations of ectodermic tissue. In the case of the pancreas a portion of the gland has an inde- pendent origin in the epithelium of the ductus choledochus. This portion ultimately unites with the main mass of the gland and its duct. The duct of Santorini sometimes remains patent, but in many cases atrophies so that the entire pancreatic secretion usually reaches the intestine through the pancreatic duct. The liver begins as a ventral downgrowth of the intestinal epi- thelium into the mesoderm of the transverse septum. This almost immediately divides into two hepatic diverticula. About the ends of these diverticula active proliferation of entodermic cells takes place, and this represents the first appearance of liver tissue. General References for Further Study. Oppel: Lehrbuch der vergleichenden mikroskopischen Anatomic Kolliker: Handbuch der Gewebelehre des Menschen. Opie : The Pancreas. Stohr: Salivary Glands, in Text-book of Histology. CHAPTER VII. THE RESPIRATORY SYSTEM. The respiratory apparatus consists of a system of passages — . nares, larynx, trachea, and bronchi, which serve for the transmission of air to and from the essential organ of respiration, the lungs. The Nares. The nares, or nasal passages, are divided into vestibular, respira- tory, and olfactory regions, the differentiation depending mainly upon the structure of their mucous membranes. The vestibular region marks the transition between skin and mucous membrane (page 180). Its epithelium is of the stratified squamous variety and rests upon a basement membrane, which is thrown into folds by papillae of the underlying stroma. The latter is richly cellular and contains sebaceous glands (page 326) and the follicles of the nasal hairs. The respiratory region is much larger than both the vestibular and olfactory regions. Its epithelium is of the stratified columnar variety. The cells of the surface layer are ciliated and are inter- spersed with goblet cells. The stroma is distinguished by its thick- ness (3 to 5 mm. over the inferior turbinates) and by the presence of networks of such large veins that the tissue closely resembles erectile tissue. It contains considerable diffuse lymphoid tissue and here and there small lymph nodules. In the stroma are small simple tubular glands lined with both serous and mucous cells. There is no submucosa, the stroma being connected directly with the perios- teum and perichondrium of the nasal bones and cartilages. The mucous membrane of the accessory nasal sinuses is similar in structure to that of the respiratory region of the nares, but is thinner and contains fewer glands. The olfactory region can be distinguished with the naked eye by its brownish-yellow color, in contrast with the reddish tint of the surrounding respiratory mucosa. The epithelium is of the stratified columnar type, and is considerably thicker than that of the respiratory 16 241 242 THE ORGANS. region. The surface cells are of two kinds: (1) sustentacular cells, and (2) olfactory cells. (1) The sustentacular cells are the more numerous. Each cell consists of three parts : (a) A superficial portion, which is broad and cylindrical, and contains pigment, and granules arranged in longi- tudinal rows. The cells have well-marked, striated, thickened free borders, which unite to form the so-called memb'rana limitans olfac- toria. (b) A middle portion which contains an oval nucleus. As the nuclei of these cells all lie in the same plane, they form a distinct narrow band, which is known as the zone of oval nuclei. (c) A thin filamentous process which extends from the nuclear portion down between the cells of the deeper layers. This process is irregular and pitted by pressure of surrounding cells. It usually forks and appar- ently anastomoses with processes of other cells to form a sort of pro- toplasmic reticulum. (2) The olfactory cells lie between the sustentacular cells. Their nuclei are spherical, lie at different levels, and are most of them more deeply placed than those of the sustentacular cells. They thus form a broad band, the zone of round nuclei. From the nuclear portion of the cell a delicate process extends to the surface, where it ends in several minute hair-like processes. From the opposite pole of the cell a longer process extends centrally as a centripetal nerve fibre. The olfactory cell is thus seen to be of the nature of a ganglion cell (see also page 450). Between the nuclear parts of the olfactory cells and the basement membrane are the basal cells. These are small nucleated elements, the irregular branching protoplasm of which anastomoses with that of neighboring basal cells and of the sustentacular cells to form the peculiar protoplasmic reticulum already mentioned. The basement membrane is not well developed. The stroma consists of loosely arranged white fibres, delicate elas- tic fibres, and connective-tissue cells. Embedded in the stroma are large numbers of simple branched tubular glands, the glands of Bow- man. Each tubule consists of a duct, a body, and a fundus. The secreting cells are large and irregular and contain a yellowish pig- ment, which with that of the sustentacular cells is responsible for the peculiar color of the olfactory mucosa. These glands were long described as serous, but are now believed to be mucous in character. They frequently extend beyond the limits of the olfactory region. THE RESPIRATORY SYSTEM. 243 The Larynx. The larynx consists essentially of a group of cartilages united by strong fibrous bands and lined by mucous membrane. The epithelium covering the true vocal cords, the free margin of the epiglottis, and parts of the arytenoid cartilages is of the stratified squamous variety with underlying papillae. With these exceptions the mucous membrane of the larynx is lined with stratified columnar ciliated epithelium similar to that of the respiratory portion of the nares. Numerous goblet cells are usually present, and the epithe- lium rests upon a broad basement membrane. On the posterior sur- face of the epiglottis many taste buds (see nerve endings, page 353) are embedded in the epithelium. The stroma is especially rich in elastic fibres. The true vocal cords consist almost wholly of longitudinal elastic fibres covered by stratified squamous epithelium. Lymphoid cells are present in vary- ing numbers. In some places they are so numerous that the tissue assumes the character of diffuse lymphoid tissue. Distinct nodules sometimes occur. Owing to the absence of a muscularis mucosae the stroma passes over with no distinct line of demarcation into the submucosa. This is a more loosely arranged, less cellular connective tissue, and con- tains simple tubular glands lined with both serous and mucous cells. Externally the submucosa merges into a layer of more dense fibrous tissue which connects it with the laryngeal cartilages and with the surrounding structures. Immediately surrounding the cartilages the connective tissue forms an extremely dense layer, the perichon- drium. Of the cartilages of the larynx, the epiglottis, the middle part of the thyroid, the apex and vocal process of the arytenoid, the carti- lages of Santorini and of VVrisburg are of the yellow elastic variety. The main body of the arytenoid, the rest of the thyroid and the cri- coid cartilages are hyaline. After the twentieth year, more or less ossification is usually found in the cricoid and thyroid cartilages. The Trachea. The walls of the trachea consist of three layers — mucosa, submu- cosa, and fibrosa (Fig. 150). The mucosa is continuous with that of the larynx, which it close- 244 THE ORGANS. ly resembles in structure. It consists of a stratified columnar ciliated epithelium, with numerous goblet cells, resting upon a broad base- ment membrane, and of a stroma of mixed fibrous and elastic tissue containing many lymphoid cells. The submucosa is not distinctly marked off from the stroma on account of the absence of a muscularis mucosae. It is distinguished from the stroma by its looser, less cellular structure, by its numerous large blood-vessels, and by the presence of glands. These are of the simple branched tubular variety and are lined with both serous and mucous cells. Some of the mucous tubules have well-marked cres- cents of Gianuzzi. The glands are most numerous between the ends of the cartilaginous rings, where they extend into the fibrosa. The fibrosa is composed of coarse, rather loosely woven connec- tive-tissue fibres embedded in which are the tracheal cartilages. ..'.,. j- bfikSH&S&lHJ iW'^^^: "?-5^% ■& '- ' ■ ' •% < « FIG. 150.— From Longitudinal .Section of Human Trachea. X 4°- (Technic 3, p. 246.) a, Epi- thelium ; />, stroma; c, cartilage ; d, fibrous coat ; e, serous tubules;/, mucous tubules ; g; glands in submucosa ; //, ducts. These are incomplete rings of hyaline cartilage shaped like the letter C (Fig. 151). They are from sixteen to twenty in number and en- circle about four-fifths of the tube, being open posteriorly. The THE RESPIRATORY SYSTEM. 245 openings between the ends of the cartilaginous rings are bridged over by a thickened continuation of the fibrous coat, strengthened by a layer of smooth muscle (Fig. 151, in). The bundles of muscle cells PIG. 151.— Transverse Section of Human Trachea through One of the Cartilage Rings. X 8. (Kolliker.) E, Epithelium of is) mucous membrane ; dr, glands ; as, gland duct ; ad, ade- noid tissue ; A', cartilage ; m, smooth muscle cut longitudinally, extending across between ends of cartilage ring. run mainly in a transverse direction, and extend across the intervals between adjacent rings as well as between their open ends. There are frequently a few bundles of longitudinally disposed cells. Outside the fibrous coat proper is a looser, more irregular con- nective tissue, which serves to attach the trachea to the surrounding structures. Blood-vessels, lymphatics, and nerves have a similar distribution in larynx and trachea. The larger vessels pass directly to the sub- 246 THE ORGANS. mucosa. From these, smaller branches pass to the different coats, where they break up into capillary networks. Lymphatics form plexuses in the submucosa and mucosa, the most superficial lying just beneath the subepithelial capillary plexus. The nerves of the larynx and trachea are derived from both cere- bro-spinal and sympathetic systems. The cerebro-spinal nerves are afferent, the dendrites of spinal ganglion cells. They form a sub- epithelial plexus from which are given off fibrils which pass into the epithelium and terminate freely among the epithelial cells. Other afferent fibres of cerebro-spinal nerves pass to the muscular coat of the trachea. Sympathetic nerve fibres form plexuses which are interspersed with minute groups of ganglion cells. Axones from these ganglion cells have been traced to the smooth muscle cells of the trachea. Sympathetic axones also pass to the glands of the trachea and larynx. On the under surface of the epiglottis small taste buds are found. TECHNIC. (i) For the study of the details of structure of the walls of the nares and larynx, fix small pieces of perfectly fresh material from different regions in formalin -M id- ler's fluid (technic 5, p. 6), harden in alcohol, stain sections with haematoxylin- eosin (technic 1, p. 17), and mount in balsam. (2) The general relations of the parts can be studied by removing the larynx, upper part of the trachea, and corresponding portion of the oesophagus of an ani- mal or of a new-born child, fixing and hardening as above, and cutting longitudinal sections through the entire specimen. (3) Trachea. — Remove a portion of the trachea and treat as in technic (1). Both longitudinal and transverse sections should be made; the longitudinal includ- ing at least two of the cartilaginous rings ; the transverse being through one of the rings. The Bronchi. The primary bronchi and their largest branches have essentially the same structure as the trachea except that the cartilaginous rings are not as complete. As the bronchi decrease in calibre, the following changes take place in their walls (Figs. 152 and 153): 0) The epithelium gradually becomes thinner. In a bronchus of medium size it has become reduced to three layers of cells, which Kcilliker describes as an outer "basal" layer, a middle "replacing" layer, and a surface layer of ciliated and goblet cells. In the smaller bronchi the epithelium is reduced to a single layer of ciliated cells. THE RESPIRATORY SYSTEM. 247 These are at first high, but become gradually lower as the bronchi become smaller, until in the terminal branches the epithelium is simple cuboidal and non-ciliated. FIG. 152.— Transverse Section through a Large and a Medium-size Bronchus of the Human Lung. X 15. (Technic 2, p. 255.) In the fibrous coat are seen the bronchial arteries and veins, a, Epithelium ; b, stroma ; c, muscularis mucosas ; d, lung tissue ; e, fibrous coat ; f, plates of cartilage. (2) The stroma decreases in thickness as the bronchi become smaller. It consists of loosely arranged white and elastic fibres. Fig. 153.— Transverse Section of Small Bronchus from Human Lung. X us- (Technic 2, p. 255.) a, Stroma ; b, epithelium ; c, muscularis mucosa? ; d, fibrous coat. There is considerable diffuse lymphatic tissue, and in some places small nodules occur, over which there may be lymphoid infiltration of the epithelium (see Tonsil, page 145). 248 THE ORGANS. (3) With decrease in thickness of the epithelium and of the stro- ma, the thickness of the mucosa is maintained by the appearance of a layer of smooth muscle. In the larger bronchi this is a continuous layer of circularly disposed smooth muscle, and lies just external to the stroma, forming a muscularis mucosae. As the bronchi become smaller the muscularis mucosas becomes thinner, discontinuous, and in the smallest bronchi is represented by only a few scattered mus- cle cells. (4) The submucosa decreases in thickness with decrease in the calibre of the bronchi. It consists of loosely arranged connective tissue. Mucous glands are present until a diameter of about 1 mm. is reached, when they disappear. (5) The cartilages, which in the trachea and primary bronchi form nearly complete rings, become gradually smaller, and finally break up into short disconnected plates (Fig. 152). These plates decrease in size and number, and are absent after a diameter of 1 mm. is reached. From the small bronchi are given off terminal bronchi. These are respiratory in character and are described with the lungs. The Lungs. The lung is built upon the plan of a compound alveolar gland, the trachea and bronchial ramifications corresponding to duct sys- tems, the air vesicles to gland alveoli. The surface of the lung is covered by a serous membrane — the paluwnary pleura — which forms its capsule, and which at the root of the lung, or hilum, is reflected upon the inner surface of the chest wall as the parietal pleura. From the capsule broad connective-tis- sue septa pass into the organ, dividing it into lobes. From the cap- sule and interlobar septa are given off smaller septa, which subdivide the lobes into lobules. The human pulmonary lobule is irregularly pyramidal, and has a diameter of from 1 to 3 cm. The amount of interlobular connective tissue is so small that no distinct separation into lobules can usually be made out. The pulmonary lobule constitutes the anatomic unit of lung structure in the same sense that the liver lobule constitutes the anatomic unit of that organ. The most superficial lobules are arranged with their bases against the pleura. Elsewhere in the lung the lobules have an irregular arrangement. THE RESPIRATORY SYSTEM. 249 The apex of each lobule is the point of entrance of a small bron- chus. This gives off within the lobule several terminal or respi- ratory bronchi i^'\g. 154, />; Figs. 155 and 156, BR). From each FlG. 154.— From Lung of an Ape. The bronchi and their dependent ducts and alveoli have been filled with quicksilver. X 15. (Kolliker, after Schulze.) b, Terminal bronchus; a, alveolar duct ; z", alveoli. terminal bronchus open from three to six narrow passages — alveolar passages or alveolar ducts (Fig. 154, a; Figs. 155 and 156, DA). The alveolar passages open into wider chambers — air passages or infundibula. The latter are irregularly pyramidal, their bases being Fig. 155.— Camera Lucida Tracing of Calf's Lung (Miller). Stippling = nuclei of epithelium and position of smooth muscle. Pulmonary artery in black. B.J?., Respiratory bronchus ; D.A., alveolar duct ; A., atrium ; A. S., air sacs. directed away from the alveolar passage. From the sides of the termi- nal bronchi, the alveolar passages, and the infundibula are given off the alveoli — air vesicles or air cells (Fig. 154, /; Figs. 155 and 1^6, AS). According to Miller a further subdivision of the alveolar passage 250 THE ORGANS. can be made. He describes the terminal bronchus as about 0.5 mm. in diameter, and as opening into from three to six narrow tubules, tflG. 156.— Camera Lucida Tracing of Section of Lung of Two and One-half Months' Old Child (Miller). Heavy black lines, smooth muscle ; pulmonary artery in black ; B.R., respiratory bronchus ; D.A., alveolar duct ; A., atrium ; A.S., air sacs. the vestibula. Each vestibulum is about o. 2 mm. in diameter, and opens into several larger, nearly spherical chambers, the atria. Each atrium communicates with a number of very narrow — 0.14 mm. — air- PIG. 157.— From Section of Cat's Lung Stained with Silver Nitrate. (Klein.) (Technic r, p. 65.) Small bronchus surrounded by alveoli, in which are seen both flat cells (respira- tory epithelium), and cuboidal cells ffcetal cells). sac passages f rom which open the air sacs. From the latter are given off on all sides, the air cells or alveoli. Alveoli are not, however, THE RESPIRATORY SYSTEM. 251 confined to the periphery of the air sacs, but are given off in small numbers from the terminal bronchus, and in constantly increasing numbers from the alveolar ducts and infundibula. The terminal bronchus. The proximal portion of the terminal or respiratory bronchus is lined by a simple columnar ciliated epithe- lium, resting upon a basement membrane. Beneath this is a richly elastic stroma containing bundles of circularly disposed smooth mus- cle cells. The epithelium becomes gradually lower and non-ciliated, Fig. 158.— Section Through Three Alveoli of Human Lung. X 235. Weigert's elastic-tissue stain (technic 3, p. 25) to show arrangement of elastic tissue, a, Alveolus cut through side walls only ; i, alveolus cut through side walls and portion of bottom or top ; c, alve- olus in which either the bottom or top is included in section. and near the distal end of the terminal bronchus there appear small groups or islands of flat, non-nucleated epithelial cells — respiratory epithelium. The alveolar passage. Here the cuboidal epithelium is almost completely replaced by the respiratory. Beneath the epithelium the walls have a structure similar to those of the distal end of the termi- nal bronchus, consisting of delicate fibro-elastic tissue with scattered smooth muscle cells. The basement membrane is extremely thin. The air passage. The epithelium of the air passage consists of two kinds of cells, respiratory cells and so-called "fa-tal" cells (see Development, page 255). The respiratory cells (Fig. 157) are some of them large, flat, non- 252 THE ORGANS. nucleated plates, while others are much smaller, non-nucleated ele- ments. The absence of nuclei and the extremely small amount of intercellular substance render these cells quite invisible in sections stained by the more common methods. The cell boundaries are best demonstrated by means of silver nitrate (technic i, p. 65). Fig. 159. — Parts of Four Alveoli from Section of Injected Human Lung. X 200. (Technics, p. 255-) a. Wall of alveolus seen on flat; c, same, but only small part of alveolarwall in plane of section ; 6, alveoli in which plane of section includes only side walls ; alveolar wall seen on edge. The "ftvtal" cells are granular, nucleated cells which are scat- tered among the respiratory cells. Their position appears to be less superficial than that of the respiratory cells, the fcetal cells lying in the meshes of the capillary network, the respiratory cells covering the capillaries. In the embryonic lung the air passages and alveoli contain only this type of cells. The alveolus is similar in structure to the alveolar passage, its walls consisting mainly of delicate elastic fibrils supporting respi- ratory and fcetal cells. Around the opening of the alveolus the elas- tic fibres are more numerous, forming a more or less definite ring. The interalveolar connective tissue, while extremely small in amount, serves to separate the alveoli from one another. Somewhat THE RESPIRATORY SYSTEM. 253 thicker connective tissue separates the alveoli of one alveolar passage from those of another. Still stronger connective-tissue bands sepa- rate adjacent lobules. Blood-vessels. — Two systems of vessels distribute blood to the lungs. One, the bronchial system, carries blood for the nutrition of the lung tissue. The other, the much larger pulmonary system, car- ries blood for the respiratory function. The bronchial artery and the pulmonary artery enter the lung at its hilum. Within the lung the vessels branch, following the branch- ings of the bronchi, which they accompany. The pulmonary vessels are much the larger and run in the connective tissue outside the bronchial walls. The bronchial vessels lie within the fibrous coat of the bronchus. A section of a bronchus thus usually shows the large pulmonary vessels, one on either side of the bronchus, and two or more small bronchial vessels in the walls of the bronchus (Fig. 152). The pulmonary lobule forms a distinct "blood-vascular unit." A branch of the pulmonary artery enters the apex of each lobule close to the lobular bronchus, and almost immediately breaks up into branches, one of which passes to each alveolar passage. From these are given off minute terminal arterioles which pass to the central sides of the alveolar passages and alveoli, where they give rise to a rich capillary network. This capillary network is extremely close- Air. / a Blood. >v c Fig. 160. — Diagram of Tissues Interposed Between Blood and Air in Alveolus. <7, Respira- tory epithelium ; 6, basement membrane and small amount of fibro-elastic tissue ; c, endothelium of capillary. meshed, and invests the alveoli on all sides. Similar networks in- vest the walls of the respiratory bronchi, the alveolar ducts, and their alveoli. All of these capillary networks freely anastomose. There are thus interposed between the blood in the capillaries and the air in the alveoli only three extremely thin layers : (i) The thin endothelium of the capillary wall ; (2) the single layer of flat respiratory epithelial plates ; and (3) the delicate basement mem- brane upon which the respiratory epithelium rests together with an extremely small amount of fibrous and elastic tissues (see diagram, Fig. 160). The veins begin as small radicles, one from the base of each alve- olus. These empty into small veins at the periphery of the lobule. 254 THE ORGANS. These veins at first run in the interlobular connective tissue away from the artery and bronchus. Later they empty into the large pul- monary trunks which accompany the bronchi. The bronchial arteries break up into capillary networks in the walls of the bronchi, supplying them as far as their respirator} 7 divi- sions, beyond which point the capillaries belong to the pulmonary system. The bronchial arteries supply the walls of the bronchi, the bronchial lymph nodes, the walls of the pulmonary vessels, and the pulmonary pleura. Of the bronchial capillaries some empty into the bronchial veins, others into the pulmonary veins. Lymphatics. — The lymphatics of the lung begin as small lymph spaces in the interalveolar connective tissue. These communicate with larger lymph channels in the interlobular septa. Some of these empty into the deep pulmonary lymphatics, which follow the pulmo- nary vessels to the lymph glands at the root of the lung. Others empty into the superficial pulmonary lymphatics, which form an extensive subpleural plexus connected with small subpleural lymph nodes, whence by means of several larger vessels the lymph is carried to the lymph nodes at the hilum. Nerves. — Bundles of meclullated and non-medullated fibres accom- pany the bronchial arteries and veins. Small sympathetic ganglia are distributed along these nerves. The fibres form plexuses in the fibrous layer of the bronchi, from which terminals pass to the muscle of the bronchi and of the vessel walls and to the mucosa. Free end- ings upon the epithelium of bronchi, air passages, and alveoli have been described. Development of the Respiratory System. The epithelium of the respiratory system develops from ento- derm, the connective-tissue elements from mesoderm. The first dif- ferentiation of respiratory system appears as a dipping clown of the entoderm of the floor of the primitive pharynx. The tubule thus formed divides into a larger and longer right branch, which sub- divides into three branches corresponding to the three lobes of the future right lung, and a smaller and shorter left branch, which sub- divides into two branches corresponding to the two lobes of the future left lung. By repeated subdivisions of these tubules the entire bronchial system is formed. The last to develop are the respiratory divisions of the bronchi with their alveolar passages and THE RESPIRA TOR Y SYS TEM. 2 5 5 alveoli. The epithelium of the air passages and alveoli is at first entirely of the foetal-cell type, the large flat respiratory plates appear- ing only after the lungs have become inflated. The foetal and respi- ratory cells of the adult lung have therefore the same embryonic origin. During the early stages of lung development the mesoder- mic tissue predominates, but with the rapid growth of the tubules the proportion of the two changes until in the adult lung the meso- dermic tissue becomes restricted to the inconspicuous pulmonary framework and the blood-vessels. TECHNIC. (1) The technic for the largest bronchi is the same as for the trachea (technic 3, p. 246). The medium size and small bronchi are studied in sections of the lung. (2) Lung and Bronchi. — Carefully remove the lungs and trachea (human, dog r or cat) and tie into the trachea a cannula to which a funnel is attached. Distend the lungs moderately (pressure of two to four inches) by pouring in formalin-Mid- ler's fluid (technic 5, p. 6), and then immerse the whole in the same fixative for twenty-four hours. Cut into small blocks, using a very sharp razor so as not to squeeze the tissue, harden in alcohol, stain thin sections with haematoxylin-eosin (technic 1, p. 17), and mount in balsam or in eosin-glycerin. The larger bronchi are found in sections near the root of the lung. The arrangement of the pulmo- nary lobules is best seen in sections near and horizontal to the surface. Sections perpendicular to and including the surface show the pulmonary pleura. (3) Respiratory Epithelium (technic 1, p. 65). (4) Elastic Tissue of the Lung (technic 3, p. 2 5)- (5) Blood-vessels.— For the study of the blood-vessels, especially of the capil- lary networks of the alveoli, sections of injected lung should be made. A fresh lung is injected (page 21; with blue gelatin, through the pulmonary artery. It is then hardened in alcohol, embedded in celloidin, and thick sections are stained with eosin and mounted in balsam. The Thyroid. The thyroid is a ductless structure built upon the general prin- ciple of a compound alveolar gland. There are usually two lateral lobes connected by a narrow band of glandular tissue, the "isthmus. " Each lobe is surrounded by a connective-tissue capsule, from which septa pass into the lobe, subdividing it into lobules. From the peri- lobular connective tissue finer strands extend into the lobules, sepa- rating the alveoli. The latter are spherical, oval, or irregular in shape, and are as a rule non-communicating. At birth most of the alveoli are empty, but soon become more or less filled with a peculiar substance known as "colloid." The alveoli are lined with a single or double layer of cuboidal epithelial cells. Two types of cells are recognized, 256 THE ORGANS. chief cells and colloid cells. It is probable that these represent dif- ferent secretory conditions of the same cell. In the secretion of colloid the chief cell seems to be first transformed into a colloid cell. The latter appears in some cases simply to pour out its colloid secretion into the lumen, after which it assumes the character of a chief cell ; in other cases the cell appears to be completely trans- formed into colloid, its place being taken by proliferation of the chief cells. In certain alveoli which are much distended with colloid the lining epithelium is flattened. The blood supply of the thyroid is extremely rich, the vessels branching and anastomosing in the connective tissue and forming dense capillary networks around the alveoli. Lymphatics accompany the blood-vessels in the connective tissue. Nerves are mainly non-medullated fibres which form plexuses around the blood-vessels and in the connective tissue surrounding the alveoli. Terminals to the secreting cells end in club-like dilata- tions against the bases or between the epithelial cells. A few affer- ent medullated fibres are found in the plexuses surrounding the blood-vessels. Development.- — The median portion of the thyroid or isthmus originates as a diverticulum from the entoderm of the primitive pharynx, the lateral lobes as diverticula from the fourth visceral cleft. These three bodies, at first independent, unite to form the thyroid and become entirely separated from the entoderm. The gland at first consists of solid cords of cells. Ingrowth of connective tissue divides these into groups or lobules, and at the same time breaks up the long tubules into short segments. Dilatation of the alveoli oc- curs with the formation of colloid. The Parathyroids. These are small ductless glands, usually four in number, which lie upon the lateral lobes of the thyroid. They consist of a vascular connective tissue and solid anastomosing cords of epithelial cells. After removal of the thyroids the parathyroids hypertrophy and ap- parently assume the function of the thyroid. THE RESPIRATORY SYSTEM. 257 TECHNIC. The thyroid and parathyroid glands are best fixed in formalin-Midler's fluid. Sections may be stained with ha-matoxylin-eosin or hcematoxylin-picro-acid-fuchsin and mounted in balsam. General References for Further Study. Miller, W. S. : Das Lungenlappchen, seine Blut- und Lymphgefasse. Councilman: The Lobule of the Lung and Its Relations to the Lymphatics. Kdlliker : Handbuch der Gewebelehre des Menschen. h \d CHAPTER VIII. THE URINARY SYSTEM. The Kidney. The kidney is a compound tubular gland. It is enclosed by ?. firm connective-tissue capsule, the inner layer of which contains smooth muscle cells. In many of the lower animals and in the human a b foetus septa extend from the j\ capsule into the gland, dividing it into a number of lobes or renculi. In some animals, e.g., ^■•/^ c the guinea-pig and rabbit, the entire kidney consists of a single lobe (Fig. 161). In the adult human kidney the division into ( ■-- ^- lobes is not complete, the pe- =r-r5 d \(" ripheral parts of the different \\raa*. lobes blending. Rarely the foetal division into lobes persists in adult life, such a kidney being known as a " lobulated kidney. " On the mesially directed side of the kidney is a depression v'j known as. the hilum (Fig. 161). f This serves as the point of en- fig. x6i.-Longitudinai section Through Kid- trance of the renal artery and of ney of Guinea-pig, including hilum and beginning of ureter, x 5. (Technic 1, exit for the renal vein and ureter. p. 273). a, Pelvis; b, papilla; c, wall of n „ prt : on a Hiviqinn of the pelvis; d, ureter;*, ducts of Bellini;/, Un Section a C11V1S1011 01 tlie cortical pyamids;^, medullary rays; //, organ into twO ZOneS is apparent cortex ; ?", medulla ; j, renal corpuscles. . to the naked eye (Pigs, ioi and 162). The outer zone or cortex has a granular appearance, while the inner zone or medulla shows radial striations. This difference in appearance between cortex and medulla is mainly due, as will be seen subsequently, to the fact that in the cortex the kidney 250 *.'f" ■■■■ g ■p THE URINARY SYSTEM. 2 59 tubules are convoluted, while in the medulla they run in parallel radial lines alternating with straight blood-vessels. The medullary portion of the kidney projects into the pelvis, or upper expanded Fig. 162. Fig. 163. FIG. 162.— Longitudinal Section of Kidney Through Hilum. a, Cortical pyramid ; 5, medul- lary ray ; c, medulla ; d, cortex ; e, renal calyx ; f, hilum ; g, ureter ; h, renal artery ; t, obliquely cut tubules of medulla ; j and k, renal arches ; /, column of Bertini ; m, con- nective tissue and fat surrounding renal vessels ; //, medulla cut obliquely ; o, papilla ; /, medullary pyramid. (Merkel-Henle.) FIG. 163. — Scheme of Uriniferous Tubule and of the Blood-vessels of the Kidney showing their relation to each other and to the different parts of the kidney. G, Glomerulus ; BC, Bowman's capsule; A", neck; PC, proximal convoluted tubule; S, spiral tubule; D, descending arm of Henle's loop ; L, Henle's loop ; A, ascending arm of Henle's loop; /, DC, distal convoluted tubule ; AC, arched tubule; SC, straight collecting tubule ; ED, duct of Bellini ; A, arcuate artery, and V, arcuate vein, giving off interlobular vessels to cortex and vasa recta to medulla ; a, afferent vessel of glomerulus ; e, efferent vessel of glomerulus; c, capillary network in cortical labyrinth; s, stellate veins; vr, vasa recta and capillary network of medulla. (Pearsol.) • beginning of the ureter (Figs. 161 and 162) in the form of papilla;. The number of papillae varies from ten to fifteen, corresponding to the number of lobes in the foetal kidney. The pyramidal seo-- 26o THE ORGANS. ment of medulla, the apex of which is a papilla — in other words, the medullary portion of a foetal lobe — is known as a medullary or Malpighian pyramid. The extensions downward of cortical substance between the Malpighian pyramids constitute the columns of Bertini or septa rents. Radiating lines — medullary rays or pyramids of Ferrein — extend outward from the base of each Mal- pighian pyramid into the cortex (Fig. 162). As the rays extend out- ward in groups they outline pyramidal cortical areas. These are known as the cortical pyramids or cortical labyrinths. The secreting portion of the kidney is composed of a large num- ber of long tortuous tubules, the uriniferous tubules. Each uriniferous tubule begins in an expansion known as Bowman s capsule (Figs. 163, B C, and 164). This encloses a tuft Fig. 164. — Diagrams Illustrating Successive Stages in Development of the Renal Corpuscle, i and 2, Approach of blood-vessel and blind end of tubule ; 3, invagination of tubule by- blood-vessels ; 4 and 5, later stages, showing development of glomerulus and of the two- layered capsule of the renal corpuscle, the outer layer being the capsule of Bowman con- tinuous with the epithelium of the first convoluted tubule. of blood capillaries, the glomerulus. Bowman's capsule and the glomerulus together constitute the Malpighian body or renal cor- puscle. As it leaves the Malpighian body the uriniferous tubule becomes constricted to form the neck (Figs. 163, N, and 164). It next broadens out into a greatly convoluted portion, the first or proxi- mal convoluted tubule. The Malpighian body, the neck, and the first convoluted tubule are situated in the cortical pyramid (Fig. 163). The tubule next takes a quite straight course downward into the medulla — descending arm of Picnic's loop (Fig. 163, D) — turns sharply upon itself — Henle's loop (Fig. 163, /.) — and passes again toward the surface — ascending arm of Henle's loop (Fig. 163) A, — THE URINARY SYSTEM. 2 6i through the medulla and medullary ray. Leaving the medullary ray, it enters a cortical pyramid (probably as a rule the same pyramid from which it took origin) to become the second or distal convoluted a J-, ■** V --sat - nSr* o«» 5 # »> ■-**>« * ftv _ -v-, s>" *» C V-A*- PlG. 165-— Malpighian Body from Human Kidney. X 280. (Technic 2, p. 273. ) a, Bowman's capsule ; b, neck ; c, first convoluted tubule ; d, afferent and efferent vessels. tubule (Fig. 163, DC). This passes into the arched tubule (AC) which enters a medullary ray and continues straight down through the medullary ray and medulla as the straight or collecting tubule (SC). During its course the collecting tubule receives other /:';- .,..,..■ ■■ © ./ Fig. 166.— Proximal Convoluted Tubules of Human Kidney. X 350. (Technic 2, p. 273.) A, Cross-section ; B, oblique section. arched tubules. As it descends it becomes broader, enters the papilla, where it is known as the duct of Bellini (ED), and opens on the surface of the papilla into the kidney pelvis. About twenty 262 THE ORGANS. ducts of Bellini open upon the surface of each papilla, their open- ings being known as the foramina papillaria. Each tubule consists of a delicate homogeneous mcmbrana propria upon which rests a single layer of epithelial cells. The shape and structure of the epithelium differ in different portions of the tubule. i. The MalpigJiian body'is spheroidal, and has a diameter of from 1 20 to 200, a. The structure of the Malpighian body can be best understood by reference to its development (Fig. 164). During the development of the uriniferous tubules and of the blood-vessels of the 1 Z 1 § \ jg&f. \ k:;. FIG. 167.— Tubules of Human Kidney. X 560. From longitudinal section. (Technic 2, p. 273.) 1, Descending arm of Henle's loop ; 2, ascending arm of Henle's loop ; 3, collecting tubule ; 4, duct of Bellini. Beneath the longitudinal sections are seen cross sections of the same tubules. kidney the growing end of a vessel meets the growing end of a tubule in such a way that there is an invagination of the tubule by the blood-vessel (see Fig. 164). The result is that the end of the ves- sel which develops a tuft-like network of capillaries — the glomerulus — comes to lie within the expanded end of the tubule, which thus forms a two-layered capsule for the glomerulus. One layer of the cap- sule closely invests the tuft of capillaries. This layer by modification of the original tubular epithelium is finally composed of a single layer of flat epithelial cells with projecting nuclei. The outer layer of the THE URINARY SYSTEM. 263 capsule lies against the delicate connective tissue which surrounds the Malpighian body. This layer consists of a similar though slight- ly higher epithelium and is known as Bowman's capsule. Between the glomerular layer of the capsule and Bowman's capsule proper is a space which represents the beginning of the lumen of the uriniferous tubule (Fig. 165), the epithelium of Bowman's capsule being directly continuous with that of the neck of the tubule. 2. The Neck. — This is short and narrow, and is lined by a few cuboidal epithelial cells. Toward its glomerular end the epithelium is transitional between the flat epithelium of Bowman's capsule and the cuboidal epithelium of the neck proper. At its other end the epithelium of the neck becomes larger and more irregular as it passes over into that lining the next division of the tubule. 3. The first ox proximal convoluted tubule (Fig. 166) measures from 40 to 70 ij. in diameter. It is lined by irregularly cuboidal or pyramidal epithelium, with very indistinct demarcation between the cells. The cytoplasm is granular, and the granules are arranged in rows, giving the cell a striated appearance. This is especially marked at the basal end of the cell where the nucleus is situated. A zone of fine striations along the free surface frequently presents somewhat the appearance of cilia. 4. The descending arm of Henle\s loop is narrow (Fig. 167, /), 10 to 1 5, a in diameter. It is lined by a simple flat epithelium. The part of the cell which contains the nucleus bulges into the lumen, and as the nuclei of opposite sides of the tubule usually alternate, the lumen is apt to present a wavy appearance in longitudinal sections. 5. He/ile's Loop. — The epithelium here changes from the flat of the descending arm to the cuboidal of the ascending arm. The exact point where the transition occurs varies. It may take place during the turn of the loop, or in either the ascending or descending arm. 6. The ascending- arm of Heule's loop (Fig. 167, 2) is broader than the descending, measuring from 20 to 30//. in diameter. Its epithelium is cuboidal with granular striated protoplasm. The cells thus resemble those of the convoluted tubule, but are smaller, more regular, and less granular. 7. The second or distal convoluted tubule has a diameter of 40 to 50 !>■. It is much less tortuous than the first convoluted tubule. Its epithelium is similar to that lining the first convoluted tubule except that it is slightly lower and less distinctly striated. 264 THE ORGANS. 8. The arched tubule has a somewhat wider lumen than the second convoluted. It is lined with a low cuboidal epithelium with only slightly granular cytoplasm. 9. The straight or collecting tubule (Fig. 167, J) has at its com- mencement at the apex of a medullary ray a diameter about the same as that of the arched tubule. As it descends it receives other arched .&. '^•V/ Fig. 168.— Cross Section Through Cortex of Human Kidney. X 60. (Technic2, p. 273.) a, Con- voluted tubules of cortical pyramid ; 6, interlobular artery ; c, medullary rays ; d, Mal- pighian bodies. tubules, and increases in diameter until in the ducts of Bellini (Fig. 167, 4) of the papilla it has a diameter of from 200 to 300/; and a widely open lumen. The epithelium is at first low and gradually increases in height. In the ducts of Bellini it is of the high colum- nar type. The cytoplasm of these cells contains comparatively few granules, thus appearing transparent in contrast with the granular cytoplasm of the ascending arms or Henle's loops and of the con- voluted tubules. The epithelium of the uriniferous tubule rests upon an apparently structureless basement membrane. Ruhle describes the basement membrane as consisting of delicate longitudinal and circular connec- tive-tissue fibrils. He regards the fibrils as merely a more regular arrangement of the interstitial connective tissue. According to THE i RINA KY Si STEM. 265 Riihle the epithelium simply rests upon the basement membrane, being in no way connected with it. In the cortex the tubules are closely packed and the amount of interstitial connective tissue is ex- tremely small. In the medulla the connective tissue is more abundant. Of the function of the different parts of the uriniferous tubule our knowledge is extremely limited. The experiments of Heiden- hain tend to prove that the urinary solids are secreted mainly or wholly by the cells of the proximal convoluted tubule and of the as- cending arm of Henle's loop, the other parts of the tubule allowing only water to pass through their epithelium. Blood-vessels (diagram, Fig. 170). — The blood supply to the kid- ney is rich and the blood-vessels come into intimate relations with the tubules. The renal artery enters the kidney at the hilum, and imme- 6 & d qm m, » \ <^> FIG. 169.— Cross Section Through Medulla of Human Kidney. X 465. (Technic 2. p. 173 ) a. Capillaries;*, collecting tubule; c, ascending arms of Henle's loops; ) a thin Fin. 175.— Cross Section of Convoluted Portion of Human Seminiferous Tubule. X 480. (Kolli- ker.) M, Basement membrane ; 2, its inner homogeneous layer, fs, its outer fibrous layer ; 5, nucleus of Sertoli cell ; sft, spermatogone ;' sc, spermatocyte ; sc', spermatocyte showing mitosis ; sf, nearly mature spermatozoon ; sf, spermatozoon free in lumen of tubule ; d, degenerating nucleus in lumen : f, fat droplets stained by osmic acid. basement membrane ; and (c) a lining epithelium. The epithelium consists of two kinds of cells, the so-called supporting ox sustentacular cells and the glandular cells proper, the spermatogenic cells. The sustentacular cells, or columns of Sertoli, are irregular, high, epithelial structures, whose bases rest upon the basement membrane, and which extend through or nearly through the entire epithelium (Fig. 176, s). Their sides show marked irregularities and depressions, due to the pressure of surrounding spermatogenic cells. The cells of Sertoli were long considered as sustentacular in character. It has recently been suggested that these cells are derived from the sper- matogenic cells, but that, instead of developing into spermatozoa, they THE REPRODUCTIVE SYSTEM. 277 undergo retrograde changes, their protoplasm mingling with the intercellular substance, their nuclei becoming lost and the cells finally disappearing. According to this theory the tuft-like arrange- ment of the spermatozoa about the ends of the Sertoli cells is due to pressure by surrounding spermatogenic cells (Figs. 176, h and 178, /)• The appearance which the spermatogenic cells present depends upon the functional condition of the tubule. In the resting state the epithelium consists of several layers of spherical cells containing h s 6 h f ( r 20 Pig. 176.— Parts of Transverse Section of three Seminiferous Tubules from Testis of White Mouse. X 600. (Szymonowicz.) s, Sertoli cell with nucleus; sp, spermatogone, resting state ; sp f , spermatogone in mitosis ; sc, spermatocyte ; s/, spermatid ; sf, spermatid de- veloping into spermatozoon ; //, head of spermatozoon ; /, tails of developing spermato- zoa ; b, blood-vessel ; c, interstitial cell ; ;«, basal membrane ;f, fat droplets. nuclei which stain with Varying degrees of intensity. In the active state several distinct layers of spermatogenic cells can be differen- tiated. These from without inward are as follows : (1) Spermatogones (Figs. 175 and 176, sf). — These are small cuboi- dal cells which lie against the basement membrane. Their nuclei are 278 THE ORGANS. spherical and rich in chromatin. By mitotic division of the sperma- togones are formed the cells of the second layer, the spermatocytes. (2) Spermatocytes (Figs. 175 and 176, sc). — These are larger spherical cells with abundant cytoplasm and large vesicular nuclei showing vari- ous stages of mitosis. They form from two to four layers to the inner side of the spermatogones, and are sometimes differentiated into spermatocytes of the Fig. 177. Fig. 178. Fig. 177.— Passage of Convoluted Part of Seminiferous Tubules into Straight Tubules and of these into the Rete Testis. (Milhalkowicz.) a, Convoluted part of tubule; 6, fibrous stroma continued from the mediastinum testis ; c, rete testis. Fig. 178. — Spermatoblast with some Adjacent Sperm Cells, from Testis of Sparrow. (From Kolliker, after Etzold.) M, Basement membrane ; s, nucleus of Sertoli cell ; sf>, sperma- togones ; sc, spermatocyte ; s( x and s/ 2 , spermatids lying along the surface of the Sertoli cell, s' and st t i- at s/ 3 are seen the nearly mature spermatozoa ; /, tuft-like arrangement of bodies of spermatids around free end of Sertoli cell, with two mature spermatozoa. first order and spermatocytes of the second order. By mitotic division of the innermost spermatocytes are formed the spermatids. (3) The spermatids (Figs. 175 and 176, st) are small round cells which line the lumen of the seminiferous tubule. They are the direct progenitors of the spermatozoa. (For details of spermatogen- esis see page 285.) THE REPRODUCTIVE SYSTEM. 279 In the actively secreting testicle spermatozoa are frequently found either free in the lumen of the tubule or with their heads among the superficial cells and their tails extending out into the lumen (Figs. 175, sf l and 178). Separating and supporting the convoluted tubules is a small amount of interstitial connective tissue in which are the blood-ves- sels and nerves. Among the usual connective-tissue elements are M. SK Fig. 179.— From Section through Human Mediastinum and Rete Testis. X 96. (Kolliker.) A, Ar- tery ; V, vein ; L, lymph space ; C, canals of rete testis ; s, cords of tissue projecting into the lumina of the tubules and so cut transversly or obliquely ; Sk, section of convoluted portion of seminiferous tubule. found groups of rather large spherical cells with large nuclei — inter- stitial cells. They are believed to represent remains of the Wolffian body (Fig. 176, c). 2. The Straight Tubule. — With the termination of the con- voluted portion, the spermatogenic tissue of the gland ends, the 2SO THE ORGANS. Fig. 180.— Part of a Cross Section through a Vas Efferens of the Human Epididymis. X 140. (Kolliker.) F, High columnar ciliated epithe- lium ; d. lower non-ciliated epithelium, present- ing appearance of a gland ; d', the same cut obliquely. remainder of the tubule constituting a complex system of excretory ducts. The straight tubule is much narrower than the convoluted, having a diameter of from 20 to 40 //. It is lined by a single layer of cuboidal cells resting upon a thin basement membrane. At the apex of the lobule the straight tub- ules become continuous with the tubules of the rete testis. 3. The Tubules of the Rete Testis. — These are irregular canals which vary greatly in shape and size. They are lined with a single layer of low cuboidal or flat epithelial cells (Fig. 179, C). The Seminal Ducts. — While the already described straight tubules and the tubules of the rete testis must be regarded as part of the complex excretory duct system of the testis, there are certain structures which are wholly outside the testis proper, which serve to transmit the secretion of the testis, and are known as the seminal ducts. On leaving the testis these ducts form the epididymis, after which they converge to form the main excretory duct of the testis, the vas deferens. The Epididymis. — From the tubules of the rete testis arise from eight to fifteen tubules, the vasa cfferentia, or efferent ducts of the testis (Fig. 173, e). Each vas efferens pursues a tortuous course, is separated from its fellows by connective tissue, and forms one of the lobules of the head of the epididymis. The epithelium of the vasa efferentia consists of two kinds of cells, high columnar ciliated cells (Fig. 180, F), and, interspersed among these, low cuboidal non- ciliated cells (Fig. 180, d). Occasionally some of the high cells are free from cilia and some of the cuboidal cells may bear cilia. The cuboidal cells lie in groups between groups of the higher cells, often giving the appearance of crypt-like depressions. These have been referred to as intra-epithelial glands. They do not, however, invagi- nate the underlying tissues. The epithelium rests upon a basement membrane, beneath which are several layers of circularly disposed smooth muscle cells. THE REPRODUCTIVE SYSTEM. 281 The vasa efferentia converge to form the vas epididymis (Fig. 181). Here the epithelium is of the stratified variety, there being two or three rows of cells. The surface cells are narrow, high, and ciliated, and their nuclei are placed at different levels (Fig. 182). The cilia are long and each cell has only a few cilia. The deeper cells are irregular in shape. The basement membrane and muscular layers are the same as in the vasa efferentia. As the vas deferens is ap- proached the muscular coat becomes thickened, and is sometimes strengthened by the addition of scattered bundles of longitudinally disposed cells. The Vas Deferens. — The walls of the vas deferens consist of four coats — mucosa, submucosa, muscularis, and fibrosa (Fig. 183). The mucosa is folded longitudinally, and is composed of a stroma and a lining epithelium. The epithelium is of the stratified columnar type with two or three rows of cells, being similar to that lining the vas epididymis. The extent to which the epithelium is ciliated va- ries greatly. In some cases the entire vas is ciliated, in others only "% ; : ■»• , ft X>si; Fig. 181.— From Cross Section through Head of Epididymis. X 35. (Kiilliker.) 6, Interstitial connective tissue ; c, sections through tubules of epididymis, showing two-layered colum- nar epithelium ; g, blood-vessel. the upper portion, in still others no cilia are present beyond the epididymis. The epithelium rests upon a basement membrane be- neath which is a fibro-elastic cellular stroma. The stroma merges without distinct demarcation into the more vascular submucosa. The muscularis consists of two strongly developed layers of 282 THE ORGANS. smooth muscle, an inner circular and an outer longitudinal (Fig. 183), which together constitute about seven-eighths of the wall of the vas. At the beginning of the vas deferens a third layer of muscle is added composed of longitudinal bundles, NjjUlljs an d situated between the inner Cih^^MMMAMi hljA hh ^^i — c circular layer and the submucosa. The fibrosa consists of fibrous tissue containing many elastic fibres. Near its termination the vas dilates to form the ampulla, the walls of which present essential- ly the same structure as those of the vas. The lining epithelium fig. 182. -vertical Section through Waii of ■ however, frequently markedly Tubules of Epididymis. X 7°°. (kolhker.) 1 J J (Fig. 181 more highly magnified.) b, Con- pigmented and the mucosa COn- nective-tissue and smooth muscle cells ; e, . , , , . , , n basai layer of epithelial cells ; /, high coi- tains branched tubular glands. umnar epithelial cells; p, pigment gran- The Seminal VesideS and ules in columnar cells ; c, cuticula ; h, cilia. Ejaculatory Ducts. — The sem- inal vesicles. The walls of the seminal vesicles are similar in structure to those of the ampulla. The epithelium is pseudo-stratified with two or three rows of nuclei and contains a yellow pigment. When the vesicles are distended the epithelium flattens out and the nuclei lie more in one plane, thus giving the appearance of an ordinary sim- ple columnar epithelium. Beneath the epithelium is a thin stroma, outside of which is an inner circular and an outer longitudinal layer of smooth muscle, both layers being much less developed than in the vas. The seminal vesicles are to be regarded as accessory genital glands. The ejaculatory ducts are lined with a single layer of columnar cells. The muscularis is the same as in the ampulla except that the inner circular layer is thinner. In the prostatic portion of the duct the muscularis is indistinct, merging with the muscle tissue of the gland. The ducts empty either directly into the urethra or into the urethra through the vesicula prostatica. Rudimentary Structures Connected with the Development of the Genital System. — Connected with the testicle and its ducts are remains of certain fcetal structures. These are : ( 1 ) The paradidymis, or organ of Giraldes, situated between the THE REPRODUCTIVE SYSTEM. 283 vessels of the spermatic cord near the testis. It consists of several blind tubules lined with simple columnar ciliated epithelium. (2) The ductus aberrans Hallcri, found in the epididymis. It is lined with simple columnar ciliated epithelium and opens into the vas epididymis. Instead of a single ductus aberrans, several ducts may be present. (3) The appendix testis (stalked hydatid or hydatid of Morgagni), in the upper part of the globus major. It consists of a vascular connective tissue surrounding a cavity lined with simple columnar ciliated epithelium. (4) The appendix epididymidis, a vascular structure, not always present, lying near the appendix testis. It resembles the latter in structure. -/ - d Fig. 183.— Cross Section of Human Vas Deferens. X 37. (Szymonowicz.) a, Epithelium ; b, stroma ; c, submucosa ; d, inner circular muscle layer ; e, outer longitudinal muscle layer ; f, fibrous layer ; £■, blood-vessels. The paradidymis and ductus aberrans Halleri probably represent remains of the embryonal mesonephros. The appendix testis and the appendix epididymidis are believed by some to be derived from the primitive kidney, by others from the embryonal duct of Muller. Blood-vessels. — Branches of the spermatic artery ramify in the mediastinum and in the tunica vasculosa. These send branches into 284 THE ORGANS. the septa of the testicle, which give rise to a capillary network among the convoluted tubules. From the capillaries arise veins which ac- company the arteries. Lymph capillaries begin as clefts in the tunica albuginea and in the connective tissue surrounding the seminiferous tubules. These connect with the more definite lymph vessels of the mediastinum and of the spermatic cord. Nerves. — Non-medullated nerve fibres form plexuses around the blood-vessels. From these, fibres pass to plexuses among the semi- niferous tubules. Their exact method of termination in connection with the epithelium has not been determined. In the epididymis are found small sympathetic ganglia. The walls of the vasa efferentia, vas epididymis, and vas deferens contain plexuses of non-medullated nerve fibres, which give off terminals to the smooth muscle cells and to the mucosa. The Spermatozoa. — The spermatozoa are the specific secretion of the testicle. They are long, slender flagellate bodies, from 50 to 70 fi in length, and are suspended in the semen, which is a secretion of the accessory sexual glands. It has been estimated that the human spermatozoa average about sixty thousand per cubic millimetre of semen. The human spermatozoon consists of (1) a head, (2) a middle piece or body, and (3) a tail or flagellum (Fig. 184). The head, from 3 to 5 :>■ long and about half that in breadth, is oval in shape when seen on flat, pear-shaped when seen on edge. It consists of chromatin derived from the nucleus of the parent cell. The body is cylindrical, about the same length as the head, and consists of a fibrillatecl central core, the axial thread, surrounded by a protoplasmic capsule. Just behind the head the axial thread pre- sents a bulbous thickening, the terminal nodi tic or end bulb, which fits into a depression in the head. The terminal nodule probably represents the centrosome. The tail consists of a main segment, from 40 to 60," in length, and a terminal segment having a length of from 5 to 10 //. The main segment has a central fibrillatecl axial thread which is continuous with the axial thread of the body. This is enclosed in a thin mem- brane or capsule continuous with the capsule of the body. The terminal segment consists of the axial thread alone. The motility of THE REPRODUCTIVE SYSTEM. 28: the spermatozoon depends entirely upon the flagellate movements of the tail. In many of the lower animals the spermatozoon has a much more complicated structure. Of the above-described parts of the spermatozoon only the head and tail can zisually be differentiated, except by the use of special metJiods and very high-power objectives. Development of the Spermatozoa. — As already noted in de- scribing the testicle, the spermatozoa are developed from the epithe- lial cells of the seminiferous tubules. The most peripheral of the tubule cells, the spermatogones (Fig. 175, sp and Fig. 1 j6, sp) are small round cells with nuclei rich in chromatin. By mitosis the spermatogone gives rise to two daughter cells, one of which remains at the periphery as a spermatogone, while the other takes up a more central position as a spermatocyte (Fig. 176, sc and Fig. 178., sc). The latter are rather large spherical cells, whose nuclei show very distinct chromatin networks. By mitotic division of the spermatocytes of the innermost row are formed the spermatids (Fig. 176, st and Fig. 17S, sf). These are small spherical cells, which line the lumen of the tubule and are the direct progenitors of the spermatozoa. In the transformation of sper- matocyte into spermatid an extremely important change takes place in the nucleus. This consists in a reduction of its chromosomes to one-half the number specific for the species (page 46). The trans- formation of the spermatid into the spermatozoon differs somewhat in different animals and the de- tails of the process must be regarded as not yet definitely determined. The nucleus of the sper- matid first becomes oval in shape, and its chromosomes become condensed into a small homogeneous mass, which forms the head of the spermatozoon. During their transformation into the heads of the spermatozoa, the nuclei of the spermatids arrange them- selves in tufts against the inner ends of the cells of Sertoli. This compound structure, consisting of a Sertoli cell and of a group of developing spermatozoa attached to its central end, is known as a spermatoblast (Fig. 178). The body or middle piece of the sperma- Fig. 1S4. — Human Spermatozoa. (Af- ter Retzius.) /, Head seen on flat ; 2, head seen on edge ; k, head ; m, body ; f. tail ; e, end piece. 286 THE ORGANS. tozoon is described by most investigators as derived from the centro- some, while the tail is a derivative of the cytoplasm. TECHNIC. (i) For the study of the general topography of the testis, remove the testis of a new-born child, make a deep incision through the tunica albuginea in order to allow the fixative to penetrate quickly, and fix in formalin-Midler's fluid (technic 5, p. 6). Antero-posterior longitudinal sections through the entire organ and in- cluding the epididymis should be stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 18) or with haematoxylin-eosin (technic 1, p. 17) and mounted in balsam. (2) The testis of a young adult is removed as soon after death as possible, is cut into thin transverse slices, which include the epididymis, and is fixed in forma- lin-Miiller's or in Zenker's fluid (technic 9, p. 7) • Select a slice which includes the head of the epididymis, cut away the anterior half or two-thirds of the testis proper in order to reduce the size of the block, and, after the usual hardening and embedding, cut thin sections through the remaining posterior portion of the testis, the mediastinum and epididymis. Stain with haematoxylin-eosin (technic 1, p. 17) and mount in balsam. (3) For the study of spermatogenesis fix a mouse's testis in chrome-acetic- osmic mixture (technic 7, p. 7). Harden in alcohol and mount thin unstained sections in balsam or in glycerin. (4) Spermatozoa. — Human spermatozoa may be examined fresh in warm nor- mal saline solution or fixed in saturated aqueous solution of picric acid and mounted in glycerin. Mammalian spermatozoa may be obtained from the vagina after intercourse, or by incision into the head of the epididymis. Technic same as for human. (5) A portion of the vas deferens is usually removed with the testis and may be subjected to technic (2) above. Transverse sections are stained with haema- toxylin-eosin and mounted in balsam. The Prostate Gland. The prostate is described by some as a compound tubular, by others as a compound alveolar gland. It is perhaps best regarded as a collection of simple branched tubular glands with dilated termi- nal tubules. These number from forty to fifty, and their ducts con- verge to form about twenty main ducts, which open into the urethra. The gland is surrounded by a capsule of fibro-elastic tissue and smooth muscle cells, the muscle cells predominating. From the capsule broad trabeadce of the same structure as the capsule pass into the gland. The amount of connective tissue is large. It is less in the prostate of the young than of the old. The hypertrophied pros- tate of age is due mainly to an increase in the connective-tissue ele- ments. The tubules have wide lumina and are lined with simple THE REPRODUCTIVE SYSTEM. 287 cuboiclal epithelium of the serous type, resting upon a delicate base- ment membrane (Fig. 185). Less commonly the epithelium is pseudo- stratified. The ducts are lined with simple columnar epithe- lium until near their terminations, where they are lined with trans- itional epithelium similar to that lining the urethra. Peculiar con- . - ! '\ t'»e i ' / 1"' it y "» 1 ■ ■ : WBM ' »? o I ■? *'" Pig. 185.— Section of Human Prostate. X 150. (Technic 1, p. 288.) a. Epithelium of tubule ; b, interstitial connective tissue ; c, corpora amylacea. centrically laminated bodies, crescentic corpuscles, or corporasamylacca, are frequently present in the terminal tubules (Fig. 185, c). They are more numerous after middle life. Through the prostate runs the prostatic portion of the urethra. Within the prostate is found the vesicula prostatica {iitriculus prostaticits — uterus masculimis). It represents the remains of a foe- tal structure, the Miillerian duct (see page 314) and consists of a blind sac with folded mucous membrane lined with a two-rowed ciliated epithelium which dips down to form short tubular glands. The prostatic secretion is serous. The blood-vessels of the prostate ramify in the capsule and tra- beculas. The small arteries give rise to a capillary network which surrounds the gland tubules. From these arise small veins, which accompany the arteries in the septa and unite to form venous plexuses in the capsule. 288 THE ORGANS. The lymphatics begin as blind clefts in the trabecular and follow the general course of the blood-vessels. Nerves. — Small groups of sympathetic ganglion cells are found in the larger trabeculae and beneath the capsule. Axones of these cells pass to the smooth muscle of the trabeculas and of the walls of the blood-vessels. Their mode of termination is not known. Timofeew describes afferent medullated fibres ending within capsular structures of flat nucleated cells. Two kinds of fibres pass to each capsule : one a large medullated fibre which loses its sheath and gives rise within the capsule to several flat fibres with serrated edges, the other small medullated fibres which lose their sheaths and split up into small varicose fibrils which form a network around the terminals of the larger fibre. Cowper's Glands. The bulbo-urethral glands, or glands of Cowper, are small, branched, tubular glands. They are lined with mucous cells. The smaller ducts are lined with simple cuboidal epithelium. They unite to form two main excretory ducts which open Into the urethra and are lined with stratified columnar epithelium consisting of two or three layers of cells. TECHNIC. (i) Fix small pieces of the prostate of a young man in formalin-Muller's fluid (technic 5, p. 6). Stain sections with haematoxylin-eosin (technic 1, p. 17) and mount in balsam. ' (2) The prostate of an old man should be treated with the same technic and compared with the above. (3) Cowper's glands. Same technic as prostate (1). The Penis. The penis consists largely of three long cylindrical bodies, the corpus spongiosum and the two corpora cavernosa. The latter lie side by side, dorsally, while the corpus spongiosum occupies a medial ventral position (Fig. 186). All three are enclosed in a common connective-tissue capsule which is loosely attached to the overlying skin. In addition each corpus has its own special capsule or tunica albuginea, about a millimetre in thickness, and composed of dense connective tissue containing many elastic fibres. The corpus spongiosum and corpora cavernosa have essentially the THE REPRODUCTIVE SYSTEM. 289 same structure, being composed of so-called erectile tissue (Fig. 187). This consists of thick trabecular of intermingled fibro-elastic tissue and bundles of smooth muscle cells, which anastomose to form a coarse- meshed network, the spaces of which are lined with endothelium. The spaces are known as cavernous sinuses, and communicate with one another, and with the blood-vessels of the penis. In the flaccid con- dition of the organ these sinuses are empty and their sides are in apposition sinuses become filled with venous blood. The arteries have thick muscular walls and run in the septa. A few of them open directly into the venous sinuses. Most of them give rise to a superficial capillary network beneath the tunica albu- ginea. From this capillary plexus the blood passes into a plexus of T n erection these F" IG - ^.—Transverse Section through Human Penis, a, Skin; b, subcutaneous tissue; c, fibrous tunic ; d, dorsal vein ; e, corpora cav- ernosa ; f, corpus spongiosum ; g, urethra. r r "i '-, .-> b &,','• '. $ ■ Fig. 1S7.— Erectile Tissue of Corpus Spongiosum of Human Penis. X 60. a, Trabecular of con- nective tissue and smooth muscle ; b, cavernous sinuses ; c, groups of leucocytes in sinus. broader venous channels in the periphery of the erectile tissue, and these in turn communicate with the cavernous sinuses. The usual direct anastomoses between arterial and venous capillaries also occur. 19 290 THE ORGANS. The blood may therefore pass either through the usual course — arte- ries, capillaries, veins — or, under certain conditions, may pass through the cavernous sinuses. This determines the flaccid or the erect con- dition of the organ. The veins arise partly from the capillaries and partly from the cavernous sinuses. They pass through the tunica albuginea and empty into the dorsal vein of the penis (Fig. 186). In the corpus spongiosum there is probably no direct opening of arteries into the sinuses. Both trabecular and sinuses are also smaller. Of the lymphatics of the penis little definite is known. The nerve endings, according to Dogiel, consist of : (a) free sensory endings, {b) deeply situated genital corpuscles, (c) Pacinian corpus- cles and Krause's end-bulbs in the more superficial connective tissue, and (d) Meissner's corpuscles in the papillae. (For details see pages 352, 353, and 354.) The glans penis consists of erectile tissue similar in structure to that of the corpus cavernosum, except that the venous spaces are smaller and more regular. The mucous membrane is very closely at- tached to the fibrous sheath of the underlying erectile tissue. A few small sebaceous glands, unconnected with hairs — the glands of Tyson — are found in the mucous membrane of the base of the glans penis. The. prepuce is a fold of skin which overlies the glans penis. Its inner surface is lined with mucous membrane. The Urethra. 1 The male urethra is divided into three parts— prostatic, mem- branous, and penile. The wall of the urethra consists of three coats — mucous, submucous, and muscular. The structure of the wall differs in the different parts of the urethra. The mucous membrane (Fig. 188) consists of epithelium and stroma. The epithelium of the prostatic part is stratified squamous (transitional), resembling that of the bladder. In the membranous part it is stratified columnar or pseudostratified. In the penile por- tion it is pseudostratified up to the fossa navicularis, where it changes 1 The female urethra, while not so distinctly divisible into sections, presents essentially the same structure as the male urethra. The epithelium begins at the bladder as stratified squamous of the transitional type, changes to a two-layered stratified or pseudostratified, and finally passes over into stratified squamous near the urethral opening. Glands of Littre" are present, but are fewer than in the male. THE REPRODUCTIVE SYSTEM. 29] to stratified squamous. The epithelium rests upon a basement mem- brane, beneath which is a thin stroma rich in elastic fibres and hav- ing papillas which are especially prominent in the terminal dilated portion of the urethra, the fossa navicularis. The stroma merges without distinct demarcation into the submucosa. The submucosa consists of connective tissue and, in the penile portion, of more or less longitudinally disposed smooth muscle. It contains a dense network of veins — cavernous veins — which give it the character of erectile tissue (Fig. 189). The muscular coat is thickest in the prostatic and membranous por- tions. Here it consists of a thin inner longitudinal and a thicker outer circular layer. A definite muscular wall ceases at the beginning of the penile portion, although circularly dis- ;_": • ._> Fig. iSS. Fig. 189. Fig. 188.— From Transverse Section of Urethra and Corpus Spongiosum, including Mucous Membrane and part of Submucosa. X 15. The dark spots represent the cavernous veins. FIG. 189. — Vertical Section through Portion of Wall of Human Male Urethra. X 350. A. Mu- cous membrane; B. submucosa ; a, epithelium; d, stroma; c, cavernous veins; , J'), At the surface of the ovary, just beneath the peritoneum, the stroma forms a rather dense layer of fibrous tissue, the tunica albuginca. At the margin of the peritoneal surface of the ovary the connective tis- sue of the peritoneum becomes continuous with the stroma of the ovary, while the flat mcsothelium of the general peritoneum is re- placed by a single layer of cuboidal cells, which covers the surface of the ovary and is known from its function as the germinal epithelium (Fig. 190, /). The parenchyma or secreting portion of the ovary consists of peculiar glandular elements, the Graafian follicles . THE REPRODUCTIVE SYSTEM. 293 The structure of the Graafian follicle can be best appreciated by studying its development. The follicles originate from the germinal s mmmm FIG. 190.— Semidiagrammatic Drawing of Part of Cortex and Medulla of Cat's Ovary. (From Schron, in Quain's "Anatomy.") 1, Germinal epithelium, beneath which is 3, the tunica albuginea ; 2, medulla, containing large blood-vessels, 4 ; 2, 2', fibrous stroma, arranged around mature Graafian follicle as its theca folliculi ; 3', stroma of cortex ; 5, small (primitive) Graafian follicles near surface ; 6, same deeper in cortex ; 7, later stage of Graafian follicle, beginning of cavity ; 8 and 8', still later stages in development of folli- cle; 9, mature follicle; a, stratum granulosum ; 6, germ hill; c, ovum; d, nucleus (ger- minal vesicle) ; e, nucleolus (germinal spot). epithelium during fcetal life. At this time the germinal epithelium is proliferating, and certain of its cells differentiate into larger EG 0^ Fig. 191. — Semidiagrammatic Drawing to show Development of Ovum from Germinal Epithelium of Ovary. (Duval.) spherical cells — -primitive ova. The primitive ova pass down into the stroma accompanied by a considerable number of the undifferen- 294 THE ORGANS. tiated cells of the germinal epithelium. A cord-like mass of cells is thus formed, extending from the surface into the stroma. These are known as Pflugers egg cords (Fig. 191, A, B, C). Each cord usually contains several ova. In some cases the differentiation JggWB Fig. 192.— Vertical Section through Cortex of Ovary of Young Girl. • 190. (Bohm and von Davidoff.) a, Germinal epithelium; b, tunica albuginea ; c, follicular epithelium; d, ovum ; e. primitive Graafian follicles in ovarian cortex ; f, granular layer of large Graafian follicle. of the ova cells does not occur upon the surface but in the cords after they have extended down from the surface. The connection of the cord with the surface epithelium is next broken so that each cord becomes completely surrounded by stroma. It is now known as an egg nest. During this process proliferation of the epithelial cells of the cords and nests has been going on, and each ovum surrounded by a layer of epithelial cells becomes separated from its neighbors (Fig. 191, D). This central ovum surrounded by a single layer of epithe- THE REPRODUCTIVE SYSTEM. 295 lial cells (follicular cells) is the. primitive Graafian follicle (Fig. 191, D, Fig. 192, and Fig. 193, a). The follicle increases in size, mainly on account of proliferation of the follicular cells, which soon form several layers instead of a single layer, but also partly on account of growth of the ovum itself (Fig. 193). The latter now leaves the centre of the follicle and takes up an eccentric position. At the same time a cavity (or several small cavities which later unite) appears near the centre of the follicle (Fig. 193, e and Fig. 190, J). This is filled with fluid which seems to be in part a secretion of the follicular cells, in part a result of their disintegration. The cavity is known as the follicular cavity or antrum, the fluid as the liquor folliculi. Lining the follicular cavity are several rows of follicular cells with granular protoplasm — the stratum granulosum. With increase in the liquor folliculi the ovum becomes still further pressed to one side of the follicle, where, surrounded by an accumulation of follicular cells, it forms a distinct projection into the cavity (Fig. 194, and Fig. 190, 8 and p). This is known as. the genu hill {discus proligcrus — cumu- lus obpJiorus). The cells of the germ hill nearest the ovum become itfS V>---: i b — i. ■'£.•■-! . ; , /■ - .,. fl &&4tff. c • '■' V fV,v •■-- •■ . <-8&ri Pig. 193.— From Section through Cortex of Ape's Ovary. X 150. (Szymonowicz.) i^! ^.--M^-^ i — e • <■ • -j§{i i — d Fig. 194.— Section through Graafian Follicle of Ape's Ovary. X 90. (Szymonowicz.) Later stage of development than Fig. 193. , ovum with clear zona pelluckla, germinal vesicle, and germinal spot; d, follicular epithelium (membraua granulosa) ; e, follicular cavity ; /, theca folliculi ; g; blood-vessel. surrounded by a double-contoured nuclear membrane, and contains a distinct chromatic network and nucleolus or germinal spot. The cytoplasm is quite easily differentiated into a spongioplasm network and a homogeneous hyaloplasm. Such ova are present in all active ovaries, i.e., during the childbearing period, but are especially nu- merous in the ovary of the infant and child (Fig. 192). With the development of the follicle the ovum increases in size and becomes surrounded by a clear membrane, the zona pellucida, believed by some to be a cuticular formation deposited by the egg cell, by others to be a product of the surrounding follicular cells. Minute canals extend into the zona pellucida from its outer surface. These contain processes of the cells of the corona radiata. A narrow THE REPRODUCTIVE SYSTEM. 297 cleft, the perivitelline space, has been described as separating the ovum from the zona pellucida. During the growth of the ovum its cytoplasm becomes coarsely granular from the development of yolk or deutoplasm granules. Immediately surrounding the nucleus, and just beneath the zona pellucida, the egg protoplasm is fairly free from yolk granules. The further maturation of the ovum, which is necessary before the egg cell is in condition to be fertilized, consists in changes in the chromatic elements of the nucleus, which result in the extrusion of the polar bodies, and apparently have as their main object the reduc- tion in number of chromosomes to one-half the number characteristic of the species. This process has been described (page 42). In many of the lower animals maturation of the ovum is completed out- side the ovary. In man and the higher animals the entire process takes place within the ovary, the second polar body being extruded just before the escape of the ovum from its follicle. The youngest of the Graafian follicles are found just under the tunica albuginea near the germinal epithelium, from which they originate (Fig. 190,5). As the follicle matures it passes deeper into the cortex. With complete maturity the follicle usually assumes macroscopic proportions — 8 to 12 mm. — and often occupies the entire thickness of the cortex, its theca at one point touching the tunica albuginea. A thinning of the follicular wall nearest the sur- face of the ovary next takes place, while at the same time an increase in the liquor folliculi determines increased intrafollicular pressure. This results in rupture of the Graafian follicle and the discharge of its ovum, together with the liquor folliculi and some of the follicular cells. An escape of blood into the follicle from the torn vessels of the theca always accompanies the discharge of the ovum. The follicle again becomes a closed cavity, while the contained blood clot becomes organized by the ingrowth of vessels from the theca, to form the cor- pus kcemorrkagicum, which represents the earliest stage in the de- velopment of the corpus luteuvi. The corpus luteum (Fig. 196), which replaces the corpus haemor- rhagicum, consists of large yellow cells — lutein cells — and of connec- tive tissue. The latter with its blood-vessels is derived from the inner layer of the theca. The origin of the lutein cells is not clear. They are described by some as derived from the connective-tissue 298 THE ORGANS. cells of the theca ; by others as the result of proliferation of the cells of the stratum granulosum. The cells have a yellow color from the presence of fatty (lutein) granules in their protoplasm, and it is to these granules that the characteristic yellow color of the corpus luteum is due. A definite cellular structure with a supporting con- nective-tissue framework thus replaces the corpus haemorrhagicum, FIG. 195.— Graafian Follicle and Contained Ovum of Cat ; directly reproduced from a photo- graph of a preparation by Dahlgren. X 235. (From "The Cell in Development and In- heritance," Prof. E. B. Wilson ; The Macmillan Company, publishers.; The ovum is seen lying in the Graafian follicle within the germ hill, the cells of the latter imme- diately surrounding the ovum forming the corona radiata. The clear zone within the corona is the zona pellucida, within which are the egg protoplasm, nucleus, and nucleolus. Encircling the follicle is the connective tissue of the theca folliculi. remains of which are usually present in the shape of orange-colored crystals of haematoidin. By degeneration and subsequent absorption of its tissues the corpus luteum becomes gradually reduced in size, loses its yellow color, and is then known as the corpus albicans. This also is mostly absorbed, being finally represented merely by a small area of fibrous tissue. Corpora lutea are divided into true corpora lutea (corpora lutea vera or corpora lutea of pregnancy) and false corpora lutea (corpora THE REPRODUCTIVE SYSTEM. 299 lutea spuria). The former replace follicles whose ova have under- gone fertilization, the latter, follicles whose ova have not been ferti- lized. The structure of both is similar, but the true corpus luteum is larger, and both it and its corpus albicans are slower in passing through their retrogressive changes, thus remaining much longer in the ovary. While the function of the corpus luteum is not known, the recent experiments of Fraenkel seem to be confirmatory of the theory ad- vanced by Born, that the corpus luteum is a gland having an internal secretion, which appears to have some influence upon the attachment of the fecundated ovum to the uterus and upon its nutrition during the first few weeks of its development. According to Fraenkel the d ■i*\ AS. C D Fig. 196. — Formation of the Corpus Luteum according to Sobotta. Four successive stages in the mouse. A, Vascular bud of tunica intima extending into the proliferating fol- licular epithelium. B, Vascular buds passing toward the central cavity ; between them the proliferating follicular cells, among which leucocytes have now appeared. C '. Later stage ; cells in distinct columns between strands of connective tissue. £>, Central cavity replaced by connective tissue resembling mucous tissue, columns broken up by anastomosis of con- nective-tissue strands. %$m : FIG. 202.— Diagram of Human Placenta at Close of Pregnancy. (Schaper.) a, Amnion ; i, chorion ; ■ mi? .Jm Fig. 208.— Longitudinal Section through Root of Human Nail and Nail Bed. X 10. (Schaper.) a, Body of nail; b, free edge; c, root of nail; d, epidermis; e, eponychium ; f, stratum germinativumof matrix ; g; folds in derma of nail bed ; A, bone of finger ; A, hyponychium. root is known as the matrix. The nail bed is bounded on either side by folds of skin, the nail wall, while between the nail wall and the nail bed is a furrow, the nail groove (Fig. 209). THE SKIN AND ITS APPENDAGES. 32 1 The nail bed consists of corium. Its connective-tissue fibres are arranged partly horizontal to the long axis of the nail, partly in a Fig. 209.— Transverse Section of Nail and Nail Bed. (Rannie.) «, Nail : a, epidermis ; p, nail wall, to inner side of which is the nail groove ; /, folds of derma ; d, nail bed. vertical plane extending from the periosteum to the nail. Papillae are not present, but in their place are minute longitudinal ridges, which begin at the matrix and, increasing in height as they pass for- . FIG. 210.— Vertical Transverse Section through Nail Body. X 280. (Szymonowicz.) a, Nail i b, stratum germinativum ; c, ridge of nail bed ; d, derma ; e, blood-vessel. ward, terminate abruptly at the end of the nail bed, beyond which are the usual papillae of the derma. The nail itself consists of two parts — an outer harder part or true nail, and an under softer part. The outer portion is hard and horny, 21 322 THE ORGANS. is developed from the stratum lucidum, and consists of several layers of clear, flat, nucleated cells. These layers overlap in such a manner that each layer extends a little farther forward than the layer above. The under softer portion of the nail corresponds to the stratum germinativum of the skin and, like the latter, consists of polygonal "prickle" cells and a stratum cylindricum resting upon a basement membrane. In the matrix where the process of nail formation is going on, this layer is thicker than elsewhere and is white and opaque from the presence of keratohyalin. The convex anterior margin of this area can be seen with the naked eye and is known as the lunula. At the junction of nail and skin, in the nail groove, the stratum corneum extends somewhat over the nail as its eponycJiium. A simi- lar extension of the stratum corneum occurs on the under surface of the nail where the nail becomes free from the nail bed. This is known as the hyponychium (Fig. 208). Growth of nail takes place by a transformation of the cells of the matrix into true nail cells. In this process the outer hard layer is pushed forward over the stratum germinativum, the latter remaining always in the same position. TECHNIC. (1) Remove two or more distal phalanges from the fingers of a new-born child and fix in absolute alcohol or in formalin-Muller's fluid (technic 5, p. 6). After fixing, the bone should be carefully removed. Both longitudinal and transverse sections are made, stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 18), and mounted in balsam. In cutting the sections it is usually best so to place the block that the knife passes through volar surface first, through nail last. (2) The cellular elements of nail do not show well in sections. For demon- strating the nail cells, boil a piece of nail in concentrated potash lye or warm it in strong sulphuric acid, scrape off cells from the softened surface, and mount in glycerin. The Hair. The hair, like the nail, is a development of the epidermis. The hair itself consists of a shaft, that portion of the hair which projects above the skin, and a root, that portion embedded within the skin. At its lower end the root presents a knob-like expansion, the hair bulb, in the under surface of which is a cup- like depression, which receives an extension of corium. This is known as the papilla. En- closing the hair root is the hair follicle. THE SKIN AND ITS APPENDAGES. 323 The Hair. — This is composed of epithelial cells arranged in three layers, which from within outward are medulla, cortex, and cuticle (Fig. 212). (1) The medulla occupies the central axis of the hair. It is ab- sent in small hairs, and in the large hairs does not extend throughout their entire length. It is from 16 to a 20 fi in diameter, and consists of from two to four layers of polygonal or cuboidal cells with finely granular, usually pigmented protoplasm and rudimentary nuclei. (2) The cortex makes up the main bulk of the hair and consists of several layers of long spindle- shaped cells, the protoplasm of which shows distinct longitudinal striations, while the nuclei appear atrophied. As these striations give the hair the appearance of being composed of fibrillae, the term '* cortical fibres " has been applied to them. In col- ored hair, pigment granules and pig- ment in solution are found in and between the cells of this layer. This pigment determines the color of the hair. In the root the cortical cells are less flattened than in the shaft. (3) The cuticle has a thickness of about 1 ,"., and consists of clear scale- like, non-nucleated epithelial cells. These overlap one another like shingles on a roof, giving to the sur- face of the hair a serrated appear- ance. The Hair Follicle. — This is also a modification of the skin. In the formation of the follicles of the finer (lanugo) hairs the epi- dermis alone, is concerned. The follicles of the larger hairs contain both epidermal and dermal elements. The latter form the connec- tive-tissue follicle, while the epidermis forms the root sheaths. FlG. 211. — Longitudinal Section of Hair and Its Follicle from Vertical Section of Scalp. (Ranvier.) a, Shaft of hair ; l>, derma; c, arrector pili muscle; d, se- baceous gland ; e, outer root sheath ; f, inner root sheath ; g; connective-tis- sue follicle ; //, vitreous membrane; t, hair bulb ; /, papilla ; j, epidermis. 324 THE ORGANS, (i) The root sheatJi consists of two sub-layers — the inner root sheath and the outer root sheath (Figs. 213, 214, and 215). (a) The inner root sheath consists of three layers, which from within outward are the cuticle of the root sheath, Huxley's layer, and Henle's layer. The cuticle of the root sheath lies against the cuticle of the hair and is similar to the latter in structure. It consists of thin scale-like overlapping cells, nucleated in the deeper parts of the sheath, non-nucleated nearer the surface (Figs. 213, 214, and 215, c). Huxley s layer lies immediately outside the cuticle of the root sheath, constituting the middle layer of the inner root sheath. It consists of about two rows of elongated cells with slightly granular protoplasm containing eleidin. In the deeper portion of the root ■ a b c these cells contain nuclei. Nearer the surface the nuclei are rudimentary or absent (Figs. 213, 214, and 215, d). Henle 's layer is a single row of clear flat cells. In the bulb these cells may contain nuclei ; elsewhere they are non- nucleated (Fig. 215, e). (b) The outer root sheath is derived from the stratum germinativum, to which it corresponds in structure. Next to the vitreous membrane is a single layer of columnar cells (stratum cylindricum). In- side of this are several layers of " prickle" cells (Figs. 213, 214, and 215,/). (2) The connective-tissue follicle con- sists of three layers — an inner vitreous membrane, a middle vascular layer, and an outer fibrous layer. (a) The vitreous or hyaline membrane is a thin homogeneous structure of the nature of an elastic membrane. It lies next to the outer root sheath and corresponds to the basement membrane of the derma (Figs. 213, 214, and 215,^). (/>) The middle or vascular layer is composed of fine connective- tissue fibres, the general arrangement of which is circular. Cellular elements are quite abundant, while elastic fibres are as a rule absent. Fig. 212.— Longitudinal Section of Hair. X 350. (Kolliker.) a, Medulla; b s cortex; c, cuticle. THE SKIN AND ITS APPEND A GES. 325 As its name would indicate, this layer is especially rich in blood- vessels (Figs. 213, 214, and 215, i). (c) The outer or fibrous layer consists of rather coarse, loosely woven bundles of white fibres, which run mainly in a longitudinal direction. Among these are elastic fibres and a few connective-tis- sue cells. i mm. FIG. 213. — Longitudinal Section of Lower End of Root of Hair, including Papilla. (Kolliker.) tf, Root of hair ; b, cuticle of hair ; c, cuticle of root sheath ; d, Huxley's layer of inner root sheath; e, Henle's layer of inner root sheath; f, outer root sheath; g; vitreous membrane ; j, connective-tissue follicle; /!>, bulb of hair; /, papilla. In the deeper portion of the root, some little distance above the bulb,' all the layers of the hair and its follicle can be distinctly seen. The differentiation of the layers becomes less marked as one passes in either direction. At about the level of the entrance of the ducts of the sebaceous glands (see p. 326) the inner root sheath disappears, and the outer root sheath passes over into the stratum germinativum of the skin, while between the outer root sheath (now stratum germi- nativum) and the hair are interposed the outer layers of the skin, stratum granulosum and stratum lucidum, when present, and stratum 326 THE ORGANS. corneum. All of these are continuous with the same layers of the skin. In the region of the bulb the outer root sheath first becomes thinner, then disappears, while the layers of the inner root sheath retain their identity until the neck of the papilla is reached, at which point the different layers coalesce. The arrcctor pili muscle (Fig. 21 1, c) is a narrow band or bands of smooth muscle connected with the hair follicle. It arises from the outer layer of the derma on the side toward which the hair slants, and is inserted into the wall of the follicle at the junction of its middle and lower thirds, the sebaceous gland being usually included between the muscle and the hair (see below). The contraction of the mus- cle thus tends to straighten the hair and to compress the gland. The sebaceous glands are with few exceptions connected with the hair follicles. They are simple or branched alveolar glands. The FIG. 214.— Transverse Section through Root of Hair and Hair Follicle. (Kolliker.) rt, Hair; b, hair cuticle; c, cuticle of root sheath; d, Huxley's layer; e, Henle's layer ; /", outer root sheath ; z', connective-tissue follicle. size of. the gland bears no relation to the size of the hair, the largest glands being frequently connected with the smallest hairs. The glands are spherical or oval in shape and each gland is enclosed by a connective-tissue capsule derived from the follicle or from the der- THE SKIN AND ITS APPENDAGES. 327 ma. Beneath the capsule is a basement membrane continuous with the vitreous membrane of the follicle. The wide excretory duct empties into the upper third of the follicle and is lined with stratified squamous epithelium continuous with the outer root sheath and stra- tum germinativum. The lower end of the duct opens into several simple or branched alveoli, at the mouths of which the epithelium becomes reduced to a single layer of cuboidal cells. In the alveoli themselves the cells are spheroidal or polyhedral, and usually fill the entire alveolus. These cells, like those lining the duct, are deriva- tives of the outer root sheath. The secretion of the gland — an oily sub- stance called sebum — appears to be the direct product of disintegration of the alveolar cells, which can usually be seen in all stages of the process of transformation of the cell into the secretion of the gland. The most peripheral cells show the least secre- tory changes, containing a few small fat droplets. The central cells and those in the lumen of the duct show the most marked changes, their proto- plasm being almost wholly converted into fat, their nuclei shrunken or dis- integrated or lost. In the middle zone are cells showing intermediary stages in the process. Shedding of hair occurs in most mammalia at regularly recurring peri- ods. In man there is a constant death and replacement of hair. In a hair about to be shed, the bulb be- comes cornified and splits up into a number of fibres. The hair next becomes detached from the papilla and from the root sheath and is cast off, the empty root sheaths collapsing and forming a cord of cells between the papilla and lower end of the shedding Fig. 215.— From Logitudinal Section through Hair and Hair Follicle. En- larged to 800 diameters. (Schafer.) A, Hair, a, Cortex of hair; b, cu- ticle of hair. B, Inner sheath, c, Cuticle of root sheath ; d, Huxley's layer ; e, Henle's layer ; f, outer root sheath ; jr, vitreous membrane; 2", connective-tissue follicle; m, fat cells. 328 THE ORGANS. hair. If the dead hair is to be replaced by a new one, there sooner or later occurs a proliferation of the cells of the outer root sheath in the region of the old papilla. From this " hair germ " the new hair is formed in a manner similar to embryonal hair formation, the new hair growing upward under or to one side of the dead hair, which it finally replaces. As to the manner in which growth of hair takes place, two views are held. According to one of these, the hair, cuticle, and inner root sheath are replenished by proliferation of the epithelial cells surrounding the papilla. These parts thus grow from below toward the surface. The oldest cells of the outer root-sheath, on the other hand, lie against the vitreous membrane, so that growth of this sheath takes place from without inward. According to the second view, the various parts of the hair and its follicle are direct derivatives of the different layers of the skin, and their growth takes place by a contin- uous process of invagination. Thus the most peripheral cells of the outer root-sheath — stratum cylindricum — pass over the papilla and turn upward to form the medulla of the hair; the deeper cells — stratum spinosum — of the outer root-sheath become continuous with the cor- tex of the hair; the stratum lucidum, with the sheath of Henle, which turns up on the hair as its cuticle; Huxley's layer, with the cuticle of the inner root-sheath. According to this view growth of hair is accomplished by continuous growth downward from the sur- face, and turning up into the hair, of these layers. TECHNIC. Pin out small pieces of human scalp on cork and fix in absolute alcohol or in formalin-Miiller's fluid (technic 5, p. 6). From one block cut sections perpen- dicular to the surface of the scalp and in the long axes of the hair and follicles. From a second block cut sections at right angles to the hair follicles, i.e. , not quite parallel to the surface of the scalp but a little obliquely. By this means not only are transverse sections secured, but if the block be sufficiently long the follicles are cut through at all levels. Sections are stained with haematoxylin-picro-acid- fuchsin (technic 3, p. 18) and mounted in balsam. Blood-vessels of the skin. From the larger arteries in the subcu- taneous tissue branches penetrate the pars reticularis of the derma, where they anastomose to form cutaneous networks. The latter give off branches, which pass to the papillary layer of the derma and there form a second series of networks, the subpapillary, just beneath the papillae. From the cutaneous networks arise two sets of capillaries, one supplying the fat lobules, the other supplying the region of the THE SKIN AND ITS APPENDAGES. 329 sweat glands. From the subpapillary networks are given off small arteries which break up into capillary networks for the supply of the papillae, sebaceous glands, and hair follicles. The return blood from these capillaries first enters a horizontal plexus of veins just under the papillae. This communicates with a second plexus just beneath the first. Small veins from this second plexus pass alongside the arteries to the deeper part of the corium, where they form a third plexus with larger, more irregular meshes. Into this plexus pass most of the veins from the fat lobules and sweat glands, although one or two small veins from the sweat glands usually follow the duct and empty into the subpapillary plexus. The blood next passes into a fourth plexus in the subcutaneous tissue, from which arise veins of considerable size. These accompany the arteries. Small arteries from the plexuses of the skin and subcutis pass to the hair follicle. The larger arterioles run longitudinally in the outer layer of the follicle. From these are given off branches which form a rich plexus of small arterioles and capillaries in the vascular layer of the follicle. Capillaries from this plexus also pass to the sebaceous- glands, the arrectores pilorum muscles, and the papillae. The lymphatics of the skin. These begin as clefts in the papil- lae, which open into a horizontal network of lymph capillaries in the pars papillaris. This communicates with a network of larger lymph capillaries with wider meshes in the subcutaneous tissue. The latter also receives lymph capillaries from plexuses which sur- round the sebaceous glands, the sweat glands, and the hair follicles. The nerves of the skin. These are mainly sensory. Motor sympathetic axones supply the smooth muscle of the walls of the blood-vessels and of the arrectores pilorum. The medullated sensory nerves are dendrites of spinal ganglion cells. The larger trunks lie in the subcutis, giving off branches which pass to the corium, where they form a rich subpapillary plexus of both medul- lated and non-medullated fibres. From the subcutaneous nerve- trunks and from the subpapillary plexus are given off fibres which terminate in more or less elaborate special nerve endings (see page 348). Their location is as follows: (1) /;/ the subcutis: Vater- Pacinian corpuscles, the corpuscles of Ruffini, and the Golgi-Mazzoni corpuscles of the finger-tip. The first two forms are most numerous in the palms and soles. (2) /;/ the derma ; Tactile corpuscles of Meissner and Wagner. These are found in the papillae, especially 330 THE ORGANS. of the finger-tip, palm, and sole. Krause's end bulbs — usually in the derma just beneath the papillae, more rarely in the papillae them- selves. (3) In the epithelium : Free nerve endings and tactile cor- puscles. Branches of the cutaneous nerves supply the hair follicles. As a rule but one nerve passes to each follicle, entering it just below the entrance of the duct of the sebaceous gland. As it enters the follicle the nerve fibre loses its medullary sheath and divides into two branches, which further subdivide to form a ring-like plexus of fine fibres encircling the follicle. From this ring, small varicose fibrils run for a short distance up the follicle, terminating mainly in slight expansions on the vitreous membrane. TECHNIC. For the study of the blood-vessels of the skin inject (technic p. 21) the en- tire hand or foot of a new-born child. Examine rather thick sections either mounted unstained or stained only with eosin. Development of the Skin, Nails, and Hair. The epidermis is, as already noted, of ectodermic origin. It con- sists at first of a single layer of cuboidal cells. This soon differen- tiates into two layers — an outer, the future stratum corneum, and an inner, the future stratum germinativum. The stratum granulosum and stratum lucidum are special developments of the stratum germi- nativum. The corium is of mesoblastic origin. It is at first smooth, the papillae being a secondary development. The nail first appears as a thickening of the stratum lucidum. This spreads until the future nail bed is completely covered. Dur- ing development the stratum corneum extends completely over the nail as its eponychium. During the ninth month (intra-uterine) the nail begins to grow forward free from its bed and the eponychium dis- appears, except as already noted. The hair also develops from ectoderm. It first appears about the end of the third foetal month as a small local thickening of the epi- dermis. This thickening is due mainly to proliferation of the cells of the stratum mucosum, and soon pushes its way clown into the underlying corium, forming a long slender cord of cells — the hair germ. Differentiation of the surrounding connective tissue of the THE SKIN AND ITS APPENDAGES. 331 corium forms the follicle wall, while an invagination of this connec- tive tissue into the lower end of the hair germ forms the papilla. The cells of the hair germ now differentiate into two layers : a central core the middle portion of which forms the hair, while the peripheral portion forms the inner root sheath ; and an outer layer which be- comes the outer root sheath. The sublayers are formed from these by subsequent differentiation. The hair when first formed lies wholly beneath the surface of the skin. As the hair reaches the sur- face its pointed extremity pierces the surface epithelium to become the hair shaft. The sebaceous gland develops as an outgrowth from the outer root sheath. This is a flask-shaped and at first solid mass of cells, which later differentiate to form the ducts and alveoli of the gland. The sweat glands first appear as solid ingrowths of the stratum germinativum into the underlying corium. The lower end of the ingrowth becomes thickened and convoluted to form the coiled por- tion of the gland, and somewhat later the central portion becomes channelled' out to form the lumen. The muscle tissue of the sweat glands, which lies between the epithelium and the basement mem- brane, is the only muscle of the body derived from the ectoderm. The Mammary Gland. The mammary gland is a compound alveolar gland. It consists of from fifteen to twenty lobes, each of which is subdivided into lobules. The gland is surrounded by a layer of connective tissue containing more or less fat. From this periglandular connective tissue broad septa extend into the gland, separating the lobes (inter- lobar septa). From the latter finer connective-tissue bands pass in between the lobules (interlobular septa). From the interlobular septa strands of connective tissue extend into the lobule where they act as support for the glandular structures proper. An excretory duct passes to each lobe where it divides into a number of smaller ducts (lobular ducts), one of which runs to each lobule. Within the latter the lobular duct breaks up into a number of terminal ducts, which in turn open into groups of alveoli. The fifteen to twenty main excre- tory ducts pass through the nipple and open on its surface. At the base of the nipple each main duct presents a sac-like dilatation, the 332 THE ORGANS. ampulla, which appears to act as a reservoir for the storage of the milk. Until puberty the gland continues to develop alike in both sexes,, but after about the twelfth year the male gland undergoes retrogres- sive changes, while the female gland continues its development. The inactive mammary gland, by which is meant the female gland up to the advent of the first pregnancy and between periods of lactation, consists mainly of connective tissue and a few scattered groups of excretory ducts (Fig. 216). Around the ends of some of the ducts are small groups of collapsed alveoli. Both ducts and alveoli are lined with a low columnar, often rather flat epithelium. The Active Mammary Gland. — Throughout pregnancy the gland undergoes extensive developmental changes and becomes func- tional at about the time of birth of the child. The microscopic ap- '. A Fig. 16.— From Section of Human Inactive Mammary Gland. X 25. (Technic 1, p. 335-) Gland: composed almost wholly of connective tissue ; few scattered groups of tubules. pearance of the active gland differs greatly from that of the inac- tive (Fig. 217). There is a marked reduction in the connective tissue of the gland, its place being taken by newly developed ducts and alveoli. The alveoli are spheroidal, oval, or irregular in shape,. THE SKIN AND ITS APPENDAGES. ojo and vary considerably in size. The alveoli are lined by a single layer of low columnar or cuboidal epithelial cells which rest upon a homogeneous basement membrane. The appearance of the cells dif- fers according to their secretory conditions. The resting cell is cu- boidal and its protoplasm granular. With the onset of secretion the cell elongates, and a number of minute fat droplets appear. These unite to form one or two large globules of fat in the free end of the cell. The fat is next discharged into the lumen of the alveolus, and •,(■..««: yvtjrz ■■}>*?'.'*& >"*> .-^'wyiL; ;■&&• '0 .v^;- cV^v&^i M Un Fig. 217. — From Section of Human Mammary Gland daring Lactation. X 50. (Stohr.) Branch of excretory duct ; b, interlobular connective tissue ; c, alveoli. regeneration of the cell takes place from the unchanged basal por- tion. . As to the number of times a cell is able to go through this process of secretion and repair before it must be replaced by a new cell, nothing definite is known. Active secretion does not as a rule take place in all the alveoli of a lobule at the same time. Each lob- ule thus contains both active and inactive alveoli. The smallest ducts are lined with a low columnar or cuboidal epithelium. This increases in height with increase in the diameter of the duct until in the largest ducts the epithelium is of the high columnar type. The secretion of the gland is milk. This consists microscopically of a clear fluid or plasma in which are suspended the milk globules. The latter are droplets of fat from 3 to 5 ;<■ in diameter, each enclosed 334 THE ORGANS. in a thin albuminous membrane which prevents the droplets from coalescing. Cells, probably leucocytes, containing fat droplets may also be present. In the secretion of the gland during the later months of pregnancy, and also for a few days following the birth of Fig. 218.— From Section of Mammary Gland of Guinea-pig during Lactation. X 500. (Osmic acid.) (Szymonowicz.) a, Basement membrane ; i, lumen of alveolus; c, tangential sec- tion of alveolus; d, fat globules. the child, a relatively large number of large fat-containing leucocytes — colostrum corpuscles — are found. Blood-vessels. — These enter the gland, branch and ramify in the interlobar and interlobular connective tissue, and finally terminate in capillary networks among the alveoli and ducts. From the capillaries arise veins which accompany the arteries. Lymphatics. — Lymph capillaries form networks among the alveoli and terminal ducts. The lymph capillaries empty into larger lym- phatics in the connective tissue. These in turn communicate with several lymph vessels which convey the lymph to the axillary glands. Nerves. — Both cerebro-spinal and sympathetic nerves supply the gland, the larger trunks following the interlobar and interlobular connective-tissue septa. The nerve terminals break up into plexuses which surround the alveoli just outside their basement membranes. THE SKIN AND ITS APPENDAGES. 335 From these plexuses, delicate fibrils have been described passing through the basement membrane and ending between the secreting cells. Development. — The development of the mammary gland is quite similar to the development of the sebaceous glands. The gland first appears as a dipping down of solid cord-like masses of cells from the stratum mucosum. The alveoli remain rudimentary until the advent of pregnancy. After lactation the alveoli atrophy, being replaced by connective tissue, and the gland returns to the resting state. After the menopause a permanent atrophy of the gland begins, fat and con- nective tissue ultimately almost wholly replacing the glandular ele- ments. TECHNIC. (1) Fix thin slices of an inactive mammary gland in formalin-Muller's fluid (technic 5, p. 6). Stain sections with hcematoxylin-eosin (technic 1, p. 17), and mount in balsam. (2) Prepare sections of an active mammary gland, as in preceding technic (1). (3) Fix very thin small pieces of an active gland in one-per-cent aqueous solu- tion of osmic acid. After twenty-four hours wash in water and harden in graded alcohols. Thin sections may be mounted unstained, or after slight eosin stain, in glycerin. General References for Further Study. Kolliker: Handbuch der Gewebelehre des Alenschen. Ranvier: Traite Technique d'Histologie. Schafer: Essentials of Histology. Spalteholz : Die Vertheilung der Blutgefasse in der Haut. Arch. Anat. u. Phys., Anat. Abth., 1893. McMurrick : Development of the Human Body. CHAPTER XI. THE NERVOUS SYSTEM. The nervous mechanism in man consists of two distinct though associated systems, the cerebrospinal nervous system and the sym- pathetic nervous system. Each of these systems is composed of a central portion (which is its centre of nervous activity) and of a per- ipheral portion (which serves to place the centre in connection with the organs which it controls). In the cerebro-spinal system the cen- tral portion is known as the central nervous system and consists of the cerebro-spinal axis, or brain and spinal cord. The peripheral portion is formed by the cranial and spinal nerves. The central por- tion of the sympathetic system consists of a series of ganglia from which the sympathetic nerves take origin. These latter constitute its peripheral portion. Histological Development. The beginning differentiation of the nervous system appears very early in embryonic life. There is first the formation of a groove or furrow in the outer embryonic layer, or ectoderm. This is known as the neural groove. On either side of this groove is an elevation — the neural fold. By the dorsal union of these folds the neural groove is converted into the neural tube. The lumen of the neural tube corresponds to the central canal of the cord and the ventricles of the brain in the adult, and it is from the ectodermic cells which form the walls of this tube that the entire nervous system is developed. At that end of the neural tube which corresponds to the head of the embryo the greatest development takes place. Here are early formed the three primary cerebral vesicles. These are known respectively as the fore brain (anterior cerebral vesicle — prosencephalon), the midbrain (middle cerebral vesicle — mesencephalon), and the hiud- brain (posterior cerebral vesicle — rhombencephalon). From the an- terior cerebral vesicle are developed the cerebral hemispheres, the 336 THE NERVOUS SYSTEM. 337 corpus striatum., the optic thalamus, and posteriorly as far as the anterior corpora quadrigemina. From the middle cerebral vesicle are developed the corpora quadrigemina and the cerebral peduncles. From the posterior cerebral vesicle are developed the cerebellum, pons, and medulla oblongata. From the remainder of the neural tube is formed the spinal cord. The wall of the neural tube is at first composed of a single layer of epithelial cells. By proliferation of these cells the epithelium soon becomes many-layered, although some of the original epithelial cells still extend through the entire thickness of the wall. Some of the cells which extend through the entire thickness of the wall of the neural tube {spongioblasts of His) increase in length as the wall increases in thickness. The inner ends of these cells form the lining of the tube, while the parts of the cells between the lumen and the nuclei tend to collapse, forming cord-like structures. The outer ends of the cells, on the other hand, become perforated and unite to form a thick network — the marginal veil of His. Of these cells, some retain this position in the adult and are known as ependymal cells; others move away from the central canal and be- come neuroglia cells. Other of the cells which form the wall of the neural tube also develop into various forms of glia cells. Still other of the cells of the neural tube are destined to become neurones, and as such are known as neuroblasts. From the outer end of the neuroblast a process grows out — the future axone. Den- drites which at this stage are absent develop later in a similar man- ner, i.e., by extensions of the cell protoplasm. The neuroblasts soon leave their original position near the central canal and pass outward along the spaces between the elongated ependymal cells. The direc- tions which these neuroblasts take seem to be determined largely by the lines of least resistance offered by the network of the marginal veil. A large number of these cells pass ventrally, their axones piercing the marginal veil and leaving the cord as the ventral root fibres. Other neuroblasts pass laterally and dorsally. The axones of these neuroblasts seem to meet such opposition in the marginal veil that they do not pierce it, but are directed upward and downward within the cord. Later becoming medullated, these axones consti- tute many of the fibres of the white matter of the cord. During the closure of the neural groove, groups of cells from the crest of each neural fold become separated from the rest of the de- 338 THE ORGANS. veloping nervous system. From these are formed the spinal ganglia. The ganglia of the sympathetic system are, according to His, formed of cells which pass out from the spinal ganglia to the positions occu- pied later by the sympathetic ganglia. According to others some of the cells of the sympathetic ganglia may be derived from cells which migrate from the neural tube along the ventral roots. Membranes of The Brain and Cord. The brain and cord are enclosed by two connective-tissue mem- branes, the dura mater and the pia mater. The dura mater is the outer of the two membranes and consists of dense fibrous tissue. The cerebral dura serves both as an invest- ing membrane for the brain and as periosteum for the inner surfaces of the cranial bones. It consists of two layers : (a) An inner layer of closely packed fibro-elastic tissue containing many connective- tissue cells, and lined on its brain surface with a single layer of flat cells ; and (b) an outer layer, which forms the periosteum and is similar in structure to the inner layer, but much richer in blood-ves- sels and nerves. Between the two layers are large venous sinuses. The spinal dura corresponds to the inner layer of the cerebral dura, which it resembles in structure, the vertebrae having their own sep- arate periosteum. The outer surface of the spinal dura is covered with a single layer of flat cells, and is separated from the periosteum by the epidural space, which contains anastomosing venous channels lying in an areolar tissue rich in fat. The pia mater closely invests the brain and cord, extending into the sulci and sending prolongations into the ventricles. It consists of fibro-elastic tissue arranged in irregular lamellae, forming a spongy tissue, the cavities of which contain more or less fluid. The outer lamellae are the most compact, and are covered on the dural surface by a single layer of flat cells. It is this external layer of the pia which is frequently described as a separate membrane, the arachnoid. The inner lamellae of the pia are more loosely arranged, are more cellular and more vascular. Especially conspicuous are large, irregu- lar cells with delicate bodies and large distinct nulcei. They lie upon the connective-tissue bundles partially lining the spaces. The Pacchionian bodies are peculiar outgrowths from the outer layer of the pia mater cerebralis, which are most numerous along THE NERVOUS SYSTEM. 339 the longitudinal fissure. They are composed of fibrous tissue, and frequently contain fat cells and calcareous deposits. Blood-vessels. — The spinal dura and the inner layer of the cere- bral dura are poor in blood-vessels. The outer layer of the cerebral dura, forming as it does the periosteum of the cranial bones, is rich in blood-vessels which pass into and supply the bones. The pia is very vascular, especially its inner layers, from which vessels pass into the brain and cord. TECHNIC. For the study of the structure of the membranes of the brain and spinal cord, fix pieces of the cord with its membranes, and of the surface of the brain with mem- branes attached, in formalin-Midler's fluid (technic 5, p. 6) and stain sections with hasmatoxylin-eosin (technic 1, p. 17). THE GANGLIA. Ganglia are collections of nerve cells which are connected with the peripheral nerves. Each ganglion is surrounded by a connective- tissue capsule which is continuous with the perineurium. From this Fig. 219.— Longitudinal Section through a Spinal Ganglion. X 20. (Stohr.) a, Ventral nerve root; b, dorsal nerve root ; c, mixed spinal nerve; d, groups of ganglion cells; f, nerve fibres ; f, perineurium ; gr, fat ; //, blood-vessel. capsule connective-tissue trabecular extend into the ganglion, forming a connective-tissue framework. Within the ganglion the nerve cells 340 THE ORGANS. are separated into irregular groups by strands of connective tissue and by bundles of nerve fibres. Ganglia are of two kinds : those of the cerebro-spinal system and those of the sympathetic system. Cerebro-Spinal Ganglia (Fig. 219). — The spinal ganglia lie on the dorsal roots of the spinal nerves between their exit from the cord and their union with the ventral roots. The cerebral ganglia occupy an analogous position relative to the cranial nerves. The ganglion cells are large and spherical (Fig. 220). Each contains a centrally located nucleus and a distinct nucleolus, and is surrounded by a capsule of flat, concentrically arranged connective-tissue cells (Fig. 220, s). Stained by Nissl's method the cytoplasm is seen to contain rather small, finely granular chromophilic bodies, which show a tendency to concentric arrangement around the nucleus. Pigmen- tation is common. According to Dogiel (Fig. 221) there are two distinct types of ganglion cells: (1) Unipolar ganglion cells, the single process of which divides, one branch entering the cord as one ^$$ '' ' l ' ■■■: s \ ; \ :-'¥^' W 1 • 1 i jw.'-v ' v 7-,. . '' & 7c ' FIG. 220.— Large Spinal Ganglion Cell from Human Spinal Ganglion showing Connective- tissue Capsule. (From 1 iarker, after von Lenhosstfk.) s, Capsule; ^.peripheral zone of clear cytoplasm ; /<■, axone hill ; «, axone ; pi, pigment. of the fibres of a dorsal nerve root, the other becoming a fibre of a peripheral nerve. (2) Unipolar ganglion cells, the single process of which almost immediately splits up into many fine medullated fibres. These remain within the ganglion and end in dense feltvvorks around THE NERVOUS SYSTEM. 341 other spinal ganglion cells. A few multipolar cells are also described as occurring in the spinal ganglia. In addition to the processes of these ganglion cells, most of which are medullated and which make Fig. 221.— Scheme of Neurone Relations within a Spinal Ganglion, according to Dogiel. (Barker.) A, Ventral root ; B, dorsal root ; C, spinal nerve ; D, ventral division, E, dor- sal division of spinal nerve ; /■", communicating branch to sympathetic ; a, spinal ganglion cell of first type, the main process of which (//) divides, one arm passing centrally as a fibre of the dorsal root, the other peripherally as an afferent fibre of the mixed spinal nerve ; b, spinal ganglion cell of second type, the axone of which (n) ends in a pericellular network around the bodies of cells of the first type; j, sympathetic fibres ending in plexuses around the bodies of cells of the second t}-pe. up the main mass of fibres of the ganglia, there are also a few fine non-medullated fibres which come from cells in adjacent sympathetic ganglia and end in arborizations around the spinal ganglion cells. Dogiel believes that these end entirely around cells of the second type. The relation of the spinal ganglion cell to the dorsal roots is de- scribed on page 355. The Sympathetic Ganglia.- — -The larger ganglia resemble the spinal ganglia in having a connective-tissue capsule and framework. 342 THE ORGANS. The cells are smaller and often densely pigmented. Each cell is sur- rounded by a capsule of flat connective- tissue cells, but the capsule is not so thick and distinct as that of the spinal ganglion cell. Most of the cells are multipolar. The fibres which traverse these ganglia are mainly of the non-medullated variety. Sympathetic ganglion cells are not confined, however, to definite ganglionic structures, but occur in ill-defined groups in certain of the viscera, e.g., in the heart and in the intestinal plexuses of Meissner and Auerbach. Groups of two or three cells, or even single cells, are also found scattered along the sympathetic nerves. Such cells show great variation in shape, size, and internal structure. TECHNIC. (i) Fix spinal and sympathetic ganglia in formalin-Midler's fluid (technic 5, p. 6). Stain sections with haematoxylin-eosin (technic 1, p. 17), or with h'aematoxylin- picro-acid fuchsin (technic 3, p. 18). (2) Fix spinal and sympathetic ganglia in absolute alcohol or in ten-per-cent formalin, and stain sections by Nissl's method (technic, p. 32). (3) See also technic 1, p. 360. THE PERIPHERAL NERVES. The peripheral nerves are divided into spinal nerves and cranial nerves, the former taking origin from the cord, the latter from higher centres. Each spinal nerve consists of two parts— a motor or effer- ent part and a sensory or afferent part. Of the cranial nerves some are purely efferent, others purely afferent, while still others consist like the spinal nerves of both efferent and afferent fibres. The efferent fibres of the spinal nerves are axones of cell bodies situated in the anterior horns of the cord (see p. 357, and Figs. 227 and 236). They leave the cord as separate bundles, which join to form the motor or efferent root. The afferent fibres are peripheral proc- esses of cell bodies situated in the spinal ganglia (p. 351 and Figs. 227, 236). These leave the ganglion and join with the fibres of the motor root to form the mixed spinal nerve (Fig. 227,/). The con- nection of the ganglion with the cord is by means of the axones of the spinal ganglion cells, which enter the cord as the posterior root (Fig. 236). Among the afferent fibres of the posterior root are also found a few efferent fibres (Fig. 227, c), processes of cells in the cord. The peripheral nerve consists of nerve fibres supported by con- nective tissue (Fig. 222). Enclosing the entire nerve is a sheath of THE NERVOUS SYSTEM. 343 dense connective-tissue, the epineurium. This sends septa into the nerve which divide the fibres into a number of bundles or fascicles. Surrounding each fascicle the connective tissue forms a fairly distinct sheath, the perifascicular sheath or perineurium. From the latter, delicate strands of connective tissue pass into the fascicle, separating the individual nerve fibres. This constitutes the intrafascicular con- Fig. 222. — From Transverse Section of Human Nerve Trunk. (Osmic acid fixation. ) (Quain.) ep, Nerve sheath or epineurium surrounding the entire nerve and containing blood-ves- sels (i>) and small groups of fat cells (/") ; per, perifascicular sheath or perineurium sur- rounding each bundle or fascicle of nerve fibres ; end, interior of fascicle showing sup- porting connective tissue, the endoneurium. nective tissue or endoneurium. In the connective-tissue layers of the perineurium are lymph spaces lined with endothelium, which com- municate with lymph channels within the fascicle. When nerves branch, the connective-tissue sheaths follow the branchings. When the nerve becomes reduced to a single fibre, the connective tissue still remaining constitutes the sheath of Henle (see Fig. 66, p. ii2). For description of medullated and non-medullated nerve fibres see pages 1 1 1 and 112. For sensory-nerve terminations see page 352 ; for motor-nerve terminations see page 357. 344 THE ORGANS. TECHNIC. Fix a medium -sized nerve, such as the human radial or ulnar, by suspending it, with a weight attached to the lower end, in formalin- Midler's fluid (technic 5, p. 6). Stain transverse sections in haematoxylin-picro-acid fuchsin (technic 3, p. 18) and mount in balsam. THE SPINAL CORD. The spinal cord encased in its membranes lies loosely in the ver- tebral canal, extending from the upper border of the first cervical vertebra to the middle or lower border of the first lumbar ver- tebra. It is cylindrical in shape and continuous above with the medulla oblongata, while below it terminates in a slender cord, the filum terminate. At two levels, one in the cervical and one in the lumbar region, the diameter of the cord is considerably increased. These are known respectively as the cervical and lumbar enlarge- ments. The spinal nerve roots leave the cord at regular intervals, thus indicating a division of the cord into segments, each segment extending above and below its nerve roots one-half the distance to the next adjacent roots. There are 3 1 segments corresponding to the 3 1 spinal nerves ; 8 cervical, 1 2 dorsal, 5 lumbar, 5 sacral, and 1 coccygeal. If the fresh cord be cut through, it is seen to consist of a central gray matter surrounded by a peripheral zone of white matter. The difference in color is due to the fact that the peripheral zone is com- posed almost entirely of medullated nerve fibres with their white myelin sheaths, while the gray matter is comparatively poor in medullated fibres, consisting mainly of nerve cell bodies and their dendritic processes. The greater vascularity of the gray matter also contributes to its color. The internal structure of the cord can be best studied by means of transverse sections taken at different levels. TECHNIC. (1) Carefully remove the cord (human if possible; if not, that of a large dog) with its membranes, cut into two or three pieces if necessary, and lay on sheet cork. Slit the dura along one side of the cord, lay the folds back, and pin the dura to the cork. Care must be taken to leave the dura very loose, otherwise it will fhitten the cord as it shrinks in hardening. With a sharp razor now cut the cord, but not the dura, into segments about 1 cm. thick. Fix for two weeks in Midler's fluid, wash in water to which a little formalin has been added, harden in THE NERVOUS SYSTEM. 345 graded alcohols. Pieces of the cord may be cut out as wanted and embedded in celloidin. Sections should be cut about 15 u in thickness. (2) For the study of the general internal structure of the cord, stain a section through the lumbar enlargement of a cord prepared according to the preceding technic (1) in hsematoxylin-picro-acid fuchsin (technic 3, p. iS) and another section through the same level in Weigert's haematoxylin (technic p. 27)- Mount both in balsam. Practical Study. Section Through the Lumbar Enlargement (Fig. 223). — The general features of the section can be best seen with the naked eye or with a low-power dissecting lens. (1) In the picro-acid-fuchsin-stained section note the shape and size of the cord, and that it is surrounded by a thin membrane, the pia *% gg*f PlG. 223.— Cross Section of Human Spinal Cord through the Fifth Lumbar Segment. X 10. (Weigert stain.) (Marburg.) a, Anterior median fissure ; b, posterior septum ; c, posterior column ; cf, lateral column ; e, anterior column ; /, cell groups of anterior horn ; £-, poste- rior horn ; //, posterior root fibres ; 2", Clarke's column and fibres entering it ; /, reticular process. In the centre of the figure is seen the central canal surrounded by the central gelatinous substance. Ventral and dorsal to the latter, but not distinguishable from it, are the ventral and dorsal gray commissures. The dorsal white commissure is seen at s, while the thick bundle of fibres at the bottom of the anterior fissure is the ventral white commissure. In the broad head of the posterior horn is a large light area, the gelatinous substance of Rolando, between which and the surface of the cord is the zone of Lissauer. Note fibres passing from the posterior columns into the gray matter of the posterior horns, especially into the column of Clarke ; the grouping^of cells in the anterior horn, and the anterior root fibres passing to the surface. mater spinalis ; the anterior median fissure, broad and shallow, into which the pia mater extends; the posterior median septum consisting of neuroglia, and over which the pia mater passes without entering. 346 THE ORGANS, The gray matter is seen in the central part of the section, stained red, and arranged somewhat in the form of the letter H. Posteriorly the gray matter extends almost to the surface of the cord as the pos- terior horns or cornua. The anterior horns are, on the other hand, short and broad, and do not approach the surface of the cord. Sur- rounding the gray matter is the white matter stained yellow. This is divided by the posterior horn into two parts, one lying between the horn and the posterior median septum, the posterior column ; the other comprising the remainder of the white matter, the antero-lateral column. This latter is again partially divided by the anterior horn and anterior nerve roots into a lateral columnand an anterior column. In the concavity between the anterior and posterior horns some proc- esses of the gray matter extend out into the white matter where they interlace with the longitudinally running fibres of the latter to form the reticular process. For the study of further details the low-power objective should be used. Gray Matter. — In the cross portion of the H is seen the central canal, usually obliterated in the adult and represented only by a group of epithelial cells. This group of cells divides the gray matter con- necting the two sides of the cord into a ventral gray commissure and a dorsal gray commissure. Immediately surrounding the epithelial cells is a light granular area composed mainly of neuroglia and known as the central gelatinous substance. Toward the surface of the cord the posterior horn expands into a broad head or caput, in which is an area similar in general appearance to that surrounding the central canal, the gelatinous substance of Rolando. The head is connected with the rest of the gray matter by a narrower neck or cervix. Note the interlacing of fibres in the reticular process; the well-defined groups of large nerve cells in the anterior horns ; the fibres which pass out from the anterior horns to the surface of the cord, anterior nerve roots (Fig. 223). White Matter. — Note the general appearance of the white mat- ter and the disposition of the supporting strands of neuroglia tissue (stained red). The neuroglia is seen to form a fairly thick layer just beneath the pia mater from which trabecular pass in among the fibres, the broadest strand forming the posterior median septum. If the section has been cut through a posterior nerve root, a strong bundle of posterior root fibres can be seen entering the white matter of the cord THE NERVOUS SYSTEM. 347 to the inner side of the posterior horn. Just ventral to the anterior .■gray commissure is a bundle of transversely-running medullated fibres — the anterior white commissure (Fig. 223). Such finer details of structure as are brought out by this stain should next be studied with the high-power objective. In the gray matter note the large multipolar ganglion cells of the anterior horn with their coarsely granular protoplasm. In the white matter note the transversely -cut medullated fibres and their marked variation in size. The shrunken axones are stained red, the usually ■somewhat broken up medullary sheaths, yellow. Neuroglia cells are not well shown by this method, but can be seen, especially in the region of the processus reticularis, with their irregular-shaped cell bodies and darkly stained nuclei. (2) In the Weigert- stained section the only element stained is the medullary sheath (Fig. 223); consequently the white matter, which contains a much larger proportion of medullated fibres than the gray matter, is stained more deeply than the latter. Note first the same general structure seen in the preceding section, the nerve fibres, how- ever, being much more clearly shown. Note the central gelatinous substance and the gelatinous substance of Rolando, conspicuous from their lack of medullated fibres. Separating the gelatinous substance of Rolando from the surface of the cord is a narrow zone, more lightly stained on account of its very fine fibres, and known as the zone of Lissauer. Note the exact mode of entrance and distribution within the cord of the posterior root fibres ; the passage of the ventral root fibres to the surface of the cord ; the already mentioned anterior white commissure ; the posterior white commissure, consisting of a few medullated fibres crossing just dorsal to the posterior gray com- missure. Note especially theplexus of fine fibres throughout the gray matter and the general interchange of fibres between the gray matter and the white matter (Fig. 223). While the general structure above described obtains throughout the cord, the size and shape of the cord, the size and shape of the gray matter, and the relative proportion of gray matter and white matter, vary in different parts of the cord, which must therefore be ■separately considered. TECHNIC. (1) From a cord prepared according to technic 1. p. 344. remove small seg- ments from each of the following levels : (1) the twelfth dorsal, (2) the mid-dorsal. 348 THE ORGANS. and (3) the cervical enlargement. The segments are embedded in celloidin, sec- tions cut 15 to 20M thick, stained by Weigert's method (page 27), and mounted in balsam. Medullated sheaths alone are stained by this method and appear dark blue or black. Practical Study. Section through the Twelfth Dorsal Segment (Fig. 224). — Note that the cord is smaller than in the lumbar enlargement and somewhat flattened dorso-ventrally ; that the amount of gray matter and white matter is diminished ; that both anterior and posterior horns are more slender, the anterior horn containing comparatively few cells. At the inner side and base of the posterior horn may be Fig. 224. — Cross Section of Human Spinal Cord through the Twelfth Dorsal Segment. X 10. (Weigert stain.) (Marburg.) a, Fibres of posterior column entering Clarke's column ; b, fibres passing from Clarke's column cells to the direct cerebellar tract ; c, Clarke's column. seen a small group of cells belonging to Clarke s column. These cells form a continuous column from the third lumbar to the seventh •cervical segments, but are most numerous in the upper lumbar and lower dorsal region. Isolated portions of the nucleus are found in the sacral and in the upper cervical cord. Medullated fibres can be seen passing into Clarke's column, where they interlace among the ganglion cells. Section through the Mid-dorsal Region (Fig. 225). — Com- pare with the lumbar sections. Note the change in shape and size; that the cord is more nearly round and smaller; that while the reduc- tion in size affects both gray matter and white matter, it is the former that shows the greater decrease. The horns are even more slender THE NERVOUS SYSTEM. 349 than in the first lumbar section, and the anterior horn contains still fewer cells. Clarke's column is present, but not so large. Fig. 225. —Cross Section of Human Spinal Cord through the Eighth Dorsal Segment. X 10. (Weigert stain.) (Marburg.) a, Reticular process ; b, Clarke's column. Section through the Cervical Enlargement (Fig. 226). — Note the marked increase in size of the cord, which affects both gray ,x&gG&BSMk9M'm8*j* FIG. 226.— Cross Section of Human Spinal Cord through Fourth Cervical Segment. X 10. (Weigert stain.) (Marburg.) Note lateral extension of anterior horn to form the lateral horn, a, Reticular process; />, Clarke's column: c, septum between column of Goll and column of Burdach. 350 THE ORGANS. matter and white matter. Depending upon the exact level at which the section is taken, the cord may be nearly round or flattened dorso- ventrally. The posterior horns remain slender while the anterior are: much broader and have lateral extensions known as the lateral horns. The reticular process is more prominent than in any of the pre- vious sections. As in the lumbar cord, the cell groups of the anterior horn are numerous and well defined. A more or less definite septum divides the posterior column into an inner part, the column of Goll,. and an outer part, the column of 'Burdock. Origin of the Fibres which Make up the White Matter of the Cord. It has already been observed that the white matter of the cord is composed mainly of medullated nerve fibres, most of which run in a longitudinal direction. From our study of the neurone it follows that each of these fibres must be the axone of some nerve cell. These cells, the axones of which form the white matter of the cord, are situated as follows : f(i) Cells outside the central nervous system (spinal A. Cells outside the spinal! ganglion cells). cord. [Extrinsic cells.)') (2) Cells in other parts of the central nervous sys- [ tern (the brain). f (3) Root cells, such as those of the anterior horn,. whose axones form the ventral root. I (4) Column cells, whose axones enter into forma- B. Cells situated in the gray | tion of the fibre columns of the cord, matter of the cord. (In-\ (5) Cells of Golgi, type II., the axones of which trinsic cells.) ramify in the gray matter. (These cells do not give rise to fibres of the white matter, but are conveniently mentioned here among the other cord cells.) (1) The Spinal Ganglion Cell and the Origin of the Posterior Columns. The fibres of these columns consist mainly of ascending and de- scending branches of the fibres, which enter the cord as the pos- terior nerve roots. Following these fibres outward, they are seen to originate in the cells of the spinal ganglia. In very early embry- onic life the group of cells which later becomes a spinal ganglion is represented by a few cctodermic cells which lie between the closing medullary plate and the external layer of the ectoderm. These cells become separated from the medullary plate by the mesoderm. At THE NERVOUS SYSTEM. 351 first round, these cells which have thus migrated from the central nervous system soon become spindle-shaped, and from each end of the spindle a process grows out : one, directed toward the surface of the body, joins the axones of the cells of the anterior horn to make up the mixed spinal nerve; the other, directed centrally, enters the cord as one of the fibres of the posterior root (Fig. 227). During its Fig. 227.— Transverse Section through Spinal Cord and Posterior Root Ganglia of an Embryo Chick. (Van Gehuchten.) a, Spinal ganglion, its bipolar cells sending their peripheral processes outward to become fibres of the mixed spinal nerve (/), their central processes into the dorsal columns of the cord as the dorsal root fibres (b~); within the posterior col- umns these fibres can be seen bifurcating and sending collaterals into the gray matter of the posterior columns, one collateral passing to the gray matter of the opposite side. The few efferent fibres of the dorsal root (c) are disproportionately conspicuous. The large multipolar cells of the ventral horns are seen sending their axones (i +- Alt S^?>£© ^ • 1(2 *ii THE NERVOUS SYSTEM. a be bo v >>r: *►, o u -> a .2 .S 5 a> r u ^ u « ^ 'S a c o. t, "" . £ £ ^. sse ^:: ■2 s S.s-g 8^ &° I 359 ^§--r:^ . « « 5 -o £ -a .- o)J3 JJ 01D— c *- a a S r S-| & « i * fi S c S S *_3f: 5 o £ .| « 8 g s .2 S * £ I * J 2£.S2"-SSj='H-grtC m be > 5 £' !I ft * T. «2> « is --* a o F-"35t »-^ c .Saw « •- -5 £ -2 ° o •£ 5 -s g 8 -a 5 ^2ftlgli^|s°u 0-S^O^e3i-a;^rtuo 5^"go- s i.2££Si38S ° | 2 -5 I - s oo S 2 8 £ Ss I 1 .5 2 S3 t S « -"s « £ « Q "So -" r- « -a -a c-S^ |.2?°«u.S , aQcBo' 00 «!; « , °££ M SS = ot!n n J o s ^ i c ? p - « " o o 360 THE ORGANS (5) Cells of Golgi Type II. The axones of these cells do not leave the gray matter, but divide rapidly and terminate in the gray matter near their cells of origin, some crossing to terminate in the gray matter of the opposite side (Fig. 236). TECHNIC. (1) For the purpose of studying the spinal ganglion cell with its processes and their relations to the peripheral nerves and to the cord, the most satisfactory mate- rial is the embryo chick of six days' incubation, treated by the rapid silver method of Golgi (technic b, p. 29). Rather thick (75^) transverse and longitudinal sec- tions are made and mounted in hard balsam without a cover-glass. Owing to the uncertainty of the Golgi reaction several attempts are frequently necessary before good sections are obtained. (2) The root cells of the anterior horn with their axones passing out of the ■cord and joining the peripheral processes of the spinal ganglion cells, to form the spinal nerves, can usually be seen in the transverse sections of the six-day embryo chick cord prepared as above, technic (1). (3) For studying the column cells of the cord, embryo chicks of from five to six ■days' incubation should be treated as in technic (1 ). Owing to the already mentioned uncertainty of the Golgi reaction, it is usually necessary to make a large number of sections, mounting only those which are satisfactorily impregnated. It is rare for a single section to show all types of cells. Some sections contain tautomeric cells, some contain heteromeric, while in very few will the hecateromeric type be found. Sections containing fewest impregnated cells frequently show collaterals to best advantage. These are seen as a fringe of fine fibres crossing the boundary line between gray matter and white matter. Practical Study. Transverse Section of Six-day Chick Embryo (Technic 1). — Using a low-power objective, first locate the cord and determine the outlines of gray matter and white matter. Observe the spinal ganglia lying one on either side of the cord (Fig. 227, a). One of the ganglia will probably show one or more bipolar cells, sending one process toward the periphery, the other toward the spinal cord. Note that the peripheral process is joined, beyond the ganglion, by fibres which come from the ventral region of the cord (fibres of the anterior root). In some specimens the latter can be traced to their origin in the cells of the anterior horn (Fig. 227, d). The union of the periph- eral processes of the spinal ganglion cells and the anterior horn fibres is seen to make up the mixed spinal nerve (Fig. 227,/). Observe the central processes of the spinal ganglion cells entering the dorsal THE NERVOUS SYSTEM. 361 column of the cord and bifurcating (Fig. 227, b). As these branches pass up and down the cord, only a short portion of each can be seen in a transverse section. Note the fibres (collaterals) passing from the white matter into the gray matter. Note in some of the sections, a little round mass just ventral and to the inner side of the spinal ganglion, in which nerve cells may be seen, and some fibres passing into or out of it. This rep- resents the beginning of the sympa- thetic system with its chain of gan- glia. Note the relation which this bears to the spinal cord and spinal ganglia. Longitudinal Section of Six- day Chick Embryo (Technic 1, p. 360). — Using a low-power objective locate gray matter and white matter and identify plane of section relative to transverse section above described. Note in the white matter longitudinal- ly-running fibres from which branches pass off into the gray matter (Fig. 237). Those of the posterior columns are the ascending and descending branches of the central processes of the spinal gan- glion cells, and the branches passing into the gray matter are their collater- als and terminals. If the section hap- pens to include the entering fibres of a posterior root, these can be seen branch- ing in the posterior columns into as- cending and descending arms (Fig. 237). The longitudinal fibres of the lateral and anterior columns are ax- ones of column cells and of cells sit- uated in higher centres (see pages 356 and 357) collaterals and terminals into the gray mattei. A B Fig. 237.— From Longitudinal Section of Spinal Cord of Embryo Chick. (Van Gehuchten.) A, White columns of cord ; B, gray matter. The cells of the gray matter (column cells) are seen sending their axones into the white matter, where they bifurcate, their ascending and descending arms becoming fibres of the white columns. The dendrites of these cells are seen ramifying in the gray matter. To the left are seen fibres (posterior root fibres) entering the white matter and bifurcating, the ascending and de- scending arms becoming fibres of the white columns. From the latter are seen fibres (collaterals) passing into the gray matter and ending in arbori- zations. These also send 362 THE ORGANS. Fibre Tracts of the Cord. The fact that the cell bodies of neurones are located in the gray matter of the brain and spinal cord, in the ganglia, and in the per- ipheral end organs of certain of the nerves of special sense, has been already referred to. In the brain and cord there are more or less definite groupings of these neurones for physiological purposes, their cell bodies being grouped together to form centres or nuclei ; their axones, following certain definite paths, known as fibre tracts or Fig. 238.— Diagram showing Fibre Tracts of the Cord, Ascending Tracts being shown on the Right Side, Descending Tracts on the Left Side. (Schafer.) /, Crossed pyramidal tract; 2, direct pyramidal tract ; j>, antero-lateral descending tract or tract of Loewenthal ',3a, bundle of Helweg ; 4, rubro-spinal tract or von Monakow's bundle; 5, comma tract; 6, column of Goll ; 7, column of Burdach ; II The Antero-lateral Descending Tract. {Anterior Marginal Bundle of Loewenthal.) — This consists of descending axones of neurones whose cell bodies are situated in the cerebellum. In the cord these fibres lie along the ventral margin, overlapping the tract of Gowers (Fig. 238, J). Investigators are not in accord as to whether there are any fibres passing without interruption from cerebellum to cord, or whether they are all interrupted in Deiter's nucleus (Fig. 264). III. Von Monakow's Tract. {Rubrospinal Tract.) — This con- sists of axones of cells situated in the red nucleus of the opposite side. In the cord the tract lies in the lateral column just ventral to the crossed pyramidal tract (Fig. 238, /f). IV. Descending Tract from Deiter's Nucleus. — This consists of axones from cells of Deiter's nucleus of the same side in the me- dulla (p. 387). Its fibres intermingle in the cord with those of Loewenthal's tract (Fig7238, p. 362). Some of its fibres have been traced to the sacral cord. V. Quadrigemino-spinal Tract. — This consists of axones of celis situated at about the junction of thalamus and mid-brain. In the cord these fibres lie along the anterior median fissure, some of them extending down into the lumbar cord. VI. Descending Tract from the Vestibular Nuclei of Both Sides. — These fibres occupy about the same position in the cord as the preceding, and both are continuations in the cord of the dorsal longitudinal fasciculus (p. 379V VII. Helweg's tract is a small triangular bundle of fibres lying 368 THE ORGANS. along the ventrolateral margin of the cord, and is traceable upward as far as the olives (Fig. 238,3* a). The origin and destination of its fibres are not definitely known. VIII. The Septo-marginal Tract. {Oval Bundle of Flechsig.) — This is a small bundle of fibres lying next the posterior septum (Fig. 238, sni). Itis probably composed of descending axones of cells in thecord. IX. The so-called "comma" tract of Schultze is a small comma-shaped bundle of descending fibres lying about the middle of the posterior column (Fig. 238, 5). It is most prominent in the dor- sal cord. Its fibres are believed by some to be descending branches of spinal ganglion cells, by others to be descending axones from cells situated in the gray matter of the cord (column cells). X. Tractus Reticulo-spinalis. — This consists of axones of cells in the formatio reticularis of the medulla. In the cord these fibres are intermingled with those of the antero-lateral ground bundles (see below). This tract includes the fasciculus of Thomas, lying in the lateral column near the gray matter. The latter originates in the reticular formation of the medulla and ends in the cervical cord. Fundamental Columns or Ground Bundles of the Cord. The ascending and descending tracts above described are known as the long fibre tracts of the cord. If the area which these tracts occupy be subtracted from the total area of white matter it is seen that a considerable area still remains unaccounted for. This area is especially large in the antero-lateral region, and extends up along the lateral side of the posterior horn between the latter and the crossed pyramidal tract (Figs. 240 and 242). A small area in the posterior column just dorsal to the posterior commissure, and extending up a short distance along the medial aspect of the horn, should also be included. These areas are occupied by the fundamental columns or sliort-fibrc systems of the conl. The fibres serve as longitudinal commissural fibres to bring the different segments of the cord into communication (Fig. 237). The shorter fibres lie nearest the gray matter and link together adjacent segments. The longer fibres lie farther from the gray matter and continue through several segments. The origin of these fi bres as axone s of cells of the gray matter, and the manner i n w hich they re-enter the gray matter as terminals and collaterals have been considered (page 358). THE NERVOUS SYSTEM. 369 From the neurones thus far studied and the tracts which their axones follow, we may determine the following general impulse path- ways in the cord : (1) The Direct Reflex Path (Fig. 239). — (a) The peripheral sen- sory neurone ; its peripheral process and end organ, the spinal gan- glion cell, its central process with collaterals terminating around motor cells of anterior horn ; (b) the. peripheral motor neurone; motor cell of anterior horn withaxone passing to muscles, etc. This is a two- neurone reflex path, chiefly uncrossed, and in most cases involving only closely adjacent segments. (2) The Indirect Reflex Path (Fig. 240). — (a) The peripheral sensory neurone as in the direct reflex, but terminating around col- umn cells of the cord, (b) The cord neurone (column cells)- — axones forming fundamental columns with collaterals and terminals to ante- FtG. 239. — Diagram Illustrating Path followed by and Neurones involved in a Simple Direct Reflex. (Van Gehuchten.) A, Sensory surface ; B, muscle ; C, spinal cord. Arrows show direction of impulse, which starts at the sensory surface, passes along first the peripheral then the central arm of the spinal ganglion cell to the dorsal columns of the cord, thence by means of a collateral or terminal to the ventral horn, where it is transferred to a motor cell. The impulse then passes along the axone of the motor cell (motor fibre of spinal nerve) to the muscle. The two neurones involved in a simple reflex are thus seen to be the peripheral sensory neurone and the peripheral motor neurone. rior horn cells of different levels, (c) The peripheral motor neurone as in the direct reflex. This is a three-neurone reflex path involving both sides of the' cord and segments above and below the segment of entrance of the stimulus. 37° THE ORGANS. (3) Direct Ascending Paths to HigJier Centres. — The peripheral sensory neurone as in the direct reflex, but with central process pass- ing as fibre of Goll or Burdachto the nucleus of one of these columns in medulla (Fig. 242). FlG. 240. — Diagram Illustrating Pathway of Compound Reflex (Van Gehuchten) Involving Three Neurones. /, Peripheral sensory neurone by means of which the impulse passes from the sensory surface to the gray matter of the cord, as in Fig. 239. In the gray mat- ter, instead of passing directly to a motor cell as in the direct reflex, the impulse is transferred to a neurone, 2, whose axone becomes a fibre to the ground bundles. Frorn the latter terminals and collaterals enter the gray matter and end around cells of the "ventral horn, whence the impulse is carried to the muscle as in the direct reflex, 3. The essential difference between the simple and the compound reflex is thus seen to be the interposition of a third neurone and the fact that a number of motor cells situated at different levels are involved. (4) Indirect Ascending Paths to Higher Centres. — {a) Peripheral sensory neurone as in direct reflex, but communicating in cord with column cells of direct cerebellar and of Gowers' tracts, (b) Column cells sending their axones to higher centres in the direct cerebellar and Gowers' tracts (Fig. 361). (5) Descending Paths from Higher Centres. — (a) The cortico- spinal motor neurone whose cell bodies are situated in the motor cor- tex, and whose axones form the pyramidal tracts. These axones terminate around anterior horn cells, those of the crossed tract in the horn of the same side, those of the direct tract in the horn of the op- posite side, (J?) The peripheral motor neurone^— anterior horn cell — its axone to muscle (Fig. 239). . In addition to the main descending paths are the other descending paths mentioned on page 367, by which an impulse may pass from higher centres to the cord. THE NERVOUS SYSTEM. 371 TECHNIC. (1) A human cord from a case in which death has occurred some time after fracture of the vertebras with resulting crushing of the cord, furnishes valuable but of course rarely available material. If death occur within a few weeks after the injury, the method of Marchi ' should be used ; if after several weeks the method of Weigert (page 27). The picture in the cord is dependent upon the fact that axones cut off from their cells of origin degenerate and are finally replaced by connective tissue. After a complete transverse lesion of the cord, therefore, all ascending tracts are found degenerated above the lesion, all descending tracts below the le- sion. The method of Marchi gives a positive picture of osmic-acid-stained degene- rated myelin in the affected tracts. The method of Weigert gives a negative pict- ure, the connective tissue which has replaced the degenerated tracts being unstained in contrast with the normal tracts, the myelin sheaths of whose fibres stain, as usual, dark blue or black. (2) Human cords from cases which have lived some time after the destruction of the motor cortex, or after interruption of the motor tract in any part of its course, may also be used for studying the descending fibre tracts. (3) The cord of an animal may be cut or crushed, the animal kept alive for from two weeks to several months, and the cord then treated as in technic 1. The most satisfactory animal material may be obtained from a large dog by cutting the cord half-way across, the danger of too early death from shock or complica- tions being much less than after complete section. (4) The cord of a human foetus from the sixth month to term furnishes good material for the study of the anterior and posterior root fibres, the plexus of fine fibres in the gray matter, the groupings of the anterior horn cells, etc. The pyram- idal tracts are at this age non-medullated and are consequently unstained in Wei- gert preparations. The Weigert-Pal method gives the best results (page 28). (5) For the study of the course of the posterior root fibres within the cord, cut any desired number of posterior roots between the ganglia and the cord and treat material by the Marchi or the Weigert method, according to the time elapsed between the operation and the death of the animal. THE MEDULLA OBLONGATA. (Including the Pons Varolii.) The medulla oblongata is the continuation upward of the spinal cord and extends from the lower limit of the pyramidal decussation below to the lower margin of the midbrain above. Externally, the medulla shows the continuation upward of the 1 Marchi's solution consists of two parts Miiller fluid and one part one-per- cent aqueous solution osmic acid. After hardening for from seven to ten days in Midler's fluid, thin slices of tissue are transferred to the Marchi solution, where they remain for about the same length of time. Sections are usually mounted, without further staining, in balsam. 372 THE ORGANS. anterior fissure and posterior septum of the cord. On either side of the anterior fissure is a prominence caused by the anterior pyramid, and to the outer side of the pyramid the bulging of the olivary body may be seen. The antero-lateral surface of the medulla is also marked by the exit of the fifth to the twelfth (inclusive) cranial nerves. The posterior surface shows two prominences on either side. The more median of these, known as the clava, is caused by the nucleus gracilis, or nucleus of the column of Goll ; the other, lying just to the outer side of the clava, is due to the nucleus cuneatus or nucleus of the column of Burdach. The central canal of the cord continues into the medulla, where it gradually approaches the dorsal surface, and about the middle of the medulla opens into the cavity of the fourth ventricle. The internal structure of the medulla considerably resembles that of the cord. This is especially true of the lower part of the medulla, the structures of which are directly continuous with those of the cord. The fibre tracts of the cord, however, assume in the medulla new directions, and in so doing break up the formation of the gray mat- ter. This and the appearance of certain new masses of gray matter and of some new fibre bundles, many of them connected with the cranial nerves, are the main factors determining the difference in structure between cord and medulla. Of the ascending tracts, the posterior columns end in the nuclei of Goll and Burdach, whence a second neurone system connects them with higher centres, the axones passing up as the fillet ; the direct cerebellar tract passes into the restiform body, while the tract of Gowers follows the course described on p. 365. Of the descending tracts, the direct and crossed pyramidal tracts are represented in the medulla by the anterior pyramids. Other de- scending tracts described on pp. 367 and 368 are also present in the medulla, passing to their nuclei of origin. Of the spinal gray matter, there are continuations which form the nuclei of termination for sensory cranial nerves, the largest mass being the extended nucleus of the spinal fifth. The anterior horns of the cord are represented in the medulla by separate masses of gray matter, which are the nuclei of origin for motor cranial nerves. Of new masses of gray matter, the most important are the nuclei of the columns of Goll and of Burdach and the olivary nucleus, which is connected with the cerebellum via its inferior peduncle. THE NERVOUS SYSTEM. 373 The cranial nerves, with the exception of the first (olfactory) and the second (optic;, are analogous, both embryologically and anatomi- cally, to the spinal nerves. The neurones which constitute the sensory portions of the cranial nerves have their cell bodies situated in ganglia outside the central nervous system. These ganglia correspond to the posterior root ganglia of the spinal nerves. The outwardly directed processes of these cells pass to their peripheral terminations as do those of the spinal ganglion cells. The central axones of these neurones enter the medulla and form longitudinal tracts of fibres in a manner quite analogous to the formation of the posterior columns by the ascending branches of the central axones of the spinal ganglion cells. The longer branches of the sensory root fibres of the cranial nerves, however, do not ascend, as do those of the spinal nerves, but turn spinalward, forming descending roots. These fibres terminate in the gray mat- ter of the medulla (terminal nuclei of the cranial nerves) in the same manner as do the spinal sensory root fibres in the gray matter of the cord and medulla. Thus the sensory root fibres of the fifth nerve form a distinct bundle known as the spinal root of the fifth ; some of the fibres of the vestibular part of the eighth nerve form another dis- tinct bundle, the descending root of the eighth ; while the descending root fibres of the ninth and tenth form the solitary fasciculus. The fibres of each of these descending roots terminate in an accom- panying nucleus. The axones of the cells of these terminal nuclei form secondary ascending tracts to higher centres, these tracts thus bearing the same relation to the cranial nerves that the fillet bears to the spinal. The motor root fibres of the cranial nerves are the axones of neurones whose cell bodies are situated in the gray matter of the medulla and parts above (motor nuclei of the cranial nerves), just as the motor root fibres of the spinal nerves are the axones of neurones whose cell bodies are situated in the gray matter of the cord (anterior horns). These motor nuclei are distributed in two series, one situ- ated near the median line, the other more laterally. In the former series are the motor nuclei of the third, fourth, sixth, and twelfth ; in the latter are the motor nuclei of the fifth, seventh, ninth, and tenth. While these nuclei are the nuclei of origin of the motor divisions of the cranial nerves, they are also the nuclei of termination for descend- / 374 THE ORGANS. ing axones of neurones of higher systems which serve to bring these peripheral motor neurones under the control of higher centres. The internal structure of the medulla can be best studied by means of a series of transverse sections. TECHNIC. The technic of the medulla is the same as that of the cord (page 344). Trans- verse sections should be cut through the following typical levels, stained by Wei- gert's method (page 27), and mounted in balsam : 1. Through the pyramidal decussation. 2. Through the sensory decussation. 3. Through the lower part of the olivary nucleus. 4. Through the middle of the olivary nucleus. 5. Through the exit of the eighth cranial nerve. 6. Through the exits of the sixth and seventh cranial nerves. i. Transverse Section of the Medulla through the Decussation of the Main Motor Tracts (Pyramidal Decussation) (Fig. 241). Compare the section with the section of the cervical cord (page 349) and note the following structures studied in the cord sections : 1. The posterior column: (a) The column of Goll (funiculus gracilis) and (b) the column of Burdach (funiculus cuneatus) remain as in the cervical cord. 2 and 3. The lateral and anterior columns : (a) The pyramidal tracts are decussating. As we pass upward from the cord, this appears as a crossing of the fibres of the crossed pyramidal tract from their position in the lateral columns to the opposite anterior column where they join the direct pyramidal tract to form the anterior pyra- mid, (b) the direct cerebellar tract, (c) the tract of Gowers, (d) the spino-tectal tract, (e) von Monakow's bundle, (/") the descending tract from Deiter's nucleus, (g) the descending tract from the vestibular nuclei, (h) thequadrigemino-spinal tract, (z) Helweg's tract, (j) the reticulo-spinal tract, and (k) the tract of Loewenthal, occupy about the same relative positions as in the cervical cord (Fig. 238). (While the general locations of these tracts should be noted, they cannot of course be differentiated in the normal adult human medulla.) 4. The posterior horn. This is larger, especially the gelatinous substance of Rolando, and is almost entirely separated from the THE NERVOUS SYSTEM. 375 rest of the gray matter, being connected with it by a very long, slen- der cervix or neck (Fig. 241). 5. The anterior horn is cut off from the rest of the gray matter by decussating pyramidal fibres. 6. The central canal and the central gelatinous substance are the same as in the cervical cord. Note also the following new structures : 7. The reticular formation ; beginning to show in this section, al- though not so well developed as higher up in the medulla. Its coarse basketwork appearance is due to a breaking-up of the late- ral gray matter by longitudinal fibres. Some of these are con- tinuations into the medulla of the lateral fundamental column fibres of the cord (tractus reticulo-spi- nalis and tractus spino-reticularis), others are the short association fibres of the medulla analogous to those of the ground bundles of the cord. 8. Decussation of the pyra- mids. This is the most important feature of the section. Bundles of fibres are seen crossing from the anterior pyramid of one side to the opposite dorso-lateral column, where they turn downward as the crossed pyramidal tract. These fibres, as already noted in the cord, are descending axones from motor cells situated in the cere- bral cortex. In the pyramidal decussation most of these fibres cross to the opposite postero-lateral region to pass down the cord as the crossed pyramidal tract (p. 366, and Fig. 236, j ; Fig. 238, /). A few remain in their original anterior position to continue down the cord as the direct pyramidal tract (p. 366, and Fig. 236, /; Fig. 23S, 2). The bundles of fibres do not cross in a transverse plane, but take a downward direction at the same time. For this reason trans- verse sections show these fibres cut rather obliquely. Because of the fact that the fibres cross in alternate bundles, the number of decussat- FlG. 241.— Transverse Section of the Medulla, at the Level of the Pyramidal Decussation. (Dejerine.) /, Posterior column; ia, col- umn of Goll ; ib, column of Burdach ; 2, lateral column ; 3, anterior column ; 4, pos- terior horn ; j, anterior horn; 7, reticular formation ; S, decussation of the pyramids \ o, dorsal root of first cervical nerve ; jo, gelatinous substance of Rolando ; x, neck of posterior horn. 376 THE ORGANS. ing fibres seen in any one section is greater on one side than on the other (Fig. 241). 9. The root fibres of the spinal accessory nerve. This is a motor nerve, its fibres being axones of cells of the anterior horn. They cross the lateral column and leave the medulla on its lateral surface. 10. The dorsal root of the first cervical nerve. 2. Transverse Section of the Medulla through the Decussation of the Fillet (Sensory Decussation) (Fig. 242). Note the following already mentioned structures: 1. Theposterior column. Both the column of Goll (, resti- form body ; 24, olivary nucleus. 378 THE ORGANS. (Fig. 242, XB). In the higher sensory levels there is usually an accessory cuneate nucleus (Fig. 243, NBa). These nuclei serve as nuclei of termination for the fibres of the pos- terior columns. With their termination in these nuclei we come to the ending of that system of fibres which we have traced from their ori- gin in the cells of the spinal ganglia. In other words, we have com- pleted the course of the spinal peripheral sensory neurone. As the fibres of the posterior columns are constantly terminating in these nuclei, there is, in passing from below upward, a constant increase in the size of the nuclei and a corresponding decrease in the size of the posterior columns, until, just below the olive, the whole of the column of Goll and most of the column of Burdach are occupied by their respec- tive nuclei (Fig. 243, NG and XB). By means of neurones whose cell bodies are situated in these nuclei and whose axones cross to the oppo- site side and pass upward to terminate in the optic thalamus (see 12, 13, 14, below), the sensory conduction path is continued brainward. 12. Internal arcuate fibres, pass ventrally and inward from the nuclei of the posterior columns to a point just below the central canal, where they form the 13. Sensory decussation, or decussation of the fillet. These fibres are axones of neurones whose cell bodies are situated in the nuclei of the posterior columns. After decussating they turn brainward, form- ing a tract of fibres known as the 14. Fillet, or median lemniscus, which lies just dorsal to the an- terior pyramid, and increases in size as we ascend through this level. 15. Spinal (descending) root of the fifth cranial nerve (trigemi- nus). This is a bundle of very fine fibres lying just external to the posterior horn, thus occupying the position of Lissauer's column in the cord. From this bundle, fibres can be seen entering the remains of the posterior horn, which, as stated above (page 377), is its termi- nal nucleus. The neurones of the latter constitute the secondary sensory (ascending) tract for the fifth nerve, as do those of the nuclei of Goll and Burdach for spinal sensory nerves. 16. The nucleus of origin of the medullary portion of the eleventh cranial (spinal accessory) nerve {Xxi) and its root fibres {XI) passing toward the surface. The following new structures are to be seen only in the higher levels of the sensory decussation (Fig. 243). THE NERVOUS SYSTEM. 379 d.n.X X/ 17. The accessory olivary nucleus; an elongated L-shaped mass of gray matter lying just dorsal to the anterior pyramid. 18. The arciform nucleus; on the surface of the medulla ventral to the anterior pyramid. 19. The solitary fascic- ulus. This shows in some of the sections as a distinct round bundle of fibres just lateral to the central gray matter. It consists of the descending or sensory root fibres of the ninth (glosso- , pharyngeal) and tenth (vagus) 'cranial nerves. The gray matter in its immediate vicin- ity is its terminal nucleus. 20 (Nxii\. The nucleus P „ _,. v / FIG. 244.— Diagram of Origm of Cranial Nerves X of origin of the twelfth cranial nerve (hypoglossal). This is a group of nerve cells lying in the ventral part of the centra: gelatinous substance, near the median line. Root fibres of this nerve may be seen passing from the nucleus to the ventral surface of the cord {XII). 21. The posterior longi- tudinal fasciculus ; a bundle of fibres situated just dorsal to the fillet. It consists of the quadrigemi.no.-' spinal tract, the descending tract from the vestibular nuclei, and other descending fibres from cells in the reticular formation. 22. The external arcuate fibres. These are often present at this level running parallel to the lateral surface of the cord just under the pia mater. They are at this level, and higher, axones of neurones whose cell bodies are situated in the lateral nucleus and in the formatio-reticu- laris. These axones pass first as internal, then as external arcuate fibres, to the restiform body, thence to the cerebellum (p. 384, 23 and Fig. 264). It is probable that some of these fibres end among cells of the arciform nucleus. 244. and XII. (Schafer.) pyr, Pyramid ; o, olivary nucleus; r, restiform body; d . V, spinal root of fifth nerve ; n.XII, nucleus of hypoglossal ; XII, hypoglossal nerve; d.n.X. XI, dorsal nucleus of vagus and spinal accessory ; n.amb, nucleus am- biguus ; f.s, solitary fasciculus (descending root of vagus and glosso-pharyngeal); f.s.n, nucleus of solitary fasciculus ; X, motor fibre of vagus from nucleus ambiguus ; g, ganglion cell of sensory root of vagus sending central arm into solitary fasciculus (f.s) and collateral to its nucleus (f.s.n.); f.s. 7i, cell of nucleus of solitary fasciculus sending axone as internal arcuate fibre to opposite side of cord (secondary vagus and glosso-pharyn- geal tract). 3 8o THE ORGANS. 23. The restiform body. This appears in the higher sensory de- cussation levels as a narrow band of fibres along the lateral margin of the medulla. (For details see page 384, 23.) 24. The olivary nucleus. This may sometimes be seen as one or JNxrn Fig. 245.— Transverse Section of the Medulla through the Lower Part of the Olivary Nucleus. (Dejerine.) /, Posterior column; 2, lateral column; 3, pyramid; Sj?/?, gelatinous sub- stance of Rolando and remains of posterior horn ; j, remains of anterior horn ; XCp, nu- cleus of the posterior [column ; 12, internal arcuate fibres; /j, sensory decussation; 14, fillet ; /j, spinal root of fifth cranial nerve ; 77, accessory olivary nuclei ; 17 a, dorsal acces- sory olivary nucleus; /, through pons at level of VIII nerve. Spacing between the different levels is not proportionate. Inland C the crusta is omitted. Neurone No. /.—Cell bodies in spiral ganglion (gang, spiralis); peripheral processes end in organ of Corti; central processes terminate principally in ventral or accessory nucleus (Nu. accessorius) and lateral nucleus (Nu. lateralis) or tuberculum acusticum; some also terminate in superior olives, (Oliva superior) and nuclei of trapezoid body (Nu. corp. trapezoidei) of same and opposite sides. Neurone No. 2 (or 3?). — Axones of cells in accessory nucleus, in superior olives, and in nuclei of trapezoid body constitute a ventral path in the lower border of the tegmentum, and form the lateral part of the lateral lemniscus (Lemniscus lateralis) or lateral fillet on the opposite side. Axones of cells in the lateral nucleus traverse the floor of the fourth ventricle as the stria; acustica?, forming a dorsal pathway, decussate and then turn ven- tral to a point dorsal to the superior olive and join the lateral lemniscus as its mesial part. Some axones also of cells in the accessory and lateral nuclei pass dorsally, looping around the restiform body, and then proceed ventrally (bundle of Held) to join the op- posite lemniscus. The lateral lemniscus passes upward to the posterior corpus quadri- geminum, some of the axones terminating en route in the nucleus of the lateral lemniscus. From cells in this nucleus some axones again join the lateral lemniscus, and a few de- cussate and then pass upward to the posterior corp. quad. The vast majority of axones of the lateral lemniscus terminate in the posterior corp. quad., but a few possibly pass on to terminate in the anterior corp. quad. (Lat. lemniscus fibre to ant. corp. quad.). Neurone No. 3 (or 4?).— Axones of cells in the gray matter of the posterior corp. quad, form its brachium (Brachium corp. quad, post.) and ascend to terminate in the internal or medial geniculate bod}- (Corp. genie, inter.). Neurone No. 4 (or 5?).— Axones of cells in the internal geniculate body pass as a part of the thalamic radiation via the posterior part of the internal capsule to the cortex of the temporal lobe of the cerebrum. The axones which constitute the ventral path (Neurones 1 and 2) form a bundle of fibres known as the trapezoid body (Corpus trapezoidium). The decussation of these is peculiar in that the dorsal axones of the bundle on one side become the ventral ones on the op- posite side; this accounts for the convergence of the axones at the median raphe. Axones of cells in the superior olive pass to the nucleus of VI nerve (reflex). There is probably also a descending path from the lateral nucleus to the spinal cord (not indi- cated). /Via n. vest dig e Oa.no. Sco-rba. J Fig. 247 b. EXPLANATION OF FIG. 247$. Fig. 247 b.— Principal Connections of the Vestibular Portion of the Auditory (VIII) Nerve. A, .Section at level of oculomotor (III) nerve; B, section through pons and medulla; C, through inferior olives; D, through spinal cord. Neurone No. 1. — Cell bodies in ganglion of Scarpa; peripheral processes end in semi- circular canals; central processes bifurcate, and ascending arms go to Deiter's nucleus (Nu. lat. d. vestib.) (/ a), to von Bechterew's nucleus (Nu. sup. n. vestib.) (/ b), and to nuclei fastigii (and other portions?) of cerebellum (/ c) ; descending arms go to nucleus of descending root (Nu. n. vestib. desc.) (id) and (collaterals ?) to principal or median nucleus (Nu. med. n. vestib.) (/ e). Neurone No. 2.— Axones of some cells in Deiter's nucleus descend (2 a, Tr. desc. nu. Deitersi) uncrossed toantero-lateral column of the cord, axones of other cells enter the posterior longitudinal fasciculus (Fasc. long, post., 2 6) of same side and descend to inte- rior column of the cord, others pass to the posterior longitudinal fasciculus of opposite side whence some (2 c) descend to anterior column of the cord, occupying a position near the anterior median fissure, while some (2d) ascend in the posterior longitudinal fascic- ulus and terminate principally in the nuclei of VI, IV, and III nerves. Axones of cells in von Bechterew's nucleus ascend (2 e), joining lateral part of posterior longitudinal fasciculus of same side, and terminate in nuclei of IV and III nerves. Axones of cells in the nucleus of the descending root probably pass in part to lateral part of reticular formation of same and opposite sides, ascending and descending (to other motor nuclei?). Axones of ceils in the median nucleus probably pass largely into the reticular formation, possibly also to the posterior longitudinal fasciculus (not indicated). Axones of cells in the nuclei fastigii of the cerebellum pass to von Bechterew's nucleus (2/) and to Deiter's nucleus (2g). The cerebellar associations intercalated between these (2 j\ 2 g) and the vestibular fibres to the cerebellum (/ c) are not known. [It is evident that impulses other than vestibular ones entering the cerebellum may also by 2/and 2,^ act indirectly upon the motor nuclei innervated by axones of the cells in Deiter's and von Bechterew's nuclei. Compare Fig. 264. J THE NERVOUS SYSTEM. 387 The fibres of the vestibular root ( VIII, v) enter above and mesial to those of the cochlear root, passing dorsally along the inner side of the restiform body to four nuclei, which cannot all be clearly seen in any one section; (a) Deiter's nucleus (lateral vestibular nucleus) (ND), situated at the end of the main bundle of root fibres, just in- ternal to the restiform body; (&) von Bechterew's nucleus (superior vestibular nucleus) situated somewhat dorsal to Deiter's nucleus in the lateral wall of the fourth ventricle; (c) the median or princi- pal nucleus of the vestibular division — a large triangular nucleus, occupying the greater part of the floor of the fourth ventricle Pig. 248. — Diagram of Origin of Eighth Cranial Nerve and some of its more Important Central Connections. (Obersteiner.) Cbll, Cerebellum; Crst, restiform body; Co, cerebral cor- tex; Py, pyramid; Ra, median raphe; Va, spinal root of fifth nerve; NVI, nucleus of sixth nerve; VI, root fibres of sixth nerve ; VII, root fibres of seventh nerve ; Rl, cochlear root of eighth nerve (axones of ceils in spinal ganglion or ganglion of Corti) passing to their terminations in the ventral cochlear nucleus, Aacc, and in the dorsal cochlear nu- cleus, Tba ; Rm, vestibular root of eighth nerve (axones of cells in Scarpa's ganglion) passing to their terminations in Deiter's nucleus, I\'D, and in the median vestibular nu- cleus, Ntr (the nucleus of von Bechterew and the spinal vestibular nucleus are not seen at this level t; S/r?n, stria? acustica? ; Tgm, tegmentum; Os, superior olivary nucleus; Ost, fibres from superior olivary nucleus to nucleus of sixth nerve; JVt, trapezoid nucleus; Ctr, trapezoid body ; Lml, lateral lemniscus or lateral fillet ; Qa, anterior corpus quad- rijreminuin ; Qp, posterior corpus quadrigeminum. (Nviii, v) ; and (d) the spinal vestibular nucleus which accompa- nies the descending fibres of the vestibular root (spinal eighth, see p. 384, 29). The fibres of the cochlear nerve are axones of bipolar cells in the spiral ganglion, or ganglion of Corti (see p. 443, Fig. 283). The 388 THE ORGANS. central processes of these cells enter the medulla as the above-de- scribed cochlear root, to terminate in arborizations among the cells of the cochlear nuclei. Most of the axones of the cells of these nuclei cross to the opposite side of the medulla, forming the second- ary cochlear tract to higher centres, known as the lateral fillet (see p. 391, 36). Some of the cochlear fibres pass both ventral and dorsal nuclei to end in the superior olivary and trapezoid nuclei. Axones from the cell's of these nuclei also join the lateral fillet. The neurones of the vestibular nerve have their cell bodies situ- ated in Scarpa's ganglion (vestibular ganglion). These cells are bipolar, their peripheral processes ending freely among the hair cells of the crista and macula acustica, their central processes forming the already mentioned vestibular root. The axones of the cells of the terminal nuclei of the vestibular root form secondary vestibular tracts, some axones going to the cerebellum and midbrain, others descending in the reticular formation, still others forming part of the posterior longitudinal fasciculus. 32. The root fibres of the seventh (facial) cranial nerve (Fig. 247, VII) and its nucleus of origin (Nvii). These can be seen in higher sections of this level. (For details see p. 390, 32.) 33. The root fibres of the sixth (abducens) cranial nerve (Fig. 247, VI) and its nucleus of origin. (For details see p. 390* 33)- 34. The acoustic striae ; in the lateral part of the floor of the fourth ventricle. These are fibres of the secondary cochlear tract from the dorsal cochlear nucleus (see p. 385, 31). 35. Transverse fibres of the pons Varolii ; crossing ventral to the pyramids (see p. 391, 35). 37. The central tegmental tract (see 37, p. 391). 6. Transverse Section through the Exits of the Root Fibres of the Sixth (Abducens) and Seventh (Facial) Cranial Nerves. The following structures seen in the preceding level have now disappeared : [8. The arciform nucleus. 22. The external arcuate fibres; unless the superficial transverse pons fibres be classed as arcuate fibres. THE NERVOUS SYSTEM. .89 23. The restiform body; now passed or passing into the cere- bellum as its inferior peduncle. 24. The olivary nucleus. 31. The cochlear portion of the auditory nerve with its nuclei. 34. The acoustic stria?. The following structures are still present : 2. The tract of Gowers, the spino-tectal tract, and von Monakow's bundle lie just ventro-lateral to the superior olivary nucleus. Deiter's tract has reached its nucleus of origin. VITar VEhr "Wy Fig. 249. — Transverse Section of the Medulla through the Exits of the Sixth and Seventh Cranial Nerves. (Dejerine). 3, Pyramid ; 4, gelatinous substance of Rolando and re- mains of posterior horn ; SR, reticular formation ; Rm, fillet ; /j, spinal root of fifth nerve; 2/, posterior longitudinal fasciculus; 2j, restiform body or inferior cerebellar peduncle; 25, fourth ventricle; VIIIv, vestibular root of eighth cranial nerve; .YviY, nucleus of origin of seventh cranial nerve ; VII7, root fibres of seventh nerve passing from nucleus of origin to floor of fourth ventricle ; Vllg, transversely cut bundle of root fibres of seventh nerve ascending in floor of fourth ventricle ; VII, root fibres of seventh nerve leaving medulla ; Nvi, nucleus of origin of sixth cranial nerve; VI, root fibres of sixth cranial nerve ; 35 bl, superficial transverse fibres of the pons ; 35 t>2, deep transverse fibres-of pons; 36, lateral lemniscus; Fee, central tegmental tract ; A"«, nucleus of the reticular formation; N-p, pontile nuclei; TV, trapezoid body; >-, median raph6; 37, pe- duncle of superior olivary nucleus; 38, superior olivary nucleus. 3. The pyramid; now occupying the middle of the pons. 4. The remains of the posterior horn. 7. The reticular formation (SR); in which are several groups of ganglion cells, the nuclei of the reticular formation {Net). 390 THE ORGANS. ventr // 12. The internal arcuate fibres; rather indefinite. 14. The fillet (Rm), now called the median lemniscua to distin- guish it from the lateral lemniscus ; much flattened dorso-ventrally lying between the reticular formation and the pons. 15. The spinal root of the fifth cranial nerve; usually broken up into several bundles. 21. The posterior longitudinal fasciculus; now a distinctly sepa- rate bundle lying next the median line near the floor of the fourth ventricle. 25. The fourth ventricle; the roof now being formed by the cerebellum. 3 1 . The root fibres of the vestibular division of the eighth nerve. 32. The root fibres of the seventh (facial) cranial nerve and its nucleus of origin. The latter consists of a fairly well-defined group of large motor cells situated deep in the reticular formation (Fig. 249, Nvii). The axones of the cells of this nucleus pass dorsally and mesially toward the floor of the fourth ventricle (VII J). Here they turn and ascend in the floor of the fourth ventricle — appearing in the section as a bundle of transversely cut fibres ( VII g) — to the genu or bend, where they turn ventro- laterally and pass to the surface ( VII). (Only portions of the course of the root fibres of this nerve can be seen in any one section.) 33. The root fibres of the sixth (abducens) cranial nerve and its nucleus of origin. The nucleus consists of a group of large motor cells lying in the floor of the fourth ventricle (Nvi) y partially surrounded by the genu of the seventh nerve. From this nucleus, fibres may be seen ( VI), passing ventrally through the reticular formation and pons of the surface. P//A w" FIG. 250. -Diagram of Origin of Sixth and Seventh Cranial Nerves. (Sehafer.) pyr, Pyramid ; cr, restiform body ; dV, spinal root of fifth nerve; Venlr.IV, fourth ventricle; VIII v, vestibular root of eighth nerve ; n. VI, chief nucleus of sixth nerve ; ;z' VI, ac- cessory nucleus of sixth nerve; VI, sixth nerve ; ?i. VII, nucleus of seventh nerve, from which the axones pass dorso-mesially to the floor of the ventricle, where they turn brain- ward, appearing as a bundle of transversely cut fibres, a VII, and ascend to the "genu," g, where they turn and pass ventro-laterally to the surface as the seventh nerve, VII. THE NERVOUS SYSTEM. 39r 35. The pons Varolii. This occupies the ventral part of the sec- tion. It consists of longitudinal fibres, transverse fibres, and gray matter (pontile nuclei). (a) The pontile nuclei {Nfi) are masses of gray matter lying among the fibres of the pons. They are nuclei of origin of the transverse pontile fibres. (p) The transverse pontile fibres, or middle peduncle of the cere- bellum, connect the pontile nuclei with the opposite cerebellar hemi- sphere. They are divided by the longitudinal fibres into (£1), superficial transverse fibres and (£2) deep transverse fibres. (e) The longitudinal fibres of the pons lie with the pyramidal tracts between the superficial and deep transverse fibres. They are mainly descending axones to the pontile nuclei from cells situated in the cerebral cortex. 37. The central tegmental tract {Fee) lies in the reticular forma- tion between the root fibres of the sixth and seventh nerves. It is probably a descending tract from higher centres to the olives. The following new structures are to be noted : 36. The lateral lemniscus or lateral fillet lies to the outer side of the reticular formation. It contains a mass of gray matter known as the nucleus of the lateral lemniscus, in which some of the fibres of the acoustic pathway are interrupted. Its fibres are mainly a sec- ondary cochlear tract, the axones of cells in the terminal nuclei of the cochlear nerve (see p. 385, 31). The fibres of the lateral lemnis- cus terminate mainly in the gray matter of the posterior corpora quad- rigemina, possibly also in the anterior corpora quadrigemina, most of them in the corpora quadrigemina of the same side, a few in opposite side. From the posterior corpus quadrigeminum, fibres (brachium of posterior corpus quadrigeminum) pass to the internal or medial genic- ulate body of the same side, whence they pass as a part of the thala- mic radiations to end in the cortex of the temporal lobe. 38. The superior olive is a mass of gray matter lying just lateral to the central tegmental tract. This nucleus, together with several other nuclei in its immediate vicinity (pre-olivary nucleus, semilunar nucleus, and trapezoid nucleus) are terminal nuclei for the secondary cochlear fibres of the trapezium (7V). Some of the transverse fibres passing through the ventral part of the tegmentum are the decussating fibres of the secondary acoustic tract (see page 388) to the lateral fillet. 392 THE ORGANS. 7. Transverse Section Through the Exit of the Root Fibres oi the Fifth (Trigeminus) Cranial Nerve (Fig. 251). The following structures seen in the last section have disappeared : 4. The posterior horn, which has been serving as the nucleus of termination for the descending root of the fifth nerve. In some of -xvJ*Y s 35 £2 < Nrt Fig. 251. — Transverse Section of the Medulla through the Exit of the Fifth Cranial Nerve. (Dejerine.) j, Pyramidal fibres and longitudinal pontile fibres; SR, reticular formation ; Km, fillet ; 7V, trapezoid body ; ij, spinal root of fifth nerve ; 27, posterior longitudinal fasciculus ; 25, fourth ventricle ;jj>6, transverse pontile fibres ;jj bi, superficial transverse pontile fibres ; jj fe, deep transverse pontile fibres ; Fee, central tegmental tract; Nrt, nucleus of the reticular formation ; Nfi, pontile nuclei ; jS, superior olivary nucleus ; V, root fibres of fifth cranial nerve; NVs, sensory nucleus of fifth cranial nerve; NVm, motor nucleus of fifth cranial nerve ; Nft, nucleus funiculi teretis ; 23, inferior cerebellar peduncle, the continuation of the restiform body ; 40, superior cerebellar peduncle ; 35 b, transverse pontile fibres forming middle cerebellar peduncle ; Lig, middle lobe of cere- bellum ; x, fibres passing to superior cerebellar peduncle. the lower sections of this level a small remnant of the posterior horn may be present. 31. The root fibres of the vestibular division of the eighth cranial nerve. 32. The root fibres and nucleus of the seventh cranial nerve. 33. The root fibres and nucleus of the sixth cranial nerve. The following structures seen in the preceding section are still present : THE NERVOUS SYSTEM. 393 2, The tract of Gowers, the spino-tectal tract, and von Mon- akow's bundle ; not distinguishable in the sections, but lying in the ventral part of the tegmentum just internal to the root of the fifth nerve. 3. The pyramid; now much broken up into bundles by the trans- verse fibres of the pons (35 b). 7. The reticular formation (SR); occupies a considerable portion of the tegmentum, its gray matter being known as the nucleus of the reticular formation {Nri). 12. Internal arcuate fibres; crossing the median raphe. 14. The fillet (Rm); much flattened dorso-ventrally, and broken up into several bundles of fibres, which lie just ventral to the reticular formation and dorsal to the deepest of the transverse pontile fibres. 15. The spinal root of the fifth nerve (see p. 378, 15)- 21. The posterior longitudinal fasciculus. 25. The fourth ventricle; somewhat narrower as it is approach- ing the iter. 35. The pons; much increased in extent. (For details see p. 39i, 35-) 36. The lateral lemniscus. (For details see p. 391, 36.) 37. The central tegmental tract (Fee) ; in about the same position. 38. The superior olive; smaller than in the last section. Note the following new structures : 39. The root fibres of the fifth cranial nerve (trigeminus). As the fifth is a mixed nerve, some of these fibres are sensory, others motor. (er) The fibres of the sensory root pass between the fibres of the pons to the floor of the fourth ventricle, where some of them termi- nate in the main sensory nucleus (NVs), while others turn spinal- ward as the descending spinal root (75), which with its nucleus (the remains of the posterior horn) has been noted in all sections of the medulla. (b) The fibres of the motor root leave the medulla just internal to those of the sensor)' root. They are axones of cells situated in two nuclei — only one of which (NVm) can be seen in this section — lying in the lateral part of the reticular formation. The cell bodies of the neurones whose axones make up the sen- sory root of the fifth nerve are situated in the Gasserian or semilunar 394 THE ORGANS. ganglion. This ganglion is analogous to the posterior root ganglion of the spinal nerve. The cells are unipolar, the single process bifur- cating as in the cells of the spinal ganglia. Their peripheral branches pass to the surface. Their central branches pierce the fibres of the pons and, reaching the floor of the fourth ventricle, bifurcate. The shorter ascending arms terminate in the main sen- sory nucleus of the fifth nerve. The long descending arms form th* cv. r cM rtmV. FlG. 252. — Diagram of Origin of Fifth Cranial Nerve. (Schafer.) G, Gasserian ganglion ; a, />, c, the three divisions of the nerve ; m.n. V, principal motor nucleus ; fi.s.n. V, principal sen- sory nucleus; d.s.ti.V, descending sensory or spinal nucleus; d.s.V, descending or spinal root ; c.V and c'.V, secondary trigeminal tracts (axones of cells in sensory nuclei); r, median raphe. descending or spinal root of the fifth nerve. The fibres of this root send collaterals into, and terminate in, the gelatinous substance of the posterior horn, which thus constitutes an extended terminal nucleus for this root (Fig. 252). The cell bodies of the neurones whose axones constitute the motor root of the fifth nerve are situated, as already noted, in two nuclei. One of these, the principal motor nucleus, has been de- scribed. The other nucleus consists of a long column of cells ex- tending from this level upward to the region of the corpora quadri- gemina. The axones from this nucleus form the descending motor THE NERVOUS SYSTEM. 395 or mesencephalic root of the fifth nerve (Fig. 253, Vd). The axones from these two nuclei join to form the motor root of the fifth nerve. 40. All three of the cerebellar peduncles can be seen in this sec- tion. The inferior peduncle (23) is the continuation into the cere- bellum of the restiform body which has been noted in all of the sections above the pyramidal decussation. (For details see p. 384, 23.) The middle peduncle (35 b) has been described in connec- tion with the transverse fibres of the pons (see p. 391, 35 b). The superior peduncles (40) form a large part of the lateral wall of the fourth ventricle. They are more conspicuous in the succeeding section. (For details see p. 397, 40.) THE MIDBRAIN— MESENCEPHALON OR ISTHMUS. Through the midbrain can be followed the further continuation upward of the main fibre tracts of the cord and medulla. Ventrally the midbrain shows a deep groove or sulcus, caused by the diver- gence of the crusta or continuation of the main motor tracts. The dorsal surface of the midbrain presents four rounded prominences, the two posterior and the two anterior corpora quadrigemina. Just dorsal to the crusta is a layer of gray matter which contains deeply pigmented nerve cells. This is known as the substantia nigra and separates the crusta from the rest of the midbrain, the parts dorsal to the substantia nigra being collectively known as the tegmentum. There are thus to be considered in studying the midbrain, the crusta, the tegmentum, and the intervening substantia nigra. Transverse Section of the Midbrain through the Exit of the Fourth Cranial Nerve (Pathetic) (Fig. 253). The following structures present in the preceding sections have disappeared : 12. The internal arcuate fibres; unless the decussating fibres of the superior peduncles of the cerebellum be regarded as such. 15. Spinal root of the fifth nerve; with its nucleus, the posterior horn. 25. The fourth ventricle; now become the iter, the roof of which is formed by the valve of Vieussens or anterior medullary vellum. 35. The pons. 30 THE ORGANS. 38. The superior olivary and trapezoid nuclei. 39. The fifth nerve, with its roots and nuclei, excepting the small descending motor (mesencephalic) root ( Vd), and its nucleus. The following structures seen in the preceding section are still present : In the crust a : 3. The pyramid (VP). The numerous bundles of pyramidal fibres seen in the last section among the transverse pontile fibres sIV Fig. 253.— Transverse Section of the Midbrain through the Exit of the Fourth Cranial Nerve. (Dejerine.) VP, Pyramid; 7, reticular formation; Rm, fillet; 21, posterior longitudinal fasciculus; 2;, iter; 36, lateral lemniscus; NRl, nucleus of lateral lemniscus; Vd, de- scending or mesencephalic root of fifth nerve ; 40, superior cerebellar peduncle ; 40 a, dor- sal decussation of superior peduncles ; 40 b, ventral decussation of superior peduncles; Nl, lateral nucleus, a band of gray matter lying between the superior peduncles and the fillet ; Ln, substantia nigra; IV, root fibres of fourth cranial nerve; xIV, decussation of root fibres of fourth cranial nerve; x, gray matter forming floor of iter; xF, tegmental decussation. now form one large bundle, the crusta. The middle three-fifths of the latter are occupied by the pyramidal tracts proper (cerebro- spinal), and by fibres to the motor nuclei of the cranial nerves; the mesial fifth, mainly by axones passing from cells in the frontal lobe to terminate in the pontile nuclei ; the lateral fifth, by fibres which probably connect the temporal lobe with the pontile nuclei. THE NERVOUS SYSTEM. 397 In the most dorsal part of the crusta are a small number of fibres which have been described as derived from the fillet and as probably passing either to the cortex or thalamus. In the tegmentum: 2. Gowers' tract is now external to the superior cerebellar pe- duncle, passing back to the cerebellum (p. 365). Von Monakow's bundle lies mesial tc the nucleus of the lateral lemniscus. 7. The reticular formation; much diminished in size, contains besides its association fibres, ascending axones from cells of the fifth nerve nuclei (secondary trigeminal tract), crossed decending tract from the tectum, and descending fibres from the superior peduncle. 14. The fillet; now a much flattened band of fibres just dorsal to the substantia nigra. 21. The posterior longitudinal fasciculus; just ventral to the gray matter of the floor of the iter. l6. The lateral lemniscus; occupying with its nucleus [NRl) the extreme dorso- lateral part of the tegmentum. 37. The central tegmental tract. 39 b. (Vd) The descending motor or mesencephalic root of the fifth nerve. (For details see p. 394.) 40. The superior cerebellar peduncles or brachia conjunctiva. These occupy the greater part of the tegmentum and can be seen de- cussating in the median line. They are composed mainly of axones from cells in the dentate nucleus and possibly in other cerebellar nuclei. Crossing to the opposite side in the decussation, these axones terminate mainly in the red nucleus and thalamus; also in the pons and medulla and possibly in the nucleus of the third nerve. Of new structures, the only ones to which special attention is called are : 41. The fourth cranial nerve (pathetic). The root fibres of this nerve are seen decussating in the roof of the iter ixIV). Some transversely cut bundles of fourth root fibres can also be seen in the lateral wall of the iter. The fibres of this nerve are axones of a group of cells which lie deep in the central gray matter. These axones pass first dorso-later- ally to about the position of the mesencephalic root of the fifth nerve when they turn and run spinalward. At the level of the anterior 39§ THE ORGANS. medullary velum they turn dorso-mesially to form the above-men- tioned decussation, after which they pass to the surface. Transverse Section of the Midbrain through the Exit of the Third Cranial Nerve (Oculomotor) (Fig. 254). The following structures seen in the preceding section have dis- appeared : 2. Gowers' tract. 39 b. The descending motor (mesencephalic) root of the fifth nerve. 4 1 . The fourth cranial nerve. The following structures are still present : 2. The spino-tectal tract lies just dorsal to the median lemniscus. Von Monakow's bundle is crossing to its nucleus of origin, the red nucleus of the opposite side. Fig. 254.— Transverse Section of the Midbrain through the Exit of the Third Cranial Nerve. (Dejerine.) j, Pyramid; 7, reticular formation of tegmentum; 74, fillet, Flp, posterior longitudinal fasciculus ; 23, iter ; j6, lateral lemniscus : not marked, but lying just dorsal to most dorsal fillet fibres, 14 ; 40, superior cerebellar peduncle; Ln, substantia nigra; a, a, anterior corpora quadrigemina ; b, brachium of anterior corpus quadrigeminum ; Cgi, internal geniculate body ; 45, red nucleus; „r, central gray matter ; Fcop, lateral gray matter of tegmentum (superior lateral nucleus of Flechsig); xF, ventral tegmental de- cussation or decussation of Forel ; xM, dorsal tegmental decussation or decussation of Meynert ; N/J/, nucleus of origin of third cranial nerve ; ///, root fibres of third nerve. 3. The crusta; its fibres . being arranged essentially as in the preceding section. THE NERVOUS SYSTEM. 399 7. The reticular formation ; less extensive. 14. The fillet; just dorsal to the substantia r/igra. 21. The posterior longitudinal (Flp) fasciculus ; not easily dis- tinguished, but lying to the ventral and lateral side of the nucleus of the third nerve. 36. The lateral lemniscus; smaller from loss of fibres, which ended in the posterior corpus quadrigeminum. The remainder of its fibres pass upward to terminate in the anterior corpus quadrigem- inum and in the lateral geniculate body. 40. The superior cerebellar peduncles; their decussation, now completed, lie one on either side of the median line. 42. The substantia nigra {Ln), between the crusta and tegmentum. The following new structures are to be noted : 43. The anterior corpora quadrigemina (a, a). (For description see p. 400). 44. The geniculate bodies, only the medial of which {Cgi) can be seen in the section ; two masses of gray matter, the medial lying dorsal, the lateral lying dorso-lateral, to the crusta. The lateral geniculate bodies are connected with the optic tracts (see Optic Nerve, p. 430). 45. The red nucleus; a large mass of gray matter lying between the substantia nigra and the posterior longitudinal fasciculus. The relation of this nucleus to the superior peduncles of the cerebellum was described in connection with the preceding section (p. 397, 40). From cells in this nucleus axones pass upward to higher centres and downward (von Monakow's bundle) to the spinal cord. 46. The root fibres and nucleus of origin of the third cranial nerve (oculomotor). The nucleus is a well-defined group of large motor cells lying in the deepest part of the central gray matter. From this nucleus bundles of fibres may be seen passing in a curved course through the reticular formation to reach the surface just to the inner side of the crusta (HI). At this level two decussations occur. The ventral (decussation of Forel) consists of the above-mentioned crossing fibres of von Mona- kow's bundle (2, p. 398); the dorsal (decussation of Meynert) con- sists largely of fibres from the tectum to the anterior corpus quadrigem- inum, which, after crossing, descend to the medulla and possibly to the cord. 400 THE ORGANS. The Corpora Quadrigemina. — The Posterior Corpora Quadrigem- ina. — These consist mainly of gray matter and are connected with parts above and below by tracts of fibres. The fibres which ascend to termi- nate in the gray matter of the posterior corpus quadrigeminum come mainly from the lateral lemniscus (for fibres which this contains see p. 391, 36). From thecellsof the gray matter of the posterior corpus quadri- geminum some axones ascend, joining the fibres of that part of the lateral lemniscus which passes by the posterior corpus quadrigeminum. These together form the inferior brachium quadrigeminum and pass to the anterior corpus quadrigeminum and to the medial corpus geniculatum. The Anterior Corpora Quadrigemina. — These consist of both gray matter and white matter. The white matter is made up mainly of fibres of the optic tracts, axones of neurones whose cell bodies are located in the retinae. The gray matter of the anterior corpora quadrigemina serves as the terminal nuclei for these axones. It probably serves also as a terminal nucleus for some axones of the lateral lemniscus — i.e., for the secondary acoustic tract. The neurones whose cell bodies are situ- ated in the anterior corpora quadrigemina send their axones mainly downward. Their destinations are not fully known. Some appear to cross through Meynert's decussation (Fig. 254) to the opposite side, where they continue spinalward, giving off collaterals and terminals to the nuclei of the third, fourth, and sixth cranial nerves. Other axones pass downward on the same side, and probably end in the pontile nuclei, thus bringing the corpora quadrigemina into connection with the oppo- site cerebellar hemisphere. The Cerebral Peduncles {crura cerebri). — These are the direct continuation brainward of the crusta and tegmentum (see sections of midbrain), the former containing the main motor tract (p. 396, 3), the latter containing the main sensory tract. As the peduncles ap- proach the basal ganglia, the substantia nigra disappears and the tegmentum lies just dorsal to the crusta. These bundles of fibres pass through the basal ganglia between the nucleus caudatus and the optic thalamus on the mesial side, and the nucleus lenticularis on the lateral side. Here they form the internal capsule, which is directly continuous above with the corona radiata through which the fibres enter the cortex cerebri. In a horizontal section through the basal ganglia, the internal capsule is seen to present a sharp bend ox genu somewhat anterior to its mid-point. This bend divides the capsule into an anterior portion and a posterior portion. The anterior portion THE NERVOUS SYSTEM. 4©i lies between the caudate nucleus internally and the lenticular nucleus externally. This part of the capsule consists mainly of fibres which connect the cortex cerebri and the optic thalamus. The posterior portion of the internal capsule lies between the lenticular nucleus on its outer side and the optic thalamus on its inner side. About the anterior two-thirds of this portion is occupied by the fibres of the pyramidal tract (including descending fibres to the motor cranial nerve nuclei). THE CEREBELLUM. General Histology of the Cerebellar Cortex. The cerebellum consists of a central portion or core of white mat- ter which extends outward into the cortex as a series of transversely disposed branching plates. These, covered by a layer of gray mat- ter, form the lamina, which can be seen on the surface, and which on transverse section present the characteristic leaf-like appearance known as the arbor vitce. Each leaflet is seen on section to consist of (i) a central core of white matter and (2) a covering of gray matter which consists of three layers : (a) an internal or granular layer, (b) an external or molecular layer, and between these (c) a layer composed of a single row of very large cells, the layer of Purkinjc cells. (1) The iv hit e matter consists of medullated nerve fibres which pass out in a radial manner into the layers of gray matter. These fibres, while apparently alike, maybe subdivided into (a) fibres which are axones of cells situated in other parts of the nervous system — these axones are passing to their terminations in the cerebellar cor- tex ; (/>) fibres which are axones of cells situated in the cerebellum (mainly axones of cells of Purkinje) — these axones pass through the white matter of the cerebellum to terminate in some other part of the nervous system ; (c) fibres which are axones of neurones entirely confined to the cerebellum. (2) The gray matter, or cortex ccrebclli, may be subdivided into: (a) an internal, granular or nuclear layer; (b) an outer molecular layer, and between the two, (c) the layer of Purkinje cells. (a) The internal, granular, or nuclear layer appears under ordinary staining methods to be composed of a mass of small, closely packed cells, each consisting of a nucleus surrounded by a small amount 26 402 THE ORGANS. of protoplasm (Fig. 255). Intermingled with these cells are medul- lated and non-medullated nerve fibres. Studied by the method of Golgi, the nerve-cell elements of this layer can be divided into (1) small granule cells and (2 ) large granule cells. The small granule FIG. 255 —From Section through the Cerebellar Cortex, Stained with Haematoxylin-eosin. (Bohm and von Davidoff.) /, Blood-vessel ; 2, dendrite of Purkinje cell ramifying' in mo- lecular layer ; j, body of Purkinje cell at junction of molecular and granular layers; 4 and j, cells of the granular layer ; 6, layer of nerve fibres (white matter). cells (Fig. 257, c) are multipolar, their short dendritic processes rami- fying in the granular layer; their axones, which are non-medullated, passing into the molecular layer. Here each axone bifurcates, the branches running parallel to the surface and to the lamina, and terminating freely. The large granule cells (Fig. 257, d) are also multipolar. Their dendrites, however, pass outward to ramify in the molecular layer, while their axones branch rapidly and form a dense network in the granular layer. The dense plexus of nerve fibres in the granular layer is formed by the processes of the cells above de- scribed, by axones and their collaterals of Purkinje cells, and by fibres which enter the layer from the central core of white matter. Reaching the boundary between the granular layer and the molecular THE NERVOUS SYSTEM. 403 layer, many of these fibres turn and pass horizontally and in a direc- tion transverse to the long axis of the convolution. From these, branches pass vertically into the molecular layer. {b) The molecular lave?' contains larger and smaller multipolar cells. Most of the dendrites of these cells pass toward the surface. The axones run horizontally in the transverse axis of the convolution (Fig. 257, b). A few collaterals pass upward. Most of the collat- erals and terminals pass downward to end in basket-like arborizations around the bodies of the Purkinje cells. For this reason these cells of the molecular layer are often called " basket cells." There are also found in this layer cells the destination of whose axones is un- known. The fibres of this layer consist of processes of already de- scribed cerebellar cells, together with fibres which come from the Fig. 256. — Cross Section of a Cerebellar Convolution Stained by Weigert's Method. (Kolli- ker.) m, Molecular layer ; A', granular layer ; , Axone ; A\ collateral. 406 THE ORGANS. about the middle of each cerebellar hemisphere. Other smaller nuclei occur in the white matter of the middle lobe. The connections of the cerebellum with other nerve centres through its superior, middle, and inferior peduncles have been de- scribed in connection with the medulla (page 395, 40). THE CEREBRUM. General Histology of the Cerebral Cortex. Each cerebral convolution, like the convolutions of the cerebel- lum, consists of a central white core covered over by a layer of gray matter, which latter constitutes the cortex cerebri. / 'd BAB FIG. 260.— From Vertical Section of Human Cerebral Cortex. Weigert stain. X 10. De- tail drawn under a magnification of sixty diameters. (Dejerine). A, Corona radiata or central core of white matter ; Zi, gray matter of cortex ; a, superficial tangential fibres ; d, deep tangential fibres; b and r, intermediate bands of tangential fibres, b sometimes known as the outer line of Baillarger, ) through the corpus callosum to convolutions of the opposite hemi- sphere — these are also fibres of association, but are conveniently called commissural fibres ; (c) to the internal capsule as fibres of the de- scending tracts — projection fibres. (2) Ascending axones of cells situated in other parts of the nervous system, which are passing to their terminal arborizations among the cells of the cortex cerebri. These fibres are: (a) Axones of cell bodies which are situated in other convolutions of the same hemisphere — association fibres; (b) axones of cell bodies which are situated in the convolutions of the opposite hemisphere — these pass through the corpus callosum — commissural fibres ; (c) axones which have come through the internal capsule from cells situated in lower centres — projection fibres ; these axones are passing to their termi- nal arborizations in the cortex. 4io THE ORGANS. In addition to the fibres of the corona ra- diata, which form dense plexuses among the nerve cells, are bundles of fibres which traverse the gray matter at right angles to the fibres of the corona. These form more or less distinct white lines, as seen with the naked eye in the fresh cortex. The outermost of these in the molecular layer have been mentioned as the superficial tangential fibres. The deep tan- gential fibres form a second "white line" (known as the outer line of Baillarger) just out- side of the layer of large pyramids. A third white line through the layer of large pyramids, the inner line of Bail- larger, is present in the greater part of the cortex. While the above-de- scribed arrangement of cells and fibres may be considered to be in gen- eral characteristic of the cerebral cortex, much variation exists in dif- ferent regions. THE XER J 'O C 'S S 1 STEM. 4 1 1 TECHNIC. (1) The general structure of the cerebellum is well brought out by staining sections of formalin-Mullers fluid-fixed material with haematoxylin-picro-acid- fuchsin (technic 3. p. 17), and mounting in balsam. (2) The arrangement of the cell layers of both cerebellum and cerebrum, as well as certain details of internal structure of the cells, can be studied in sections of alcohol or formalin-fixed material stained by the method of Nissl (technic, p. 28). (3) The distribution of the medullated nerve fibre of either the cerebellar or cerebral cortex is best demonstrated by fixing material in Midler's fluid (technic 4, p. 5) or in formalin-AIuller 1 s fluid, and staining rather thick sections by the Wei- gert or Weigert-Pal method (technic, pp. 25 and 26). (4) The external morphology of the cerebellar and cerebral neurones and the relations of cell and fibre can be thoroughly understood only by means of sections stained by one of the Golgi methods (technic, pp. 27 and 28). Especially in the case of the cerebellum, sections should be made both at right angles, and longi- tudinal to the long axis of the convolution. Golgi preparations from embryonic material and from the brains of lower animals furnish instructive pictures. Fig. 263.— Diagram showing- the Most Important Direct Paths which an Impulse follows in passing from a Sensory Surface to the Cerebral Cortex and from the latter back to a Mus- cle ; also some of the cranial-nerve connections with the cerebral cortex. A, Sensory cortex ; B, motor cortex ; C, level of third nerve nucleus ; D, level of sixth and seventh nerve nuclei ; E, level of sensory decussation ; P. level of pyramidal decussation ; G, spi- nal cord. Fro?n Periphery to Cortex. Neurone No. /.— The Peripheral Sensor}' Neurone: i, Spinal ; cell bodies in spinal ganglia ; sensory end-organ, 6', peripheral arm of spinal ganglion cell ; central arm of spi- nal ganglion cell as fibre of dorsal root to column of Goll or of Burdach, thence to nu- cleus of one of these columns in the medulla. r" 1( Cranial (example, fifth cranial nerve, trigeminus) ; cell bodies in Gasserian ganglion ; sensory end organ ; peripheral arm of Gasserian ganglion cell ; central arm of Gasserian ganglion cell to medulla as sensory root of fifth nerve, thence to terminal nuclei in medulla. Neurone No. 2.-2, Spinal connection— ce\\ body in nucleus of Goll or of Burdach ; ax- one passing as fibre of fillet to thalamus. Vz, cranial nerve connection (trigeminal), cell body in one of trigeminal nuclei in medulla, axone as fibre of secondary trigeminal tract to thalamus. A'euroue No. j. — j, Cell bod}' in thalamus, axone passing through internal capsule to termination in cortex. Prom Cortex to Periphery. Neurone No. 4. — 4, Cell body in motor cerebral cortex ; axone through internal cap- sule and crusta to O) motor nuclei of cranial nerves, (£) by means of pyramidal tracts to ventral horns of spinal cord. Neurone No. 5.— 5, Spinal, Cell body in ventral horn of cord ; axone as motor fibre of ventral root through mixed spinal nerve to muscle. Neurone No. 5. — Cranial — V 6 , Cell body in motor nucleus of trigeminus; axone pass- ing to muscle as motor fibre of fifth nerve. II 1^, Peripheral motor neurone of third nerve — oculomotor. f'/ 5 . Peripheral motor neurone of sixth nerve — abducens. V1I§, Peripheral motor neurone of seventh nerve^ facial. Nil.,, Peripheral motor neurone of twelfth nerve— hypoglossal. 1i2 THE ORGANS. The Pituitary Body. The pituitary body or hypophysis cerebri consists of two lobes which are totally different both in structure and in origin. The Anterior Lobe. — This is the larger, and is glandular in character. It is of ectodermic origin, developing as a diverticulum from the primitive oral cavity. Its mode of development is that of a compound tubular gland, the single primary diverticulum undergoing repeated division to form the terminal tubules. The original diver- ticulum ultimately atrophies and disappears, leaving the gland en- tirely unconnected with the surface. The gland is enclosed in a connective-tissue capsule, from which trabeculse pass into the organ forming its framework. The gland cells are arranged in slightly convoluted tubules and rest upon a basement membrane. Between the tubules is a vascular connective tissue. Some of the gland cells are small cuboidal cells with nuclei at their bases and a finely granular basophile protoplasm {chief cells). Others, somewhat less numerous than the preceding, are larger polygonal cells with centrally placed nuclei and protoplasm containing coarse acidophile (eosinophile) gran- ules (chromophile cells). While presenting different appearances and usually described as two kinds of cells, it is probable that chromophile cells and chief cells represent merely different functional conditions of the same cell. Some alveoli in the posterior portion of the lobe fre- quently contain a colloid substance similar to that found in the thyroid. As in all ductless glands, the blood supply is rich and the rela- tions of capillaries to gland cells are extremely intimate, dense net- works of capillaries surrounding the alveoli on all sides. The Posterior Lobe. — This, like the anterior, is surrounded by a connective-tissue capsule which sends trabecular into its substance. In the human adult the lobe consists mainly of neuroglia with a few scattered cells, which probably represent rudimentary ganglion cells. In the human embryo and in many adult lower animals, the nervous elements are much more prominent and more definitely arranged. Thus Berkley describes the posterior lobe of the pituitary body of the dog as consisting of three distinct zones : (i) An outer zone of three or four layers of cells resembling ependymal cells. Connective-tissue septa from the capsule separate the cells into irregular groups. (2) A middle zone of glandular epithelium, some of the cells of which are arranged as rather indefinite alveoli which Nerve. k L Nuclei (Secondary ieir Distribution. Tertiary Tracts. I. Olfactorius tc _ t0 H m bic lobe. II. Opticus j s et spinalis, etc., to o-spinal nerves (reflex). . iccipital cortex cerebri. III. Oculomotornccipital cortex cerebri. IV. Trochlearis . V. Trigeminus, (a) sensory . . . (b) motor. al part formatio reticu- liscus to thalamus Thalamic radiations to central cortex cerebri. Thalamic radiations to central cortex cerebri VI. Abducens. . . VII. Facialis. (a) motor (£>) sensory . . . . VIII. Acusticus. (a) vestibularis . I i ong . fasc to midbrain VI, IV, and III (reflex;, st. long. fasc. to vent. jr spinal nerves (reflex), liar formation to vent, tor spinal nerves (reflex) 1 to ventro-lateral cols. al nerves (reflex) (from pncephalic to midbrain VI, IV, and III (reflex) w's). spinalis (reflex), collate- ] Nucl. lemniscus •. sup I lat. (reflex), jrming arming I Corp. quadrige- ; late- ! minum infe- rius (reflex). __, _, . ig and Corp. genicula- IX. Glossopharyng, i ate . | tum i nte r- (a) sensor>- j num . (b) motor. . . . X. Pneumogastria (a) sensory . . (b) cochlearis (b) motor. XI. Accessorius (a) medullary port (b) spinal portion XII. Hypoglossus. Spinales. (a) motor. (b) senson r Thalamic radiations to tem- poral cortex cerebri. lerves (reflex). laris dorsalis (see peri- bral pathway, Fig. 264). I aris ventralis (see peri- bral pathway, Fig. 264). I 1 to thalamus Thalamic radiations to central cortex cerebri. TABLE OF CRANIAL AND SPINAL NERVES. I. Olfactorius. II. Opticus. . . III. Oculomotorius. IV. Trochlearis . V. Trigeminus, (a) sensory. . . (6) motor VI. Abducens. . VII. Facialis. (a) motor (6) sensory . . . . VIII. Acusticus. (a) vestibularis (b) cochlearis . Cells of Origin. Olfactory epithelial cells. Ganglion cells of retina . . Nuc.eus nerve III in midbrain. Nucleus nerve IV in isthmus. . Ganglion Gasseri Distribution of Peripheral Axones. Motor nucleus V, and mesen- cephalic nucleus. Nucleus VI in medulla Nucleus VII in medulla Gang, geniculi Gang, of Scarpa Gang . spiralis . IX. Glossopharyngeus (a) sensory (b) motor X. Pneumogastricus . (a) sensory (b) motor XI. Accessorius. (a) medullary portion (b) spinal portion. . . . XII. Hypoglossus Spinales. (a) motor (b) sensory Gang, jugulare sup. and gang, petrosum. Motor nucleus — ant. continua- tion nucl. ambig. Gang, iugulare and gang, plex- iform. or nodosum. (a) nucl. dorsalis (alae cineren?) in medulla. (6) nucl. ventralis (ambiguus) in medulla. = Xa. Anterior horn cells, ist to 5th or 7th cerv. segments. Nucl. XII in medulla Anterior horn cells of cord Gang, spinales eye muse, levator palpebral sup., and radix brevis gang, ciliaris. Mus. obliquus superius. Rami ophthal., max. and mandib. to skin of head and face, epi- thelium of mouth and nose, teeth, meninges. Muscles of jaw, etc. (massetericus, temporalis, pterygoideus digas- tricus, tens, veli pal., tens, tym- pani, mylohyoideus). Muse, rectus lateralis. Muscles of face, etc. Ant. part of tongue Ampulla? semicirc. canals, utricu- lus, sacculus. Organ of Corti Post, part of tongue (taste), muc. memb. of pharynx, etc. Muscles of pharynx. Mucous membranes pharynx, oesophagus, lungs, heart, and other viscera. Muscles of larynx. Smooth muscles of viscera. Muse, sternocleidomastoideus and trapezius. Muscles of tongue. Muscles. Periphery of body. Central Axones (Root -Fi- bres and Central Tracts). Fila olf actoria Nervus and (crossed) tractus opticus. Tractus spinalis V Fasc. solitarius Tractus vestib. spinalis. Fasc. solitarius. Fasc. solitarius. Posterior root collaterals. Posterior root collaterals. Posterior root, posterior col- umns of cord. Terminal Nuclei. Cells in bulbus olfactorius. . . . Corpus quadrigeminum ante- rius. Corpus geniculatum externum Pulvinar thalami Sensory nucleus V and nucleus tractus spinalis (substan. gelat. Rolando). Nucleus fasciculi solitarii. Nucl. tract, vestib. spinalis. Nucleus medialis (' ' princi- pal") Nucleus lateralis (Deiter's). Nucleus superior (von Bech- terew's). Nucleus fastigii cerebelli. Nucl. accessorius Nucl. oliv. sup. (reflex). Nucl. corporis trapezoidei Tuberculum acusticum. Nucleus fasc. solitarii. Nucl. fasciculi solitarii. Motor spinal nuclei (reflex). Tautomeric and heteromeric cells of cord. Column of Clarke Tautomeric and heteromeric cells of cord. Nucl. columns Goll and Bur- dach. Axones of Terminal Nuclei (Secondary Tracts) and Their Distribution. Tertiary Tracts. Tractus olfactorius, etc., to limbic lobe. Tractus tecto-bulbaris et spinalis, etc., to motor nuclei cerebro-spinal nerves (reflex) Optic radiations to occipital cortex cerebri Optic radiations to occipital cortex cerebri To motor nuclei (reflex). Crossed tract in dorsal part formatio reticu- laris to thalamus. Crossed tract via lemniscus to thalamus To cerebellum? f Crossed asc. in post. long, fasc to midbrain and nucl. nerves VI, IV, and III (reflex). Crossed desc. in post. long. fasc. to vent, col. cord and motor spinal nerves (reflex). Uncrossed in reticular formation to vent. t col. cord, and motor spinal nerves (reflex) Uncrossed vest. -spinal to ventro-latera! cols. cord and motor spinal nerves (reflex) (from Deiter's). Uncrossed vest. -mesencephalic to midbrain and nuclei nerves VI, IV, and III (reflex) (from von Bechterew's). Cerebello-bulbaris et spinalis (reflex). Corpus trapezoid, (collate- 1 Nucl. lemniscus rals to nucl. oliv. sup. lat. (reflex). [reflex]), crossing, forming lemniscus lateralis. Corpus trapezoid., forming i Corp. quadrige- opposite lemniscus late- f minum infe- ralis. I rius (reflex). Striae acusticse crossing and Corp. genicula- forming lemniscus late- 1 turn inter- ralis. J num. Motor nuclei spinal nerves (reflex). Tractus spino-cerebellaris dorsalis (see peri- phero-cerebello-cerebral pathway, Fig. 264). Tractus spino-cerebellaris ventralis (see peri- phero-cerebello-cerebral pathway, Fig. 264). Lemniscus principalis to thalamus Thalamic radiations to central cortex cerebri. Thalamic radiations to central cortex cerebri Thalamic radiations to tem- poral cortex cerebri. s a •Jl c £ fcfl o s a O d (LI bo > u ci ft o Si O s G C4 W rrt 4 a, « g « « » « t s » - * S £ ^ '•US - c CO O 0) > u ■a 0/ CD 6 J2 cS ft cu a) a ft o "3 _ 5 a! S o is i> be 3 5 p o .3 3 .O co d '> CO o 3 C 3 iS cd a O > - cS O 1) 3 o flj 5^ cd 5 S -o | ?' & g oj o ~ £ c C to °i .. CO d r- O - .5 •O 3 n P « O c ■ O ° 2 C O CD d 3 O .3 rt p O •- -C w * -~ cS > = 3 jj; 3 u > 5 o > « '5, J2 v. 3 " — - ~ X -i 3 3 CD > a CS r - !S O ,3 'E CO 3 3 1) CO 3 a) ft is o CO 3 S o 3 x; •a cu CS 6 CS si O si P o 9 CS a> o CO as 5 | :j _B >, CS J3 3 1. ^ 3 .a f 3 'Z cS 3 O o *j ? ^2 o THE ORGANS OE SPECIAL SENSE. 41/ (2) Anterior elastic membrane or membrane of Bowman. (3) Substantia propria cornese. (4) Posterior elastic membrane or membrane of Descemet. (5) Posterior endothelium or endothelium of Descemet. (1) The anterior epithelium (Fig. 267, /) is of the stratified squamous type and consists of from four to eight layers of cells. The deepest cells are columnar and rest upon the anterior elastic mem- brane. The middle cells are polygonal and are connected by short Fig. 265.— Diagram of Eyeball showing Coats. (Merkel-Henle.) a, Sclera ; b, chorioid ; c, retina ; d, cornea; e, lens ;/, iris; g; conjunctiva ; k, ciliary body ; /", sclero-corneal junc- tion and canal of Schlemm ; j\ fovea centralis ; £, optic nerve. intercellular bridges. The surface cells are flat. Along the margin of the cornea the epithelium is continuous with that of the conjunc- tiva (Fig. 270). (2) The anterior clastie membrane (Fig. 267, 2) is a highly developed basement membrane, its anterior surface being pitted to receive the bases of the deepest epithelial cells. It is apparently homogeneous, and while called an elastic membrane, does not con- 27 4i8 THE ORGANS. form chemically to either fibrous or elastic tissue. By means of special technic, a fibrillar structure has been demonstrated. (3) The substantia propria (Fig. 267, j>) constitutes the main bulk of the cornea. It consists of connective tissue the fibrils of which are doubly refracting and are cemented together to form bun- dles and lamellae. In the human cornea the lamellae are about sixty FIG. 266.— Vertical Section through Sclera, Chorioid, and Pigment Layer of Retina. (Merkel- Henle.) A, Sclera; £, chorioid; C, pigment layer of retina ; d, lamina suprachorioidea ; e. Haller's layer of straight vessels; /, choriocapillaris ; g; vitreous membrane. in number. The lamellae are parallel to one another and to the sur- face of the cornea, but the fibres of adjacent lamellae cross one another at an angle of about twelve degrees. The lamellae are united by cement substance. Fibres running obliquely through the lamellae from posterior to anterior elastic membranes hold the la- mellae firmly together. They are known as perforating or arcuate fibres. Between the lamellae are irregular flat cell spaces which commu- nicate with one another and with the lymph spaces at the margin of :he cornea by means of canaliculi. Seen in sections vertical to the surface of the cornea, these spaces appear fusiform. In the spaces are the connective-tissue cells of the cornea or corneal cor- puscles. These are flat cells corresponding in shape to the spaces and sending out processes into the canaliculi (Figs. 268 and 269). (4) The posterior elastic membrane or membrane of Descemet (Fig. 267, 4) resembles the anterior, but is much thinner. Like the anterior, it does not give the chemical reaction of elastic tissue. (5) The posterior endothelium or endothelium of Descemet (Fig. 267, 5) consists of a single layer of flat hexagonal cells, the nuclei of which frequently project slightly above the surface. The cornea contains no blood-vessels. THE ORGANS OE SPECIAL SENSE. 419 The Chorioid. — This is made up of four layers which from without inward are as follows (Fig. 266) : (1) The lamina suprachorioidea. (2) The layer of straight vessels — Haller's layer. (3) The capillary layer — choriocapillaris. (4) The vitreous membrane — lamina citrea — membrane of Bruch. (1) The lamina suprachorioidea (Fig. 266, d) is intimately con- nected with the lamina fusca of the sclera and consists of loosely arranged bundles of fibrous and elastic tissue among which are scat- tered pigmented and non-pigmented connective-tissue cells. Numer- ous lymph spaces are found between the bundles of connective tissue and between the lamina suprachorioidea and lamina fusca. The latter are known as the perichoroidal lymph spaces (Fig. 270). (2) The layer of straight vessels (Fig. 266, e) consists of fibro-elastic tissue containing numerous pigmented and non - pigmented cells, support- ing the large blood-vessels of. the layer. The latter can be seen with the naked eye, and, running parallel straight courses, give to the layer a striated appearance. The arteries lie to the inner side. The veins which are larger than the arteries converge toward four points — vence vorticose? — one in each quadrant of the eyeball. A narrow boundary zone, rich in elastic fibres and free from pigment, limits this layer internally. It is much more highly developed in some of the lower animals than in man. Formed of connective-tissue bundles in ruminants and horses, it is known as the tapetum fibrosa m, while in the carnivora its structure — several layers of flat cells — gives it the name of the tapetum ccllulosum. (3) The choriocapillaris (Fig. 266, f) consists of connective tissue supporting a dense network of capillaries, which is most dense Fig. 267. — Vertical Section of Cornea. (Merkel-Henle.) /, Anterior epithe- lium ; 2, anterior elastic membrane ; S, substantia propria cornea? ; ./, posterior elastic membrane ; j. pos- terior endothelium. 42C THE ORGANS. in the region of the macula lutea. This layer is usually described as free from pigment, although it not infrequently contains some pig- mented cells. (4) The vitreous membrane (Fig. 266, g) is a clear, apparently m Fig. 268. — Section of Human Cornea cut Tangential to Surface— X 350 (technic 9, p. 75) — showing corneal cell spaces (lacuna;) and anastomosing canaliculi. structureless membrane about two microns thick. Its outer surface is grooved by the capillaries of the choriocapillaris, while its inner surface is pitted by the retinal epithelium. x I ^iijjfEi^Jjit^ ^-T'V S / /"■"' . . -"■'■ ... .'' t "I i \ \ ■ i \ V-'C •.■r;:"..--;>o„J.i. j v.„ --•4... « f'A — Fig. 269. — Section of Human Cornea cut Tangential to Surface— X 350 (technic 8, p. 75) — showing corneal cells and their anastomosing processes. The Ciliary Body. — This is the anterior extension of the cho- riridand consists of the ciliary processes and the ciliary muscle (Fig. THE ORGANS OF SPECIAL SENSE. 421 270.). It extends from the ora serrata (a wavy edge which marks the anterior limit of the nervous elements of the retina — see Retina) to the margin of the iris (see below). The ciliary processes (Fig. 270), from seventy to eighty in num- ber, are meridionally-running folds of the chorioid from which are given off numerous irregular secondary folds. The processes begin low at the ora serrata, gradually increase in height to about 1 mm., Jff Cornea — nr.n-n! Anterior chamber Iris Pars iridica retime Ciliary process 3mm _lg*>™ £A\Y - ° / ' ,r ^* K $\ Ligamentum •Spaces of Fontana Conjunctiva pectinatum iridis Circular fibres of ciliary muscle RaJial fibres of ciliary muscle Pars ciliaris reticse Perlchorioidal lymph space VfcVw\:\'M o Zonule of Zinn Fig. 270. — Vertical Section through Human Sclero-corneal Junction. (Cunningham.) and end abruptly at the margin of the iris. The ciliary processes consist of connective tissue containing many pigmented cells and supporting numerous blood-vessels. Invaginations lined with clear columnar epithelium have been described as ciliary glands. The ciliary folds are covered by the vitreous membrane, and internal to the latter is a continuation forward of non-nervous elements of the retina — pars ciliaris retina (Fig. 270). This consists of two layers of columnar epithelial cells, the outer layer being pigmented, the inner non-pigmented. 422 THE ORGANS. k-J ', The ciliary muscle (Fig. 270) is a band of smooth muscle which encircles the iris. It lies in the outer anterior part of the ciliary- body, and on cross section has a generally triangular shape. It is divisible into three groups of muscle cells : [a) An inner circular group near the base of the iris — circular muscle of Miiller; (b) an outer meridional group lying next to the sclera and known as the tensor chorioideae, and (c) a middle radial group. The meridional and radial groups both take origin in the posterior elastic lamina of the cornea, the former passing backward along the margin of the sclera to its insertion in the ciliary body near the ora serrata, the latter radiating fan-like to a broad insertion in the ciliary body and processes. The ciliary body is closely attached to the sclero-corneal junction by the ligamentinn pectinatum (Fig. 270), a continuation of the pos- terior elastic lamina of the cornea. Within the ligament are spaces {spaces of Font ana) lined with en- dothelium. These are apparently lymph spaces, and communicate with each other, with similar spaces around the canal of Schlemm, and with the anterior chamber. The canal of Schlemm (Fig. 270) is a venous canal which encircles the cornea, lying in the sclera close to the corneal margin. Instead of a single canal there may be several canals. The Iris (Fig. 271). — This rep- resents a further continuation for- ward of the chorioid. Its base is attached to the ciliary body and ligamentum pectinatum. From this point it extends forward as a dia- phragm in front of the lens, its centre being perforated to form the pupillary opening. It is deeply pigmented, and to its pigment the color of the eye is due. Four layers may be distinguished, which from before backward are as follows (ij The anterior endothelium. \T =5=^ b ok^ Fig. 271. — Vertical Section through Iris. (Merkel-Henle.) a, Anterior endothelium ; b, stroma or substantia propria; c, vitreous membrane; d, pigment layer ; v, blood-vessel. THE ORGANS OF SPECIAL SENSE. (2) The stroma. (3) The vitreous membrane. (4) The pigmented epithelium. (1) The anterior endothelium is a single layer of pigmented cells continuous with the posterior endothelium of the cornea (Fig. 271, a). (2) The stroma is divisible into two layers : an anterior reticular layer, containing many cells, some of which are pigmented, and a A B FiG. 272.—^, Scheme of retina as shown by the Golgi method. B, Vertical section of retina to show layers as demonstrated by the hasmatoxylin-eosin stain. (Merkel-Henle.) B.—/, Layer of pigmented epithelium ; 2, layer of rods and cones; 3, outer limiting layer; 4, outer nuclear layer ; j, outer molecular layer, 6, inner nuclear layer ; 7, inner molecular layer; £, layer of nerve cells; 9, layer of nerve fibres; /o, inner limiting layer. A.— j, Pigment layer ; 2, processes of pigmented epithelial cells extending down between rods and cones; j, rods; 4, red-ceil nuclei and rod fibres; J, cones; 6, cone fibres; 7, bipolar cells of inner nuclear layer ; 8, ganglion cells of nerve-cell layer ; 9, larger ganglion cells of nerve-cell layer ; jo, fibres of optic nerve forming layer of nerve fibres ; // and /2, types of horizontal cells; /j, 74, ij, and /6, types of cells the bodies of which lie in the inner nuclear layer; /-, efferent optic-nerve fibre ending around cell of inner nuclear layer; /S, neuroglia cells ; jg, Muller's fibre ; 20, rod-bipolar cell of inner nuclear layer. vascular layer, the vessels of which are peculiar in that their walls contain almost no muscle, but have thick connective-tissue sheaths. In the posterior part of the stroma are bundles of smooth muscle. Those nearest the pupillary margin encircle the pupil forming its sphincter muscle, while external are scattered radiating bundles forming the dilator muscle. +24 THE ORGANS. (3) The vitreous membrane is continuous with, and has the same structure as the membrane of Bruch. (4) The pigmented epithelium (Fig. 271, d) consists of several layers of cells and is continuous with the pars ciliaris retinae. Ex- cept in albinos, both layers are pigmented. The Retina. — The retina is the nervous tunic of the eye. It lines the entire eyeball, ending only at the pupillary margin of the iris. Its nervous elements, however, extend only to the ora scrrata, which marks the outer limit of the ciliary body (Fig. 270). The nervous part of the retina is known as the pars optica retina, the non-nervous extension over the ciliary processes as the pars ciliaris retince, its further continuation over the iris as the pars iridica retince. Modifications of the optic portion of the retina are found in the region of the macula lutea and of the optic nerve entrance. The Pars Optica Retince. — This is the only part of the retina directly concerned in the reception of impulses, and may be re- garded as the extremely complex sensory end-organ of the optic nerve. It is divisible into ten layers, which from without inward are as follows (Fig. 272) : (1 ) Layer of pigmented epithelium. (2) Layer of rods and cones. (3) Outer limiting membrane. ^ Layer of neuro-epithelium. (4) Outer nuclear layer. (5) Outer molecular layer. (6) Inner nuclear layer. (7) Inner molecular layer. (8) Layer of nerve cells. (9) Layer of nerve fibres. (10) Inner limiting membrane. The layer of pigmented epithelium (Fig. 272, B, 1) consists of a single layer of regular hexagonal cells (Fig. 20, p. 59). The nuclei lie in the outer part of the cell, while from the inner side thread-like projections extend down between the rods and cones of the layer next internal. The pigment has the form of rod-shaped granules. Its distribution seems to depend upon the amount of light being admitted to the retina. When little or no light is being admitted, the pigment is found in the body of the cell, the processes being wholly or almost free from pigment; when the retina is ex- posed to a bright light, some of the pigment granules pass down Ganglionic layer. THE ORGANS OF SPECIAL SENSE. 425 into the processes so that the pigment becomes more evenly distrib- uted throughout the cell. The layer of rods and cones and the outer nuclear layer (Fig. 272, B, 2, 4) are best considered as subdivisions of a single layer, the neuro-epithelial layer. This consists essentially of two forms of neuro-epithelial elements, rod visual cells and cone visual cells. These, with supporting connective tissue, constitute the layer of rods and cones and the outer nuclear layer, the separation into sub- layers being due to the sharp demarcation between the nucleated and non-nucleated parts of the cells and the separation of the two parts by the perforated outer limiting membrane. The rod visual cell (Fig. 272, A, f) consists of rod, rod-fibre, and nucleus. The rod (Fig. 272, A, j) is a cylinder from 30 to 40 ,u in length and about 2 ,a in diameter. It is divisible into an outer clear portion, which contains the so-called "visual purple" and an inner granular portion. At the outer end of the latter is a fibrillated ellipsoidal body, much more distinct in some of the lower animals, the ellipsoid of Krause. At its inner end the rod tapers down to a fine fibre, the rod fibre, which passes through a perforation in the outer limiting membrane into the outer nuclear layer, where it ex- pands and contains the nucleus of the rod visual cell. These nuclei are situated at various levels in the fibre and constitute the most conspicuous element of the outer nuclear layer (Fig. 272, B, f). The cone visual cell (Fig. 272, A, 5, 6) consists of cone, cone- fibre, and nucleus. The cone (Fig. 272, A, j) is shorter and broader than the rod, and like the latter is divisible into two parts. The outer part is short, clear, and tapering, the inner part broad and granular, and like the rod contains a fibrillated ellipsoid body. The cone fibre (Fig. 272, A, 6) is much broader than the rod fibre, passes completely through the outer nuclear layer and ends in an expansion at the mar- gin of the outer molecular layer. The nucleus of the cone cell usu- ally lies just beneath the outer limiting membrane. The remaining layers of the retina must be considered in relation on the one hand to the neuro-epithelium, on the other to the optic nerve. The inner nuclear layer (Fig. 272, B, 6) and the layer of nerve cells (Fig. 272, B, 8) are composed largely of nerve-cell bodies, while the two molecular layers (Fig. 272, B, 5, 7) are formed mainly of the ramifications of the processes of these cells. In the inner nuclear layer are two kinds of nerve elements, rod bipolar cells and 426 THE ORGANS. cone bipolar cells. The bodies of these cells with their large nuclei form the bulk of this layer. From the rod bipolars (Fig. 272, A, 26) processes {dendrites) pass outward to ramify in the outer molecular layer around the terminations of the rod fibres. From the cone bi- polars (Fig. 272, A, 7) similar processes (dendrites} extend into the outer molecular layer where they ramify around the termination of the cone cells. Two other forms of nerve cells occur in the inner nuclear layer. One is known as the horizontal cell (Fig. 272, A, 12). Its processes ramify almost wholly in the outer molecular layer. The other lies along the inner margin of the inner nuclear layer and sends its dendrites into the inner molecular layer (Fig. 272, A, ij, 14, 1 j, and 16). Many of these latter cells appear to have no axone and are consequently known as amacrine cells. The outer molecular layer is thus seen to be formed mainly of terminations of the rod and cone visual cells, of the dendrites of the rod and cone bipolars, and of the processes of the horizontal cells. From the cone bipolar a process {axone) extends inward to ramify in the inner molecular layer, while from the rod bipolar a process {axone} passes inward through the inner molecular layer to terminate around the cells of the nerve-cell layer. The layer of nerve cells (Fig. 272, B, 8) consists for the most part of large ganglion cells whose dendrites ramify in the inner mo- lecular layer, and whose axones pass into the layer of nerve fibres and thence into the optic nerve. Some small ganglion cells are also found in this layer, especially in the region of the macula lutea (see page 427). The inner molecular layer is thus seen to be composed mainly of the processes {axones) of the rod and cone bipolars and of the dendrites of the ganglion cells of the nerve-cell layer. The layer of nerve fibres (Fig. 272, B, 9) consists mainly of the axones of the just-described ganglion cells, although a few centrifu- gal axones of brain cells (Fig. 272, A, ij) are probably intermingled. The outer and inner limiting layers or membranes (Fig. 272, B, j, 10) are parts of the sustentacular apparatus of the retina, being connected with the cells or fibres of Miiller (Fig. 272, A, ip and Fig. 273). The latter form the most conspicuous elements of the supportive tissue of the retina. They are like the nerve elements proper, of ectodermic origin and are elongated cells which extend through all the retinal layers, excepting the layer of rods and cones THE ORGANS OF SPECIAL SENSE. 427 and the pigment layer. The inner ends of the cells, which are coni- cal and fibrillated, unite to form the inner limiting membrane (Fig. 273, Id). Through the inner molecular layer the cell takes the form of a narrow stalk with numerous fringe-like side fibrils (Fig. 273, 7). This widens in the inner nuclear layer, where cup-like depressions in the sides of the Midler's cell are caused by the pressure of the sur- rounding nerve cells (Fig. 273, b). This wide portion of the cell in the inner nuclear layer contains the nucleus (Fig. 273, a). In the outer molecular layer the cell again becomes narrow (Fig. 273, 5) and in the outer nuclear layer broadens out into a sponge-like reticulum (Fig. 273, ./), which supports the rod and cone bipolars. At the inner margin of the layer of rods and cones the protoplasm of the Midler's cells spreads out and unites to form the so- called outer limiting membrane (Fig. 273, ?), from which delicate fibrils {fibre baskets) pass outward between the rods and cones. In addition to the Midler's cells, which are neuroglia elements, spider cells also occur in the retina (Fig. 272, A, 18). The retina of the macula lutca presents certain peculiarities. Its name is derived 1 from the yellow pigment which is distrib- FlG - »73--Two Mailer's Fibres J ± ° from Retina of Ox showing uted diffusely through the inner layers, ex- tending as far out as the outer molecular layer. The ganglion-cell layer and the inner nuclear layer are thicker than in other parts of the retina. In the layer of rods and cones there is a gradual reduction in the number of rods, while the number of cones is correspondingly increased. In the centre of the macula is a depression, the fovea centralis. As the retina approaches this area it becomes greatly thinned, little remaining but the layer of cone cells and the somewhat thickened layer of pigmented epithelium. At the ora serrata the nervous elements of the retina cease. The Relation to Layers of Retina. (Ramon y Cajal.) j, Outer limiting- layer ; 4, outer nu- clear layer \j, outer molecular layer ; 6, inner nuclear layer ; 7, inner molecular layer ; ,?, layer of nerve celis ; 9, layer of nerve fibres; /o, inner limiting layer; a, nucleus; f>, cup-like depression caused by pressure from surrounding cells. 428 THE ORGANS. £ f non-nervous retinal extension over the ciliary body {pais ciliaris re- tina:) and over the iris {pars iridica retina) have been described in connection with the ciliary body and iris. The Optic Nerve. — The optic nerve (Fig. 274, d) is enclosed by two connective-tissue sheaths, both of which are extensions of the brain membranes. The outer dural sheath (Fig. 274, a) is continuous with the dura mater of the brain pos- teriorly, while anteriorly it blends with the sclera. The inner pial sheath (Fig. 274, /;) is an extension of the pia mater and is separated from the outer sheath by the sub- dural space (Fig. 274, c). The pial sheath is divisible into two sub- layers : an outer fibrous layer (the so- called arachnoid), and an inner vascular layer. These two layers are separated by a narrow space, the subar- achnoid space. The optic nerve fibres, in passing through the sclera and cho- rioid, separate the connec- tive-tissue bundles so that they form a lattice-work, the already mentioned lamina cribrosa (Fig. 274, h). The optic nerve fibres are medullated, but have no neurilemma. As they pass through the lamina cribrosa the medullary sheaths are lost, the fibres reaching the retina as naked axones. Relations of Optic Nerve to Retina and Brain. The rod and cone visual cells are the neuro-epithelial beginnings of the visual tract (Fig. 272, A, j, ./, 5, and 6). By their expanded bases in the outer molecular layer, the rod and cone cells comrau- P'lG. 274. — Section through Entrance of Optic Nerve into Eyeball. CMerkel-Henle.) a, Dural sheath; b, pial sheath, inner and outer layers; c, space be- tween inner and outer layers of pia mater ; , membrane of Reissner ; d, membrana tectoria or membrane of Corti ; /", spiral prominence;^, organ of Corti ; //, spiral ligament; i, basilar membrane (outer portion— zona pectinata— covered by cells of Claudius); z, stria vascularis; v, external spiral sulcus; r, crista basilaris ; s, membranous spiral lamina ; .r, bony spiral lamina; m, spiral limbus ; w, internal spiral sulcus ; o, medullated peripheral processes (dendrites) of cells of spiral ganglion passing to the organ of Corti; /, spiral ganglion; q. blood- vessel. bony cochlear canal and following its spiral turns from the vestibule, where it is connected with the saccule through the canalis reuniens, 444 THE ORGANS. to its blind ending in the cupola. It is triangular in shape on trans- verse section, thus allowing a division of its walls into upper, outer,, and lower (Fig. 283, Dc). The upper or vestibular wall is formed by the thin membrane of Reissuer (Fig. 283, b) which separates the cochlear duct from the scala vestibuli. The membrane consists of a thin central lamina of connective tissue covered on its vestibular side by the vestibular en- dothelium, on its cochlear side by the epithelium of the cochlea. The outer wall of the cochlear duct is formed by the spiral liga- ment, which is a thickening of the periosteum. The outer part of the spiral ligament consists of dense fibrous tissue, its projecting part of more loosely arranged tissue. From it, two folds project slightly into the duct. One, the crista basilaris (Fig. 283, r), serves for the attachment of the membranous spiral lamina; the other, the spiral prominence (Fig. 283, f), contains several small veins. Be- tween the two projections is a depression, the external spiral sulcus (Fig. 283, v). That part of the spiral ligament between the spiral prominence and the attachment of Reissner's membrane is known as the stria vascularis (Fig. 283, ,er). It is lined with granular cuboidal epithelial cells, which, owing to the absence of a basement mem- brane, are not sharply separated from the underlying connective tis- sue. For this reason the capillaries extend somewhat between the epithelial cells, giving the unusual appearance of a vascular epithe- lium. The lower or tympanic wall of the cochlear duct has an extremely complex structure. Its base is formed by the already mentioned bony and membranous division wall between the scala media and the scala. tympani (bony spiral lamina and membranous spiral lamina). The bony spiral lamina has been described (page 442). The membranous spiral lamina consists of a substantia propria or basilar membrane, its tympanic covering, and its cochlear covering. The basilar membrane (Fig. 283) is a connective-tissue mem- brane composed of fine straight fibres which extend from the bony spiral lamina to the spiral ligament. Among the fibres are a few connective-tissue cells. On either side of the fibre layer is a thin, apparently structureless membrane. The tympanic covering of the basilar membrane consists of a thin layer of connective tissue — an extension of the periosteum of the spiral lamina — covered over by a single layer of flat endothelial cells. THE ORGANS OF SPECIAL SENSE. 445 The cochlear covering of the basilar membrane is epithelial. Owing to the marked difference in the character of the epithelium, the basilar membrane is divided into an outer portion, the zona pectinata (Fig. 283, i) and an inner portion, the zona tecta (Fig. 283, s). The epithelium of the former is of the ordinary columnar type; that of the latter is the highly differentiated neuro-epithelium of Corti's organ. The Organ of Corti. — The spiral organ or the organ of Corti (Fig. 283, g, and Fig. 284) is a neuro-epithelial structure running the entire length of the cochlear canal with the exception of a short Umbos membrana tectoria outer hair-cdls nervejibres inner rod vas basilar outer cells of Deiters stpiialc membrane rod Fig. 284. — Semidiagrammatic Representation of the Organ of Corti and Adjacent Structures. (Merkel-Henle.) a. Cells of Hensen ; b, cells of Claudius; c, internal spiral sulcus; x, Nuel's space. The nerve fibres (dendrites of cells of the spinal ganglion) are seen pass- ing to Corti's organ through openings (foramina nervosa) in the bony spiral lamina. The black dots represent longitudinally-running branches, one bundle lying to the inner side of the inner pillar, a second just to the outer side of the inner pillar within Corti's tunnel, the third beneath the outer hair cells. distance at either end. It rests upon the membranous portion of the spiral lamina, and consists of a complex arrangement of four different kinds of epithelial cells. These are known as: (1) pillar cells, (2) hair cells, (3) Deiter's cells, and (4) Hensen's cells (Fig. 284). (1) The pillar cells are divided into outer pillar eel/sand inner pillar cells. They are sustentacular in character. Each cell con- sists of a broad curved protoplasmic base which contains the nucleus, and of a long-drawn-out shaft or pillar which probably represents a highly specialized cuticular formation. The end of the pillar away from the base is known as the head. The head of the outer pillar 446 THE ORGANS. presents a convexity on its inner side, which fits into a corresponding concavity on the head of the inner pillar, the heads of opposite pillars thus "articulating" with each other. From their articulation the pillars diverge, so that their bases which rest upon the basilar mem- brane are widely separated. There are thus formed by the pillars a series of arches known as Corti 1 s arches, enclosing a triangular canal, Corti's tunnel. This canal is filled with a gelatinous substance and crossed by delicate nerve fibrils. As the outer pillar cells are the larger, they are fewer in number, the estimated number in the human cochlea being about forty-five hundred of the outer cells and about six thousand of the inner cells. (2) The hair cells or auditory cells lie on either side of the arches of Corti, and are thus divided into inner hair cells and outer hair cells. Both inner and outer hair cells are short, cylindrical elements which do not extend to the basilar membrane. Each cell ends below in a point, while from its free surface are given off a number of fine stiff hairs. The inner hair cells lie in a single layer against the inner side of the inner pillar cells, one hair cell resting upon about every two pil- lars. The outer hair cells lie in three or four layers to the outer side of the outer pillar cells, being separated from one another by susten- tacular cells, the cells of Deiter, so that no two hair cells come in contact. (3) Deiter s Cells (Fig. 284). — These like the pillar cells are sus- tentacular. Their bases rest upon the basilar membrane, where they form a continuous layer. Toward the surface they become separated from one another by the hair cells. The long slender portions of the Deiter's cells, which pass in between the hair cells, are known as pJialangeal processes. Between the innermost of the outer hair cells and the outer pillar is a space known as Nuel 's space (Fig. 284, x). (4) Hcnsens Cells (Fig. 284, a). — These are sustentacular cells, which form about eight rows to the outer side of the outermost Dei- ter's cells. These cells form the outer crest of Corti's organ and consequently have a somewhat radial disposition, their free surfaces being broad, their basal ends narrow. They decrease in height from within outward, and at the end of Corti's organ become continuous with the cells of Claudius (Fig. 284, b), the name given to the coch- THE ORGANS OF SPECIAL SENSE. 447 lear epithelium covering the basal membrane to the outer side of Corti's organ. The phalangeal processes of the Deiter's cells are cemented to- gether and to the superficial parts of the outer pillars in such a man- ner as to form a sort of cuticular membrane, the lamina reticularis, through which the heads of the outer hair cells project. This mem- brane also extends out as a cuticula over the cells of Hensen and of Claudius. The Mcmbrana Tcctoria. — This is a peculiar membranous struct- ure attached to a projection of the bony spiral lamina known as the spiral limbns (Fig. 284), the concavity beneath its attachment being the internal spiral sulcus (Fig. 284, c). The membrane is non-nu- cleated and shows fine radial striations. It bridges over the internal spiral sulcus and ends in a thin margin, which rests upon Corti's organ just at the outer limit of the outer hair cells. Blood-vessels. — The arteries consist of two small branches of the auditory — one to the bony labyrinth, the other to the membranous labyrinth. The latter divides into two branches — a vestibular and a cochlear. The vestibular artery accompanies the branches of the auditory nerve to the utricle, saccule, and semicircular canals. It supplies these parts, giving rise to a capillary network, which is coarse meshed except in the cristas and maculae, where the meshes are fine. The cochlear artery also starts out in company with the auditory nerve, but accompanies it only to the first turn of the coch- lea. Here it enters the modiolus where it gives off several much coiled branches, the glomerular arteries of the cochlea. Branches from these pierce the vestibular part of the osseous spiral lamina and supply the various structures of the cochlear duct. The veins ac- company the arteries, but reach the axis of the modiolus through foramina in the tympanic part of the bony spiral lamina. Lymphatics. — The scala media contains endolymph and is in communication with the subdural lymph spaces by means of the en- dolymphatic duct, the endolymphatic sac, and minute lymph channels connecting the latter with the subdural spaces. The perilymph spaces — scala tympani and scala vestibuli — are connected with the pial lymph spaces by means of the perilymphatic duct. Lymph spaces also surround the vessels and nerves. These empty into the pial lymphatics. Nerves. — The vestibular branch of the auditory nerve divides into 44« THE ORGANS. branches which supply the saccule, utricle, and semicircular canals, ■where they end in the maculas and cristae as described on page 437. The ganglion of the vestibular branch is situated in the internal auditory meatus. The cochlear branch of the auditory nerve enters the axis of the modiolus, where it divides into a number of branches which pass up through its central axis. From these, numerous fibres radiate to the bony spiral laminae, in the bases of which they enter the spiral ganglia (Fig. 283, p). The cells of the spiral ganglia are peculiar, in that while of the same general type as the spinal ganglion cell they maintain their embryonic bipolar condition (see page 351) throughout life. Their axones follow the already described course through the modiolus and thence through the internal auditory meatus to their terminal nuclei in the medulla (see page 387). Their dendrites become medullated like the dendrites of the spinal ganglion cells and pass outward in bundles in the bony spiral laminae (Fig. 283, 0, and Fig. 284). From these are given off branches which enter the tympanic portion of the lamina, where they lose their medullary sheaths and pass through the foramina nervosa (minute canals in the tympanic part of the spiral lamina) to their terminations in the organ of Corti. In the latter the fibres run in three bundles parallel to Corti's tunnel. One bundle lies just inside the inner pillar beneath the inner row of hair cells (Fig. 284). A second bundle runs in the tunnel to the outer side of the inner pillar (Fig. 284). The third bundle crosses the tunnel (tunnel-fibres) and turns at right angles to run between the cells of Deiter beneath the outer hair cells (Fig. 284). From all of these bundles of fibres are given off delicate terminals which end on the hair cells. Development of the Ear. The essential auditory part of the organ of hearing, the mem- branous labyrinth, is of ectodermic origin. This first appears as a thickening followed by an invagination of the surface ectoderm in the region of the posterior cerebral vesicle. This is known as the auditory pit. By closure of the lips of this pit and growth of the surrounding mesodermic tissue is formed the otic vesicle or otocyst, which is completely separated from the surface ectoderm. Diver- ticula soon appear passing off from the otic vesicle. These are three THE ORGANS OF SPECIAL SENSE. 449 in number and correspond respectively to the future endolymphatic duct, the cochlear duct and the membranous semicircular canals. Within the saccule, utricle, and ampullae special differentiations of the lining epithelium give rise to the maculae and cristas acusticae. Of the cochlear duct the upper and lateral walls become thinned to form Reissner's membrane and the epithelium of the outer wall, while the lower wall becomes the basilar membrane, its epithelium undergoing an elaborate specialization to form the organ of Corti. Of the cochlea, only the membranous cochlear duct develops from" the otic vesicle; the scala vestibuli, scala tympani, and bony cochlea developing from the surrounding mesoderm. The mesodermic con- nective tissue at first completely fills in the space between the coch- lear duct and the bony canal. Absorption of this tissue takes place, resulting in formation of the scala tympani and scala vestibuli. During the differentiation of the above parts a constriction ap- pears in the body of the primitive otic vesicle. This results in the incomplete septum which divides the utricle from the saccule. The middle ear is formed from the upper segment of the pharyn- geal groove, the lower segment giving rise to the Eustachian tube. The external ear is developed from the ectoderm of the first branchial cleft and adjacent branchial arches. The tympanic mem- brane is formed from the mesoderm of the first branchial arch, its outer covering being of ectodermic, its inner of entodermic origin. TECHNIC. (1) For the study of the general structure of the pinna and walls of the exter- nal auditory meatus, material may be fixed in formalin-Muller's fluid (technic 5, p. 6) and sections stained with haematoxylin-eosin (technic 1. p. 17). In sections of the wall of the cartilaginous meatus the ceruminous glands may be studied, material from children and from new-born infants furnishing the best demonstra- tions of these glands. (2) For the study of the inner ear the guinea-pig is most satisfactory on account of the ease with which the parts may be removed. Remove the cochlea of a guinea-pig with as much as possible of the vestibule and semicircular canals and fix in Flemming's fluid (technic 7. p. 7). A small opening should be made in the first turn of the cochlea in order to allow the fixative to enter the canal. After forty-eight hours the cochlea is removed from the fixative and hardened in graded alcohols (pageS). The bone is next decalcified, either by one of the methods men- tioned on page 9 or in saturated alcoholic solution of picric acid. If one of the aqueous decalcifying fluids is used, care must be taken to carry the material through graded alcohols. Embed in celloidin or paraffin, cut sections through the long axis of the modiolus, through the utricle and saccule, and through the semicircular canals. Stain with hasmatoxylin-eosin and mount in balsam. 29 4SO THE' ORGANS. (3) The neurone relations of the crista?, maculae, and cochlear duct can be demonstrated only by means of the Golgi method. The ear of a new-born mouse or guinea-pig furnishes good material. The cochlea together with some of the base of the skull should be removed and treated by the Golgi rapid method (page 29). Sections should be thick and must of course be cut through undecalcified bone. Good results are difficult to obtain. The Organ of Smell. The olfactory organ consists of the olfactory portion of the nasal mucosa. In this connection it is, however, convenient to describe briefly the olfactory bulb and the olfactory tract. TJic Olfactory Mucosa. — This has been described (page 241). The peculiar olfactory cells there described are not neuro-epithelium but are analogs of the spinal ganglion cell, being the only example FIG. 285.— Diagram of Structure of Olfactory Mucosa and Olfactory Bulb. (Ram6n y Cajal.) be, Bipolar cells of olfactory mucosa; sm, submucosa; ethm, cribriform plate of ethmoid; a, layer of olfactory fibres; off, olfactory glomeruli; vie, mitral cells; ep, epithelium of olfactory ventricle. in man of the peripherally placed ganglion cell found in certain lower animals. Each cell sends to the surface a short dendrite which ends in several short, stiff, hair-like processes. From its opposite end each cell gives off a longer centrally directed process (axone), which as a fibre of one of the olfactory nerves passes through the cribriform THE ORGANS OF SPECIAL SENSE. 451 plate of the ethmoid (Fig. 285, etJuii) to its terminal nucleus in the olfactory bulb (Fig. 285). The Olfactory Bulb. — This is a somewhat rudimentary structure analogous to the much more prominent olfactory brain lobe of some of the lower animals. It consists of both gray matter and white matter arranged in six fairly distinct layers. These from below up- ward are as follows: (a) The layer of olfactory fibres; (a) the layer of glomeruli; (c) the molecular layer; (d) the layer of mitral cells ; (e) the granule layer; (/) the layer of longitudinal fibre bundles. Through the centre of the last-named layer runs a band of neuroglia which represents the obliterated lumen of the embryonal lobe. The relations of these layers to the olfactory neurone system are as fol- lows : The layer of olfactory fibres (Fig. 285, a) consists of a dense plexiform arrangement of the axones of the above-described olfactory cells. From this layer the axones pass into the layer of olfactory glomeruli where their terminal ramifications mingle with the den- dritic terminals of cells lying in the more dorsal layers, to form dis- tinctly outlined spheroidal or oval nerve-fibre nests, the olfactory glomeruli (Fig. 285, og). The latter mark the ending of neurone system No. I. of the olfactory conduction path. The molecular layer contains both small nerve cells and large nerve cells. These send their dendrites into the olfactory glomeruli. The smaller cells belong to Golgi Type II. (see page no) and appear to be association neurones between adjacent glomeruli. The axones of the larger cells, the so-called brush cells, become fibres of the olfactory tract. Of the mitral cells (Fig. 285, me), the main dendrites end in the olfactory glomeruli, while their axones, like those of the brush cells, become fibres of the olfactory tract. In addition to the fibres which pass through it (axones of mitral and of brush cells), the granular layer contains numerous nerve cells. Many of these are small and apparently have no axones (amacrine cells). Their longer dendrites pass toward the periphery, their shorter dendrites toward the olfactory tract. Larger multipolar cells, whose axones end in the molecular layer, also occur in the granular layer. The layer of longitudinal fibre bundles consists mainly of .the centrally directed agones of the mitral and brush cells. These fibres 452 THE ORGANS. run in distinct bundles separated by neuroglia. Leaving the bulb they form the olfactory tract by means of which they pass to their cerebral terminations. The brush cells and mitral cells with their processes thus consti- tute neurone system No. II. of the olfactory conduction path. TECHNIC. (i) Carefully remove the olfactory portion of the nasal mucosa (if human material is not available, material from a rabbit is quite satisfactory). This may be recognized by its distinctly brown color. Fix in Flemming's fluid (technic 7, p. 6), or in Zenker's (technic 9, p. 7). Stain thin vertical sections with hsematoxy- lin-eosin (technic 1, p. 17) and mount in balsam. (2) For the study of the nerve relations of the olfactory cells material should be treated by the rapid Golgi method (page 27). The Organ of Taste. The organ of taste consists of the so-called taste buds of the lin- gual mucosae. These have been mentioned in connection with the papillae of the tongue (page 186) and under sensory end-organs (page 353)- The taste buds are found in the side walls of the circumvallate papillae (page 185). of some few of the fungiform papillae, in the mu- cosa of the posterior surface of the epi- glottis, and especially in folds (foliate papillae) which occur along the postero- lateral margin of the tongue. The taste bud (Fig. 286) is an ovoid epithelial structure embedded in the epi- thelium and connected with the surface by means of a minute canal, the gustatory canal (Fig. 286, a), the outer and inner ends of which are known respectively as the outer and inner taste pores. Each taste bud consists of two kinds of cells, neuro-epithelial cells or gustatory cells and sustentacular cells (Fig. 286). The gustatory cells are long, delicate, spindle-shaped cells which occupy the centre of the taste bud, each ending externally in a cilium-like proc- ess, which, usually projects through the inner pore. The inner end Pig —Taste-bud from Side Wall of Circumvallate Papilla. CMerkel-Henle.) «, Taste-pore ; by nerve fibres, some of which en- ter the taste-bud— intragemin- al fibres, while others end freely in the surrounding epithelium — intergeminal fibres. THE ORGANS OF SPECIAL SENSE. 453 of the cell tapers down to a fine process, which may be single or branched. The sustentacular cells are long, slender cells which form a shell several cells thick around the gustatory cells. Sensory termi- nals of the glosso-pharyngeal nerves (Fig. 286, b) end within the taste buds in a network of varicose fibres — intrageminal fibres. Other sen- sory terminals of the same nerve end freely in the epithelium between the taste buds. These are finer and smoother than the intrageminal fibres and are known as intergeminal fibres (Fig. 286). TECHNIC. (1) The general structure of the taste buds is shown in the sections of tongue (technic, p. 186). (2) For the study of the nerve terminals the method of Golgi should be used (page 29). General References for Further Study. Schwalbe : Lehrbuch der Anatomie der Sinnesorgane, 18S7, Kolliker : Handbuch der Gewebelehre des Menschen. Ramon y Cajal: La re'tine des vertebres. La Cellule, ix., 1893. McMurrich : The Development of the Human Body. INDEX. Abducens nerve, 388 Absorption, 218 of fat, 220 Accessory nasal sinuses, 242 olivary nucleus, 378, 381, 385 Achromatic element of intranuclear network, 40 spindle, 44 Acid aniline dyes, 17 cells, 200, 219 Acidophile granules, 91 Acini, 177 Acoustic striae, 388 Adipose tissue, 79 Adrenal, 270 blood-vessels of, 272 capsule of, 270 cortex of, 271 development of, 272 lymphatics of, 272 medulla of, 272 nerves of, 272 structure of, 270 technic of, 273 Adventitia of arteries, 125 of lymph vessels, 133 of veins, 128 Afferent peripheral nerves, 342 Agminated follicles, 208 Air cells, 250 passages, 249 sacs, 250 vesicles, 249 Alcohol, as a fixative, 5 dilute, as a fixative, 5 -ether celloidin, 10 for hardening, 8 graded, 8 Ranvier's, 4 strong, as a fixative, 5 Alimentary canal, 179 Alimentary canal, development of , 238 endgut, 210 foregut, 195 headgut, 180 midgut, 204 Altmann's granule theory of proto- plasmic structure, 38 Alum-carmine, 16 for staining in bulk, 19 Alveolar ducts, 249 glands, 177 Alveoli, 249, 250 Amacrine cells, 426 Amitosis, 43 Amoeboid movement, 42 Amphipyrenin, 40 Amphophile granules, 91 Ampullse, 440 of Thoma, 150 Anaphase, 45 Aniline dyes, acid, 17 basic, 16 Anistrophic line, 97 substance, 97 Annular terminations, 354 Annuli fibrosi, 130 Anterior horns, 346, 372, 375, 377, 381 root or motor cells of, 356 technic of, 356 marginal bundle of Loewenthal, 367 median fissure, 345 medullary vellum, 395 pyramids, 366, 381, 382, 3S5, 389 white commissure, 347 Antero-lateral ascending tract, 365 Antrum, 294 Anus, 214 Aorta, 125 Appendix epididymidis, 283 testis, 2S3 455 456 INDEX Appendix vermiformis, 212 Arachnoid membrane, 338 of optic nerve, 428 Arbor vitas, 401 Arborescent terminations, 354 Arborizations, terminal, no Archoplasm, 41 Arciform nucleus, 378, 381, 384, 385, 388 Arcuate fibres, external, 379, 381, 384, 385, 388 internal, 378, 381, 383, 385, 39°. 393 Arrector pili muscle, 326 Arrectores pilorum, 329 Arteriae arciformes, 266 Arteries, 122 adventitia of, 125 aorta and other large, 125 arcuate, 266 arteriole, 123 bronchial, 253, 254 coats of, 122 development of, 132 elastic tissue of, 125 greater arterial circle of iris, 43 2 hepatic, 233 interlobular, 266 intima of, 124 large, like the aorta, 125 lesser arterial, of the iris, 432 media of, 125 medium-sized, 123 phrenic, 267 precapillary, 123 pulmonary, 253 recurrent, 267 renal, 258 small, 123 suprarenal, 267 technic of, 128 vasa vasorum, 128 Arteriole, 123 Articular cartilages, 169 Articulations, 169; see Joints diarthrosis, 169 synarthrosis, 169 synchondrosis, 169 syndesmosis, 169 technic of, 1 70 Ascending fibre tracts of spinal cord, 364 antero-lateral, 365 direct cerebellar, 365 Gowers', 365 posterior columns, 364 spino-tectal, 366 tract of Flechsig, 365 tractus spino-cerebellaris centra- lis, 365 tractus spino-cerebellaris dorsalis, 365 . Association fibres, 409 Atresia of follicle, 300 Atria, 250 Attraction sphere, 40 Auditory canal, 437 hairs, 441 nerve, 385 cochlear branch of, 386 vestibular branch of, 386 pit of, 448 Auerbach's plexus, 206, 211, 217, 342 Auricle, 437 muscle of, 130 Auriculo- ventricular ring, 130 Axis cylinder, 105, 112, 354 Axolemma and neurilemma, relation of, 113 Axone, 105, no ascending arm of, 355 collaterals of, 356 descending arm of, 355 development of, 337 medullated, 112 non-medullated, in Axone-hill, no Baillarger, inner line of, 406, 410 outer line of, 406, 410 Balsam, Canada, for mounting, 20 Barker, reference to, 108 Bartholin, glands of, 312 duct of, 223 Basal granule, 63 Basic aniline dyes, 16 Basket cells, 182, 403 Basophile granules, 91 Bellini, duct of, 261 Bertini, columns of, 260 INDEX 457 Bethe, concerning continuity of ax- olemma and neurilemma, 113 Betz, cells of, 40S Bioblasts, 38 Bipolar nerve cells, 106 Bladder, urinary, 269 Blastoderm, 50 Blastomeres, 49 Blocking, 10 Blood, 89 corpuscles, 89 crenation of red cells, 90 development of, 92 erythrocytes of, 89 haemoglobin of, 89 Jenner's stain for, 26 leucocytes of, 90 platelets, 92 red cells of, S9 smears, technic of, 94 stroma of, 89 technic of, 93 vascular unit, 253 white cells of, 90 Blood-islands, 92, 132 Blood-vascular unit, 253 Blood-vessel system, 121 arteries, 122 capillaries, 121 development of, 132 heart, 129 lining of, 121 technic of, 12S, 131 veins, 126 Blood-vessels, 121 lymph channels of, 128 nerves of, 128 technic of, 128 Body cavities, 133 Bone, breakers, 163 decalcification of, 9 formers, 161 Bone marrow, 156 technic of, 160 red, 156 cells of, 156 eosinophile cells of, 158 erythroblasts of, 157 fat cells of, 1 58 mast cells of, 158 multinuclear cells of, 157 Bone marrow, red, myelocytes of, 156 myeloplaxes of, 157 non-nucleated red blood cells- of, 156 nucleated red blood cells of,. yellow, 158 gelatinous, 158 Bone tissue, 86 cells of, S7 cementum, 190 intercellular substance of, 87 lacunas and canaliculi of, 87 lamellae of, 87 technic of, 87 Bones, 152 blood-vessels of, 159 cancellous or spongy, 152 cells of origin of, 162 circumferential lamellae of, 155, development of, 160 growth of, 167 hard or compact, 152 Haversian canals of, 154 lamellae, 154 interstitial lamellae of, 155 lacunae, origin of, 162 lymphatics of, 159 nerves of, 159 nutrient canal, 159 foramen, 159 vessels, 159 perforating fibres, 155 fibres of Sharpey, 156 periosteum of, 155 technic of, 160 developing bone, 168 Volkmann's canals, 155, 159 Bony spiral lamina, 442 Borax-carmine, alcoholic solution, iq> Born's theory of corpus luteum, 299 Bowman, capsule of, 260, 263 glands of, 242 membrane of. 417 sarcous elements of, 97 Brachia conjunctiva, 397 Brachium of posterior corpus quad- rigeminum, 391 Brain, 406; see Cerebrum 458 INDEX Brain, membranes of, 338 arachnoid, 338 blood-vessels of, 339 cerebral dura, 338 dura mater, 338 pia mater, 338 technic of, 339 Pacchionian bodies of, 338 relation to optic nerve, 428 sand, 415 Bridges, intercellular, 57 Bronchi, 246 blood-vessels of, 253 cartilages of, 248 development of, 254 nerves of, 254 primary, 246 respiratory, 249 structure of walls of, 246 technic of, 255 terminal, 249, 251 Bruch, membrane of, 419 Brunner's glands; 209 Bulbus olfactorius, 45 1 oculi ,416; see Eyeball Burdach, column of, 350, 364 Bursa?, 172 Butschli's theory of protoplasm struct- ure, 38 •Cajal, cells of, 407 Cajeput oil for clearing sections, 20 Calcification centre, 161, 164 zone, 166 •Canal, lacrymal, 433 Canaliculi of bone, 87 Canalis communis, 440 Canalized fibrin, 310 Cancellous bone, 162 Capillaries, 121 chyle, 217 development of, 132 technic of, 128 Capillary network, 122 Capsule of Bowman, 260, 263 of Glisson, 231 Carbol-xylol for clearing specimens, 20 Cardiac glands, 200 ■Carmine alum, 16, 19 borax, 19 gelatin, 22 Carmine, neutral, 17 picro-, 18 Carotid gland, 134 Cartilage, 83 cells, 83 classification of, 84 development of, 86 elastic, 85 epiphyseal, 168 fibrous, 85 hyaline, 84 perichondrium of, 86 technic of, 86 thyroid, 243 tracheal, 244 Wrisburg's, 243 Caryochromes, 108 Cavernous sinuses, 289 Cavity of embryonic vesicle, 49 Cell, the, 37 body of, 38 centrosome of, 40 function of, 41 irritability of, 41 membrane of, 39 metabolism of, 41 motion of, 42 nucleolus of, 40 nucleus of, 39 primary germ layers of, 49 reproduction of, 43 structure of, 37 technic of, 50 typical, structure of, 37 vital properties of, 41 Cell column of the lower extremity, 357 of the upper extremity, 357 Cell division, direct, 43 indirect, 43 Cell islands of Langerhans, 229 function of, 229 Opie's theory of, 230 origin of, 229 structure of, 229 technic of, 231 Celloidin, alcohol-ether, 10 clove-oil, 1 1 embedding, 10 sections, 13 Cells, acid, 200, 219 adelomorphous, 199 INDEX 459 Cells, air, 249 amacrine, 426 auditory, 446 basal, 242 basket, 182, 403 blood, 89 bone, 87 brush, 451 centro-acini, of Langerhans, 227 centro-tubular, 227 chief, 199, 256, 414 chromophile, 414 colloid, 256 compound tactile, 352 connective-tissue, 68 corneum, 318 daughter, 46 decidual, 307 Deiter's, 445, 446 delomorphous, 199 ectodermic, 336 eosinophile, 15S epithelial, 57 extrinsic, 350 fixed, 68 foetal, 252 ganglia, 339 goblet, 205. 211 Golgi, Type I., no Golgi, Type II., no, 40S, 451 granule, 402 gustatory, 452 hair, 441, 445 hecatomeric, 358 Hensen's, 445 heteromeric, 358 intrinsic, 350 Kupffer's, 237 Langerhans', 227 Leydig's, 434 liver, 234 lymphoid, 79 lutein, 297 marrow, 156 mast, 68, 92, 15S mesamceboid, 56 mitral, 451 mossy, 1 15 mucous, 181, 205 nerve, 105, 350; for classification see Nerve cells Cells, neuroblasts, 337 neuro-epithelial, 441 of Claudius, 446 of oral glands, 181 olfactory, 242 oxyntic, 199 Paneth's, 207 parietal, 200 peptic, 199 pillar, 445 plasma, 68 prickle, 318, 324 primitive ora, 293 Purkinje, 401, 403 replacing, 60, 201, 206 respiratory, 251 resting, 44 Sertoli's, 276, 285 signet-ring, 80 simple tactile, 352 single primitive, 43 spermatids, 278, 285 spermatocytes, 278, 285 spermatogenic, 276 spermatogones, 277, 285 spider, 115 sustentacular, 228, 242, 276, 401, 45 2 tactile, 352 tautomeric, 358, 365 wandering, 68, 206 Cementing glycerin mounts, 19 Cementum, 187, 190, 193 Central canal, 346, 375, 377, 381 gelatinous substance, 346, 347, 375. 377- 381 nervous system, 336; see Ner- vous system (cerebrospinal) spindle, 44 tegmental tract, 388, 391. 393 Centro-acinar cells of Langerhans, 227 Centrosome, 40 Centrosphere, 40 Cerebellar peduncles, 395 Cerebello-olivary fibres, 381, 383 Cerebellum. 401 arbor vita?. 401 basket cells of, 403 cells of, 401. 402. 403. 404 climbing fibres of, 404 cortex of, 401 460 INDEX Cerebellum, dentate nucleus of, 405 general histology of, 401 gray matter of, 401 laminae of, 401 middle peduncle of, 391 peduncles of, 395, 406 Purkinje cells of, 401, 403 technic of, 411 vermis, 365 Cerebral convolutions, 406 hemispheres, development of, 336 peduncles, 400 reside, 336 Cerebro-spinal ganglia, 34.0 technic of, 342 Cerebro-spinal nervous system, 336; see Nervous system {cerebrospinal) Cerebrum, 406 ; see also Cortex cerebri convolutions of, 406 cortex of, 406 histology of, 406 membranes of, 338; see also Brain technic of, 411 Ceruminous glands, 437 Cervical enlargement of cord, 344 segments of cord, 344 Cervix, 304 epithelium of, 304, 305 of posterior horns, 346 ovula Nabothi, 305 plica? palmatas, 304 technic of, 314 Cheeks, mucous membrane of, 180 Chemotaxis, 42 Chiasma, optic, 430 Chloride of gold for staining connec- tive-tissue cells, 25 Choledochus ductus, 240 Choriocapillaris, 419 Chorion, 308 Chorionic villi, 309 Chorioid, the, 419 choriocapillaris of, 419 ciliary body of, 420 fissure, 436 Haller's layer of, 419 iris, 422 lamina citrea, 419 suprachorioidca, 419 perichorioidal lymph spaces of , 4 1 9 Chorioid, plexus, 384, 385 tapetum cellulosum of, 419 fibrosum of, 419 venas vorticosse of, 419 vitreous membrane of, 420 Chorioid plexus, 384, 385 Chromatic element of intranuclear net- work, 40 Chromatin, 40 Chrome-silver method of Golgi, 25 Chromophilic bodies, 108 significance of, 109 Chromosomes, 44 Chyle vessels, 217 Ciliary artery, 431 body, 420 blood-vessels of, 432 canal of Schlemm, 421 ligamentum pectinatum, 421 muscles of, 421 ora serrata of, 421 pars ciliaris retinas, 421 processes of, 421 spaces of Fontana, 421 movement, 42 muscle, 422 plexus, 433 processes, 421 Circulatory system, 121 blood-vessel system, 121 development of, 132 lymph- vessel system, 132 Circumferential lamellae, 155 Circumvallate papillae, 184 Clarke's columns, 348, 363, 365 Claudius, cells of, 446 Clava, the, 372 Clearing specimens before mounting, 20 Clefts of Schmidt-Lantermann, 113 Climbing fibres, 404 Clitoris, 312 Closed skein, 45 Clouet's canal, 431 Clove-oil celloidin, 1 1 Coccygeal glands, 134 segments of spinal cord, 344 Cochlea, 442 bony spiral lamina of, 442 cupola of, 442 hamulus of, 442 INDEX 461 Cochlea, helicotrema, 443 membranous spiral ligament of, 442 modiolus of, 442 scala tympani, 443 vestibuli, 443 spiral ligament of, 442 Cochlear duct, 443 basilar membrane of, 444 crista basilaris, 444 external spiral sulcus, 444 membrane of Reissner, 444 organ of Corti, 445 spiral prominence of, 444 stria vascularis, 444 zona pectinata, 445 tecta, 445 Ccelom, 133 Cohnheim's field, 98 Collaterals, no technic of, 360 Colloid, 255, 414 Colostrum corpuscles, 334 Column cells, 358 hecateromeric, 358 heteromeric , 358 tautomeric, 358 technic of, 360 Column of Burdach, 350, 374, 376 of Goll, 350, 374, 376 Columnar rectales, 214 Columns of Bertini, 260 of spinal cord, 346, 34S, 350, 356, 35 7 Comma tract of Schultze, 368 Commissural fibres, 409 Compound tactile cells, 352 Conduction path, 362 cortico-spino-peripheral, 362 cortico-spino-peripheral motor, 367 sensory, 365, 383 Cone association neurones, 429 fibres, 425 -visual cell, 425 Cones, layer of rods and, 425 Conjunctiva, 434 end bulbs of, 353 Connective tissue, 67 adipose or fat, 79 areolar, 7 1 Connective tissue, basement substance of, 71 bone, 86 canaliculi of, 68 cartilage, 83 cells of, 68 characteristics of, 67 chlorid-of-gold method for de- monstrating cells of, 25 classification of, 67 elastic, 72 embryonal, 75 fibres of, 69 elastic, 69 fibrillated, 69 reticular, 69 white, 69 yellow, 69 fibrillar, 68 formed, 71 histogenesis of, 67 interalveolar, 252 intercellular substance of, 69 intrafascicular, 172, 343 lacunas of, 68 lymphatic, 79 loose, 71 Mallory's stain for, 26 mucous, 75 neuroglia, 88 periglandular, 331 reticular, 77 retinaculas cutis, 317 staining cells of, 25 technic for, 74, 77, 79, S3 theories of development of fibres of, 72 Constrictions of Ranvier, 113 Corium, 315; see Derma Cornea, the. 416 anterior elastic membrane of, 417 corneal corpuscles of, 41S endothelium of Descemet of, 418 epithelium of, 417 layers of, 416 membrane of Bowman of, 417 of Descemet of, 418 perforating or arcuate fibres of, 41S posterior elastic membrane of, 41 S substantia propria of, 418 462 INDEX Corneal corpuscles, 41 S Cornua, 346 Corona radiata, 295, 400, 409 Corpora amylacea, 287 cavernosa, 288 lutea of pregnancy, 298 spuria, 299 vera, 298 quadrigemina, 366, 395, 400 anterior, 395, 400 development of, 337 posterior, 395, 400 Corpus albicans, 298 callosum, 409 haBmorrhagicum, 297 Highmori, or mediastinum testis, 274 luteum, 297 theory of, 33, 299 quadrigeminum, anterior, 366 spongiosum, 288 striatum, development of, 337 Corpuscles, blood, 89 colostrum, 334 corneal, 418 crescentic, 387 Golgi-Mazzoni, 329 of Grandry, 352 of Meissner, 290, 329, 353 of Merkel, 352 Pacinian, 354 Ruffini, 329 tactile, 329, 330, 352 Vater-Pacinian, 329 Wagner, 329 Cortex cerebelli, 401; see also Cerebel- lum Cortex cerebri, 406; see also Cerebrum association fibres of, 409 barren or molecular layer of, 407 cells of Betz, 408 of Cajal, 407 of Golgi, Type II., 408 of Martinotti, 408 commissural fibres, 409 corona radiata of, 409 deep tangential fibres of, 408 layer of polymorphous cells, 407, 409 of pyramidal cells of, 407 projection fibres, 409 Cortex cerebri, superficial tangential fibres of, 407 Cortical labyrinths, 260 pyramids, 260; see also Kidney Cortico-spinal motor neurone system, 367 -spino-peripheral conduction path, 362 -spino-peripheral motor conduc- tion path, 367 Corti's arches, 446 ganglion, 387 organ, 445 ; see also Organ of Corti tunnel, 446 Cotyledons, 309 Cowper's glands, 288 structure of, 288 technic of, 288 Cox-Golgi method of staining, 30 Cranial nerves, 373; see also Nerves, cranial Crenation, 90 Crescentic corpuscles, 287 Crescents of Gianuzzi, 182, 223, 244 Crista acustica, 442 basilaris, 444 Crossed pyramidal tracts, 366, 374, 375 Crura cerebri, 400 Crusta, 39, 395, 396, 398 fibres of, 396 pyramid of, 396 Crypt of Lieberkiihn, 207 Cumulus oophorus, 295 Cupola, 442 Cupula, 442 Cuticle, 317; see Epidermis Cuticula, 39 dentis, 190 Cuticular membrane, 58, 193, 205 Cystic duct, 237 Cytoplasm, 38 of nerve cells, 107 Cytoreticulum, 38 Daughter cells, 46 chromosomes, 45 stars, 45 Decalcifying, 9 fluids, 9 INDEX. 46; Decidua basalis, 307 capsularis, 307 graviditatis, 307 menstrualis, 306 placentalis subchorialis, 310 reflexa, 307 serotina, 307 vera, 307 Decidual cells, 307 Decolorizing fluid for Weigert's haema- toxylin, 28 Decussation of fillet, 378, 381 optic, 430 of Forel, 399 of Meynert, 399 of pyramids, 375, 377 sensory, 376, 378, 381 Dehiscent glands, 177 Deiter's cells, 445 nucleus, 367, 387 descending tract from, 367, 339 Delafield's haematoxylin, 182 Dendrites, the, 105, no Dental canals, 188, 193 germ, 191 papilla, 191 periosteum, 191 Dental ridge, 191 sac, 191 sheath, Neumann's, 189 Dentate nucleus, 405 Dentinal fibres, 188 pulp, 187 Dentine, 189 Derma, or corium, 315 corpuscles of Meissner, 353 papillte, compound, 316 nerve, 316 simple, 316 vascular, 316 pars papillaris. 316 reticularis, 315 pigmentation of, 319 Descemet, endothelium of, 41S membrane of, 418 Descending fibre tracts of the spinal cord, 366; see Fibre tracts of spinal cord (descending) Deutoplasm, 39 Diapedesis, 92 Diarthrosis, 169 Diaster, 46, 47 Digestive system, 179 alimentary tract of, 179 development of, 238 endgut ,210 foregut. 195 gall bladder, 237 headgut, 180 larger glands of, 221 liver, 231 midgut, 204 mouth, 1 So pancreas, 225 salivary glands, 221 Direct cerebellar tract, 365, 374, 377, 380 pyramidal tract, 366 Discus proligerus, 295 Dissociation of tissue elements, 4 Distal convoluted tubule, 263 Dogiel's end plates, 433 theory of structure of spinal gan- glion, 340 Dorsal accessory olivary nucleus', 382, 384 gray commissure, 347 segments of spinal cord, 344 Dorso-lateral ascending tract, 365 spino-cerebellar fasciculus, 365 Duct systems of glands, 177 Ducts, aberrans Halleri, 283 alveolar, 249 Bartholini's, 223 Bellini's. 261 choledochus, 240 cochlear, 443 common, 237 excretory, of glands, 177 cystic. 237 endolymphatic, 440 ejaculatory, 2S2 Gartner's, 300 hepatic, 233, 237 Miillerian, 287, 314 nasal, 434 of sweat glands, 319 oviduct, 301 pancreatic, 226 pronephritic, 313 reuniens, 440 464 INDEX. Ducts, Santorini's, 226, 240 secondary pancreatic, 226 seminal, 280 Stenoni's, 222 utriculo -saccular, 440 vas deferens, 281 vasa efferentia, 2S0 Wharton's, 223 Wirsung's, 226 Wolffian, 313 Dura mater, 338 blood-vessels of, 339 technic of, 339 Dyes, aniline, 16 nuclear, 14 plasma, 17 JEar, external, 437 auricle, 437 blood-vessels of, 438 ceruminous glands of, 437 ear drum, 438 external auditory canal, 437 lymphatics of, 438 nerves of, 438 pinna, 437 tympanic membrane, 438 internal, 439 ampulla, 440 blood-vessels of, 447 canalis communis, 440 cochlea, 443 ducts of, 440 ductus reuniens, 440 endolymph of, 439 •endolymphatic duct, 440 sac, 440 fenestra ovalis, 439 rotunda, 440 lymphatics of, 447 membrana tectoria, 447 membranous labyrinth, 439, 440 nerves of, 447 •organ of Corti, 445 osseous labyrinth of, 439 perilymph of, 439 ■saccule, 441 semicircular canals, 440 utricle, 441 utriculosaccular duct, 440 Ear, internal, vestibule, 439, 440 middle, or tympanum, 438 fenestra rotunda of, 438 ossicles of, 439 Ear drum, 438 wax, 437 Ebner's glands, 185 hydrochloric-salt solution, 9 Ectoderm, 49 tissue, derivations from, 55, 331 Ectodermic cells, 350 Efferent fibre system of cord, 366 peripheral nerves, 342 Egg nest, 294 Ehrlich, Greek-letter granules of, 9 1 Ejaculatory ducts, 282 Elastic cartilage, 85 tissue, 72 Weigert's stain for, 25 Elastin, 70 Eleidin, 318 Ellipsoid of Krause, 425 Ellipsoids of spleen, 148 Embedding, 10 celloidin, 10 paraffin, 10 Embryonal tissue, 75 Enamel, 187, 190, 193 cells, 192 fibres, 190, 193 organ, 191, 192, 193 prisms, 190 lines of Retzius of, 190 End bulbs, 353 of Krause, 186, 330 Endgut, 210 large intestine, 210 rectum, 214 vermiform appendix, 212 Endocardium, 130 Endochondral ossification, 163 Endolymph, 439 Endolymphatic sac, 440 Endomysium, 172 Endoncurium, 343 Endoplasm, 39 Endosteum, 159 Endothelial tube, 132 Endothelium, 64 Entoderm, 50 tissue derivations from, 55 INDEX. 465 Eosin, 17 -glycerin, 19 -haematoxylin stain, 17 Eosinophile granules, 91 Ependymal cells, 337 Epiblast, 49 Epicardium, 131 Epicranium, 162 Epidermis (or cuticle), 317 corneum cells of, 318 eleidin, 318 keratin, 318 keratohyaline granules, 318 mitosis of cells of, 319 pareleidin, 319 pigmentation of, 319 prickle cells of, 318 stratum corneum of, 318 cylindricum of, 317 germinativum of, 317 granulosum of, 318 lucidum of, 318 Malpighii of, 317 mucosum of, 317 spinosum of, 318 Epididymis, 275 cells of, 280 vas epididymis of, 281 vasa efferentia of, 280 Epidural space, 338 Epimysium, 171 Epineurium, 343 Epiphyseal cartilage, 168 Epithelium, 57 basal membrane of, 57 ciliated, 63 classification of, 58 cuboidal, 59 cuticular membrane of, 58 general characteristics of, 57 germinal, 292 glandular, 64 histogenesis of, 57 intercellular bridges of, 57 lens, 430 membrana propria of, C7 neuro-, 64 pigmented, 64 pseudo-stratified, 60 replacing cells of, 60 respiratory, 251 30 Epithelium, simple, 58 columnar, 58 pseudo-stratified, 60 squamous, 58 stratified, 60 columnar, 61 squamous, 60 transitional, 62 surface, of mucous membranes, 178 syncytium, 309 tactile cells of, 352 technic of, 65 Eponychium, 322 Epoophoron, 300 Erectile tissue, 289 Erythroblasts, 157 Erythrocytes, 89 Erythrosin, 17 Eustachian tube, 439 Excretory substances in cells, 39 Exoplasm, 39 External arcuate fibres, 379, 381, 384 385, 388 External ear, 437; see Ear, external Eye, the, 416; see Organ of vision Eye, pineal, 415 Eyeball (or bulbus oculi), 416 blood-vessels of, 431 chorioid of, 419 ciliary body of, 420 cornea of, 416 iris of, 422 lymphatics of, 432 nerves of, 433 retina of, 424 sclera of, 416 technic of, 436 Eyelashes, 434 Eyelid, the, 434 blood-vessels of, 434 conjunctiva of, 434 epidermis of, 434 glands of, 434 of Mall, 434 lymphatics of, 435 Meibomian! glands of, 434 muscles of, 434 nerves of, 435 tarsus of, 434 technic of, 437 466 INDEX. Facial nerve, 388 Fallopian tube, 301; see Oviduct ampulla of, 301 blood-vessels of, 302 coats of, 301 development of, 314 fimbriated extremity of, 301 isthmus of, 301 lymphatics of, 302 nerves of, 302 ovarian extremity, 301 technic of, 302 False corpora lutea, 298 Fascicles of muscle, 171 of nerves, 343 Fasciculi, 362 Fasciculus, of Thomas, 368 posterior longitudinal, 376, 379, 381, 384, 385, 390, 393, 397 solitarius, 378, 381, 384, 385 ventrolateralis superficialis, 365 Fat, absorption of, 220 technic of, 221 blood supply of, 82 development of, 81 gobules, 220 osmic-acid stain for, 26 subcutaneous, 317 tissue, 79 technic of, 83 Fat-droplets in cells, 39 Fat-lobules, 81 Fauces, mucous membrane of, 180 Female genital organs, 292 pronucleus, 47 Fenestra ovalis, 439 rotunda, 439 Ferrein, pyramids of, 260 Fertilization of the ovum, 46 Fibre baskets, 427 systems, 362 efferent, 366 main motor, 366 short, 358, 368 tracts of spinal cord, 358, 362 technic of, 371 tracts of cord (ascending), 364 antero-lateral, 365 direct cerebellar, 358, 365 Gowers', 358, 365 posterior columns, 364 Fibre tracts of cord (ascending), spino-tectal, 366 tract of Flechsig, 365 tractus spino-cerebellaris centralis, 365 tractus spino-cerebellaris dorsalis, 365 tracts (descending), 366 anterior marginal bundle of Loewenthal, 367 antero-lateral, 367 comma tract of Schultze, 368 crossed pyramidal, 366 descending tract from Dei- ter's nucleus, 367 descending tract from the vestibular nuclei of both sides, 367 direct pyramidal, 366 Helweg's, 367 origin of tracts, 356 oval bundle of Flechsig, 367 pyramidal, 366 quadrigemino-spinal, 367 rubeo-spinal, 367 septo-marginal, 368 tract of Tiirck, 366 tractus cortico-spinalis, 366 tractus reticulo-spinalis, 368 Von Monakow's tract, 367 tracts (fundamental, or ground bundles), 368 descending paths from higher centres, 370 direct ascending paths to higher centres, 370 direct reflex path, 369 indirect ascending paths to higher centres, 370 indirect reflex path, 369 technic for, 371 Fibres, 453 calcified, 161 cone, 425 dentinal, 188 development of connective-tissue, 72 enamel, 190 genioglossal, 183 intergeminal, 453 intrageminal, 453 INDEX. 467 Fibres, lens, 430 Mallory's method of staining connective-tissue, 26 mantle, 44 Miiller's, 426 nerve, 105; see also Nerve fibres medullated, in non-medullated, 112 neuroglia, 115 of areolar tissue, 71 of bone, 87 of developing muscle, 103 of formed connective tissue, 7 1 of Remak, 112 of Sharpey, 156 olfactory, layer of, 451 perforating or arcuate, of cornea, 418 of Sharpey, 156 picro-acid-fuchsin for staining connective-tissue, 18 rod, 425 tendon, 71, 172 tunnel, 448 voluntary muscle, 96, 99 Weigert's method for staining elastic, 25 method for staining nerve, 26 white or fibrillated, 69 yellow or elastic, 69, 72 Fibrillar connective tissue, 68 Fibroblasts, 72 Fibrous cartilage, 85 Filar mass, 38 Filiform papillae, 184 Fillet, the, 372, 378,381,385,390, 393 Filum terminale, 344 Fissure, anterior median, 45 chorioid, 436 of Rolando, 366 Fixation, 5 by injection, in toto, 5 Fixatives, 5 Flechsig, oval bundle of , 367 tract of, 365 Flemming's fluid, 6 Foam theory of protoplasm structure, 3 8 Fcetal cells, 252 structures, of genital system, 2S2, 312 Fcetal structures, appendix epididy- midis, 2S3 testis, 283 ductus aberrans Halleri, 283 organ of Giraldes, 282 paradidymis, 2S2 Foliate papillae, 452 Follicle, Graafian, 293 Follicles, agminated, 208 solitary, 202, 208, 211 Follicular cavity or antrum, 295 Folliculi linguales, 145; see Tonsils Fontana, spaces of, 422 Foramina nervosa, 448 Forebrain, 336 Foregut, the, 195 general structure of walls of the gastro-intestinal canal, 196 oesophagus, 195 stomach, 198 Forel, decussation of, 398 Formalin, as a fixative, 5 for macerating, 4 -bichromate method, 30 Formalin-Midler's fluid, 6 Fossa navicularis, 291 Fourth ventricle, 381, 382, 384, 385, 39°> 393 Fovea centralis, 427 Fraenkel's theory of corpus luteum, 299 Fuchsin, 16 Function of cells, 41 Fundamental columns of spinal cord, 36S descending paths from higher centres, 370 direct ascending paths to higher centres, 370 direct reflex paths, 369 indirect ascending paths to higher centres, 370 indirect reflex path, 369 Fundus glands, 199 Fungiform papilla?, 1S4 Funiculus cuneatus, 374 gracilis, 374 teres, nucleus of, 385 Gage's haematoxylin, 14 Gall-bladder, 237 coats of, 237 468 INDEX. Gall-bladder, mucous membrane of, 237 rugae of, 237 Galvanotaxis, 42 Ganglia, 339 cerebral, 340 Gasserian, 393 spinal, 340 spiral, 387, 448 structure of, 340 sympathetic, 341 Ganglion cells, 340 classification of, 340 of Corti, 387 of Scarpa, 388 Gasserian ganglion, 393 Gastric crypts, 198 glands, 198, 199 pits, 198 Gastro-intestinal canal, general struct- ure of the walls of, 196 Gelatin, carmine for injecting, 22 Prussian blue, for injecting, 22 Gelatinous marrow, 158 substance of Rolando, 346, 347 Genio-glossal fibres, 183 Genital ridge, 314 system, 274; see also Reproduc- tive system development of, 312 rudimentary structures con- nected with development of, 282, 300, 312 Gentian violet, 16 Genu, or bend, 400 Germ hill, 295 layers, 49 tissues derived from, 55 Germinal spot, 296 vesicle, 46 Gianuzzi, crescents of, 182, 223, 244 Girald&s, organ of, 282 Gland cells, 174 Glands, 1 74 acini of, 1 77 adrenal, 270 alveolar, J75, 176 compound, 177 simple, 177 alveoli of, 175, 177 axillary, 334 Glands, Bartholini's, 312 Bowman's, 242 Brunner's, 209 cardiac, 200 carotid, 134 cells of, 174 ceruminous, 437 classification of, 175 coccygeal, 134 compound, 175 corpus luteum, 297 Cowper's, 288 epithelium of, 174 excretory ducts of, 174 dehiscent, 177 ductless, 177 Ebner's, 185 fundus of, 199 gastric, 198 general structure of, 174 haemolymph, 139 internal secreting, 177 interstitial tissue of, 175 intraepithelial, 280 kidney, 258 lacrymal, 433 large, of digestive system, 221 Lieberkiihn's, 207 lingual, 182 Littre^s, 290, 291 liver, 231 lobes of, 175 lobules, 175 lymph, 134 Mall's, 434 mammary, 331 Meibomian, 177, 434 mixed, 181 mucous, 181 of the oral mucosa, 181 ovary, 292 pancreas, 225 parathyroids, 256 parenchyma of, 175 parotid, 222 penis, 288 peptic, 199 pineal, 415 prostate, 286 pyloric, 191, 201 racemose, 1 75 INDEX. 469 Glands, reticular, 177 saccular, 175 salivary, 221 sebaceous, 319, 326 secreting portions of, 174 serous, 181 simple, 175 spleen, 146 sublingual, 223 submaxillary, 223 suprarenal, 270 sweat, 319 tarsal, 434 thymus, 142 thyroid, 255 tonsils, 144 tubular, 175 compound, 177, 258 simple branched, 176, 288 simple coiled, 176 simple straight, 176 Tyson's, 290 uterine, 306 Glandulee sudoriparae, 319 vestibulares majores, 312 minores, 312 Glandular epithelium, 174 Glans penis, 290 Glenoid ligaments, 169 Glisson, capsule of, 231 Globus major, 275 minor, 275 Glomerulus of kidney, 260 blood-vessels of, 266 olfactory, 451 Glosso-pharyngeal nerve, 379 Glycerin for mounting specimens, 19 Glycogen granules, 235 Gold chlorid for staining connective- tissue cells, 25 Golgi bichlorid method for nerve tis- sue, 30 cell, Type I., no cell, Type II., no, 408 method, bichlorid, 30 chrome-silver, 25 Cox modification of, 30 formalin bichromate, 30 mixed, 30 rapid, 29 silver, for nerve tissue, 29 Golgi method, slow, for nerve tissue, 29 Golgi-Mazzoni corpuscles, 329 Golgi net, 115 Golgi, organs of, 354 Goll, column of, 350, 364 Gowers' tract, 365, 376, 380, 382, 385, 389 Graafian follicles, 293 antrum of, 294 corona radiata, 295 cumulus oophorus, 295 development of, 293 discus proligerus, 295 egg nest, 294 epithelium of, 293 follicular cavity of, 295 germ hill of, 295 liquor folliculi, 295 ovum of, 292 Pfliiger's egg cords, 294 primitive Graafian follicle, 295 primitive ova, 293 rupture of, 297 stratum granulosum, 295 technic of, 303 theca folliculi, 295 tunica fibrosa, 295 tunica vasculosa, 296 Graded alcohols, 7 Grandry, corpuscles of, 352 Gray matter, 344 Greek-letter granules of Ehrlich, 91 Ground bundles of spinal cord, 368 Gums, mucous membrane of, 180 Gustatory canal, 452 H.-emalum, Mayer's, 15 Hsematin, 15 Haematoidin, crystals of, 298 Hsematoxylin, 14 and eosin, for staining double, 17 and picro-acid fuchsin, iS Delafield's, 15 Gage's, 14 Heidenhain's, 15 Mallory's stain, 26 Weigert's, 16, 27 Haemoglobin, S9 Hasmolymph nodes, 139 blood sinuses of, 139 blood-vessels of, 141 4/0 INDEX. Hasmolymph nodes, cells of, 140 marrow-lymph, 140 spleno-lymph, 140 technic of, 141 Hair, 322 arrector pili muscle of the, 326 blood-vessels of, 329 bulb, 322 cells of the, 323, 327 connective-tissue follicle of, 324 cortex of, 323 cortical fibres of, 323 cuticle of, 323, 324 Henle's layer of, 324 Huxley's layer of, 324 prickle cells of, 324 root sheath of, 324 development of the, 330 excretory duct of, 327 eyelashes, 434 follicle, 323 germ, 328 growth of the, 328 inner root sheath, 324 lanugo, the, 323 layers of the, 323, 324 lymphatics, 329 medulla of, 323 nerves of, 330 outer root sheath, 324 papilla of, 322 prickle cells, 324 root of the, 325 root sheath, 324 sebaceous glands of the, 326 sebum of the, 327 shaft of, 322 shedding of the, 327 stratum cylindricum, 324 technic of the, 328 Hair cells, 446 Halleri, ductus aberrans, 283 Haller's layer, 419 Hamulus, 442 Hardening, 7 celloidin-embcdded specimens, 10 clove-oil celloidin-embedded spec- imens, 1 1 Hassal's corpuscles, 143 Haversian canals, 154 Haversian canals, development of, 166 fringes, 177 lamella?, 154 spaces, 166 systems, 154 development of, 167 Headgut, 180 mouth, 180 pharynx, 194 teeth, 187 tongue, 183 Hearing, organ of, 437; see Ear Heart, 129 annuli fibrosi, 130 auricular muscle, 130 auriculo-ventricular ring, 130 blood-vessels of, 131 development of, 132 endocardium of, 130 epicardium of, 131 lymphatics of, 131 muscle, 100; see Involuntary striated muscle myocardium of, 130 nerves of, 131, 354 technic of, 131 valves of, 131 Hecateromeres, 358 Heidenhain, concerning the kidney, 265 demilunes of, 182 Heidenhain 's hematoxylin, 15 Heisterian valve, 237 Helicotrema, 443 Heller's plexus, 215 Helweg, tract of, 367, 374 Henle's layer, 324 loop, 260, 263 sheath, 113, 343 Hensen's cells, 446 line, 97 Hepatic artery, 233 cells, 234 cords, 236 duct, 233, 237 Heteromeres, 358 Hilum of liver, 231 of kidney, 258 Ilindbrain, 336 His, marginal veil of, 337 spongioblasts of, 337 INDEX. 47i Histogenesis, 55 Horizontal cells, 426 Howship's lacunae, 163 Huxley's layer, 324 Hyaline cartilage, 84 Hyaloid canal, 431 membrane, 431 Hyaloplasm, 38 Hydatid of Morgagni, 314 Hydrochloric acid for decalcifying, 9 Hyoglossal fibres, 183 Hypoblast, 50 Hypoglossal nerve, 379 Hyponychium, 322 Hypophysis cerebri, 412; see also Pit- uitary body Implantation cone, no Incisures of Schmidt-Lantermann, 113 Inferior brachium quadrigeminum, 400 cerebellar peduncle, 400; see Res - tiform body Infundibula, 249 Injecting, 22 apparatus, 22 double, 23 separate organs, 23 whole animals, 23 Inner bulb, 354 Innervation of muscles, 357 Intercellular bridges of epithelium, 57 bridges of muscle tissue, 96 substance, 56 of connective tissue, 69 silver-nitrate method of staining. 25 Interfilar mass, 38 Intermediate cartilage, 168 lamella?, 155 Intermedio-lateral column, 357 Internal arcuate fibres, 378, 381, 383, 385. 39°> 393 Internode, 113 Interstitial lamella?, 155 Intestine; see Small intestine, 204; Large intestine, 210 Intestines, development of, 239 Intima, 124 of arteries, 124 of lymph vessels, 133 Intima of veins, 127 Intracartilaginous ossification, 163 Intrafascicular connective tissue, 172, 243 Intramembranous ossification, 161 Intranuclear network of typical cell, 40 Involuntary striated muscle (heart) , 1 00 Cohnheim's field, 10 1 McCallum's view's, 100 membrane of Krause, 10 1 muscle columns of Kolliker, 101 nerves of, 354 sarcoplasm of, 10 1 technic of, 104 smooth muscle, 95 intercellular bridges of, 96 Iodine, to remove mercury, 8 Iris, the, 422 greater arterial circle, 432 layers of the, 422 lesser arterial circle, 432 muscles of the, 423 vitreous membrane, 424 Irritability of cells, 41 Islands, blood, 92, 132 of Langerhans, 229 Isolated smooth muscle cells, 95 technic of, 104 Isotrophic line, 97 substance, 97 Isthmus of thyroid gland, 255 Iter . 395- 397 Jenner's blood stain, 26 Joint capsule, 169 Joints, 169; see Articulations Karyokinesis, 43 Karyolysis, 31S Karyoplasm, 40 Karyosomes, 40 Katabolism, 41 Keratin, 31S Keratohyaline granules, 318 Kidney, the, 25S arteriae arciformes, 266 blood-vessels of, 265 Bowman's capsule of, 260 columns of Bertini, 260 convoluted tubules of, 263 472 INDEX. Kidney, cortex of, 258 cortical pyramids of, 260 development of, 312 duct of Bellini, 261 glomerulus of, 260 Henle's loop, 263 interlobar arteries of, 265 hilum of, 258 labyrinths of, 260 lobulated, 258 lymphatics of, 267 main excretory duct of, 268 Malpighian body, 262 pyramid, 260 medulla of, 258 medullary (or Malpighian) pyra- mid, 260 rays, 260 nerves of, 268 papillae of, 259 pelvis of, 259 pyramids of Ferrein, 260 renal artery, 258 corpuscle, 260 vein, 258 renculi or lobes of, 258 septa renis, 260 single lobe of, 258 technic of, 273 uriniferous tubule, 260; see also Uriniferous tubule Kidney-pelvis, 268 blood-vessels of, 268 calyces of, 268 cells of, 268 coats of, 268 development of, 313 lymphatics of, 269 nerves of, 269 technic of, 273 Kolliker, muscle columns of, 98 Krause, ellipsoid, 425 end-bulbs, 186, 290, 330, 433 line of, 97 Kupft'er, cells of, 237 Labia minora, sebaceous glands of , 319 Labyrinth, membranous, 439 osseous, 439 Lacrymal apparatus, 433 nasal duct of, 434 Lacrymal canal, 433 gland, 433 blood-vessels of, 434 excretory ducts of, 433 lymphatics of, 434 nerves of, 434 sac, 433 Lacteals, 220 Lacunas, 83, 87 origin of, 162 Lamellae, circumferential, 155 Haversian, 154 intermediate, 155 interstitial, 155 Lamina, bony spiral, 442 citrea, 419 cribrosa, 416, 428 fusca, 416 reticularis, 447 suprachorioidea, 419 Laminae of cerebellum, 401 Langerhans, cell islands of, 229 centro-acinar cells of, 227 centro-tubular cells of, 227 Lantanin, 40 Lanugo, 323 Large intestine, 210 Auerbach's plexus, 211 blood-vessels, 215 coats of, 210 development of, 239 gland tubules, 211 lineae coli, 211 lymphatics, 217 nerves, 217 plexus of Meissner, 211 technic of, 220 Larynx, the, 243 blood-vessels of, 245 cartilages of, 243 arytenoid, 243 cricoid, 243 epiglottis, 243 Santorini's, 243 thyroid, 243 Wrisburg's, 243 cells of, 243 epithelium of, 243 lymphatics, 246 nerves, 246 perichondrium, 243 INDEX. 473 Larynx, technic, 246 Lateral lemniscus, 391 393-397. 399 Lemniscus, 378, 381 Lens, 430 canal of Pettit, 431 capsule, 430 epithelium, 430 hyaloid membrane, 431 suspensory ligament, 430 zonula ciliaris, 430 zonule of Zinn, 430 Leopold, concerning pregnant uterus, 307 Leucocytes, 90 migratory, 206 of milk, 334 Lieberkiihn, crypts of, 207 glands of, 207 Ligament, glenoid, 169 membranous spiral, 442 spiral, 442 structure of, 72 suspensory, 430 Ligamentum pectinatum, 422 Linese coli, 211 Lines of Retzius, 190 Lingual glands, 182 tonsils, 145 Lingualis, genio-glossus fibres of, 183 hyoglossus fibres of, 183 longitudinal fibres of, 183 styloglossus fibres of, 183 transverse fibres of, 183 Linin, 40 Liquor folliculi, 295 Lissauer, zone of, 347, 364 Littre\ glands of, 291 Liver, the, 231 blood supply of, 233 capillary network, 233 capsule of Glisson, 231 cells of, 234 of Kupffer, 237 central vein of, 233 connective tissue, 231 cords of liver cells, 236 development of, 240 glycogen granules, 235 Heisterian valve, 237 hepatic artery, 233 cords, 236 Liver, hepatic duct, 233 hilum, 231 intralobular secreting tubules, 234. lobules, 232 lymphatics, 237 main ducts, 237 nerves, 237 portal canal, 234 vein, 233 radiate fibres, 237 reticulum, 237 septa, 232 sublobular vein, 233 technic of, 238 tubule of, 236 Lobulated kidney, 258 Loewenthal, anterior marginal bundle of, 367 Loop of Henle, 260, 263 Lumbar enlargement of spinal cord,. 344 segments of cord, 344 Lungs, the, 248 air cells, 250 sacs, 250 vesicles, 249 alveolar ducts, 249 alveoli of, 250 atria of, 250 blood-vessels of, 253 capsule of, 248 cells of, 251 development of, 254 epithelium of, 251 infundibula of, 249 interalveolar connective tissue of, 252 lobes of, 248 lobules of, 248 lymphatics of, 254 Miller's subdivisions, 249 nerves of, 254 pulmonary lobule, 253 pleura of, 248 respiratory bronchi, 249 epithelium, 25 1 technic of, 255 terminal bronchi of, 249, 251 vestibula of, 250 Lunula, 322 Lutein cells, 297 474 INDEX. Luteum, corpus, 297 Lymph capillaries, 133 glands, 135; see Lymph nodes nodes, 135 blood-vessels of, 138 capsule of, 135 connective tissue of, 135 cords of, 136 cortex of, 136 germinal centre of, 136 lymphatics of, 138 medulla of, 136 nerves of, 138 nodules of, 136 reticular connective tissue of, 137 sinuses of, 136 technic of, 138 nodule, 79, 136 germinal centre of, 136 paths of the eye, 432 spaces, 133 pericellular, 133 vessel system, 132 development of, 133 lymph capillaries, 133 lymph spaces, 133 technic of, 133 vessels, coats of, 133 structure of, 132 Lymphatic organs, 135 haemolymph nodes, 139 lymph nodes, 135 spleen, 146 technic of, 138, 141, 143 thymus, 142 tonsils, 144 tissue, 79 Lymphocytes, 90 Lymphoid cells, 79 tissue, 137 Macerating fluids, 4 Maceration, 4 Macula acustica, 441 lutea, 427 fovea centralis, 427 Male genital organs, 274 Mall, glands of, 434 concerning splenic pulp, 149 Mallory's hasmatoxylin stain, 26 Malpighian bodies, 146 body of kidney, 260, 262 development of, 262, 313 pyramid, 260; see Kidney Mammary gland, 331 active, 332 alveoli of active, 332 ampulla of, 332 blood-vessels of, 334 cells of, 333 colostrum corpuscles, 334 development of, 335 ducts of, 331 of nipple, 331 inactive, 332 interlobar septa of, 331 interlobular septa of, 331 lobular ducts of, 331 lymphatics of, 334 milk of, ^S3 nerves of, 334 secretion of, 333 structure of, 331 technic of, 335 Mantle fibres, 44 Marchi's solution, 371 Marginal bundle of Loewenthal, 367 Marginal veil of His, 337 Marrow, 156; see Bone marrow lymph nodes, 140 Martinotti, cells of, 408 Mast cells, 69, 92 Matrix of nail, 320 Maturation, 46 of ovum, 297 Mayer's hasmalum, 15 McCallum, concerning heart muscle, 101, 103 Media, of arteries, 125 of lymph vessels, 133 of veins, 127 Medial column, 357 Median lemniscus, 378, 381, 383, 390 raphd, 378, 381 septum, posterior, 345 Mediastinum testis, 274 Medulla oblongata, 371 accessory olivary nucleus, 378, 38t, 385 _ accessory olives, 384 acoustical stria;, 388 INDEX. 475 Medulla, anterior column of, 374, 376, 3S0 ground bundles of, 376 horns of, 372, 375, 377, 381 pyramid of, 381, 382, 385, 389 arciform nucleus of, 378, 381, 384, 385. 388 ascending tracts of, 372 -central canal of, 375, 377, 381 gelatinous substance of, 375, 377. 381 gray matter of, 381 tegmental tract of, 388, 391, 393 cerebellar peduncles of, 395 cerebello-olivary fibres, 382, 383 chorioid plexus, 384, 385 clava, 372 column of Burdach, 374, 376 of Goll, 374, 376 compared with spinal cord, 372 crossed pyramidal tract, 374, 375 cranial nerves of, 373 decussation of fillet, 378, 3S1 of pyramids, 375, 377 Deiter's nucleus, 387 tract, 389 descending or spinal root of ves- tibular portion of eighth nerve, 384, 385 tracts of, 372 from Deiter's nucleus, 374, 381, 382, 385 from the vestibular nu- clei, 374, 376, 379 development of, 337 direct cerebellar tract, 374, 377, 3S0 direct pyramidal tract, 375 dorsal accessory olivary nucleus, 382, 3S4 nucleus of ninth cranial nerve, 382, 384, 385 nucleus of tenth nerve, 382, 384, 385 root of first cervical nerve, 375 external arcuate fibres of, 379, 381, 384, 3S5, 388 Medulla, fillet of, 372, 378, 381, 383, 385- 39°. 393 fourth ventricle of, 381, 382, 384, 385. 390, 393 funiculus cuneatus, 374 gracilis, 374 gelatinous substance of Rolando, 374 general structure of, 371 Gowers' tract, 376, 380, 382, 385, 389 gray matter of, 372 Helweg's tract, 374 internal arcuate fibres of, 378, 381, 383, 385, 390, 393 structure, 372 lateral column, 374, 376, 380, 382, 385 fillet, 388 lemniscus, 391, 393 medial column, 357 median lemniscus, 378, 381, 383, 39° raphe\ 381 nuclei of the floor of the ventricle, 381 of the thalamus, 383 of posterior columns, 376, 377- 381 nucleus, accessory cuneate, 378 ambiguus, 3S2, 384, 385 cuneatus, 377 gracilis, 377 of the column of Burdach, 372, 377 of the column of Goll, 372, 377 of the fifth spinal nerve, 372 of funiculus teres, 385 of origin of eleventh cranial (spinal-accessory) nerve, 378 of origin of twelfth cranial (hypoglossal) nerve, 379, 3S1, 3S4, 385 of restiform body, 384 olivary nucleus, 380, 381, 3S4, 385 peduncles of, 395 pons Varolii, 391, 393 posterior columns of, 374, 376,380 4/6 INDEX. Medulla, posterior horns of, 374, 377, 381, 382, 385, 3S9 longitudinal fasciculus, 376, 379, 381, 384, 385. 39o, 393 pyramidal tracts, 374 quadrigemino-spinal tract, 374, 379 restiform body, 379, 380, 381, 384, 385 reticular formation, 375, 377, 381, 383. 385. 389. 393 reticulo-spinal tract, 374 root fibres and nucleus of origin of sixth {abducens) cranial nerve, 388, 399 root fibres and nucleus of origin of seventh {facial) cranial nerve, 388, 390 fibres and nuclei of eighth {auditory) cranial nerve, 385, 386, 390 fibres of ninth {glosso-pharyn- geal) and tenth {vagus) cranial nerves, 379, 382, 384, 385 fibres of eleventh {spinal- accessory) cranial nerve, 376, 377- 378 fibres of twelfth {hypoglossal) cranial nerve, 379, 381 secondary cochlear tract, 388 vestibular tract, 388 sensory tract for fifth nerve, 381 section through decussation of fillet, 376 through exit of eighth nerve, 385 through exit of root fibres of fifth cranial nerve, 392 through exits of root fibres of sixth and .seventh cranial nerves, 388 through lower part of oliv- ary nucleus, 380 through middle of olivary nucleus, 382 through pyramidal decussa- tion, 374 sensory decussation of, 376, 378, 378 Medulla, sensory and motor root fibres- of the fifth nerve, 393 solitary fasciculus, 378, 381, 384, 385 spinal (descending) root of fifth cranial nerve, 378, 381, 384, 385. 39°. 393 spino-tectal tract, 374, 381, 382,. 385, 389 superior olive of, 391, 393 technic of, 374 terminal nucleus of the descend- ing (sensory) root fibres of the fifth nerve, 377 tract of Gowers, 374, 380, 382,. 385, 388, 393 of Helweg, 374 of Loewenthal, 374, 381 transverse fibres of pons Varolii, 388 Von Monakow's bundle, 377,. 381, 382, 385, 389 Medullary rays, 260 sheath, 110, 113 vellum, anterior, 395 Medullated nerve fibres, Weigert's stain for, 27 Meibomian glands, 177, 434 Meissner, corpuscles of, 290, 329,. 353 plexus of, 209, 211, 218, 342 Membrana elastica externa, 125 elastica interna, 124 preformativa, 193 propria, 57 tectoria, 447 Membrane, cuticular, 193 of Bowman, 417 of Descemet, 418 of Krause, 97, 101 of Reissner, 440 peridental, 191 synovial, j 70 Membranous cochlea, 443 labyrinth, 439, 440 spiral lamina, 442, 444 ligament, 442 Meninges, 338 Mercuric chlorid as a fixative, 7 Merkel's corpuscles, 352 Mesamceboid cells, 56 INDEX. 477 Mesencephalic root of fifth {trigem- inus) cranial nerve, 395, 396, 397 Mesencephalon, 336 Mesenchyme, 56 Mesoblast, 50 Mesoderm, 50 tissue derivations from, 56 Mesonephros, 313 derivations from, 283, 301 Mesothelium, 56, 64 Metabolism of cells, 41 Metanephros, 313 Metaphase, 45 Metaplasm, 39 Method of embryology for studying fibre tracts of the cord, 363 of pathology for studying fibre tracts of the cord, 363 Methyl blue, 16 green, 16 violet, 16 Meynert, decussation of, 398, 400 Micron, 13 Microtome, 13 Midbrain, 336, 395 anterior corpora quadrigemina of, 395- 399. 400 medullary vellum, 395 brachia conjunctiva, 397 cerebral peduncles, 400 corona radiata, 400 crura cerebri, 400 crusta, 395, 396, 398 fillet, 397, 399 fourth cranial nerve, 397 ventricle, 395 geniculate bodies of, 399 Gowers' tract of, 397, 398 inferior brachium quadrigemi- num, 400 internal arcuate fibres, 395 iter, 395 lateral lemniscus, 397, 399 mesencephalic root of fifth nerve, 397 posterior corpora quadrigemina, 395, 400 longitudinal fasciculus, 397, 399 pyramid of, 396 red nucleus of, 399 Midbrain, reticular formation, 397, 399 root fibres and nucleus of origin of third (oculomotor) cranial nerve, 399 section through exit of fourth (pathetic) cranial nerve, 395 through exit of third (oculo- motor) cranial nerve, 398 spino-tectal tract, 398 substantia nigra, 395, 397. 399 superior cerebellar peduncles of, 397. 399 tegmentum, 395, 397 Von Monakow's bundle, 397, 398 Middle ear, 438; see Ear, middle Midgut, 204 small intestine, 204 Milk, 333 cells of, 334 colostrum corpuscles of, 334 Milk teeth, 191, 192 Miller's theory of lung subdivisions, 249 Minot, concerning the pregnant uterus, 3°7 Miton, 38 Mitosis, 43 method of demonstrating by Flemming's fluid, 7 Modiolus, 442 Monaster, 45 Mononuclear leucocytes, 90 Morgagni, hyatid of, 314 Motion of cells, 42 Motor cells of anterior horns, 356 Motor end plate, 357 nuclei, 357 peripheral nerves, 342 Mounting, 19 celloidin specimens, 20 in balsam, 20 in glycerin, 19 paraffin sections, 20 Mouth, the, 180 blood-vessels of, 182 end bulbs in mucous membrane, 353 glands of, 1S1 lymphatics of, 1S2 478 INDEX. Mouth, mucous membrane of, 1S0 nerves of, 182, 353 technic of, 183 Mucin, 75 Mucous glands, 181 Mucous membranes, 178 basement membrane of, 178 end bulbs in, 352 general structure of, 178 muscularis, 178 nerves of, 352 stroma of, 178 submucosa of, 178 surface epithelium, 178 tactile cells of, 352 corpuscles of, 352 tunica propria of, 178 Mucous tissue, 75 Miiller, circular muscle of, 422 Miiller's fibres, 426 fluid, 6 Miillerian ducts, 287, 314 Multipolar nerve cells, 106 Muscle, arrector pili, 326 auricular, 130 cells, 95 ciliary, 421, 422 circular, of Miiller, 422 -columns of Kolliker, 98 -discs, 97 fibrillae, 97 of sweat glands, 331 spindles, or neuro-muscular bun- dles, 354 Muscle-tendon junction, 172 organs of Golgi in, 354 peripheral nerve terminations in, 354 Muscle tissue, 95 classification of, 95 development of, 102 heart, 100 histogenesis, 103 intercellular bridges of, 96 involuntary smooth, 95 striated, 100 technic of, T03 voluntary striated, 96 Cohnheim's fields, 98 end bulbs of, 354 Hensen's line, 97 Muscle tissue, voluntary striated, Krause's line, 97 muscle columns of Kolliker,98 discs, 97 spindles, 354 nerves of, 354 Pacinian corpuscles of, 354 Rollett's theory, 98 sarcolemma, 96 technic of, 104 white and red fibres, 99 Muscles, voluntary striated, 171 capsule of, 171 endomysium, 172 epimysium, 171 fascicles of, 171 growth of, 173 intrafascicular connective tissue of, 172 perifascicular sheath, 172 perimysium, 172 Muscular system, 171 blood-vessels of, 173 bursae of, 172 lymphatics of, 173 nerves of, 173 technic of, 173 tendons of, 72, 172 tendon sheaths of, 172 voluntary muscle, 171 Muscularis mucosa of mucous mem- branes, 178 Musculature of intestine, 96 Myelin, 113 Myelocytes, 156 Myeloplaxes, 157 Myentericus, plexus, 217 Myoblast, 103, 173 Myocardium, 130 Myotome, 103 Nabothi, ovula, 305 Nails, the, 320 cells of the, 322 development of, 330 eponychium of, 322 growth of, 322 hyponychium, 322 lunula of, 322 matrix of, 320 structure of, 320 INDEX. 479 Nails, technic of, 322 Nail-bed, 320 groove, 320 wall, 320 Nares, 241 cells of, 242 basal, 242 olfactory, 242 sustentacular, 242 development of, 254 membrana limitans olfactoria, 242 stroma of, 242 structure of, 241, 242 olfactory region, 241 respiratory region, 241 vestibular region, 241 technic of, 246 zone of oval nuclei, 242 of round nuclei, 242 Nasal duct, 434 Nerve cells, 105; see also Neurone amacrine, 426 anterior horn, 370 basket, 403 Betz', 408 bipolar, 106, 350 brush, 451 Cajal's, 407 caryochromes, 108 column, 350, 357, 369 cone-visual, 425 ectodermic, 350 ependymal, 337 extrinsic, 350 ganglion, 339, 347 glia, 33 7 Golgi, Type I., 1 10 Type II., no, 350, 356, 408, hecateromeric, 358 heteromeric, 358 horizontal, 426 in gray matter of cord, 350 intrinsic, 350 large granule cells, 402 Martinotti's, 408 mitral, 451 mossy, 1 1 5 motor, of the anterior horn, 356 iiller's, 426, 427 Nerve cells, multipolar, 106, 356, 403 neuroblasts, 337 neuroglia, 347 of motor area of cerebral cortex, 356 outside the spinal cord, 350, 356 peripheral motor, 369, 370 sensory, 369, 370 polymorphous, 409 Purkinje, 401, 403 pyramidal, 407 rod-visual, 425 root, 350, 356 small granule, 402 somatochromes, 108 spider, 115 spinal ganglion, 350 sympathetic ganglion, 336, 341 tautomeric, 358, 365 technic for, 360 unipolar, 106 Nerve endings, 352; see Peripheral nerve terminations fibres, 105 association, 409 climbing, 404 commissural, 409 cone, 425 deep tangential, 408 felt works of, 354 layer of, of retina, 426 medullated, in of cerebellum, 401 motor end plates of, 357 non-medullated, 112 origin of, of white matter of cord, 350 posterior branches of, 350 projection, 409 rod and cone, 425 superficial tangential, 407 terminations, 352 motor, 351 sensory, 352 tissue, 105 Golgi methods of staining, 29 technic for, 116 Nerves, cranial, 373 motor and sensory nuclei of, 373 4So INDEX. .Nerves, cranial, third (oculomotor), 399 root fibres and nucleus of origin of third, 399 fourth (pathetic), 397 root fibres and nucleus of origin of fourth, 397 fifth (trigeminus), 378, 381, 392, 393 mesencephalic root of fifth, 395- 396, 397 sensory and motor root fibres of fifth, 393 spinal root of fifth, 378, 381 sixth (abducens), 388 nucleus of origin of sixth, 388, 39° root fibres of sixth, 388, 390 seventh (facial), 388 nucleus of origin of seventh, 388, 390 root fibres of seventh, 388, 390 eighth (auditory), 385, 386, 387 cochlear branch of eighth, 386, 387 nuclei of eighth, 386, 387 root fibres of eighth, 384, 385, 386, 387 vestibular branch of eighth, 386, 387 ninth (glosso-pharyngeal), 379- 382 •descending or sensory root fibres of the ninth, 379 ■dorsal nucleus of ninth, 382, 384 motor nucleus of ninth, 382 root fibres of ninth, 382, 384 tenth (vagus), 379, 382 descending or sensory root fibres of tenth, 382, 384 dorsal nucleus of tenth, 382, 384 motor nucleus of tenth, 382, 384 root fibres of tenth, 382, 384 eleventh (spinal accessory), 378 nucleus of origin of eleventh, 378 root fibres of eleventh, 378 Nerves, cranial, twelfth (hypoglossal), 379- 381, 384 nucleus of origin of twelfth, 379- 381, 384, 385 root of twelfth, 384 root fibres of twelfth, 379, 381, 384 terminal nuclei of, 373 mixed spinal, 351 peripheral, 342 spinal, anterior, motor or efferent roots of, 357 sensory, or afferent portions, 35i Nervous system, the, 336 anterior cerebral vesicle, 336 corpora quadrigemina, 337 cerebral hemispheres, 336 cerebro-spinal, 336 corpus striatum, 337 development of, 336 ectoderm, 336 forebrain, 336 ganglia of, 339 hindbrain, 336 medulla oblongata, 337 mesencephalon, 336 midbrain, 336 middle cerebral vesicle, 336 neural fold, 336 groove, 336 tube, 336 optic thalamus, 337 pons Varolii, 337 posterior cerebral vesicle, 336 prosencephalon, 336 rhombencephalon, 336 spinal cord, 337 sympathetic, 336 Nervous system, cerebrospinal, the, 336 brain, 336 cerebellum, 401 cerebro-spinal axis, 336 cerebrum, 406 cranial nerves, 336 ganglia of, 340 isthmus, 395 medulla oblongata, 371 membranes of brain and cord, 338 INDEX. 481 Nervous system, cerebrospinal, mesen- cephalon, 305 midbrain, 395 peripheral nerves, 342 pineal body, 415 pituitary body (hypophysis cerebri), 414 pons Varolii, 37 spinal cord, 336, 344 nerves, 336 sympathetic, the, 336 ganglia, 336, 341 origin of, 338, 361 sympathetic nerves, 336 Neumann's dental sheath, 189 Neural fold, 336 groove, 336 tube, 336 cells of, 337 ependymal cells, 337 marginal veil of His, 337 neuroblasts of, 337 neuroglia cells of, 337 spongioblasts of His, 337 Neuraxone, 105 Neurilemma, no, 113 and axolemma,- relation of , 113 Neurite, 105 Neuroblasts, 105, 115, 337 Neuro-epithelium, 64 amacrine cells, 426 cone bipolar cells, 426 visual cells, 425 horizontal cells, 426 rod bipolar cells, 425 visual cells, 425 Neurofibrils, 107 Neuroglia, 88, 115 mossy cells of, 115, 337 Midler's cells, 427 neuroblasts of, 103, 115, 337 spider cells, 115, 337 spongioblasts of, 115, 337 Neurokeratin network, 113 Neuromuscular bundles, 354 Neurone, the, 105 axone of, 105 cell body of, 105 chromophilic bodies, 107, 10S contact theory of, 115 continuity theory of, 115 31 Neurone-cord, 369 Neurone, cortico-spinal motor, 370 dendrites of, 1 10 development of, 105 neurofibrils of, 107 Nissl, special method of technic for, 32 nucleolus, the, 106 nucleus, the, 106 perifibrillar substance, 107 physiological significance, 114 pigment in, no protoplasmic processes, no system, 362 afferent, 365 cortico-spinal, 367 cortico-spinal motor, 367 efferent, 357, 366 main motor, 366 No. I., 429 No. I., cone neurones of, 429 No. I., horizontal neurones of, 429 No. I., rod neurones of, 429 No. II., 430 No. III., 430 peripheral motor, 357 sensory, 365, 377 second, 365 spino-peripheral, 362, 377 spino-peripheral motor, 367 Neurones, cone association, 429 rod association, 429 Neuroplasm, 112 -. Neutral carmine, 1 7 Neutrophile granules, 92 Nipple, 331 Nissl method for staining nerve cells, 3 2 Nissl, concerning chromophilic bodies, 109 Nitric acid for decalcifying, 9 Nodes of Ranvier, 113 Normoblasts, 157 Nuclear dyes, 14 alum-carmine, 16 basic aniline, 16 combination of Gage's and Mayer's formulas, 16 Delafield's haematoxylin, 15 Gage's hematoxylin, 14 482 INDEX. Nuclear dyes, haematoxylin, 14 Heidenhain's hematoxylin, 15 Mayer's hasmalum, 15 Nuclear fluid, 40 sap, 40 structures, method of demon- strating by Flemming's fluid, 6 Nuclein, 40 Nucleolus of typical cell, 40 false, 40 Nucleoplasm, 40 Nucleoreticulum, 40 Nucleus, the, 39 accessory olivary, 382, 384, 385 arciform, 378, 381, 384, 3 8 5 caudatus, 400 Deiter's, 387 dentate, 405 dorsal accessory olivary, 382, 384 cochlear, 386 dorsalis, 365 funiculi cuneati, 364, 374 gracilis, 364, 374, 377 lateral vestibular, 387 lenticular, 401 median, of vestibular nerve, 387 membrane of, 40 network of, 39 of acoustic tubercle, 386 of a typical cell, 39 of column of Burdach, 364, 372, 374, 377 of Goll, 364, 37 2 - 374, 377 of funiculus teres, 385 of origin, 363 olivary, 372, 380, 381, 384, 385 pre-olivary, 391 pulposus, 169 red, 399 resting, 46 semilunar, 391 spinal vestibular, 387 superior vestibular, 387 trapezoid, 391 ventral cochlear, 386 von Bechterew's, 387 Nuel's space, 446 Nutrient canal, 159 foramen, 159 vessels, of bone, 159 Oculomotor nerve, 399 Odontoblasts, 188, 193 (Esophagus, the, 195 coats of, 195 glands of, 196 technic of, 196 Oil of origanum cretici for clearing specimens, 20 Olfactory bulb, 451 cells of, 451 granule layer, 451 layer of glomeruli, 451 of longitudinal fibre bundles, 451 of mitral cells, 451 of olfactory fibres, 451 olfactory glomeruli of, 451 organ, 450 olfactory bulb of, 451 olfactory mucosa of, 241, 45o technic of, 452 tract, 451 Olivary nucleus, 379, 381, 384, 385 Opie, concerning the pancreas, 230 concerning the cell-islands of Langerhans, 230 Optic cup, 435 nerve, 428 arachnoid of, 428 dural sheath of, 428 lamina cribrosa, 428 pial sheath, 428 relation to retina and brain, 428 subarachnoid space, 428 subdural space, 428 technic of, 437 stalk, 345 thalamus, 378 development of, 337 vesicle, 435 Ora serrata, 421, 424 Oral glands, cells of, 181 crescents of Gianuzzi, 182 demilunes of Heidenhain, 182 mixed, 181 mucous, 181 serous, 181 technic of, 183 Orange G, 17 INDEX. 4*3 Organ of Corti, 445 cells of Claudius of, 446 Corti 's arches, 446 Corti 's tunnel, 446 Deiter's cells, 446 hair or auditory cells, 446 Hensen's calls, 446 lamina reticularis of, 447 Nuel's space of, 446 phalangeal processes, 446 pillar cells, 445 Organ of Giraldes (paradidymis), 282, 313 Organs of Golgi, peripheral nerve terminations in, 354 Organ of hearing, 437 auditory pit of, 448 blood-vessels of, 447 development of, 448 ear, external, 437 ear, internal, 438 ear, middle, 438 ^ lymphatics, 447 nerves, 447 otic vesicle or otocyst, 448 technic of, 449 Organ of smell, 450 ; see Olfactory organ Organ of taste, 452 cells of, 452 foliate papillae, 452 gustatory canal, 452 intergeminal fibres of, 453 intrageminal fibres of, 453 taste buds, 186, 353, 452 technic of, 453 Organ of vision, 416 blood-vessels of, 431 development of, 435 eyeball or bulbus oculi, 416 eyelid, 434 lacrymal apparatus, 433 lens, 430 neurone systems of, 429, 430 optic nerve, 428 technic of, 436 Organs, the, 119 Organs of special sense, 416 olfactory organ, 450 organ of hearing, 437 of taste, 452 of vision, 416 Orth's fluid, 6 Osmic acid as a fixative, 6 action on fat, 6 on myelin, 6 stain for fat, 26 Ossicles of middle ear, 439 Ossification centres, 161 endochondral, 163 intracartilaginous, 163 intramembranous, 161 subperichondral, 166 subperiosteal, 166 Osteoblasts, 161 Osteoclasts, 163 Osteogenetic tissue, 161 Otocyst, 448 Otolythic membrane, 441 Otoliths, 441 Oval bundle of Flechsig, 368 Ovary, the, 292 blood-vessels of, 300 cortex of, 292 egg nest, 294 epoophoron, 300 Fallopian tube, 292, 301 germinal epithelium of, 292 Graafian follicles, 293 hilum of, 292 lymphatics of, 300 medulla of, 292 nerves of, 300 oviduct, 292, 301 ovum, 292, 296 paroophoron, 300 Pfliiger's egg tubes or cords, 294 primitive ova, 293 secretion of, 292 structure of, 292 technic of, 302 tunica albuginea, 292 zona vasculosa, 292 Oviduct, the, 301; see Fallopian tubes Ovula Nabothi, 304 Ovum, the, 43, 292, 296 atresia of follicle, 300 cells of, 296 deutoplasm granules, 297 development of, 296 fertilization of, 46 germinal spot, 296 4 8 4 INDEX. Ovum, maturation of, 297 perivitelline space, 297 segmentation of, 48 yolk granules of, 297 zona pellucida of, 296 Pacchionian bodies, 338 Pacinian bodies, 353 inner bulk of, 354 corpuscles, 290, 354 Palate, mucous membrane of, 180 Palatine tonsils, 144; see Tonsils Pancreas, the, 225 blood-vessels of, 230 cell-islands of Langerhans, 229 centro-acinar cells of Langerhans, 227 development of, 240 duct of Santorini, 226 of Wirsung, 226 epithelium of ducts, 226 intracellular secretory tubules of, lobes of, 225 [229 lobules of, 225 lymphatics of, 230 nerves of, 230 secondary excretory duct of, 226 sustentacular cells of, 228 technic of, 231 terminal tubules of, 226 zymogen granules of, 226 Paneth, cells of, 207 Panniculus adiposus, 317 Papillae, circumvallate, 184 compound, 316 filiform, 184 fungiform, 184 nerve, 316 simple, 316 vascular, 316 Paradidymis, or organ of Giraldes, 282 Paraffin embedding, 12 apparatus for, 1 2 section-cutting, 13 sections, staining and mounting of, 20 Paramiton, 38 Paranuclem, 40 Paraplasm, 30 Parathyroids, 256 technic of, 257 Pareleidin, 319 Paroophoron, 300, 313 Parotid gland, 222 development of, 240 Stenoni's duct of, 222 technic of, 225 Parovarium, 313 Pars ciliaris retinas, 421, 424 iridica retinae, 421, 424 optica retinas, 424 papillaris, 316 reticularis, 315 Pathetic nerve, 395 Peduncle, inferior, 395 middle, 395 superior, 395 Peduncles, cerebral, 400 Pellicula, 39 Penicillus, 148 Penis, 288 arteries of, 289 cavernous sinuses, 289 corpora cavernosa of, 288 corpus spongiosum of, 288 erectile tissue. 289 glans. 290 glands of Tyson of, 290 lymphatics. 290 nerve endings of, 290 prepuce of, 290 sebaceous glands of, 319 technic of, 292 tunica albuginea of, 288 Peptic glands, 199 Perforating fibres, 156 of cornea, 418 of Sharpey, 156 Periaxial sheath, 112 Pericardial cavity, 133 Perichondrium of bone, 163 of cartilage, 86 Perichorioidal lymph spaces, 419 Pericranium, 162 Peridental membrane, 191 Perifascicular sheath, 172, 343 Perifibrillar substance, 107 Perilymph, 439 Perimysium, 172 Perineurium, 339, 343 Periosteal buds, 164 Periosteum, 155 INDEX. 4S5 Periosteum, dental, 191 Peripheral afferent neurone system, 365 efferent neurone system, 357 motor neurone system, 357 neurones of spinal cord, 357 neurones of cranial nerves, 368 nerves, 342 afferent part of, 342, 351 cranial, 342 efferent part of, 342 endoneurium of, 343 epineurium of, 343 fascicles of, 343 intrafascicular connective tissue of, 343 medullated fibres of, 111, 112, 343 motor or efferent, 342 motor nerve terminations, 357 non-medullated fibres of , 1 1 1 , 112, 343 perifascicular sheath of, 343 perineurium of, 343 sensory or afferent, 342 sensory nerve terminations, 352 structure of, 342 technic of, 344 nerve terminations, 352 annular, 354 end bulbs, 352 free endings, 355 in penis, 290 in genital corpuscles, 280 in heart muscle, 354 in mucous membrane of mouth and conjunctiva, 353 in muscle-tendon junctions, 354 in skin, 316, 352 in smooth muscle, 354 in voluntary muscle, 354 Krause's end-bulbs in penis, 290 Meissner's corpuscles in pa- pilla; of penis, 290 muscle spindles, 354 Peripheral nerve terminations, muscle- tendon organs of Golgi, 354 neuromuscular bundles, 354 Pacinian bodies, 353 Pacinian corpuscles of penis, 290 Rufhni's theory of, 354 spinal, 342 spiral, 354 tactile cells, 352 corpuscles, 352 meniscus, 352 taste buds, 353 sensory neurone system, 365 Peritoneal cavity, 133 Perivitelline space, 297 Permanent teeth, 192 Petit, canal of, 431 Peyer's patches, 208 Pfliiger's egg tubes or cords, 294 Phagocytes, 92, 141 Phagocytosis, 92 Phalangeal processes, 446 Pharyngeal tonsils, 146; see Tonsils Pharynx, the, 194 blood-vessels of, 194 lymphatics of, 194 nerves of, 194 technic of, 194 Pia mater, 338 blood-vessels of, 339 cerebralis, 338 Pacchionian bodies, 338 spinalis, 338 technic of, 339 Picric acid as a fixative, 7 Picro-acid-fuchsin, 18 Picro-carmine, 18 Pigment granules in cells, 39 in nerve cells, no, 342 Pillar cells, 445 Pineal body, 415 brain sand of, 415 technic of, 415 Pineal eye, 415 Pinna, 437 Pituitary body, 412 anterior lobe of, 412 infundibulum of, 415 posterior lobe of, 412 technic of, 415 486 INDEX. Placenta, 308 blood-vessels of, 31c canalized fibrin, 310 cell patches, 309 chorionic villi, 309 cotyledons, 309 fastening villi, 309 foetalis, 308 free or floating villi of, 309 lymphatics of, 311 membrana chorii of, 308 nerves of, 311 septa of, 310 subchorionic placental decidua, 310 syncytium of, 309 technic of, 314 uterina, 310 villi of, 308 Plasma dyes, 17 eosin, 17 neutral carmine, 17 Plasmosome, 40 Plastids, 38 Plastin, 38 Pleural cavity, 133 Pleuroperitoneal cleft, 133 Plexus annularis, 433 Auerbach's, 206, 211, 217 Heller's, 215 Meissner's, 209, 211, 218 myentericus, 217 Plicae palmatae, 304 Polar bodies, 47 rays, 44 Polymorphonuclear leucocytes, 91 Polynuclear leucocytes, 91 Pons Varolii, 371, 391, 393 longitudinal fibres of, 391 pontile nuclei of, 391 pyramid of, 389 transverse fibres of, 388, 391 Posterior columns of spinal cord, 364 column of Burdach, 364 of Goll, 364 distribution of fibres of, 364 nucleus funiculi cuneati, 364 funiculi gracilis, 364 of the column of Burdach, 364 of the column of Goll, 364 Posterior columns of spinal cord, ori- gin of fibres of, 350, 355 zone of Lissauer, 364 Posterior horns, 346, 374, 380, 382, 38S. 389 longitudinal fasciculus, 376, 379, 381, 384, 385, 390, 393, 397 median septum, 345 nerve root, 346 root fibres, 346 tract, or terminal zone of Lis- sauer, 364 Potassium hydrate, as a macerating fluid, 4 Precapillary artery, 123 Pre-olivary nucleus, 391 Prepuce, 290 Preserving, 8 Primary germ layers, 49 renal vesicles, 313 Primitive ova, 293 Projection fibres, 409 Pronephritic or Wolffian ducts, 313 Pronephros, 313 Pronucleus, 47 Prophase, 43 Prosencephalon, 336 Prostate gland, 286 blood-vessels of, 287 capsule of, 286 corpora amylacea of, 287 crescentic corpuscles of, 287 epithelium of, 287 lymphatics of, 288 Miillerian duct, 287 nerves of, 288 technic of, 288 trabecular, 286 uterus masculinus, 287 utriculus prostaticus, 287 vesicula prostatica, 287 Protoplasm, 37 theories of structure of, 38 Protoplasmic movement, 42 processes, 105, 110 radiation, 40 Proximal convoluted tubule, 263 Prussian blue gelatin as an injecting fluid, 22 Pseudopodia, 42 Purkinje cells, 401, 403 INDEX. 487 Pyloric glands, 199, 201 Pyramid, cortical, 260 of Ferrein, 260 Malpighian, 260 Pyramidal decussation, 366 tracts.. 366 anterior pyramids, 366 crossed pyramidal, 366 direct pyramidal, 366 pyramidal decussation, 366 of Tiirck, 366 Pyrenin, 40 Ranvier's alcohol as a macerating fluid, 4 nodes, 113 Raphe\ of semicircular canals, 442 median, 378, 381 Rectum, 214 columnae rectales, 214 technic of, 220 Reissner, membrane of, 444 Remak, fibres of, 112 Renal corpuscle, 260, 262 development of, 262, 313 Renculus, 258 Replacing cells, 60 Reproduction of cells, 43 Reproductive system, 274 development of, 282, 300, 312 rudimentary structures con- nected with the, 282 female organs, 292 clitoris, 312 Fallopian tube, 301 ovary, 292 oviduct, 301 placenta, 308 urethra, 290 uterus, 303 vagina, 311 vestibule, 312 male organs, 274 Cowper's glands, 288 ejaculatory ducts, 282 penis, 288 prostate gland, 286 seminal ducts, 280 vesicle, 282 seminiferous tubule, 275 testis, 274 Reproductive system, male organs, urethra, 241 Respiratory cells, 252 epithelium, 252 Respiratory system, 241 bronchi, 246 development of, 254 larynx, 243 lungs, 448 nares, 241 trachea, 243 Restiform body, 379, 380, 381, 384, 385 Rete testis, tubules of, 280 vasa efferentia, 280 Reticular formation, 375, 377, 381, 383. 385. 389- 393.397 glands, 177 process, 346, 350 tissue, 77 Retina, 424 blood-vessels of, 431 cells of, 424, 425, 426, 427 ellipsoid of Krause, 425 fibre-baskets of, 427 fovea centralis, 427 ganglionic layer of, 424 horizontal cells of, 426 inner limiting membrane of, 426 inner molecular layer, 425 inner nuclear layer, 425 layer of nerve cells, 426 of nerve fibres, 426 of neuro-epithelium, 424 of pigmented epithelium, 424 of rods and cones, 425 macula lutea, 427 Midler's cells and fibres, 426 ora serrata, 424 outer limiting membrane, 424 outer molecular layer, 425 outer nuclear layer, 425 pars ciliaris retina?, 424 iridica retinas, 424 optica retinae, 424 relation to optic nerve, 428 rod and cone cells of, 425 visual purple of, 425 Retinaculae cutis, 3 1 7 Retzius, lines of, 190 Rhombencephalon, 336 Ribboning paraffin sections, 14 488 INDEX. Rod association neurones, 429 fibres, 425 Rod- visual cells, 425 Rods, layer of rods and cones, 425 Rolando, fissure of, 366 gelatinous substance of, 346, 347 Rollett's theory of striated muscle, 98 Root cells, 356 Ruffini, corpuscles of, 329 theory of nerve terminations, 354 Rugae, 19S Riihle, concerning the uriniferous tubule, 264 Saccule, 441 and utricle, 441 auditory hairs of, 441 macula acustica, 441 neuro-epithelial cells of, 441 otolithic membrane of, 441 otoliths of, 441 sustentacular cells of, 441 Sacral segments of spinal cord, 344 Safranin, 16 Salivary corpuscles, 145 glands, 221 blood-vessels of, 223 capsule of, 221 development of, 240 ducts of, 222 interstitial tissue, 222 lobes of, 222 lobules of, 222 lymphatics of, 224 minute structure of, 181 nerves of, 224 parenchyma of, 222 parotid, the, 222 secretory tubules of, 222 . sublingual, the, 223 submaxillary, the, 223 trabecular of, 222 technic of, 225 tubules of, 222 Santorini, duct of, 226, 240 Sarcolemma, 96 Sarcoplasm, 97 Sarcostyles, 173 Sarcous elements of Bowman, 97 Scala media, 443 tympani, 443 Scala vestibuli, 443 Scarpa's ganglion, 388 Scheme of neurone relations of spinal cord, 359 Schlemm, canal of, 421 Schmidt-Lantermann segments, 113 Schultze, comma tract of, 368 Schwalbe, lymph paths of, 432 Schwann, sheath of, 110.. 113 Sclera, the, 416 lamina cribrosa of, 416 fusca of, 416 Scrotum, skin of, 316 Sebaceous glands, 319 development of, 331 of glans penis, 319 of labia minora, 319 of margin of lips, 319 of prepuce, 319 Sebum, 327 Secondary cochlear tract, 388 trigeminal tract, 397 vestibular tract, 388 Secretion, 218 Secretory tubules, 222, 223 Golgi method of demonstrating, 25 of parietal cells of stomach, 201 Section cutting, 13 celloidin specimens, 13 paraffin specimens, 13 Section staining, 14 Segmentation cavity, 49 Segmentation of ovum, 48 Semen, 284 Semicircular canals, 440, 442 crista acustica of, 442 cupula of, 442 raphe" of, 442 semilunar fold of, 442 Semilunar nucleus, 391 Seminal ducts, 280 epididymis, 280 vas deferens, 281 vasa efferentia, 280 vesicles, 282 Seminiferous tubules, 275 cells of, 276 columns of Sertoli, 276 convoluted portion of, 275 glandular cells of, 276 INDEX. 489 Seminiferous tubules, rete testis, 275, 280 spermatids, 278 spermatocytes, 278 spermatogenic cells, 276 spermatogones, 277 spermatozoa, 279 straight portion of, 279 supporting cells of, 276 sustentacular cells of, 276 technic of, 286 Sensory conduction path, 365, 383 decussation, 376, 378, 381 peripheral nerves, 342 Septa renis, 260; see Kidney Septo-marginal tract, 368 Septum linguae, 183 Serial sections, 14 Serous membranes, 133 Sertoli, cells of, 276, 285 columns of, 276 Sharpey's fibres, 156 Sheath of Henle, 113, 343 medullary, no, 113 of Schwann, no, 113 perifascicular, 172 Silver-nitrate method of staining inter- cellular substance, 25 Skein, 43 Skeletal system, 152 articulations, 169 bone-marrow, 156 bones, 152 cartilages, 168 Skin, 315 blood-vessels of, 315 color of, 319 corium, 315 corpuscles of Grandry, 352 of Meissner, 353 of Ruffini, 329 cuticle, 327 derma, 315 development of, 330 eleidin of, 318 epidermis of, 317 Golgi-Mazzoni corpuscles of, 329 hair follicles of, 323 kerato-hyalin granules, 318 Krause's end-bulbs, 330 lymphatics of, 329 Skin, Merkel's corpuscles of, 352 mitosis of cells of, 319 nerves of, 329, 352 of scrotum, 316 Pacinian bodies of, 353 panniculus adiposus of, 317 pareleidin, 319 pars papillaris, 316 pars reticularis, 315 peripheral nerve terminations in, 352 prickle cells of, 318 retinaculae cutis, 317 sebaceous glands of, 319 subcutaneous tissue of, 316 sweat glands (glandulae sudorip- arae), 319 sweat pores of, 319 tactile cells of, 352 tactile corpuscles, 330, 353 technic of, 320 for blood-vessels of, 330 Vater-Pacinian corpuscles of, 329 Skin and its appendages, 315 development of, 330 hair, 322 mammary gland, 331 nails, 320 Small intestines, 204 Auerbach's plexus, 210, 217 blood-vessels of, 215 Brunner's glands of, 209 cells of, 205 chyle capillaries of, 217 coats of, 205 crypts of Lieberkiihn, 207 development of, 239 lymphatics, 217 Meissner 's plexus, 209, 218 muscle of, 209 nerves of, 217 Peyer's patches of, 208 technic of, 220 valvulas conniventes of, 204 villi of, 204, 205, 216 Smooth muscle, 95; see Involuntary muscle Sodium hydrate as a macerating fluid, 4 Solitary fasciculus, 378, 381, 384, 385 follicles, 202, 20S, 211 Somatochromes, 108 49Q INDEX. Spermatids, 278, 285 Spermatocytes, 278, 285 Spermatogenesis, 2S5 Spermatozoa, 46, 284 development of, 46, 285 structure of, 284 technic of, 2 86 Spider cells, 115 Spinal accessory nerve, 378 Spinal cord, 344 anterior columns of, 346 horns of, 346 marginal bundle of Loewen- thal, 367 median fissure, 345 nerve roots of, 346 white commissure of, 347 antero-lateral columns of, 346 antero-lateral ascending tract, 358 antero-lateral descending tract, 367 arachnoid membrane of, 338 ascending tracts of, 358, 364 cell column of lower extremity, 357 cell column of upper extremity, 357 cells of the gray matter of, 350 of Golgi, Type II., 360 column, 358 root, 358 central canal of, 346 gelatinous substance, 346, 347 cervical enlargement of, 344 segments of, 344 Clarke's column of, 348 coccygeal segments of, 344 column of Burdach, 350, 364 of Goll, 350, 364 cells, 357 hecateromeric, 358 heteromeric, 358 tautomeric, 358, 365 comma tract of Schultze, 368 conduction paths of, 362, 365, 367 cornua of, 346 crossed pyramidal tract, 366 descending paths from higher centres, 370 Spinal cord, descending tract from Deiter's nucleus, 367 from vestibular nuclei, 367 direct ascending paths to higher centers, 358, 370 pyramidal tract, 356, 359,. 366 reflex path, 369 dorsal gray commissure, 346 segments of, 344 dura mater of, 338 efferent fibre systems of, 366 fasciculi, 362 fibre tracts of, 362 filum terminale of, 234 fundamental columns of, 358,. 368 ganglion cells of, 347 gelatinous substance of Rolando,. 346, 347 Gowers' tract, 358 gray matter of, 344, 346 ground bundles of, 358, 368 Helweg's tract, 367 indirect ascending paths to higher centres, 369 indirect reflex paths, 369 interchange of fibres, 347 intermedio-lateral column, 357 lateral column of, 346 longitudinal section of six days' chick embryo, 361 lumbar enlargement of, 244 segments of, 344 main motor fibre systems of, 366 medial column of, 357 medullated fibres of, 347, 348 membranes of, 338 arachnoid, 338 blood-vessels of, 339 dura mater, 338 pia mater, 338 technic of, 339 mixed spinal nerve, 357 multipolar ganglion cells of, 347,. 356 neuroglia cells, 347 tissue, 346 neurone systems of, 362, 365, 367 origin of fibres of white matter, 350 INDEX. 491 Spinal cord, origin of posterior col- umns of, 350 oval bundle of Flechsig, 368 peripheral motor or efferent neu- rone system, 357 sensory of afferent neurone system, 364, 365 pia mater of, 338, 345 plexus of fine fibres, 347 posterior columns of, 346, 364 horns of, 346 median septum, 345 nerve roots, 346 root fibres, 346, 347 pyramidal tracts, 359, 366 quadrigemino-spinal tract, 367 reticular process of, 346, 350 rubro-spinal tract, 367 sacral segments of, 344 scheme of neurone relations of, 359 section through cervical enlarge- ment, 349 through lumbar enlargement, 345 through mid-dorsal region of, 348 through twelfth dorsal seg- ment, 348 segments of, 344 septo-marginal tract, 368 shape of, 345 short fibre systems of, 358, 368 size of, 345 technic of, 342, 347, 360, 371 tractus cortico-spinalis, 366 reticulo-spinalis, 368, 375 spino-cerebellaris centralis, 365 spino-cerebellaris dorsalis, 365 transverse section of six days' chick embryo, 360 ventral gray commissure, 346 Von Monakow's tract, 367 white commissure of, 347 white matter of, 344, 346 zone of Lissauer, 347 Spinal ganglia, 340 development of, 338 technic of, 342 Spinal ganglion cells, ascending arms from central processes of, 356 centrally directed arm of, 355 classification of, 340 collaterals from, 356 descending arms from central processes of, 356 development of, 351 Dogiel's classification, 340 ectodermic origin of, 350 peripheral arms of, 351 relation to dorsal roots, 355 structure of, 340 technic of, 360 Spino-cerebellar tract (dorsal), 365 -cerebellar tract (ventral), 365 -peripheral motor neurone sys- tem, 367 -tectal tract, 366 Spiral ganglion, 448 ligament, 442 prominence, 444 terminations, 354 Spireme, 43 Spleen, 146 blood-vessels of, 148 cavernous veins of, 148 cells of, 149 connective-tissue of, 146 cords of, 148 corpuscles of, 147 ellipsoids of, 148 lymphatics of, 150 Mall's theory of vascular channels, of pulp, 149 Malpighian bodies of, 146 nerves of, 150 pulp of, 146, 1 48 cords of, 148 spindles of, 14S technic of, 150 Splenic corpuscles, 146 Spongioblasts, 115 of His, 337 Spongioplasm, 38 Staining, 14 double with hasmatoxylin-eosin, i? in bulk, iS methods, special, 25 special neurological, 27 492 INDEX. Staining paraffin sections, 20 sections, 1 7 double with haematoxylin- eosin, 1 7 triple with haematoxylin- picro-acid-fuchsin, 18 Stains, nuclear dyes, 14 plasma dyes, 17 Stalked hydatid, 314 Stapes, 439 Stenoni, duct of, 222 Stomach, 198 acid cells of, 200 adelomorphous cells of, 199 Auerbach's plexus, 211 blood-vessels of, 215 cardiac glands of, 200 chief cells of, 199 delomorphous cells of, 199 development of, 239 epithelium of, 198 gastric crypts of, 198 glands of, 198, 199 pits of, 198 lymphatics of, 217 mucous membrane of, 198 muscular coat of, 203 nerves of, 217 parietal cells of, 200 peptic cells of, 199 glands of, 199 pyloric glands of, 199, 201 rugae of, 198 secretion of, 218 solitary follicles of, 202 technic of, 203 Stomata, 133 Stratum fibrosum, 169 granulosum, 295 synoviale, 170 Streaming of protoplasm, 42 Stria vascularis, 444 Stroma of mucous membranes, 178 of the red blood cell, 89 Styloglossal fibres, 183 Subchorionic placental decidua, 310 Subcutis, 316, 329 Sublingual gland, 223 development of, 240 duct of Bartholin of, 223 technic of, 225 Submaxillary gland, 223 development of, 240 duct of Wharton of, 223 technic of, 225 Submucosa, 178 Subperichondrial ossification, 166 Subperiosteal ossification, 166 Substantia nigra, 395, 397 propria corneas, 418 Sulcus, external spiral, 444 Superior cerebellar peduncles, 395, 397 olive, 391, 393 Supra-renal, 270 Suspensory ligament, 430 Sustentacular cells, 228 Sweat glands, 319 development of, 331 ducts of, 319 muscle tissue of, 331 Sweat pore, 319 Sympathetic ganglia, 341 cells of, 342 development of, 338 in Auerbach's plexus, 342 in Meissner's plexus, 342 pigmentation of cells of, 342 structure of, 342 technic of, 342 Sympathetic nervous system, 336; see Nervous system (sympathetic) Synarthrosis, 169 Synchondrosis, 169 Syncytial tissue, 102 Syncytium, 102, 309 Syndesmosis, 169 Synovial membrane, 170 villi, 170 Tactile cells, 352 corpuscles, 329, 330 of Meissner, 329 of Wagner, 329 Tapetum cellulosum, 419 fibrosum, 419 Tarsal glands, 434 Tarsus, 434 Taste buds, 186, 353, 452 Tautomeres, 358 Teasing, 4 Technic, general, 3 Tectum, 366 INDEX. 493 Teeth, 187 blood-vessels of, 191 cementum of, 187, 190, 193 crown of, 187 dental canals, 188 germ, 191 papilla, 191 periosteum, 191 ridge, 191 sac, 191 dentinal fibres, 188 pulp, 187 dentine of, 187, 188 development of, 191 common dental germ, 191 cuticular membrane, 193 dental papilla, 191 ridge, 191 enamel organ, 191 special dental germ, 191 technic of, 193 Tomes' process, 193 enamel of, 187, 190, 193 cells, 192, fibres, 190, 193 organ, 191, 192, 193 prisms, 190, 193 interglobular spaces, 189, 193 lymphatics of, 191 milk, 191, 192 nerves of, 191 Neumann's dental sheath, 189 odontoblasts of, 188, 193 peridental membrane, 191 permanent , 192 pulp cavity, 1S7, 193 root of, 187 technic of, 193 Tomes' granular layer, 190 Tegmentum, 395, 397 brachia conjunctiva, 397 central tegmental tract, 397 descending motor or mesenceph- alic root of fifth nerve, 397 fillet, 397 fourth cranial nerve, 397 Gowers' tract, 397 lateral lemniscus, 397 posterior longitudinal fasciculus, 397 reticular formation, 397 Tegmentum, superior peduncles, 397 Von Monakow's bundle, 397 Telophase, 46 Tendon, structixre of, 71 sheaths, 172. Tendon-muscle junction, 172 organs of Golgi in, 354 peripheral-nerve terminations in, 354 Tenon, capsule of, 432 Tensor chorioideae, 422 Terminal arborizations, no bronchus, 251 nucleus, 363 zone of Lissauer, 364 Testis, 274 blood-vessels of, 283 corpus Highmori, 274 ducts of, 280 epididymis of, 275 lobules of, 274 lymphatics of, 284 mediastinum, 274 nerves, 284 secretion of, 284 semen, 284 seminal ducts of, 280 seminiferous tubules of, 275 spermatozoa, 276, 284 technic of, 286 tunica albuginea of, 274 vaginalis, 274 vasculosa, 274 Theca folliculi, 295 Thermotaxis, 42 Thionin, 16 Thoma, ampulla of, 150 Thomas, fasciculus of, 368 Thoracic duct, 133 Thrombocytes, 92 Thymus, 142 blood-vessels of, 143 development of, 142 Hassan's corpuscles, 143 lymphatics of, 143 nerves of, 143 technic, 143 Thyroid, 255 blood-vessels of, 256 colloid of, 255 development of, 256 494 INDEX. Thyroid, isthmus of, 255 lymphatics of, 256 nerves of, 256 technic, 257 Tissue elements, dissociation of, 4 Tissues, 53 adipose, 79 blood, 89 bone, 86 cartilage, S3 classification of, 55 connective, 67 elastic, 72 epithelial, 57 erectile, 289 examination of fresh, 4 fat, 79 histogenesis of, 55 lymphatic, 79 muscle, 95 nerve, 105 osteogenetic, 161 subcutaneous, 316 Toluidin blue, 16 Tomes' granular layer, 190 process, 193 Tongue, 183 blood-vessels of, 185 circum vallate papillae, 184 connective tissue of, 183 Ebner's glands, 185 end-bulbs of Krause, 186 filiform papillae, 184 fungiform papilla?, 184 glands of, 182 longitudinal fibres of, 183 lymph follicles of, 145, 185 spaces of, 186 muscles of, 183 nerves of, 186 papilla? of, 184 septum linguae, 183 taste buds, 186, 353 technic of, 186 transverse fibres of, 183 vertical fibres of, 183 Tonsils, 144 blood-vessels of, 146 crypts of, 144 development of, 146 lingual; folliculae linguales, 145 Tonsils, lymphatics of, 146 nerves of, 146 palatine or true, 144 pharyngeal, 146 salivary corpuscles of, 145 technic of, 146 Trachea, 243 blood-vessels of, 245 cartilages of, 244 lymphatics of, 246 muscle cells of, 245 nerves of, 246 technic of, 246 Tract, antero-lateral ascending, 365, 372 antero-lateral descending, 367 Burdach's, 350, 364, 376, 379 comma, of Schultze, 368 cortico-spinalis, 366 crossed pyramidal, 366, 374, 375 descending, from Deiter's nucleus, 3 6 7. 3 8 9 from vestibular nuclei, 367 direct cerebellar, 365, 374, 377,. 380 pyramidal, 366 dorso-lateral ascending, 365 fundamental or ground bundles, 368 Flechsig's, 365 Goll's, 350, 374, 376 Gowers', 358, 365, 376, 380, 382, 385, 389 Helweg's, 367, 374 Lissauer's, 364 marginal bundle of Loewenthal, 367 oval bundle of Flechsig, 367 posterior, 364 pyramidal, 366 quadrigemino-spinal, 367 reflex, 369 reticulo-spinalis, 368 rubro-spinal, 367 secondary cochlear, 388 vestibular, 388 septo-marginal, 368 short fibre, 358, 368 spinal cord, 346, 348, 350, 356, 357. 362 spino-cerebellaris centralis, 365 INDEX. 495 Tract, spinocerebellar is dorsalis, 365 spino-tectal, 366, 374, 3 8l > 3 8z < 385 Turck's, 366 Von Monakow's, 367, 374, 377, 381, 382, 385, 389, 397 Tractus, see Tract Transitional leticocytes, 91 Trapezoid nucleus, 391 Trapezium, 391 Trigeminus nerve, 378, 392 True axone, 355 corpora lutea, 298 Tubules, arched, 261, 264 collecting, 261, 264 distal, 261, 263 first or proximal, 260, 263 second or distal, 261, 263 seminiferous, 275 straight, 261, 264 uriniferous, 260 Tunica albuginea, of ovary, 292 of penis, 288 of testis, 274 dartos, 316 fibrosa, 285 propria, of mucous membranes, 178 vaginalis, 274 vasculosa, 274, 296 Tiirck, tract of, 366 Tympanic membrane, 438 Tympanum, 43S; see also Ear, mid- dle Tyson, glands of, 290 Ultimate fibrillae, 97 Unipolar nerve cells, 106 Ureter, 269 blood-vessels of, 268 coats of, 268 development of, 313 glands of, 268 lymphatics of, 269 nerves of, 269 technic of, 273 Urethra, female, 290 glands of Littre" of, 290 Urethra, male, 290 blood-vessels of, 291 fossa navicularis, 291 Urethra, glands of Littre' of, 291 technic of, 292 Urinary bladder, 269 blood-vessels of, 269 development of, 312 epithelium of, 269 fibrous layer of, 269 lymphatics of, 269 muscular layers of, 269 mucous membrane of, 269 nerves of, 270 technic of, 273 Urinary system, 258 development of, 312 kidney, 258 ureter, 268 urinary bladder, 269 Uriniferous tubule, 260 arched tubule of, 261, 264 ascending arm of Henle's loop, 260, 263 Bowman's capsule, 260 descending arm of Henle's loop, 260, 263 development of, 262 duct of Bellini, 261 first or proximal convoluted, 260 263 glomerulus, 260 Henle's loop, 260, 263 Malpighian body, 260, 262 membrana propria of, 262 neck of, 263 renal corpuscle, 260 second or distal convoluted, 261, 263 straight or collecting, 261, 264 Uterus, 303 blood-vessels of, 310 coats of, 303 decidual cells of, 307 development of, 312; see also Reproductive system, develop- ment of lymphatics of, 311 masculinus, 2S7 muscle cells of, 303 muscle tissue of, 303 stratum submucosum, 303 supra-vasculare, 303 vasculare, 303 49 6 INDEX. Uterus, nerves of, 311 placenta, 308 mucosa of menstruating, 305 of pregnant, 307 of resting, 304 stage of menstruation proper, 306 of preparation, 305 of reparation, 306 technic of, 314 with placenta in situ, technic of, 3 I 4 Utricle, 441 ; see also Saccule and utri- cle Utriculosaccular duct, 440 Utriculus prostaticus, 287 Uvula, mucous membrane of, 180 Vagina, 311 blood-vessels of, 312 coats of, 310 lymphatics of, 312 nerves of, 312 rugae of, 311 technic of, 314 Vagus nerve, 379 Valve, Heisterian, 237 of Vieussens, 395 Valves of heart, 131 of veins, 127 Valvulae conniventes, 198, 204 Van Gehuchten's theory of peripheral arm of ganglion cell, 355 Vas deferens, 281 ampulla of, 286 technic of, 286 Vas epididymis, 281 Vasa efferentia, 281 vasorum, 128 Vascular papilla?, 316 Vascular system, 121; see also Circu- latory system Vater-Pacinian corpuscles, 329 Veins, 126 adventitia of, 128 arcuate, 267 central, 233 coats of, 126 development of , 132 intima of, 127 media of, 1 27 portal, 233 Veins, stellate, of Verheyn, 267 technic of, 128 valves of, 127 vasa vasorum, 128 vena? vorticosae, 419 Vena? vorticosae, 419 Ventricle, fourth, 381, 382, 384, 385, 39o, 393 muscle of, 130 Verheyn, stellate veins of, 267 Vermiform appendix, 212 coats of, 213 lymph nodules of, 214 technic of, 220 Vermis of cerebellum, 365 Vesicle, air. 249 anterior cerebral, 336 middle cerebral, 336 optic, 435 otic, 448 posterior cerebral, 336 Vesicula prostatica, 282, 287 Vestibular ganglion, 388 nucleus, 387 descending tract from, 374, 376, 379 Vestibule, 312, 440 ductus reuniens of, 440 endolymphatic duct, 440 saccule of, 440, 441 utricle of, 440, 441 utriculosaccular duct of, 440 Vieussens, valve of, 395 Villi, 204, 205, 216 lacteals of, 220 synovial, 170 Visual purple, 425 Vitreous body of the eye, 431 Cloquet's canal, 431 hyaloid canal of, 431 Vitreous membrane of chorioid, 420 of iris, 424 Vocal cords, 243 Volkmann's canal, 155, 159 Voluntary striated muscle, 96; see Muscle, striated, voluntary Von Bechterew's nucleus, 387 Von Monakow's bundle, 367, 374, 377, 381, 382, 385, 389, 397 Wagner, corpuscles of, 329 INDEX. 497 Washing after fixation, 8 Weigert's elastic-tissue stain, 25 haematoxylin, 16 method of staining medullated nerve fibres, 27 Weigert-Pal method, 28 Wharton's duct, 223 White matter, 334 Wirsung, duct of, 226 Wolffian body, 313 ridges, 312 Wrisburg, cartilage of, 243 Xylol and cajeput oil for clearing, 20 -balsam, 20 Xylol-paraffin for embedding, 12 Zenker's fluid for decalcifying, 5 for fixation, 7 Zinn, zonule of, 430 Zona pectinata, 445 pellucida, 49, 296 tecta, 445 Zone of Lissauer, 347 of oval nuclei, 242 of round nuclei, 242 Zonula ciliaris, 430 Zonule of Zinn, 430 Zymogen granules, 226 technic of, 231 32 COLUMBIA UNIVERSITY LIBRARIES This book is due on the date indicated below, or at the expiration of a definite period after the date of borrowing, as provided by the library rules or by special arrangement with the Librarian in charge. DATE BQ&frYlGD* i ' L,r DATE DUE DATE BORROWED DATE DUE 61949 C28 ( 842 ) MSO V^OJc >\S