Columbia ©ntoertfitj intIifCttpoflemg0rk College of ^fjpsiictans! ano burgeons From the Library of PROFESSOR PHILIP HANSON HISS 1868-1913 Donated by Mrs. Philip Hanson Hiss Digitized by the Internet Archive in 2010 with funding from Columbia University Libraries http://www.archive.org/details/textbookofhistol1904bail 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 PROFUSELY ILLUSTRATED NEW YORK WILLIAM WOOD & COMPANY MDCCCCIV Copyright, IWi, By WILLIAM WOOD AND COMPANY THE PUBLISHERS' PRINTING COMPANY NEW YORK PREFACE. 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 and 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. PAGE General Techxic. General Considerations. .......... 3 Examination of Fresh Tissues. ......... 3 Dissociation of Tissue Elements, 4 Teasing. ............. 4 Maceration = 4 Fixation. ............. 4 Hardening, 7 Preserving, ............. 7 Decalcifying. S Embedding, 9 Celloidin Embedding 9 Paraffin Embedding, . . . . . . . . . .11 Section Cutting 12 Celloidin Sections. ........... 12 Paraffin Sections, . . . . . . . . . c .12 Staining, . 13 Nuclear Dyes, 13 Plasma Dyes, . . . . . . . . . . -15 Staining Sections, 16 Staining in Bulk, 17 Mounting. ............. 17 Staining and Mounting Paraffin Sections, 19 Injection. ............. 20 CHAPTER II. Special Staining Methods. Silver Nitrate Method of Staining the Intercellular Substance, Chlorid of Gold for Demonstrating Connective-tissue Cells. Weigert's Elastic Tissue Stain. ....... Golgi"s Chrome-silver Method for Staining Secretory Tubules. v VI CONTENTS. Mallory's Hematoxylin Stain for Connective Tissue, Osmic Acid Stain for Fat, JenneFs Blood Stain, ....... 24 24 24 CHAPTER III. Special Neurological Staining Methods Weigert's Method of Staining Medullated N Weigert-Pal Method, Golgi Methods of Staining Nerve Tissue, Slow Method, Rapid Method, Mixed Method, Formalin-bichromate Method, Bichloride Method, Golgi-Cox Method, Nissl's Method General References on Technic, . erve Fibres, 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 Cell-division, Fertilization of the Ovum, . Technic, .... References for further study, 33 33 34 35 35 36 37 37 37 37 38 38 38 38 39 39 39 42 46 47 CONTENTS. vn PART III.— THE TISSUES. CHAPTER I. PAGE Histogenesis— Classification 5 1 Tissues Derived from Ectoderm, 51 Tissues Derived from Entoderm, 5 1 Tissues Derived from Mesoderm, S 2 CHAPTER II. Epithelium (Including Mesothelium Histogenesis, General Characteristics, Classification, Simple Epithelium, Simple Squamous, . Simple Columnar, . Pseudostratified, Stratified Epithelium, . Stratified Squamous, Transitional, Stratified Columnar, Modified Forms of Epithelium, Ciliated Epithelium, Pigmented Epithelium, . Glandular Epithelium, . Neuro-epithelium, . Mesothelium and Endothelium. Technic, .... and Endothelium), 53 53 53 54 54 54 54 56 56 56 57 5S 59 59 60 60 60 60 61 CHAPTER III. The Connective Tissues, ... 63 Histogenesis, 63 General Characteristics, 63 Classification, 63 Fibrillar Connective Tissue, 64 Areolar Connective Tissue, 67 Formed Connective Tissue, 67 Development, ........... 68 Elastic Tissue 6S Technic for Fibrillar and Elastic Tissue, ...... 70 Embryonal and Mucous Tissue, . . . . . . . • • 7 1 Technic, 73 Recticular Tissue, 73 Till COA'TENTS. Lymphatic Tissue. Technic for Reticul Fat Tissue, Technic. . Cartilage. Hyaline, . Elastic, . Fibrous. . Technic, . Bone Tissue. Technic, . Neuroglia. ir and Lymphatic Tissue, PAGE 75 75 75 79 79 8o Si 8i 82 82 83 84 CHAPTER IV. The Blood, 85 Red Blood Cells, ' 85 White Blood Cells, 86 Blood Platelets, 88 Development, ............ S8 Technic, 89 CHAPTER V. Muscle Tissue. Involuntary Smooth Muscle, Voluntary Striated Muscle, . Involuntary Striated Muscle, Development of Muscle Tissue, Technic, .... 9' 91 92 96 98 99 CHAPTER VI. Nerve Tissue, .... IOI The Neurone, .... IOI General Structure, IOI The Cell Body, . IOI The Nucleus, . 102 The Cytoplasm, [03 Neurofibrils, l°3 Perifibrillar Substance, 103 Chromophilic Bodies, 104 The Dendrites, 106 The Axone, .... 106 Non-Medullated Axone (Non Medullated Nerve Fibres), 107 Medullated Axones (Medullated Nerve Fibres), 10S Theories as to Physiology of the Neurone, 1 10 Neuroglia, 1 1 1 '1 e< Imic. .......... 1 12 General References, "3 CONTENTS. IX PART IV.— THE ORGANS. CHAPTER I. The Circulatory System. The Blood-vessel System, General Structure, Capillaries, Arteries, . Veins, Te clinic, . The Heart. Technic, . Development of the Blood-vessel System, The Lymph-vessel System, Lymph Capillaries, Lymph Spaces. Serous Membranes. Technic, . The Carotid Gland, The Coccygeal Gland. . Technic. . General References on Circulatory System, PAGE 17 17 17 17 :iS [24 J 5 27 128 [28 [29 -9 :-'• CHAPTER II. Lymphatic Organs. . The Lymph Nodes. Technic. . Hannolymph Nodes, Technic. . The Thymus, Technic. . The Tonsils. . The Palatine Tonsils, The Lingual Tonsils, The Pharyngeal Tonsils Technic, . The Spleen, . Technic. . General References. 131 131 134 jj 137 13S 139 140 140 141 '4- 14- 14- 146 147 CHAPTER III. The Skeletal System. The Bones, Bone Marrow, Red Marrow, . Yellow M arrow . 14S 1 48 152 >5- 154 x CONTENTS. PAGE Technic. i$P Development of Bone 159 Intracartilaginous Development, 157 Intramembranous Development, 159 Subperiosteal, i (| - Growth of Bone, 163 Technic, 164 The Cartilages, 164 Articulations, 165 Technic -. . . 166 General References, 166 CHAPTER IV. The Muscular System. A Voluntary Muscle, 167 Tendons, 168 Tendon Sheaths and Bursa?, 168 Growth of Muscle, 169 Technic, 169 CHAPTER V. Glands and the General Structure of Mucous Membranes, . .170 Glands — General Structure and Classification, 17° General Structure of Mucous Membranes, 174 CHAPTER VI. The Digestive System 175 Anatomical Divisions, i/5 The Headgut, 176 The Mouth 176 The Mucous Membrane of the Mouth, 176 Glands of the Oral Mucosa, '77 Technic, ..... '70 The Tongue, ..... '79 Technic, ..... [82 The Teeth &3 Development of the Teeth, [87 Technic, ..... ,89 The Pharynx, 190 Technic 190 'J he Foregut, 191 The (Esophagus, i'H Technic 192 General Structure of the Walls of the Gastn >-intestina 1 Canal, 192 The Stomach, 194 Technic. ...... • 199 CONTENTS. xi PAGE The Midgut, 20c The Small Intestine, 20c The Endgut, 206 The Large Intestine, 206 The Rectum, 210 Blood-vessels of the Stomach and Intestines, 211 Lymphatics of the Stomach and Intestine, . . . . . -213 Secretion and the Absorption of Fat, 214 Technic, . . . . . . . . . . . .216 The Larger Glands of the Digestive System, 217 The Salivary Glands, 217 The Parotid 218 The Sublingual, . . . . . . . . . .219 The Submaxillary, . . . . . . . . . .219 Technic, . . . . . . . . . . . .221 The Pancreas, . . . . . . . . . . .221 Technic, 227 The Liver, 227 Excretory Ducts of the Liver, ....... 233 The Gall-bladder, 233 Development of the Digestive System, 234 Technic, ............. 235 General References. 236 CHAPTER VII. The Respiratory System 237 The Nares, 237 The Larynx, ............ 239 The Trachea, ............ 239 Technic, . 242 The Bronchi, ............ 242 The Lungs, 244 Development of the Respiratory System, 250 Technic, . . . . . . . . . . . . 251 The Thyroid, 251 The Parathyroids, 252 Technic, 253 General References, ........... 253 CHAPTER VIII. The Urinary System, 254 The Kidney, 254 The Kidney-Pelvis and Ureter, 264 The Urinary Bladder, 265 The Adrenal, 268 Technic. 269 General References, 269 Xll CONTENTS. d Ejaculatory Connected vvi Spermatozoon Ducts, th the D CHAPTER IX. The Reproductive System. Male Organs. The Testis, The Seminal Ducts, The Epididymis. The Yas Deferens. The Seminal Vesicles an Rudimentary Structures Genital System, The Spermatozoon, Development of the 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, ( ieneral References, Connected w evelopment o ith the Development o TAGE 270 270 270 276 276 277 the f the CHAPTER X. Tin-: Skix and its Appendages, The Skin, .... Technic, .... I he Nails, .... Technic, The Hair, .... I >■< In) ic, .... Development of Skin, Nails, and 1 In Mammary Gland, . Technic, .... ( reneral References, 1 lair, CONTENTS. xm CHAPTER XI. the Posterior Col al Nervous System 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 umns, .... Afferent Nerve Terminations, Cells Situated in Other Parts of the Centr 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 General Structure, . Technic, ..... Practical Study, Transverse Section through Pyramidal Decussation, Transverse Section through Sensory Decussation, Transverse Section through Lower Part of Olivary Nucleus Transverse Section through Middle of Olivary Nucleus. Transverse Section through Exit of Cranial Nerve VIII., Transverse Section through Exits of Cranial Nerves VI. and 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, The Cerebrum, ...... General Histology of the Cerebral Cortex, Technic, ...... Pons Varolii), VII 33 2 333 334 335 335 338 338 340 340 340 34i 346 346 348 35 2 35 2 353 356 356 358 360 362 364 366 367 367 369 37° 37°' 37i 376 378 381 3 S 4 388 39i 39i 391 391 394 396 397 397 402 402 407 XIV CON TENTS. PAGE The Pituitary Body 410 The Pineal Body. 411 Technic, . . . . . . . . . . . . .411 General References, 411 CHAPTER XII. Tin: Organs of Special Sense, The Organ of Vision, . The Eyeball, . The Optic Nerve, . The Relations of Optic Nerv 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, Index, to Retina and Bra 412 412 412 424 424 429 43° 43 > 43- 433 433 434 435 444 445 446 448 44S 449 449 45i 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, such as thin membranes, e.g., omentum and mesentery, may be ex- amined fresh in some inert fluid, as normal salt solution (0.75 per cent aqueous solution sodium chorid). Most tissues and organs, however, require more or less elaborate preparation to render them suitable for microscopic examination. Tissues too dense and thick to be readily seen through with the microscope must be rendered more transparent. This is accomplished by pulling the tissue apart into fine shreds, teasing, or by cutting it into thin slices, section ait- ting. 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 be first killed in such a manner as to preserve as nearly as possible the living tissue relations, fixation. If too soft it must be hardened, or if, as is the case with bone, it is too hard, it must be softened by dissolving out the mineral salts, decalcification. If very thin sec- tions 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 consistence. This is known as embedding. Again, 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 which stain different elements with different degrees rf intensity. This is known as differential or selective staining. Only the more common procedures used in the preparation of 3 4 HISTOLOGICAL TECH NIC. tissues for microscopic study are described in this 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, (2) maceration. (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 tis- sue elements, thus allowing them either to fall apart or to be more easily dissociated by teasing. The most commonly used macerating fluids are : (a) Ranvicrs 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 to by teasing. Ranvier's alcohol is an especially satisfactory macerating fluid for epithelia. (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 epithelial 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 speci- mens may then be preserved in the potassium acetate solution, in glycerin, or in 50-per-cent alcohol. This dissociating fluid is largely used for muscle cells and fibres. II. Fixation. Fixation consists in so treating a tissue as to preserve as nearly as possible its living structure. This is usually accomplished by means of chemicals in solution, the solution being known as a fixa- GENERAL TECHNIC. 5 tive. In fixation small pieces of tissue should be placed in large quantities of the fixative. Organs or even bodies may be fixed in toto by injecting the fixative through an artery and allowing it to escape through a vein. After fixation by injection the specimen should be placed in a large quantity of the same fixative. The time required for fixation depends upon the character of the tissue and upon the fixative used. The following are the fixatives in most common use : (i) Strong Alcohol (96 per cent). — This is used to fix small pieces of tissue. It is a rapid fixative requiring from two to twenty- four hours. The tissue is at the same time hardened so that at the end of this time it is ready for embedding. As good fixation is de- pendent upon keeping the alcohol up to its full strength, it is best to change the alcohol after from two to four hours. (2) Dilute Alcohol (30 to 80 per cent). — This is probably the most used of all fixatives. It does not give satisfactory results, causing, as a rule, much shrinkage. (3) Formalin (2-per-cent to 10-per-cent aqueous solutions). — Fix- ation is accomplished in from six to twenty-four hours. Formalin is a quick fixative of good penetrating powers. It acts better when used in mixtures with other fixatives than when used alone. (4) Muller's Fluid. Potassium bichromate 2.5 gm. Sodium sulphate 1.0 gm. Water 100.0 c.c. Tissues remain in this fluid from a week to several months. Large quantities of the fixatives should be used and frequently renewed. Muller's fluid finds one of its most important uses in fixing nerve tissue. Good fixation of fairly large pieces of tissue may be obtained. (5) Formalin- Mailer s Fluid {Ortlis Fluid). — In this fluid a 2.5- per-cent aqueous solution of formalin replaces the water of the pre- ceding formula. It should be freshly prepared, or, if more conven- ient, a double-strength Muller's fluid and a 5-per-cent formalin solu- tion maybe kept in stock. Orth's fluid can then be made by taking equal parts of each. The action of this fixative is similar to that of Muller's fluid. It has the advantage of fixing more quickly and of possessing greater penetrating power. It is one of the best general fixatives, and is also largely used for fixing nerve tissue. 6 HISTOLOGICAL TECH NIC. (6) Osmic Acid. — This, in a i-per-cent aqueous solution, is a quick fixative of poor penetrating power. Very small pieces of tis- sue 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. (;) Flemmings 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. As this is an osmic-acid mixture, small pieces of tissue should be used and the solution should be freshly made. Flemming's fluid is one of the best fixatives for nuclear struc- tures, and is of especial value in demonstrating mitotic figures. Tis- sues should remain in the fluid for about three days. (8) Mercuric Chlorid. — This may be used either in saturated aqueous solution or in saturated solution in 0.75-per-cent salt solution. Fixation is complete in from twelve to twenty-four hours. A saturated solution of mercuric chlorid in 5-per-cent aqueous solution of glacial acetic acid also gives excellent results. (9) Zenker s Fluid. Potassium bichromate 2.5 gm. Sodium sulphate 1.0 gm. Mercuric chlorid 5.0 gm. Hydric acetate 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 hydric acetate added at time of using. Fixation requires from two to twenty-four hours. (10) Picric acid \& an excellent fixative for cytoplasm. It may be used in : (a) Saturated aqueous solution, requiring subsequent hardening in alcohol without washing in water; (/;) saturated solu- tion of picric acid in i-per-cent aqueous solution of acetic acid; (c) saturated solution of picric acid in 2-per-cent aqueous solution of sulphuric acid. GENERAL TECHNIC. 7 III. Hardening. Most fixatives are also hardening agents if their action be pro- longed. This is, however, often detrimental. For this reason it is customary, after fixation is complete, to transfer the specimens, with or without washing, to a second fluid for the purpose of hardening. The most commonly used hardening agent is alcohol. When strong alcohol is used as a fixative no washing is neces- sary. The alcohol should, however, be changed. After fixation in dilute alcohol, hardening should be accomplished by carrying the tissue through successively stronger alcohols ending with 96 per cent. This is known as hardening by means of "graded alcohols." The first alcohol should be 40 per cent to 50 per cent, the second 70 per cent, the third 80 per cent, the last 96 per cent. With very delicate tissues the gradations may be less rapid. In each of the alcohols the specimens should remain for from twelve to twenty-four hours. After fixation with formalin, specimens may be either carried through graded alcohols or transferred at once to strong alcohol. At least forty-eight hours is required for hardening formalin-fixed tis- sues. Specimens fixed in osmic acid should be washed from one to two hours in running water, then carried through graded alcohols. Tissues fixed in any of the solutions containing chromic acid or potassium bichromate should be thoroughly washed in running water and hardened in graded alcohols. This hardening is best done in the dark. After mercuric chlorid and Zenker's fluid fixations the tissue is thoroughly washed and passed through graded alcohols. W r hen 80- per-cent alcohol is reached, a small quantity of iodin is added to the alcohol to remove all trace of the mercury. As the alcohol be- comes clear, more iodin is added until the alcohol remains slightly tinged. The specimen is then transferred to strong alcohol. 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. HISTOLOGICAL TECH NIC. V. Decalcifying. Tissues, such as bone and teeth which contain lime salts, require further treatment before sections can be cut. The object is to dis- solve out the lime salts. This is known as decalcification. Tissues to be decalcified must be first fixed and hardened. For bone, fixation in formalin-Muller's fluid and hardening in graded al- cohols 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 completion of decalcification can be determined by passing a needle through the specimen or by cutting it with a scal- pel. The time required varies with the size and hardness of the specimen and the decalcifying fluid used. (i) 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 consists 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 frequently changed. When decalcification 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 1 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 maybe satisfactorily decalcified in Zenker s fluid (see fixatives, page 6), or in the following : Picric acid 1 part. Chromic acid J part. Clacial acetic acid 5 parts. GENERAL TECHNIC. VI. Embedding. Most hardened tissues are still too soft to be easily cut into the thin sections desirable 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. (i) Alcohol-Ether Celloidin. — Three solutions should be made. Solution No. J. Thick celloidin- — a 5 -per- cent solution of cel- loidin in equal parts alcohol and ether. Solution No. 2. Medium celloidin — made by diluting solution No. 3 with an equal volume of equal parts 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, twelve to twenty-four hours. Equal parts alcohol and ether, twelve to twenty-four hours. Thin celloidin, twenty-four hours or longer. Medium celloidin, twenty-four hours or longer. 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 days, or even weeks, in thin or medium celloidin. The celloidin must now be Jiardened and the specimen blocked. By the latter is meant fastening the embedded specimen to a block of wood or other suitable material which may be clamped in the microtome (see section cutting). The specimen may be taken from the thick celloidin, considerable of the latter adhering to the speci- men, quickly pressed upon a block of wood or vulcanized fibre, al- io HISTOLOGICAL TECIINIC. lowed to harden five to ten minutes in air and then immersed in 8o-per-cent alcohol. The alcohol gives an even hardening of the celloidin, attaching the specimen firmly to the block. 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, ac- cording to the amount of celloidin, after which it should be im- mersed 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 clipping for a moment in thick cel- loidin, and then pressing firmly clown 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 may usually be accomplished by first thor- oughly drying the specimen and then dipping it in equal parts alco- hol and ether. This softens the celloidin, after which the specimen is dipped in thick celloidin and blocked. (2) Clove-oil Celloidin. — A more rapid impregnation of the tissue may be obtained by means of what is known as clove-oil celloidin. Celloidin 30 gin. Clove oil. 100 gin. Ether 400 gm. Alcohol, absolute 20 gm. 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 and transferred from strong alcohol to the clove-oil celloidin. From six to twelve hours is sufficient to impregnate small pieces of tissue. The tissue is now taken from the celloidin, placed directly upon a wooden or vulcanized block, and immersed in chloroform. The celloidin har- dens in from three to five minutes, and is then ready for sectioning. Extremely thin sections may be cut. GENERAL TECHNIC. H 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 in pure chloroform. Paraffin Embedding. 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 whose melting-point is 62 ° C. For paraffin embedding a thermostat or paraffin oven is necessary in order that a constant temperature may be maintained. The hardened tissue is first completely dehydrated in absolute alcohol. It is then transferred to some solvent of paraffin. For this purpose xylol, chloroform, or a mixture of 1 part chloroform to 2 parts absolute alcohol may be employed. Depending upon whether xylol or chloroform is used, the tissue is next transferred to a saturated solution of paraffin in xylol or in chloroform. Tissues should remain in xylol-paraffin from one-half to one hour; in chloro- form-paraffin from one to three hours. The tissue is next trans- ferred to melted paraffin, where it remains from one to three hours, according to its size and density. Tissues should be allowed to remain in the melted paraffin only long enough for complete impreg- nation, and the paraffin should be kept at the lowest temperature consistent with complete fluidity. Except for very delicate tissues, the xylol-paraffin and chloro- form-paraffin maybe omitted, the specimen being transferred directly from xylol or chloroform to melted paraffin. For hardening the paraffin a very convenient 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 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 12 HISTOLOGICAL TECH'S I C. paraffin has become hard. After the paraffin has hardened any ex- cess 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 8o-per-cent alcohol and the specimens are removed by means of a camel's-hair brush to a dish of 8o-per-cent alcohol where they may be kept for some time if desired. When celloidin sections tear or when very thin sections 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 ft 1 or even 3 ) Neutral Carmine. Carmine . 1 gm. Liquor ammonii caustici 5 c.c. Distilled water 50 c.c. The last two ingredients are first mixed, and the carmine then added. This solution is allowed to remain in an open vessel for about three days, or until the odor of ammonia has disappeared, after which it is filtered. (r) Acid Aniline Dyes. — Of these acid fuchsin, erythrosin, and orange G are most used. They may be prepared and kept in stock in the same manner as the basic aniline dyes (see above). Ery- throsin is of especial value for sections which take the eosin stain poorly. 16 HISTOLOGICAL TECHNIC. Staining Sections. It is often of advantage to stain the different tissue elements different colors. This may be accomplished either by staining suc- cessively with several dyes, or by a single staining with a mixture of dyes. The following are the methods in most common use : (i) Staining Double with Hematoxylin and Eosin. — Sec- tions are first washed in water. They are then stained with haema- toxylin (solutions I, 2, 4, or 5, pp. 13, 14) from three to ten min- utes. After being thoroughly washed in water, they are dehydrated in strong alcohol and transferred to the alcoholic eosin solution (page 15). Most sections stain in from two to five minutes. P>y this method nuclei are stained blue or purple, cell bodies and intercellular substances, red. (2) Staining with Picro-Acid Fuchsin. Acid fuchsin, i-per-cent aqueous solution 5 c.c. Picric acid, saturated aqueous solution 100 c.c. This solution usually stains in from one to three minutes. Occa- sionally a longer staining is required. Cell bodies including muscle cells and fibres are stained yellow by the picric acid, connective-tis- sue fibres red by the fuchsin. After staining, the sections are washed thoroughly in several alcohols. (3) Triple Staining with Hematoxylin and Picro-Acid Fuchsin. — This is the same as the preceding except that before staining with picro-acid fuchsin, the sections are overstained in hematoxylin (solutions 1, 2, 4, or 5, pp. 13, 14). The usual pur- ple of hsematoxylin-stained nuclei is changed to brown by the action of the picric acid. Care should be taken that the sections do not remain too long in the picro-acid fuchsin, or the hematoxylin may be completely removed. After staining, sections are washed thor- oughly in several alcohols. If sections overstain with fuchsin, the staining solution may be diluted with water; if sections are understained with fuchsin, more fuchsin may be added. If the picric-acid stain is not sufficiently intense, the alcohol in which the sections are subsequently washed should be tinged with picric acid. (4) Staining with Picro-Carmine. Ammonium carminate 1 gm. Distilled water 35 c.c. Picric acid, saturated aqueous solution 15 c.c. GENERAL TECH NIC. 17 The ammonium carminate is first dissolved in the water, after which the saturated aqueous solution of picric acid is added with constant stirring. The mixture is then allowed to stand in an open vessel for two days, when it is filtered. This fluid stains nuclei and connective tissue red, cell protoplasm yellow. Staining in Bulk. By this is meant the staining of blocks of tissue before cutting into sections. The method is much less used than formerly. It is slower than section staining and more difficult to control. Blocks of the hardened tissue are transferred to the stain from water or alcohol according to the solvent of the stain. Alum-carmine and borax-carmine are the most used general bulk stains. (1) Alum-Carmine. Carmine 0.5 to 1 gm. Ammonia alum, 4-per cent aqueous solution ... .... 100 c.c. After mixing the ingredients the solution should be boiled fifteen minutes, and after cooling, enough sterile water added to replace that lost by evaporation. The time required for staining depends upon the size of the specimen. There is, however, little danger of over- staining. After washing out the excess of stain with water the spec- imen is dehydrated and embedded in the usual way. (2) Borax-Carmine, Alcoholic Solution. Carmine 3 gm. Borax 4gm. Water 93 c.c. After mixing the above, add 100 c.c. 70-per-cent alcohol, allow the mixture to settle ; then filter. About twenty-four hours is required to stain blocks 0.5 cm. in diameter. Larger blocks require longer staining. IX. Mounting. It is often desirable to make permanent preparations or "mounts" of the stained specimens. The most satisfactory media for mounting specimens are glycerin and Canada balsam. (1) Glycerin. — Sections may be transferred to glycerin from either water or alcohol. In the case of double-stained specimens — 1 8 HISTOLOGICAL TECHNIC. haematoxylin-eosin — the glycerin should be tinged with eosin, as the pure glycerin abstracts the eosin from the tissues. In many cases satisfactory eosin staining may be obtained by simply placing the haematoxylin-stained specimens in glycerin strongly tinged with eosin (eosin-glycerin). The specimen in a drop of glycerin is trans- ferred to the glass mounting slide, the excess of glycerin removed with filter paper or with a pipette and a cover-glass applied. Glycerin mounts must be cemented to exclude air. A satisfac- tory cement is gold-size, or a thick solution of gum shellac in alcohol to which a little castor oil has been added. Both cover-glass and slide must be cleaned free from glycerin before the cement is applied. A camel's-hair brush is used, and a ring of cement is painted around the cover in such a manner as to seal the cover to the slide. (2) Balsam. — This is the most satisfactory general mounting medium. It has an advantage over glycerin in drying down perfectly hard and thus needing no cement, and in preserving colors more permanently. Its disadvantage is that its refractive index is so high that it sometimes obscures the finer details of structure, especially of unstained or slightly stained specimens. Specially prepared Canada balsam is dissolved either in xylol or in oil of cedar, the solution being made of any desired consistence. Xylol balsam dries much more quickly than does the oil-of-cedar balsam. Preparatory to mounting in balsam, stained sections must be thoroughly dehydrated and then passed through some medium which is miscible with both alcohol and balsam. This medium, which at the same time renders the section transparent, is known as a clearing medium. For celloidin specimens the most satisfactory are : (i) Oil of origanum cretici. (2) Carbol-xylol (xylol, 100 c.c. ; carbolic acid crystals, 22 gm.), followed by pure xylol. (3) Xylol and cajeput oil, equal parts. After clearing, the section is transferred by means of a section- lifter to a glass mounting slide. It is then blotted firmly with filter paper to remove the excess of oil. Care must be taken to have the filter paper several layers thick in order that the oil may be com- pletely removed. The specimen should also be blotted firmly, giv- ing the oil time to soak into the paper. These two precautions are GENERAL TECHNIC. 19 necessary to prevent the section adhering to the paper instead of to the slide. After blotting, a drop of balsam is placed upon the centre of the specimen and a cover-glass applied. Paraffin Sections. — The technic of staining and mounting paraffin sections differs from that of celloidin sections. This is due mainly to the fact that while celloidin is transparent and may remain per- manently in the specimen, paraffin is opaque and must be dissolved out before the section is fit for microscopic study. Bulk staining with carmine (page 17) is frequently used for specimens which are to be embedded in paraffin. Sections may be counter-stained if desired. The following are the steps to be followed in staining and mount- ing paraffin sections : 1 . To attach sections to slide : Place a drop of egg albumen (equal parts white of egg and glycerin to which a little carbolic acid may be added for preserving) on a slide, and spread it out thin with the finger. Place a few drops of distilled water on the slide. Float sections on the water. Warm gently to allow sections to flatten— must not melt paraffin. Pour off excess of water, holding the end of the ribbons to pre- vent them floating off. Stand slides on end in water-bath twelve to twenty-four hours to evaporate water. 2. To remove paraffin : Place slide in xylol three to five minutes. 3. To stain sections : Place slide in fresh xylol three minutes. Transfer to absolute alcohol. Transfer to 90-per-cent alcohol. Transfer to 80-per-cent alcohol. Transfer to 50-per-cent alcohol. Transfer to distilled water. Stain with any aqueous stain. Wash in water. Transfer to 50-per-cent alcohol. Transfer to 80-per-cent alcohol 20 HISTOLOGICAL TECHNIC. Transfer to 90-per-cent alcohol. Transfer to absolute alcohol. Transfer to xylol. Mount in xylol-balsam. If an alcohol stain is used instead of an aqueous one, the carrying down and up through the graded alcohols may be omitted. If it is desired to stain double with eosin-haematoxylin (page 16) use the above technic, the hematoxylin being jhe stain. The alco- holic eosin stain is used before final transfer to absolute alcohol. X. Injection. For the study of the distribution of the blood-vessels in tissues and organs, it is often necessary to make use of sections in which the blood-vessels have been injected with some transparent coloring matter. The injecting fluid most commonly used is a solution of colored gelatin. The gelatin solution is prepared by soaking 1 part gelatin in from 5 to 10 parts water — the proportion depending upon the consistence desired — and when soft, melting on a water-bath. Various dyes are used for coloring the gelatin, the most common being Prussian blue and carmine. Prussian blue gelatin is prepared by adding saturated aqueous solution Prussian blue to the gelatin solution, the proportions de- pending upon the depth of color desired. Both solutions should be at a temperature of 6o° C. After thoroughly mixing, the blue gela- tin is filtered through cloth. Carmine gelatin is prepared by first dissolving 1 gm. carmine in 30 c.c. distilled water. To this is added ammonia until the mix- ture becomes a dark cherry red. A 10-per-cent aqueous solution of acetic acid is next added, drop by drop, with constant stirring until the mixture becomes neutral. The carmine and gelatin solutions, both being at about 60 ° C.,are now mixed in the desired proportions. If the carmine injection mass is alkaline, it diffuses through the walls of the vessels; if acid, there is a precipitation of the carmine which may interfere with its free passage through the capillaries. If, however, the alkaline carmine and gelatin be first mixed, and the 10-per-cent acetic acid solution be then added as directed above, the precipitated granules are so fine, even with an acid reaction, that they readily pass through the capillaries. The precipitation of the car- GENERAL TECHNIC. 21 mine in the shape of coarser granules is of advantage when it is desired to have an injection mass which will fill the arteries or veins only, without passing over into the capillaries. The injecting apparatus consists of a vessel which contains the injection mass, and some means of keeping the latter under a con- stant but easily varied pressure. With the vessel is connected a tube ending in a cannula, through which the injection is made. A very simple apparatus consists of a shelf which can be raised and lowered, and upon which the vessel stands. The tube connect- ing with the cannula may be attached to a faucet in the vessel or to a bent glass tube which passes into the top of the vessel and acts on the principle of a siphon. In a somewhat more elaborate apparatus the injection mass is placed in a closed vessel, and this is connected with a second vessel containing air compressed by means of an air pump. Accurate regulation of the pressure may be obtained by connect- ing the injection vessel with a manometer. If the injection is to occupy considerable time, a hot-water bath in which the gelatin may be kept at an even temperature is also nec- essary. Whole animals or separate organs may be injected. For inject- ing a whole animal, the animal, which is usually a small one such as a guinea-pig, rat, mouse, or frog, is chloroformed, the tip of the heart is cut away and a cannula is inserted through the heart into the aorta. This is first connected with a tube leading to a bottle containing warm normal saline solution. Pressure is obtained in the same manner as above described for the injection mass. By this means the entire arterial and venous systems are thoroughly washed out until the return flow from the vena cava is perfectly clear. The cannula is next connected with the tube from the vessel containing the injection mass, the pressure being only sufficient to keep the liquid flowing. When the injection mass flows easily and freely from the vena cava, the vessel is tied and the pressure is increased slightly and continued until the color of the injection mass shows clearly in the superficial capillaries. The aorta is now tied and the animal immersed in cold water to solidify the gelatin. After the gelatin becomes hard, the desired organs are removed and fixed and hardened in the usual way. Sections of injected material are usuall) cut rather thick, that the vessels may be traced the greater distance 22 HISTOLOGICAL TEC ff NIC. Better results are frequently obtained by injecting separate organs. This is accomplished by injecting through the main artery of the organ {e.g., the lungs through the pulmonary, the kidney through the renal). The injection is best done with the organ in situ, although it may be accomplished after the organ has been removed. The method is the same as given above for injecting an animal in toto. The so-called double injection by means of which, an attempt is made to fill the arteries with an injection mass of one color (red), while the veins are filled with an injection mass of another color (blue) often gives pretty, but usually inaccurate pictures, it being as a rule impossible to confine each injection mass to one system. Double injection is accomplished by first washing out the vessels with normal saline and then connecting the artery with the red gela- tin, the vein with the blue gelatin, and injecting both at the same time, the pressure driving the saline out of the vessels into the tis- sues. The difficulty is that either the arterial injection carries over into the veins, or the venous injection carries over into the arterfes. A somewhat more accurate method is first to inject the veins with an injection mass in which the coloring matter is in the form of granules too large to pass through the capillaries, and then to inject the arte- ries and capillaries in the usual manner. This method is especially useful in demonstrating the vessels of the kidney, liver, and gastro- intestinal canal. CHAPTER II. SPECIAL STAINING METHODS. Of these only the more common will be described. (i) Silver Nitrate Method of Staining Intercellular Substance. — After first washing, the tissue, e.g., omentum or cornea, is placed in a from 0.2 to 1 per cent solution of silver nitrate, where it is kept in the dark for a half-hour or more according to the thick- ness and density of the tissue. The specimen is then washed in water, transferred to 80-per-cent alcohol and placed in the direct sunlight until it assumes a light brown color. It is then placed in fresh 80-per-cent alcohol for preservation. (2) Chlorid of gold in 1 -per-cent aqueous solution is used in the same manner for demonstrating connective-tissue cells and their finer processes. (3) Weigert's Elastic Tissue Stain. — This is prepared as follows : Fuchsin 2 gra. Resorcin 4 g m • Water 200 c.c. These are boiled for five minutes, during which 25 c.c. of liquor ferri sesquichlorati are stirred in. The result is a precipitate which should be filtered out after the liquid has become cool. After dry- ing, 200 c.c. of 95 -per-cent alcohol are added to the filtrate and boiled until the latter dissolves. Lastly, 4 c.c. of hydric chlorid are added to the solution. Sections should remain in the stain thirty minutes, after which they are washed in alcohol until the stain ceases to be given off. (4) Golgi's Chrome-Silver Method for Demonstrating Secretory Tubules. — Small pieces of perfectly fresh tissue, e.g., liver, are placed in the following: Potassium bichromate. 4-per-cent aqueous solution 4 vols. Osmic acid, 1 -per-cent aqueous solution 1 vol. After three days they are transferred without washing to a 0.75- 23 24 HISTOLOGICAL TECHNIC. per-cent aqueous solution of silver nitrate, which should be changed as soon as a precipitate forms. The specimens remain in the second silver solution from two to three clays, after which they are rapidly dehydrated, embedded in celloidin, and cut into rather thick sections. (5) Mallory's Hematoxylin Stain for Connective Tissue. — Thin sections are placed for from two to ten minutes in a ten- per-cent aqueous solution of phosphomolybdic acid. They are then washed in distilled water and transferred to : Phosphomolybdic acid, 10-per-cent aqueous solution . 100.0 c.c. Distilled water 200.0 c.c. Hematoxylin crystals 1.75 gm. Carbolic-acid crystals 5.00 gm. The phosphomolybdic acid and water are first mixed, after which the hasmatoxylin and carbolic acid are added. After staining from ten to twenty minutes the sections are washed in distilled water, placed for five minutes in 50-per-cent alcohol, then in strong alcohol, cleared in xylol and mounted in xylol balsam. (6) Osmic Acid Stain for Fat. — For this purpose osmic acid is used in a 1 -per-cent aqueous solution. The method is especially useful for demonstrating developing fat, fatty secretions (mammary gland), and fat absorption (small intestine). Very small bits of the tissue are placed in the osmic-acid solution for from twelve to twenty- four hours. They are then hardened in graded alcohols, embedded in celloidin, and the sections mounted in glycerin. (7) Jenner's Blood Stain. Water-soluble eosin — Griibler, 1 -per-cent aqueous solu- tion ... 100 c.c. .Methylene blue — pure — Griibler, i-per-cent aqueous solu- tion 100 c.c. M ix, and after standing twenty-four hours filter. The filtrate is dried at 65 C, washed, again dried and powdered. To make the staining solution, 0.5 gm. of the powder is dissolved in 100 c.c. pure methyl alcohol. Blood smears stain in from two to five minutes. They are then washed in water, dried, and mounted in balsam. This solution acts as a fixative as well as a stain. CHAPTER III. SPECIAL NEUROLOGICAL STAINING METHODS. Weigert's Method of Staining Medullated Nerve Fibres. Material is fixed in one of the following fluids : (a) Miiller's fluid (page 5). (&) Potassium bichromate, 5 -per- cent aqueous solution. (c) Formalin, 10-per-cent aqueous solution. {d) Formalin, 1 volume; potassium bichromate, 5-per-cent aque- ous solution, 9 volumes. In Miiller's fluid or in plain potassium bichromate solution a hardening of from ten days to three weeks is required ; in formalin or formalin- bichromate for a week to ten days is sufficient. The specimens are then hardened in graded alcohols, embedded in cel- loidin, and sections cut in the usual way. Material fixed in formalin should be placed for several days in either a 5-per-cent aqueous solution of copper bichromate or in the following : Chrome alum 1 gra. Potassium bichromate 3 gm. Water... 100 c.c. before hardening in alcohol. Sections from material fixed in any of the chrome salt solutions are placed for from twelve to twenty-four hours in a saturated aqueous solution of neutral cupric acetate diluted with an equal volume of water. From the fact that it forms some combination with the tis- sue whereby the latter is enabled better to take up the stain used, this solution is known as a mordant and the process as mordanting. After mordanting, the sections are washed in water and trans- ferred to the following staining fluid : Hematoxylin crystals 1 gm. Alcohol, 95 per cent 10 c.c. Lithium carbonate— saturated aqueous solution ......... 1 c.c. Water go c.c. This solution must either be freshly made before using or the 2.5 26 HISTOLOGICAL TECH NIC. hematoxylin may be kept in io-per-cent alcoholic solution, the lith- ium carbonate in saturated aqueous solution, and the staining fluid made from these as needed. Sections remain in the hematoxylin solution from two to twenty- four hours, the longer time being required for staining the finer fibres of the cerebral and cerebellar cortices. They are then washed in water and decolorized in the following : Potassium ferricyanid ..., 2.5 gm. Sodium biborate 2.0 gm. Water ... 300.0 c.c. While in the decolorizer, sections should be gently shaken or moved about with a glass rod to insure equal decolorization. In the decolorizer the sections lose the uniform black which they had on removal from the hematoxylin. They remain in the decolorizing fluid until the gray matter becomes a light gray or yellow color, in sharp contrast to the white matter which remains dark. Sections are then washed in several waters to remove all traces of decolorizer and dehydrated in alcohol. Weigert-Pal Method. — In this modification of the Weigert method, sections are mordanted in a 3- to 5-per-cent aqueous solution of potassium bichromate instead of in the copper acetate solution. After rinsing in water the sections are stained in hematoxylin as in the ordinary Weigert method. They are then washed and trans- ferred to a 0.2 5-per-cent solution of potassium permanganate, where they remain from one-half to two minutes, after which they are again washed and placed in the following: Oxalic acid 1 gm. Potassium sulphite ... 1 gm. Water 200 c.c. In this solution differentiation takes place, the medullary sheaths remaining dark, while the color is entirely removed from the rest of the tissue. If the section is still too dark, it may again be carried through the permanganate and oxalic-acid solutions until sufficiently decolorized. All formalin fixed material is best stained by the Weigert-Pal method. An intensification of the stain, especially of the very fine fibres, may sometimes be obtained by placing the sections for several minutes in a o. 5-per-cent aqueous solution of osmic acid before de- colorizing:. SPECIAL NEUROLOGICAL STAINING METHODS. 27 Golgi Methods of Staining Nerve Tissue. The Golgi methods in most common use at present are the fol- lowing : (1) Golgi Silver Methods. — (a) Slow Method. — Blocks of tis- sue are placed for several months in a 3-per-cent aqueous solution of potassium bichromate. Small pieces of the tissue are then trans- ferred to a 0.75-per-cent aqueous solution of silver nitrate, where they remain for from one to three days. The only method of determining whether the tissue has been sufficiently long in the bichromate is to try at intervals small bits of the tissue in the silver solution until a satisfactory result is secured. (b) 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 : Osmic acid, i-per-cent aqueous solution ... 1 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. Pieces of tissue should be tried each day until a satisfactory result is obtained. Further treatment is the same as described in the slow method. (c) 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). (d) Formalin-bichromate Method. 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 So parts. Subsequent treatment with silver is the same as in the previously described method. The results resemble those of the slow method. The specimens maybe kept in strong alcohol. The method is satis- factory only for the adult cerebrum and cerebellum. (2) Golgi Bichlorid Method. — Material remains for several months in the potassium bichromate solution (see Slow Silver 28 HISTOLOGICAL TECHNIC. Method), after which it is transferred to a saturated aqueous solu- tion of mercuric chlorid for from four to twelve months or longer. The degree of impregnation must be determined by frequently test- ing the material. 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 40 parts. After mixing the above, add Potassium chromate, 5-per-cent aqueous solution 16 parts. Tissues remain in this fluid for from two to five months. 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, a) and mounted in xylol-balsam. After the rapid method, specimens should be 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 18). 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 sub- sequent staining with an aniline dye and final decolorization in alcohol. 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 8), 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 SPECIAL NEUROLOGICAL STAINING METHODS. 29 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 ! granules f karyosome or | net-knob S hyaloplasm spongioplasm linin network nucleoplasm . attraction-sphere " centrosome ~ plastids -* chromatin network -- nuclear membrane -- nucleolus - vacuole FIG. 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 33 34 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 proteid, plastin. Several theories are held as to the ultimate structure of proto- plasm (Fig. 2). According to one theory, protoplasm is homogene- a ous, having no definite struc- ture. Formerly quite generally accepted, this view is now held by but few cytologists. Other investigators con- sider protoplasm as made up of (i) a fibrillar element, which may occur 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 substance which fills in the meshes of the reticulum or separates the fibrils (interfilar mass or paramiton) (Fig. 2, a). Altmann's granule theory considers protoplasm as com- posed of fine granules embed- ded in a gelatinous intergran- ular substance. Altmann believed that these granules represented the ultimate vital elements, and for this reason gave them the name of bioblasts (Fig. 2, />). According to Butchli, protoplasm is a foam or emulsion. The appearance of a reticulum he considered due to the fact that each lit- tle foam space forms a complete cavity filled with fluid, it being the cut sides of these spaces which give the reticular appearance on section (Fig. 2, c). Fig. 2. — Diagram Illustrating Theories of Proto- plasmic Structure, a, Fibrillar theory; £, granule theory ; c, " foam " theory. (The gen- eral structure of cell body and nucleus corre- sponds.) The formed element of protoplasm, whether reticular or fibrillar in structure, is known as spongioplasm, the homogeneous element as liyaloplasm (Fig. i). Peculiar bodies known as plastids (Fig. i) are of frequent occurrence in vegetable cells, but are also found in some THE CELL. 35 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 pro- toplasm of the cell or waste products of cellular activity. To such protoplasmic "inclusions" the terms deutoplasm, paraplasm, meta- plasm, 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 endoplasm, the latter as cxoplasm. When the exoplasm forms a distinct limiting layer, but blends imperceptibly with the rest of the protoplasm, it is known as the critsta. 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 is covered by a membrane, it is known as the cuticula. 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 unes- sential part of the cell, the nucleus is now known to be most inti- mately associated with cellular activities. It is not only essential 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 as far as reproductive powers are concerned. 36 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 struc- tural 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 : (/?) The nuclear membrane (amp hipy renin). This forms a limit- ing membrane separating the nucleus from the cell protoplasm. It is wanting in some nuclei. (It) The intranuclear network, or nucleoreticulum, consists of a chromatic clement (nuclein or chromatin) and of an acliromatic cle- ment (linin). At nodal points of the network there are often consider- able accumulations of chromatin. These nodal points, at first thought to be nucleoli, are now known as false nucleoli, or karyosomes. In- stead 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 (lantanin). These are differentiated from chromatin by the fact that they are most susceptible to acid dyes, while chromatin takes basic dyes. (c) The nucleolus or plasmosomc (paranuclein-pyrenin) is a small spherical body within the nucleus. It stains intensely with basic dyes. Its function is unknown. (d) Nucleoplasm (karyoplasu/, 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 radiation fibrils, the centrosphere {attraction sphere, protoplasmic ra- THE CELL. 37 diation, and arcJioplasvi). The main significance of the centrosome is in connection with cell division, under which head it will be further considered (page 40). 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 stimulation — irritability. 4. To move — motion. 5. To produce other cells — reproduction. 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 protection for more delicate underlyi ng structures. Other cells — gland cells — in addition to maintaining their own nutrition produce specific sub- stances (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, consequently, unstable molecular combinations. By reduction of these unstable combinations, molecules of greater stability and less complexity are formed. This results in the transformation of poten- tial into kinetic energy, and the expenditure of this energy is ex- pressed in the 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- 38 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 :"- : >.. "I FlG. 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 usually evidenced by the movement of minute granules present in the protoplasm, by changes in the position of the nucleus, etc. (c) Ciliary movement. This is the whipping motion possessed by little hair-like processes called cilia, which project from the sur- faces of some cells. Certain cells which are specialized for the particular purpose of motion, as e.g. the muscle cell, possess such powers of contraction that they are able to move not only themselves but other parts with which they are connected. This power of contractility is dependent upon the spongioplasm, the hyaloplasm playing a more passive role. In muscle cells the highly developed contractile powers appear to be due to the excessive development and peculiar arrangement of the spongioplasm. THE CELL. 39 5. Reproduction. — The overthrow of the long-held biological fal- lacy of spontaneous generation was soon followed by the downfall of a similar theory regarding the origin of cells. We now know that all cells are derived from cells, and that the vast number and com- plex of cells which together form the adult human body are all de- rived from a single primitive cell, the ovum. Reproduction of cells takes place in two ways, by direct cell divi- sion or amitosis, and by indirect cell division or mitosis. In both amitosis and mitosis the division of the cell body is preceded by division of the nucleus. Direct Cell-Division — Amitosis (Fig. 4). — In this form of cell-division the nucleus divides into two daughter nuclei without any apparent preliminary changes in its structure. The division of the nucleus may or may not be followed by division of the cell body. This form of cell-division is uncommon in higher animals where Flemming considers it a degenerative phenomenon rather than a nor- mal method of cell-increase. It is a common method of cell-division in the protozoa. Indirect Cell-Division — Mitosis (Fig. 5). — In this form of cell division also the nucleus divides into two daughter nuclei, but only after having undergone certain characteristic changes in structure. On account of their complexity it is con- venient for purposes of de- scription to divide these changes into stages or phases. Thus we recognize in mitosis (a) the prophase; {b) the metaphasc ; (c) the anaphase; (d) the telophase. (a) The Prophase (Fig. 5, £j C ]J\ This is the Sta°"e F IG - 4-— Epithelial Cells from Ovary of Cockroach. showing Nuclei dividing Amitoticallv. (Wheeler.) of preparation on the part of the nucleus for division. It is marked by the following changes in the nucleus : 1 . The chromatic part of the intranuclear network becomes changed into a twisted skein or spireme. This is formed of a single long thread of chromatin or of several shorter threads (Fig. 5, B). 40 THE CELL. 2. During these changes in the network, the nucleolus and nuclear membrane disappear and the centrosome and its surrounding attrac- tion sphere increase in size. 3. The centrosome next divides into two equal parts. These two daughter centrosomes, each surrounded by its attraction sphere, move apart but remain connected by fine fibrils, probably derived FIG. 5.— Diagrams of Successive Phases of Mitosis. A, Resting cell, with reticular nucleus and true nucleolus; c, attraction sphere with two centrosomes. B, Early prophase. Chromatin forming continuous thread — the spireme; nucleolus still present ; a, amphiaster ; the two centrosomes connected by fibrils of achromatic spindle. C\ Later prophase. Segmentation of spireme to form the chromosomes; achromatic spindle connecting centrosomes ; polar rays ; mantle fibres ; fading of nuclear membrane. />. End of prophase. Monaster — mitotic figure complete; ep, chromosomes arranged around equator of nucleus ; fibrils of achromatic spindle connecting centrosomes; mantle fibres passing from centrosomes to chromosomes. from the linin (Fig. 5, />'). These fibrils form the central ox achro- matic spindle. Two other sets of fibrils radiate from each centrosome — one, known as the polar rays, passes out toward the periphery of the cell; the other, known as the mantle fibres, extends from the centrosome to the chromosomes (Fig. 5, C). 4. The spireme next breaks up into a number of segments — THE CELL. 41 chromosomes (Fig. 5, C). These arrange themselves regularly around the equator of the nucleus, forming loops, the closed ends of which are directed centrally. This is known as the closed skein, mother star, or monaster (Fig. 5, D). The number of chromosomes varies for different species of plants and animals, but is fixed and charac- teristic for a given species. FIG. 5.— Diagrams of Successive Phases of Mitosis. E, Metaphase. Longitudinal cleavage; splitting of chromosomes to form daughter chromo- somes, ep ; n, cast-off nucleolus. F, Anaphase. Daughter chromosomes passing along fibrils of achromatic spindle toward centrosomes; division of centrosomes ; if, interzonal fibres or central spindle. G, Late anaphase. Formation of diaster ; beginning division of cell body. H, Telophase. Reappearance of nuclear membrane and nucleolus ; two complete daughter cells, each containing a resting nucleus. (E. B. Wilson, " The Cell," The Macmillan Co.) (o) Metaphase (Fig. 5, E). This marks the beginning of actual division of the nucleus. Each chromosome splits longitudinally (longitudinal cleavage) into two daughter chromosomes. (c) Anaphase (Fig. 5, F, G). — An equal number of daughter chromosomes now travels along the fibrils of the achromatic spindle — apparently under the influence of the mantle fibres — toward each daughter centrosome. In this way are formed two daughter stars, 42 THE CELL. the mitotic figure being known at this stage as the diastcr (Fig. 5, G). 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 onc-lialf 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- somc, 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. 43 and the formation of tzvo 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 datigJitcr 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 projtucleus, 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 one-half 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 daughter centrosomes. 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 diaster. 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 ; /, tail ; e, end piece. 44 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- matozoon with centrosonie centrosome \ ,. chromosome of female pronucleus chromosome of male pronucleus centrosome chromosome ,•>' from female pronucleus ^__3/ • •-'• / — 'chromosome from male pronucleus - — centrosome Fig Diagram of Fertilization of the Ovum. (The somatic number of chromosomes being four.) (From Bohm and von Davidoff, after Boveri.) 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. 45 are known as blastomeres. Soon, however, these cells become spread out and at the same time separated into two primary germ /avers The outer of these is known as the ectoderm or epiblast, the inner as SEGMENTA- TION CAVITV. FIG. S.-Secrmentation of the Ovum. (From Gerrish, after van Keneden.) ^ T Z™ZlT 7£%* f l 0m fil ' St diVisi ° n ° f fertilized ™° i *< fa»«.ll stage ; ,, « . toderm ;*«««- tt?//j, entoderm. "y germ layers. Outer celts, ex.- 46 * THE CELL. the entoderm or hypoblast. 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- ectoderm ' . t- ' mesoderm Q;f£ v. ^_ i'V/'.^, .; ■■-,~^ r ^ rr ^^0& :V - nP - >-\ MM '^ X ■%&'■ V : ' : 'M'% $& - " ^?" S 't $i -^ entoderm. rs^^a^vS:'' ''' ' ^ : -T" __ '_ ".' _. Oi 'V- ^'" ^ ' J "" Q Fig. 9. — Two Primary Germ Layers. (From McMurrich, after Bonnet.) hol tinged with iodin. After from twelve to twenty-four hours the alcohol is de- canted and the tube partly tilled with alum-carmine solution (page 15). 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. 5, 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 amoebae 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 ol blood, human, or, better, from one of the cold-blooded THE CELL. 47 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. 6), 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 hematoxylin, technic 3. p. 14. 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 46) 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 Hassel's corpuscles of thymus. 51 52 THE TISSUES. Mesoderm. — This layer early splits into three sub-layers : Mesothclium. — The cells of this layer form tissues resembling epithelium. 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. Mesenchyme. — From the cells of this layer 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. Mesamccboici Cells. — From these are derived the 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 two or more tissues for the performance of a particular 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 tis- sues 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 tissues. (2) Connective tissues. (3) Blood. (4) Muscle tissue. (5) Nerve tissue. 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 Characteristics. — 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 " intercellular bridges''' (see Fig. 14, p. 57). 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. 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 ox membrana 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 53 54 THE TISSUES. C7iticuhu, which unite to form a continuous membrane, the cuticidar 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 connected with 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; (b) columnar; (c) pseu- dostratified. (2) Stratified Epithelium. — (a) Squamous; (b) transitional; (c) columnar. Special forms of the above-mentioned types are known as : (a) ciliated epithelium ; (b) pigmented epithelium ; (c) glandular epi- thelium ; (a 7 ) neuro-epithelium. (3) Mesothelium and Endothelium. 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 sub- stance. 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, occurring as respi- ratory epithelium in the lungs (non-nucleated), in Bowman's capsule of the kidney glomeruli, in the descending arm of Henle's loop of the uriniferous tubule, the pigmented cells of the retina, and the posterior surface of the anterior lens capsule. (b) Simple columnar epithelium consists of a single layer of elon- gated cells. The bases of the cells are usually separated from the EPITHELIUM. 55 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. — Simple Squamous Epithelium. Section of cat's lung, stained with silver nitrate. (Klein.) The outlines of the non-nucleated simple squamous epithelial cells are shown bv wavy black lines. 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 cuboidal. Simple columnar epithelium lines the gastro-intestinal canal, the uriniferous tubule (excepting the de- ^"'""V^f --\-~- *r--— ,-- ' -'-•"-' "- -~~- -- -i Fig. n. — Simple Columnar Epithelium from Human Small Intestine. scending arm of Henle's loop), simple tubular glands, the ducts of some compound tubular glands, the smaller bronchi, and the mem- branous and penile portions of the male urethra. 56 THE TISSUES. In simple columnar epithelium, in addition to the single row of epithelial cells, there are found lying near the basement membrane, between the bases of the epithelial cells, small, spherical or irregular cells, which frequently show mi- tosis and which are known as re- placing cells. They appear to de- FiG. 12 .-DiagramofPseudostratifledE P i- velo P into columnar epithelial theiium, showing Nuclei situated at Dif- cells as they are needed to replace ferent Levels. older cells. if) Pseudostratified epithelium is a form of simple columnar epi- thelium, in which, from crowding of the cells, the nuclei have come to lie at different levels, thus giving the appearance of stratification. 2. Stratified Epithelium. In stratified epithelium the cells are arranged in more than one layer. (a) Stratified squamous epithelium is developed from simple epithelium by the growth of new cells between the old cells and the FLAT SURFACE CELLS - i~-~f~?^i -'*■'■ POLYHEDRAL CELLS CUBOIDAL CELLS BASEMENT MEMBRANE SUBEPITHELIAL CONNECTIVE TISSUE m y&*rs; «'K V ,'•—'. -mv - FIG. 16. — Stratified Columnar Epithelium from the Human Male Urethra. X 400. free from papillae. The surface cells are large and may contain two or three nuclei. Their free surfaces are flat, while their under sur- 58 THE TISSUES. faces show depressions due to pressure from underlying cells. The cells of the deeper layers are polygonal, or irregularly cuboidal. '■■■'■■■■ ':■--' :"'■':' ;>■;■'■ ' Si ' il^Hsl >J '• rJ* Fig. 17. -Stratified Columnar Ciliated Epithelium from the Human Trachea. X 400. This form of epithelium lines the bladder, ureter, pelvis of the kid- ney and prostatic portion of male urethra. U) Stratified Columnar Epithelium.- — Only the surface cells are columnar, the deeper cells being irregular in shape. The surface cells frequently send long processes down among the underlying cells. The free surface is often marked by a well-developed cuticula. § ^pga m ,"'• $$$(:<■: - m ■■ l"> ■>% l - H f V •■■■■ Fig. 18.— Isolated Ciliated Cells and Goblet Cells from Dog's Trachea. X 700. Some epithelia of this type are ciliated. Stratified columnar epithe- lium is found in the larynx, nose, palpebral conjunctiva, largest of the gland ducts, the vas deferens and part of the male urethra. EPITHELIUM. 59 Modified Forms of Epithelium. (a) Ciliated Epithelium. — In this form of epithelium, fine hair-like processes — cilia — extend from the surface of the cell. These cilia vary from twelve to twenty-five for each cell and may be short as in the trachea or long as in the epididymis. There is usually a well- defined cuticula from which the cilia appear to spring. According to Apathy, the cilia extend through the cuticulse, giving to the latter a striated appearance (Fig. 19). Just beneath the cuticula each cilium shows a swelling — the basal granule. Lenhossek considers these granules centrosomes. The intracellular extensions of the cilia converge toward the nucleus, and are continuous with the reticular or fibrillar structure of the cell body. The motion of cilia is wave-like, the wave always Fig. 19. Fig. 20. FiG. 19. — Ciliated Epithelial Cell from Intestine of Mollusk (Engelmann), showing, a, cuti- cula, b, basal granules, and c, intracellular extensions of cilia. PIG. 20. — Pigmented Epithelial Cells from the Human Retina (X 350), showing different de- grees of pigmentation. The clear spots in the centres of the cells represent the unstained nuclei. passing in the same direction. Various explanations of ciliary motion have been given. The most plausible is that it is due to the contractile powers of the spongioplasm. Cilia are confined to the surface cells of simple columnar and stratified columnar epithelium. Simple columnar ciliated epithelium occurs in the smaller bronchi, uterus, Fallopian tubes and central canal of the spinal cord. 6o THE TISSUES. Stratified columnar ciliated epithelium occurs in large bronchi, trachea, larynx, nose, Eustachian tube, vas deferens and epididymis. (b) Pigmented Epithelium consists of cells the cytoplasm of which contains brown or black pigment. It is usually present in the form of spherical or rod-like granules. Examples of it are seen in the pigmented epithelium of the retina and in the pigmented cells of the deeper layers of the epidermis in colored races (Fig. 20). (e) Glandular Epithelium. — This forms the essential or secreting element of glands and is mostly of the simple cylindrical variety. The different kinds of glands and their epithelia will be described among the organs. (d) Neuro-epithelium. — This is a highly specialized form of epithelium which occurs in connection with the end organs of nerves, under which heading it will be described. 3. Mesothelium and Endothelium. While recognizing the present tendency toward considering those tissues formerly classified as endothelium, as simple squamous epithe- lium, the correctness of the newer classification still remains sub Fig. 21.— Mesothelium from Oment.im of Dog Treated according to Technic 7, p. 62. X 350. Black wavy lines indicate the intercellular cement substance. The mesothelial cells cover the strands of connective tissue, the fibres of the latter being visible through the transparent cell bodies. judice and, so long as this is the case, we prefer to retain the cer- tainly much more convenient classification of Minot, which coincides EPITHELIUM. 6 1 with his subdivision of the mesoblast. According to this classifica- tion, for those tissues which resemble epithelium in structure and which are derived from the mesenchyme, the term endothelium is FIG. 22. — The Endothelium of a Small Blood-vessel. Silver nitrate stain. X 350. retained. The term mesothelium is used for those tissues which re- semble epithelium and which are derived from the mesothelium. Mesothelium and endothelium are similar in structure. Each consists of thin flattened cells with clear or slightly granular proto- plasm and bulging oval or spherical nuclei. The edges of the cells are usually wavy or serrated. The cells are united by an extremely small amount of intercellular " cement " substance, which is usually indistinguishable except by the use of a special technic. Endothelium forms the walls of the blood and lymph capillaries and lines the entire blood-vessel and lymph-vessel systems. Mesothelium lines the body cavities — the pleura, the pericardium and the peritoneum. TECHNIC. 1. Simple Squamous Epithelium. — That of the lung may be demonstrated by injecting with silver solution (technic 1, p. 23) through a bronchus and then im- mersing the tissue in the same solution. The lungs of young kittens furnish espe- cially satisfactory material. 2. Simple Columnar Epithelium. — A piece of small intestine, human or animal, is pinned out flat on cork and fixed in formalin-Midler's fluid (technic 5, p. 5). Sections are cut perpendicular to the surface, stained with hematoxylin and eosin (technic 1. p. 16) and mounted in glycerin, tinged with eosin (page 18). Little elevations known as villi project from the inner surface of the intestine. These are covered by a single layer of columnar epithelial cells. The cuticulae and cuticular membrane are usually well shown. Among the simple cylindrical cells are seen large clear or slightly blue-stained cells. These are known from Uieir secretion as mucous cells, from their shape as goblet cells, and are classed as modified epithelium of the glandular type. These should be studied in their va- rious stages of secretion, from the cell in which only a small amount of mucus is present, near the outer margin, to the cell whose protoplasm is almost wholly re- placed by mucus. Some cells will be found in which the surface has ruptured and the mucus can be seen pouring out of the cell. 62 THE TISSUES. 3. Stratified Squamous Epithelium. — The cornea furnishes good material for the study of stratified squamous epithelium. An eye is removed from a freshly killed animal and the cornea cut out and fixed informalin-Miiller's fluid. Sections are cut perpendicular to the surface, and treated as in the preceding. The cells are laid down in from six to eight layers. The oesophagus may be used instead of the cornea, its mucous membrane being lined by a somewhat thicker epithelium. 4. Transitional Epithelium. — This is conveniently studied in the mucous mem- brane of the bladder. Technic same as 2. p. 61. 5. Stratified Columnar Epithelium. — A portion of trachea from a recently killed animal is treated according to same technic. The surface cells are ciliated so that this specimen also serves to demonstrate that type of modified epithelium. Isolated cells or clumps of cells may be obtained from the trachea in the manner described in technic 3. p. 46. 6. Pigmented Epithelium. — Fix a freshly removed eye in formalin-Muller's fluid (page 5). After hardening, cut transversely and remove the vitreous and retina. The pigmented cells remain attached to the inner surface of the choroid, and may be removed by gently scraping. They may be preserved and mounted in glycerin. 7. Mesothelium. — Part of the omentum of a recently killed animal is removed and washed in water, care being taken not to injure the tissue in handling. The water is then replaced by a 1 to 500 aqueous solution of silver nitrate. After half an hour the specimen is removed from the silver, washed in water, transferred to 80-per-cent alcohol and placed in the sunlight until it becomes of a light brown color. It is then preserved in fresh So-per-cent alcohol. The nuclei may be stained with hematoxylin (stain 5. p. 14). The specimen should be mounted in glycerin. Wavy black lines indicate the intercellular cement substance. The nuclei of the mesothelial cells are stained blue, those of the underlying connective- tissue cells a paler blue. It must be borne in mind in studying this specimen that the strands or trabeculae of the omentum are not composed of mesothelium, but of fibrous connective tissue, and that the flat mesothelial cells merely lie upon the surface of the connective-tissue strands. 8. Endothelium may be demonstrated by removing the bladder from a recently killed frog, distending it with air and subjecting it to the same technic. By this means the intercellular substance of the endothelium of the blood-vessels of the bladder wall is stained and the outlines of the cells are thus shown. CHAPTER III. THE CONNECTIVE TISSUES. Histogenesis. — All of the connective tissues, with the single exception of the connective tissue peculiar to the nervous system (neuroglia), are developed from the sub-layer of the mesoblast known as the mesenchyme. The mesoderm consists at first wholly of round or polygonal cells. With the division of the mesoderm into its three sub-layers, the cells of the mesenchyme gradually become more and more separated from one another by the interposition of a fluid intercellular substance. This intercellular substance is a product of the cell and is at first homogeneous or granular. The appearance presented at this stage is that of irregular, branching, anastomosing cells, lying in a semi- fluid ground substance. This is embryonic connective tissue. With further changes in both cells and intercellular substance, but mainly in the latter, embryonic connective tissue differentiates to form the adult types of connective tissue. General CJiaracteristics. — A characteristic of the connective tis- sues is the predominance of the intercellular substance. In this re- spect the connective tissues differ markedly from epithelial tissues. Moreover, it is the intercellular substance and not the cells which determines the physical character of the tissue. The division of connective tissue into its various sub-groups is also based upon struc- tural differences in the intercellular substance. Classification. — The connective tissues may be classified as follows : i. Fibrillar connective tissue, including areolar tissue. 2. Elastic tissue. 3. Embryonal and mucous tissue. 4. Reticular tissue. 5. Lymphatic or adenoid tissue. 63 64 THE TISSUES. 6. Fat tissue. r (a) Hyaline. 7. Cartilage. X (b) Elastic. L (c) Fibrous. 8. Bone tissue. 9. Neuroglia. I. Fibrillar Connective Tissue. Fibrillar connective tissue, also known as white fibrous tissue or connective tissue proper, consists of cells and fibres lying in a base- ment or ground substance. The elements of fibrillar tissue may be classified as follows : f (a) Fixed cells. Ce]ls J (b) Wandering cells. (c) Plasma cells. (d) Mast cells. white or fibrillated, (fl\ Fibres 2. Intercellular substance. -J I yellow or elastic. (b) Ground or basement substance. I. Connective-Tissue Cells. — (a) Fixed connective-tissue cells are flat, irregularly stellate cells with many branches (Fig. 25). The nucleus lies in the thickest part of the cell. The cytoplasm is usu- ally clear or slightly granular. Each cell lies in a cell space or lacuna. From the cell spaces minute channels (canaliculi) extend in all directions to unite with canaliculi from adjoining spaces (Fig. 24). Delicate cell processes extend into the canaliculi and there anastomose with processes from other cells (Fig. 25). Owing to the extreme sensitiveness of the protoplasm of the connective-tissue cell to most fixatives, its usual appearance is that of a minute amount of cytoplasm shrunken down around a nucleus. (b) Wandering cells are not properly a part of the connective- tissue structure. They are amoeboid white blood cells (see page 86) which have passed out from the vessels into the tissues. (c) Plasma Cells. — These cells occur mainly near the smaller blood-vessels. Their protoplasm is finely granular and stains with basic aniline dyes. They frequently contain vacuoles. Small plasma cells are about the size of leucocytes, which they closely resemble. Large plasma cells are larger than leucocytes and richer in proto- plasm. THE CONNECTIVE TISSUES. 65 (d) Mast cells are spherical or irregular-shaped cells, found like the preceding in the neighborhood of the blood-vessels. Their protoplasm contains coarse granules which stain intensely with basic aniline dyes. They are believed by some investigators to be connected with the formation of fat ; by others to represent a stage in the devel- opment of the fixed connective-tissue cell. Connective-tissue cells may be pigmented (Fig. 26). In such cells the cytoplasm is more or less filled with brown or black pig- 4 P b , ■.. J FIG. 23. — Fibrillar Connective Tissue (Areolar Type) from Subcutaneous Tissue of Rabbit (technic 2, p. 70). X 500. a, Fixed connective-tissue cell; b, fibriilated fibres ; c, elastic fibre with curled broken end ; d, elastic fibres showing Y-shaped branching. ment granules. In man pigmented connective tissue-cells occur in the skin, choroid and iris. 2. The Intercellular Substance.— (V?) Fibres. White or fibrii- lated fibres are bundles of extremely fine fibrillar (0.5 ,". in diam- eter) (Fig. 23). The fibrillar lie parallel to one another and are united by a small amount of cement substance. The fibrillar do not branch. The fibre bundles, on the other hand, branch dichotomously and anastomose. White fibres, on boiling, yield gelatin. Yellow or clastic fibres are apparently homogeneous, highly re- fractive fibres, varying in diameter from 1 to 10 p. (Fig. 23V They branch and anastomose, forming networks. The smaller fibres are 5 66 THE TISSUES. round on cross section, the larger flattened or hexagonal (Figs. 28 and 29). Their elasticity is easily demonstrated in teased specimens by curling of the broken ends of the fibres (Fig. 23). On boiling ;. ■ /inn FIG. 24.— Section of Human Cornea Cut Tangential to Surface, X 350 (technic 9, p. 71), to show Connective-tissue Cell Spaces (Lacunas) and Anastomosing; Canaliculi. they yield elastin. While apparently homogeneous when subjected to the usual technic, Mall describes an elastic fibre as composed of a |. Fix a small piece of any good-sized tendon in formalin-Muller's fluid (page 5). After a week, harden in alcohol, embed in celloidin, and make longitudinal THE CONNECTIVE TISSUES. 71 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 choroid coat of the eye. Fix an eye in formalin-Midler's fluid (see page 5). cut in half, remove choroid and retina and pick off the dark shreds which cling to the outer surface of the choroid 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 13). 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. 8.. Connective-Tissue Cells to show Anastomosing Processes. — Stain a cornea with gold chlorid (see page 23). 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 23). 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 tissue (red). 3. Embryonal and Mucous Tissue. Embryonal and mucous tissue represent an early differentiation from the general type of embryonic connective tissue. They consist according to their age of oval, fusiform, or irregular branching and anastomosing cells, lying in a matrix, which is just beginning to show evidences of a fibrillar structure. By some histologists the term "embryonic" connective tissue is limited to the stage of fusi- form cells with slightly fibrillar matrix (Fig. 30), the term " mucous " tissue being applied to an embryonic form of connective tissue in which irregular branching and anastomosing cells lie in a slightly fibrillated matrix which gives the chemical reaction for mucin (Fig- 3i)- Much variation exists as to the shape and size of the cells in em- bryonal and mucous tissue. This is due to the fact that these cells represent transition stages in the development of the adult connec- 7* THE TISSUES. tive-tissue cell. Thus in embryonic connective tissue, 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 Foetal Pig. X 600. (Technic 1, p. 73.) 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. m 1 • Fig. 31.— Mucous Connective Tissue from Umbilical Cord of Eight-inch Foetal Pig. X 600. (Technic 2, p. 73.) THE CONNECTIVE TISSUES. 73 TECHNIC. r. 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 6, p. 9), hardened in alcohol and stained for twelve hours in alum-carmine (technic Z>, p. 15;. 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 three to four months human foe- tus, or of a nine-inch foetal pig is fixed in formalin-Midler's fluid (page 5). hard- ened in alcohol, and transverse sections stained with haematoxylin-eosin (technic 1, p. 16) 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. 7^.) The nuclei belong to flat connective-tissue cells which lie upon the fibres of the reticulum, their cell bodies being- invisible. themselves 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- 1 In some lower animals and in the embryos of some higher animals, such wholly cellular reticular tissues are found. 74 THE TISSUES. tion of the impossibility of differentiating on a morphological basis between certain forms of epithelial and of connective-tissue cells, <^ >"^m i ® ,5(8) * -*at -,^- ; -. m 'v"'" ® # •iT\ % t" © i© Fig. 33. — Diffuse Lymphatic Tissue from Human Lymph Node. X 600. (Technic, p. 75.) these cells were classified where their histogenesis properly places them, as connective-tissue cells. Reticular connective tissue differs in chemical composition from FIG. 34.— Circumscribed Lymphatic Tissue from Human Lymph Node. X 450. (Technic, P. 75) both fibrous and elastic tissue. Reticular connective tissue forms the framework of adenoid tissue and of bone marrow. Fibrils giving THE CONNECTIVE TISSUES. 75 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 lym- phoid cells, the latter filling the meshes of the reticulum. Lym- phoid cells are small spherical cells. Each cell has a single nucleus which almost fills the cell. 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-Miiller's fluid (technic 5, p. 5). and stain very thin sections with hematoxylin and picro-acid-fuchsin (technic 3, p. 16). 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 or 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. 37). 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 76 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 25. (Technic 1, p. 79.) b Young Fat from Human Subcutaneous Tissue (Child). X 175. (Technic r, p. 79.) a. Interlobular connective tissue ; /■>, fixed connective-tissue cell ; c, fat cells; d, artery ; e, nucleus of fat cell and remains of cytoplasm ("signet ring'"). THE CONNECTIVE TISSUES. 77 animals, are spherical (Fig. 36). In large masses of adult fat, the closely packed cells are subjected to pressure and are polyhedral (Fig. 37). Adult 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. Fat 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 d c FIG. 37.— Adult Fat Tissue from Human Subcutaneous Tissue. X 175- (Technic 1, p. 79.) 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 in 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. 78 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 Foetal Pig. X 75. (Technic 2, p. 79.) a, Arteriole breaking up into capillary network ; b, embryonal con- nective tissue ; c, embryonal fat lobules developing around blood-vessels. Fig. 39 —Developing Fat Tissue from Subcutaneous Tissue of Five-inch Fcetal Pig. (Technic 2, p. 79. j a, Arteriole breaking up into capillary network; 6, embryor. X 35°- yonal con- nective tissue, embryonal cells from which fat cells are developing; c, capillaries ; fat droplets stained black. Cells are seen in all stages of transition from embryonal connec- tive-tissue cells containing a few fat droplets to cells in which the cytoplasm is almost completely replaced by fat. THE CONNECTIVE TISSUES. 79 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. i. Fat Tissue. — Human subcutaneous fat as fresh as possible is fixed in forma- lin-Miiller's fluid (technic 5, p. 5), hardened in alcohol and embedded in celloidin. Sections are stained with haematoxylin and picro-acid-fuchsin (technic 3, p. 16). 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. 24), 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 lacuna? are present in some of the lower animals and have been described in human cartilage. They 8o 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. 4?3 .-•■ m& ■■■■ * mi ^1 ules ; S, polvnuclear leucocvte, blood cell appears homogeneous. Bv the „„', . )„„ ' ,„„„>,,,«, '„ fit o j containing eosmophne gran- USe of special methods, this apparently tiles; 9, mononuclear leucocyte, containing basophile granules. homogeneous substance can be separated into (a) a color-bearing proteid — hcemoglobin, and (b) a stroma, the latter representing the protoplasm of the cell. It is the haemoglobin S5 86 THE TISSUES. which gives color to the corpuscles. Haemoglobin is a complex proteid, and is held in solution or in suspension in the stroma. The red blood cells are soft and elastic, and are easily twisted to accommodate themselves to the smallest capillaries. The red blood cell is extremely susceptible to changes in the plasma. Thus even slight evaporation of the plasma results in osmosis between the now denser surrounding fluid and the contents of the cell. This causes fluid to leave the cell, with the result that the latter becomes spheroidal and irregularly shrunken, with minute knob-like projections from its surface. This is known as crenation of the red cell. The addition of water to blood, thus decreasing the specific gravity of the plasma, has the opposite effect, resulting in swelling of the cell. It also causes solution of the haemoglobin, which leaves the cell, the latter then appearing colorless. Dilute acetic acid causes swelling and fading of the red cells, with the for- mation of prismatic crystals of haemoglobin. The red blood cells number about 5,000,000 per cubic millimetre of blood. 2. White blood cells (leucocytes) (Fig. 45, 4. to 9 inclusive) are colorless nucleated structures which have a generally spherical shape, but which are able to change their shape on account of their powers of amoeboid movement. They have a diameter of from 5 to 10 //, and are much less numerous than the red cells, the proportion being about one white cell to from three hundred to six hundred red cells. This proportion is, however, subject to wide variation. Leucocytes may be classified as follows : {a) Lymphocytes ; {b) mononuclear leucocytes; (c) transitional leucocytes; (d) polymor- phonuclear or polynuclear leucocytes. (a) Lymphocytes (Fig. 45, 4). — These vary in diameter from 5 to 8 ',)■, and are sometimes subdivided into small lymphocytes and large lymphocytes. The nucleus is spherical, stains deeply, and al- most completely fills the cell, the cytoplasm being confined to a nar- row zone around the nucleus. Lymphocytes constitute about 20 per cent of the white blood cells. (/>) Mononuclear leucocytes (Fig. 45, 5 and 9) are of about the same size as large lymphocytes. The nucleus, however, stains more faintly and is smaller, while the cytoplasm is greater in amount. From 2 per cent to 4 per cent of the white cells are mononuclear leucocytes. THE BLOOD. 87 {c) Transitional leucocytes (Fig. 45, <5 ) occur in about the same numbers as the preceding, and are of about the same size. There is relatively more cytoplasm, and the nucleus, instead of being spherical, is crescentic or horseshoe or irregular in shape. These cells represent a transitional stage between the mononuclear and the polymorphonuclear and polynuclear varieties. (d) Polymorphonuclear and polynuclear leucocytes (Fig. 45, /, 8) constitute about 70 per cent of the white blood cells. Their size is about the same as that of the mononuclear form, but they are somewhat more irregular in shape. The appearance of the nucleus is characteristic. In the polymorphonuclear form the nucleus con- sists of several round, oval, or irregular nuclear masses connected with one another by cords of nuclear substance. These cords are frequently so delicate as to be distinguished with difficulty. The polynuclear form is derived from the polymorphonuclear by breaking down of the connecting cords, leaving several separate nuclei or nuclear segments. The protoplasm of leucocytes is granular, and these granules pre- sent very definite reactions when subjected to certain aniline dyes. Aniline dyes may be divided into acid, basic, and neutral, accord- ing to whether the coloring matter is an acid, a base, or a combina- tion of an acid and a base. Upon the basis of their reaction to these dyes, Ehrlich divides these granules into five groups, which he designates by the first five letters of the Greek alphabet. «- Granules {acidopJiilc, or, because the most common acid dye used is eosin, eosinophile — Fig. 45, 8). These are coarse, sharply defined granules which stain intensely with acid dyes. Eosinophile cells are mainly of the polynuclear and polymorphonuclear types. More rarely transitional forms contain eosinophile granules. They are actively amoeboid. Eosinophile cells constitute from 1 per cent to 4 per cent of the leucocytes of normal blood. Under certain pathological conditions the number of eosinophile leucocytes is greatly increased. ,3- Granules (amphophile). These are very fine granules, which react to both acid and basic dyes. ,3-Granules are not found in nor- mal human blood. They are found in the blood cells of some of the lower animals. ^-Granules ipasophile) are small granules which stain with basic 88 THE TISSUES. dyes. They occur in the so-called Mastzellen, which are of rare occurrence in normal blood. They are present in certain pathologi- cal conditions, and are-found normally in the blood cells of some of the lower animals, and in some of the cells of connective tissue. "-Granules {basophile) are small granules, which stain with basic dyes (Fig. 45, 9). They are found mainly in the mononuclear leucocytes. ^-Granules (ueutrophitc) react to mixtures of acid and basic dyes. £-Granules are the most common of all granules, occurring in most of the polynuclear and polymorphonuclear forms, being thus present in about 68 per cent of all white blood cells (Fig. 45, y). Through their powers of amoeboid movement leucocytes are able not only to pass through the walls of the vessels — diapcdesis — and out into the tissues, but to wander about more or less freely in the tissues. Both inside and outside of the vessels the leucocytes have an important function to perform in the taking up and disposal of waste and foreign particles. This is known as phagocytosis, and the cells thus engaged are known as phagocytes. Phagocytosis plays an extremely important role both in normal and in certain pathological processes. 3. The blood platelets (thrombocytes) are minute round or oval bodies about 2 ;>. in diameter. They are clear (colorless), and are described by some as containing chromatin granules, by others as having distinct nuclear structures. They may be separated by the action of a 10-per-cent saline solution into two elements. — one hya- line, the other granular. They are said to possess amoeboid properties and to be concerned in the coagulation of the blood. They number about 200,000 per cubic millimetre. Development of the Blood. At an early stage of embryonic development certain mesoblastic cells of the area vasculosa, which surrounds the embryo, become arranged in groups known as blood islands. It is from these " islands " that both blood and blood-vessels develop. The periphe- ral cells arrange themselves as the primitive vascular wall, within which the central cells soon become free as the first blood corpuscles. By union of the blood islands, vascular channels are formed, inside of which are the developing blood cells. At this stage the formed ele- THE BLOOD. 89 ments of blood consist almost wholly of nucleated red cells. These undergo mitotic division and multiply within the vessels. Two views are held in regard to the manner in which the embryonic nu- cleated red cell gets rid of its nucleus in becoming the non-nucleated red cell of the adult. According to one the nucleus is absorbed within the cell ; according to the other the nucleus, as a whole, is extruded. In early embryonic life especially active proliferation of red cells occurs in the blood-vessels of the liver. This has led to the consider- ing of the liver as a blood-forming organ. The liver cells themselves, however, take no actual part in the formation of blood cells, the blind pouch-like venous capillaries of the liver, with their slow-moving blood currents, merely furnishing a peculiarly suitable place for cellular pro- liferation. Before birth the splenic pulp and bone marrow become blood-forming organs. In the adult the bone marrow is probably under normal conditions the main if not the sole seat of red-cell for- mation. During fcetal life the number of nucleated red cells constantly diminishes while the number of non-nucleated red cells increases. At birth there are usually but few nucleated red cells in the general circulation, although even in the adult they are always found in the red bone marrow. The earliest embryonic blood contains no white cells. The origin of the leucocytes is not well understood. It seems probable that the earliest leucocytes are derived like the red cells from the cells of the blood islands of the area vasculosa. Later they are formed in widely distributed groups of cells, lymph nodules, which are found in various tissues and organs. These cells enter the circulation as lymphocytes. According to some, the mononuclear, transitional, polymorphonuclear and polynuclear forms are later stages in the development of these cells. According to others, the poly- morphonuclear and polynuclear forms are derived from the myelo- cytes of bone marrow. The origin of the blood platelets is not known. It is possible that they represent extrusion products of the blood cells. TECHNIC. (1) Fresh Blood.— Prick a finger with a clean needle. Touch the drop of blood to the centre of the slide and cover quickly. For immediate examination of fresh 90 THE TISSUES. blood no further preparation is necessary. Evaporation may be prevented by cementing or by smearing a rim of vaseline around the cover-glass. (2) Blood Smears. — From the same or a second prick take up a drop of blood along the edge of a mounting slide. Quickly place the edge against the surface of a second slide and draw the edge across the surface in such a manner as to leave a thin film or smear of blood. Allow the smear to become perfectly dry and stain by technic 7, p. 24. By this method the acidophile granules are stained red, basophile granules purple, and neutrophile granules a reddish-violet. (3) Good results may also be obtained by fixing the dried smear for half an hour in equal parts alcohol and ether and staining first in a strong alcoholic solu- tion of eosin, then in a rather weak aqueous solution of methylene blue. CHAPTER V. MUSCLE TISSUE. While protoplasm in general possesses the property of contrac- tility, it is in muscle tissue that this property reaches its highest de- velopment. Moreover, in muscle this contractility is along definite FIG. 46. — Isolated Smooth Muscle Cells from Human Small Intestine. X 400. (Technic 1, p. 99.) Rod-shaped nucleus surrounded by area of finely granular protoplasm; longi- tudinal striations of cytoplasm. directions, and is capable of causing motion, not only in the cell itself, but in structures outside the cell. Muscle may be classified as : (i) Involuntary smooth muscle ; (2) voluntary striated muscle ; (3) involuntary striated muscle or heart muscle. 1. Involuntary Smooth Muscle. — This consists of long spindle- shaped cells (Fig. 46) . The length of the cell varies from 30 to 200 ;x, B Fig. 47 —Apparent Intercellular Bridges of Smooth Muscle. A, From longitudinal section of intestine of guinea-pig ; B, from transverse section of intestine of rabbit. X 420. a. Nerve cell ; d, end of muscle cell. (Stohr.) its width from 3 to 8 :>, except in the pregnant uterus, where the cells frequently attain a much greater size. At the centre of the cell, which 91 92 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 d SP^PS-SW'";' ~™-'K%'--.~;y>pi&i&$ ' '..-.'.•-'■".:V,-J''"-'>.W" ''■- 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. 100.) 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); rf, 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 [t in length and from 10 to 60 // in di- ameter. Each muscle fibre consists of (a) a delicate sheath, the sarco- lemma, enclosing (/;) the muscle substance proper, in which lie (c) the muscle nuclei. The sarcolemma is a clear, apparently structureless, membrane, MUSCLE TISSUE. 93 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 JibrillcB 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 fibrillse, of which the muscle fibre is composed. These fibrillar 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 51). The light band is composed of a singly refracting (isotrophic) substance, the dark band of a doubly refracting (anisotro- phic) substance. Through the middle of the light band runs a fine dark (anisotrophic) line {Krause' ' s line), while an even finer light (iso- trophic) line (He/iscjt'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 fibrillcB, transversely into muscle discs. What is known as the sarcous element of Bozv- mau 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 TTf 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.) ;//, 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 ; ;/, muscle nucleus ; J, sar- colemma; fl, space be- tween sarcolemma and muscle substance. 94 THE TISSUES. throughout the fibre. On cross section irregular trabecular of sar- coplasm are seen extending in from the sarcolemma (Fig. 52). These separate the fibrillar 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 Cohnheim s. field (Figs. 51 and 52). The contractile clement of the fibre, the fibrillce, is anisotrophic, I b Fig. Fig. 51. Fig. 50.— Portion of Striated Voluntary Muscle Fibre. X 350. (Technic 4, p. 100.) 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, Fibrillar ; 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 fibrillae ; b, sarcous element ; c, Krause's line, d, Hensen's line ; e, Cohnheim's field ; /", muscle disc. the sarcoplasm isotrophic; the former, therefore, appears dark, the latter light by reflected 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. 95 ing. Between the fibrillar 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 contract less rapidly than white, but are less easily fatigued. In man white fibres are in the large majority, red fibres never occurring Fig. 5 2. d c Fig. 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 ; l\ 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 wholly 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). 9 6 THE TISSUES. 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- : ^~" ~ :^SA untary muscle. Like the former, it is com- , z l ._; -j posed of cells. Like the latter, it is both ■ -- — 7 'i longitudinally and transversely striated. Heart- muscle cells are short, thick cylinders. These |gp are joined end to end to form long fibres. I S0. ; |n ':'.- By means of lateral branches the cells of one '_.• £r ■' 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 fibrillar. 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 fibrilUe united by cement substance. From the central mass of sarcoplasm which surrounds the nucleus, strands radiate toward the periphery. These strands, anastomosing, ! " " ' iiii ^Krfe wMM ['.'it Fin. 54. Semidiagram- matic Illustration of Kndings 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. 97 separate the fibrillar into columns, the muscle columns of Kollikcr. 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 Krause. 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 r'l Fig. 55. Fig. 56. FlG. 55.— Two Muscle Fibres from Upper End of Human Sartorius, to show connection of muscle and tendon. X 350. (Gage.) ?n, Muscle fibres; /, tendon fibres. FlG. 56. — Muscle Cells from the Human Heart (technic 6, p. 100), 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- 98 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- a c b ! ; ! m §/© Fig. 57. — Section of Heart Muscle. X 350. (Technic 7, p. 100.) a, Cells cut longitudinally; b T cells cut transversely (only three nuclei have been included in the plane of section) ; c, connective-tissue septum. ments and the fact that the longitudinal fibrillar 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 51), 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 formative cell changes its shape,. MUSCLE TISSUE. 99 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 myotonics. 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, and the new nuclei have migrated 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 38) 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 fibrillar representing nothing more than a specialisation 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 15). This is poured off, IOO 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 15). After twelve hours' staining, wash and transfer to eosin-glycerin. (3) Sections of Smooth Muscle. — Fix small pieces of intestine in formalin- Muller's (technic 5, p. 5) or in Zenker's fluid (technic 9, p. 6). Thin transverse or longitudinal sections are stained with hasmotoxylin-eosin (technic 1, p. 16). 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-Miiller's fluid or in Zenker's fluid (page 6). Thin sections are stained with hajmatoxylin-picro-acid-fuchsin (technic 3, p. 16) 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). (■j) 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 hamiatoxylin-eosin (technic 1, p. 16), or hasmatoxylin-picro-acid-fuchsin (technic 3. p. 16). 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. Such processes, often extending long distances from the cell body proper, constitute one of the most striking features of nerve-cell structure. It is these processes which 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 ivith 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 102 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, pyriform ; of the cells of the ventral horns of the cord, irreg- ularly stellate. According to the num- Fig. 5 8. 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 neurofte. (Barker), c, Cytoplasm of cell body containing chromophilic bodies, neurofibrils, and perifibrillar substance ; n, nucleus ; 7/', nucleolus ; d, dendrites; a /i, axone hill free from chromophilic bodies; ax, axone ; sf, side fibril (collateral) ; ;//, medullary sheath ; 11 R, node of Ranvier where side branch is given off ; si, neurilemma and incisures of Schmidt ; in', striated muscle fibre ; tel, motor end plate. PlG. ,. — Large Motor Ganglion Cell from Ventral Horn of Spinal Cord of Ox, showing Chromophilic Bodies. (From Barker, after von Lenhossek.) a, Pigment ; /;, axone ; c, axone hill ; d, dendrites. ber of processes given off, nerve cells are often referred to as unipolar, 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. 103 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- ■ Y.'V M m I § ■'■', A Fig. 60. — Ganglion Cells, Stained by Bethe's Method, showing Neurofibrils. A, Anterior horn cells (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 109). 104 THE TISSUES. (3) The chromopkilic 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 28). When sub- jected 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 caryochromes, and apparently consist wholly of neurofibrils and perifibrillar substance. Other cells react both as to their nuclei and as to their cell bodies, to the Nissl stain. These cells are known as somatochromes. Taking 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 cytoplasm is composed of two dis- tinct elements : (a) a clear, unstained ground substance, and, scattered through this, (/;) deep blue-staining masses, the chromopkilic bodies (Fig. 59). These bodies are granular in character and differ in shape, size, and arrangement. They may be large or small, regular or irregular in shape, may be arranged in rows or in an ir- regular manner, may be close together, almost filling the cell body, or quite separated from one another. Present- ing these variations in different 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 basrs of classification. 1 It is important to note in studying the nerve cell by this method Fig. 61. — Pyramidal Cell from Cere- bral Cortex of Mouse. (After Ra- mon y CajalJ Golgi cell type I. d, Cell body giving off main or apical dendrite ; ' d -■—. —■d b — B A Fig. 63. Fig. 64. FlG. 63.— Non-medullated Nerve Fibres with Neurilemma, only the nuclei of which can be seen. X 300. FlG. 64. — A, Fresh Medullated Nerve Fibre from Sciatic Nerve of Guinea-pig' (X 700I. show- ing relative size of axone and medullary sheath. B, Medullated Nerve Fibre from Hu- man Cauda Equina (X 700) (technic 4, p. 113), showing shrunken axone. ii, Axone ; b, medullary sheath ; c, node of Ranvier ; d, neurilemma ; //, incisures of Schmidt ; i, 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. ioS THE TISSUES. k _. — d ■:- (b) Non-medullated axones with a neurilemma— fibres of Remafc. In these the axone is surrounded by a delicate homogeneous, nucleated sheath, the neurilemma or sheath of Schwann (Fig. 6$). These axones are described by some writers as having no true neurilemma, but merely a discontinuous covering of flat connective-tissue cells, which wrap around the axone and corre- spond to the endoneurium of the nerve trunk ., d e (see page 339). I 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 axolemma ox periaxial sheath (Fig. 65). Fig. 65. Fig. 66. FIG. 65.- Diagram of Structure of a Med- ullated Xerve Fibre, showing two differ- ent views as to relations of neurilemma and axilemma and their behavior at the nodes of Ranvier. rSzymonowicz.) a, Neurofibrils; b, cement substance; r, axone ; d, incisure of Schmidt ; e, nucleus of neurilemma ; /, medullary sheath ; g, sheath of Schwann ; //, axone ; 1, axilem- 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 ; /<, axone; r, sheath of Henle ; d, nuclei of Henle's sheath ; e, nucleus of neurilemma. NERVE TISSUE. 109 (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 80 to 600 '.i- 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, Schmidt-Lantermann segments, by clefts which pass from the neurilemma to the axolemma or axone, and are known as the clefts or incisures of Schmidt-Lantermann (Fig. 64, B). On boiling medullated nerve fibres in alcohol and ether a fine network is brought out in the medullary sheath, the neurokeratin network. 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 Schwann (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 to 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 HO 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 10,'-', the largest from 10 to 20,^. 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 of 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 arc probably a differentiation of the spongioplasm, while the perifibrillar substance and chromophilic bodies are specializations of the hyaloplasm. NERVE TISSUE. i i i 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 extracellluar 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 tissues, neu- roglia is of ectodermic origin, being developed from the ectoder- mic cells which line the embryonic neural canal. These cells, at first morphologically identical, soon differentiate into neuroblasts 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 central portion con- taining the nucleus and, radiating out from this, delicate, straight, un branched processes. Jllossy cells also have a central portion con- taining the nucleus, from which pass off rough, thick, branching arms. As in the nerve cell, the processes of neuroglia cells do not anastomose, but form a network of interlacing fibrils for the support I 12 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 Weigert, what are in Golgi preparations apparently processes of the cells, are entirely separate neuroglia fibres, the neu- FlG. 67.— A, Mossy Cell from Human Cerebral Cortex. Golgi method. (Delafield and Prud- den.) B, Three Spider Cells from Cat's Cerebral Cortex. (RetziusJ 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 cell body, and that the apparently separate fibrils are really a part of the proto- plasm of the neuroglia cell. TECHNIC. (1) Pieces of the cerebral cortex are stained by one of the C.olgi 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 he covered. Sections are cut from 75 to 100/' in thickness, cleared in carbol- xylol or oil ol origanum and mounted iii balsam. This section shows only the ex- ternal morphology of the neurone. It is also to be used for studying the different varieties Oi neuroglia cells as demonstrated by Golgi's method (see ])a,t;e 27). NERVE TISSUE. 113 (2) Thin transverse slices from one of the enlargements of the spinal cord are fixed in absolute alcohol. Thin sections (5 to 10/-/) are stained by Nissl's method (page 28) 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-Muller's fluid (technic 5, p. 5), and hardened in alcohol. Small strands are stained twenty minutes in strong picro-acid-fuchsin solution (technic 2, p. 16), 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 Biood. Wood : Laboratory Guide to Clinical Pathology. Prenant, Bouin et Maillard : Traite d'Histologie. Barker: The Nervous System. Van Gehuchten : Le Systeme nerveux de l'homme. Bethe : Allgemeine Anatomie und Physiologie des Nervensystem. 8 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 ; (/>) 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 ; (c) 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 16//. in diameter. Its wall consists of a single layer of endothelial cells. 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- 117 n8 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. 62), 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, ti, Capillaries ; b, nuclei of capillary endothelium ; c, precapillary arteries ; ) an outer composed of mixed fibrous and elastic 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. 12/ 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 Jieart 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 350.) 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-M tiller's fluid (technic 5. p. 5). Cut transverse and longitudinal sections; stain with haematoxylm-eosin (technic 1. p. 16) 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 128 THE ORGANS. by filling with formalin-Miiller'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 tnbe, 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. 129 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- ventitia which is strengthened by bundles of longitudinal smooth muscle. Lymph 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 canalicnli of the cornea and of bone (pages 64 and 83). 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 cxlom, body cavity, or plcnroperitoneal 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. 62. 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. 9 130 THE ORGANS. (2) The Thoracic Duct. — Remove a portion of the thoracic duct, fix in for- malm-Miiller's fluid (technic 5. p. 5), and stain sections with haematoxylin-eosin (technic 1. p. 16). 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 253. 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- omv. 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 * 8 f FIG. 74. — Section through Entire Human Lymph Node, including- Hilum. X 15. (Technic 1, p. 134.) Dark zone, cortex ; light central area, medulla, a, Lymph nodule of cortex ; />, germinal centres; c, trabeculae containing blood-vessels; d, capsule; e. hilum ; f\ lvmph sinus of medulla ; g, lymph cords of medulla ; //, lymph sinuses of medulla and cortex. septa constitute the connective-tissue frameivork 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, 131 132 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 medulla (Fig. 74). Within the compartments formed by the capsule and the septa is the lymphatic tissue (for structure see page 75). 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 n6dule 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 due 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 lymph sinus — which is bridged over by reticular tissue containing compara- LYMPHATIC ORGANS. 133 tively few lymphoid cells (Fig. 75). These sinuses form a contin- uous system of anastomosing channels throughout the node. The reticular connective tissue (page 73), 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- priK 3,° 6 a '■ t J M^'^? & -f \ FlG. 75.-Sectioii through Cortex and Portion of Medulla of Human Lymph Node. X 350. (Technic 2, p. 134.) a, Capsule ; b, lymph sinus ; c, cells showing mitosis ; d, germinal cen- tre ; e, trabecular ; f, blood-vessel ; g, lymph cords : A, medulla ; i, cortex. cially those of the medulla, and in specimens in which the cells have been largely washed out or removed by maceration, the reticular structure is well shown. The lymphoid tissue proper, as represented by the lymph nodules and. anastomosing lymph cords, is thus, as it were, suspended in the meshes of a reticulum which is swung from the capsule and trabecu- lar. As both nodules and cords are everywhere separated from cap- sule and trabeculae by the sinuses, and as these latter serve for the passage of lymph through the node, it is seen that the lymphatic tis- 134 THE ORG Ays. sue of the node is broken up in such a manner as to be bathed on all sides by the circulating lymph. In addition to the definitely formed lymph nodes and the well- defined collections of lymph nodules, such as those of the tonsil or of Peyer's patches, small nodules or groups of lymphoid cells have a wide distribution throughout the various organs. While many of these collections of lymphatic tissue are inconspicuous, still the ag- gregate of lymph tissue thus distributed is by no means inconsider- able. The most important will be described in connection with the organs in which they occur. Blood-vessels. — Those which enter the hilum carry the main blood supply to the organ. Most of the arteries pass directly into the lym- phatic tissue, where they break up into dense capillary networks. Some of the arteries, instead of passing directly to the lymphatic tis- sue, follow the septa, supplying these and the capsule, and also send- ing branches to the surrounding lymphatic tissue. A few small vessels enter the capsule along the convexity of the organ and are distributed to the capsule and to the larger septa. Lymphatics. — The afferent lymph vessels enter the node on its convex surface opposite the hilum, penetrating the capsule, and pour their lymph into the cortical sinuses. The lymph passes through the sinuses of both cortex and medulla, and is collected by the efferent lymph vessels which leave the organ at the hilum. Within the node the lymph comes in contact with the superficial cells of the nodules and of the lymph cords. These cells are constantly passing out into the lymph stream so that the lymph leaves the node much richer in cellular elements. Nerves are not abundant. Hoth medullated and non-medullated fibres occur. Their exact modes of termination are not known. TECHNIC (r) Remove several lymph nodes from one of the lower animals (ox, cat, dog, rabbit), fix in formalin-Miiller's fluid (technic 5, p. 5), and harden in alcohol. Cut thin sections through the hilum, stain with luvmatoxylin-eosin (technic 1, p. id), or with hematoxylin - picro- acid - fuchsin (technic 3. p. 16), and mount in balsam. (2) Expose a chain of lymph nodules {e.g., the cervical or inguinal of a re- cently killed dog or cat). Insert a small cannula or needle into the uppermost node and inject formalin-Muller's fluid until the node becomes tense. By now slightly increasing the pressure the fluid maybe made to pass into the second node, and so through the entire chain. The nodes are then carefully dissected out and LYMPHATIC ORGANS. 135 placed for twenty-four hours in formalin- Aluller's fluid, then hardened in alcohol. Sections are cut through the hilum, stained with haematoxylin-eosin or with hasma- toxylin-picro-acid-fuchsin and mounted in balsam. Near the centre of the chain are usually found nodes in which the lymph sinuses are properly distended. The most proximal nodes are apt to be overdistended. but for this very reason are often excellent for the study of the reticular tissue from which most of the cells have been washed out, especially in the medulla. (3) Human lymph nodes may be treated by either of the above methods. Owing to the coalescence of their cortical nodules their structure is apt to be less easily demonstrable than in the lower animals. Haemolymph Nodes. These are lymphoid structures which ciosely resemble ordinary lymph nodes, but with the essential difference that their sinuses are blood sinuses instead of lymph sinuses. Each node is surrounded by a capsule of varying thickness, com- posed of fibro-elastic tissue and smooth muscle cells. From the FIG. 76.— Section through Human Haemolymph Gland, including Hilum, showing capsule, trabeculas, sinuses filled with blood, and lymph nodules. ( Warthin.) capsule trabecules of the same structure pass down into the node, forming its framework (Fig. 76). Beneath the capsule is a blood sinus, which may be broad or narrow, and usually completely sur- 136 THE ORGANS. rounds the node. Less commonly the sinus is interrupted by lym- phoid tissue extending out to the capsule. From the peripheral sinus branches extend into the interior of the node, separating the 0£2 : FIG. 77.— Section through Superficial Portion of Human Haemolymph Gland (Marrow-lymph Gland). (Warthin.) Capsule, trabeculae, and parts of two adjacent nodules; sinuses filled with blood ; among the lymph cells are large multinuclear cells resembling those of marrow, nucleated red blood cells, etc. lymphoid tissue into cords or islands. The relative proportion of sinuses and lymphoid tissue varies greatly, some nodes being com- posed almost wholly of sinuses, while in others the lymphoid tissue predominates. There is usually a fairly distinct Jiihim. In many glands no differentiation into cortex and medulla can be made. Where there are a distinct medulla and cortex the peripheral lymphoid tissue is arranged in nodules as in the ordinary lymph node. Re- ticular connective tissue crosses the sinuses and supports the cells of the lymph nodules and cords (Fig. 77). The cellular character of the lymphoid tissue has led to the sub- division of hx'iuolymph nodes into splcnolymph nodes and marrow- lymph nodes. In the splcnolymph node the lymphoid tissue resembles that of the ordinary lymph node or of the spleen. In the marrow- lymph node, which is the much less common form, the lymphoid tissue resembles red marrow. There are no distinct nodules, and LYMPHATIC ORGANS. 137 there is a quite characteristic distribution of small groups of fat cells. The most numerous cells are eosinophiles and mast cells (see page 87). Polynuclear leucocytes and large leucocytes with a single lobulated nucleus are less numerous. The very large multinuclear cells of red marrow are also found, but usually in small numbers. Large phagocytes containing blood pigment and disintegrating red blood cells are found in both forms of hasmolymph nodes, but are most numerous in the splenolymph type. In nodes which have a brownish color when fresh, these phagocytes frequently almost com- pletely fill the sinuses. Further classification of haemolymph nodes has been attempted, but is unsatisfactory, owing to the large number of transitional forms. Thus many nodes are transitional in structures between the haemolymph node and the ordinary lymph node, between the spleno- lymph node and the marrow-lymph node, and between the spleno- lymph node and the spleen. Under normal conditions the haemolymph nodes appear to be concerned mainly in the destruction of red blood cells ; possibly also in the formation of leucocytes. Under certain pathological con- ditions they probably become centres for the formation of red blood cells. Blood-vessels. — An artery or arteries enter the node at the hilum, and break up within the node into small branches, which communi- cate with the sinuses where the blood comes into intimate association with the lymphoid tissue. From the sinuses the blood passes into veins, which leave the organ either at the hilum or at some other point on the periphery. The course which the blood takes in pass- ing through the haemolymph node is thus apparently similar to that taken by the lymph in passing through the ordinary lymph node. The relation of the haemolymph node to the lymp]iatic system is not known, and like ignorance exists as to its innervation. TECHNIC. Same as for lymph nodes (technic 1, p. 134). The nodes are found in greatest numbers in the prevertebral tissue, and are often difficult to recognize. Fixing the tissues in 5-per-cent formalin aids in their recognition as it darkens the nodes while bleaching the rest of the tissues. 138 THE ORGANS. The Thymus. The thymus is an organ of foetal and early extra-uterine life, reaching in man its greatest development at the end of the second year. After this age it undergoes a slow retrograde change into fat and connective tissue, until by the twentieth year scarcely a vestige of glandular tissue remains. The thymus originates in the ectoderm and begins its foetal exist- ence as a typical epithelial gland. Into this- epithelial structure mesodermic cells grow and differentiate into lymphatie tissue. This almost completely replaces the epithelial tissue, only rudiments of which remain. Morphologically the fully developed thymus consists of lobes and lobules (Fig. 78). The whole gland is enclosed in a single con- nective-tissue capsule, and the lobes are separated from one another — / FIG. 78.— From Section of Human Thymus, showing 1 parts of five lobules and interlobular < 20. (Technic, page 139.) a, Cortex ; /', medulla; c, interlobular septum. by strong extensions of capsular tissue. Smaller connective-tissue septa extend into the lobes, subdividing them into lobules. From the perilobular connective tissue, septa extend into the lobule, sepa- rating it into a number of chambers. Each lobule consists of a cor- tical portion and a medullary portion. The cortex consists of nodules LYMPHATIC ORGANS. 139 of compact lymphatic tissue similar to those found in the lymph node. These occupy the chambers formed by the connective-tissue septa. The medulla consists of a more diffuse lymphatic tissue with no connective-tissue septa. In the medulla are found a number of spherical or oval bodies com- posed of concentrically arranged epithelial cells. These are known as HassaV s corpuscles (Fig. 79), and represent the only remains of the original glandular epithelium. The cen- tral cells of the corpuscles are usually spherical and contain nuclei, while the peripheral ' a cells are flat and non-nucleated. " - c- Unlike the Other lymphatic Fig. 79.— Hassal's Corpuscle and S mall Portion , -, , -, -, , r of Surrounding- Tissue, X 600. (Seetechnic organs, the lymph nodules of below.) the thymus contain no germinal centres. Mitosis can, however, usually be seen in the lymphoid cells. Nucleated red blood cells also occur in the thymus. The thymus must therefore be considered one of the sources of lymphoid cells and of red blood cells. Blood-vessels. — The larger arteries run in the connective-tissue septa. From these, smaller intralobular branches are given off, which break up into capillary networks in the cortex and medulla. The capillaries pass over into veins. These converge to form larger veins, which accompany the arteries. Of the lymphatics of the thymus little is known. They appear to originate in indefinite sinuses within the lymphoid tissue, whence they pass to the septa, where they accompany the blood-vessels. Nerves. — These are distributed mainly to the walls of the blood- vessels. A few fine fibres, terminating freely in the lymphatic tissue of the cortex and of the medulla, have been described. TECHNIC. Fix the thymus of a new-born infant in formalin-Miiller's fluid (technic 5, p. 5), and harden in alcohol. Stain sections with haematoxylin-eosin (technic 1, p. 16), or with haematoxylin-picro-acid-fuchsin (technic 3. p. 16), and mount in balsam . 140 THE ORGANS. The Tonsils. The Palatine Tonsils or True Tonsils. — These are compound lymphatic organs, essentially similar in structure to the lymphatic organs already described. The usual fibrous capsule is present only over the attached surface, where it separates the tonsil from sur- rounding structures. From the capsule, connective-tissue trabecules extend into the substance of the organ and branch to form its frame- work. The free surface of the tonsil is covered by a reflection of the stratified squamous epithelium of the pharynx (Fig. 80). The epithe- y Fig. 80.— Vertical Section of Dor's Tonsil through Crypt. X 15. (Szymonowicz.) a, Lymph nodule; f>, epithelium of crypt; c, blood-vessel; d, crypt; e, connective-tissue capsule; /", mucous glands ; g, epithelium of pharynx. lium is separated from the underlying lymphatic tissue of the tonsil by a more or less distinct basement membrane. At several places on the surface of the tonsil deep indentations or pockets occur. These are known as the crypts of the tonsil (Fig. 80), and are lined through- LYMPHATIC ORGANS. 141 out by a continuation of the surface epithelium. Passing off from the bottoms and sides of the main or primary crypts are frequently several secondary crypts, also lined with the same type of epithelium. Beneath the basement membrane is the lymphoid tissue of the tonsil. This consists of diffuse lymphatic tissue in which are found FlG. 81. — Vertical Section through Wall of Crypt in Dog's Tonsil, showing lymphoid infiltra- tion of epithelium. X 150. (Bohm and von Davidoff.) a, Leucocytes in epithelium ; b, space in epithelium filled with leucocytes and changed epithelial cells ; c, blood-vessel ; d, epithelium beyond area of infiltration ; e, basal layer of cells. nodules of compact lymphatic tissue similar to those in the lymph node. Each nodule has a germ centre, where active mitosis is going on, and a surrounding zone of more densely packed cells. The nodules have a fairly definite arrangement, usually forming a single layer beneath the epithelium of the crypts. At various points on the surface of the tonsil, and especially in the crypts, occurs what is known as lymphoid infiltration of the epithelium (Fig. 81). This consists in an invasion of the epithelium by the underlying lymphoid cells. It varies from the presence of only a few lymphoid cells scat- tered among the epithelial, to an almost complete replacement of epi- thelial by lymphoid tissue. In this way the latter reaches the sur- face and lymphoid cells are discharged upon the surface of the tonsil and into the crypts. These cells probably form the bulk of the so- called salivary corpuscles. The Lingual Tonsils — Folliculi Linguales. — These are small lymphatic organs situated on the dorsum and sides of the back part of the tongue, and are similar in structure to the true tonsils. Into their crypts frequently open the ducts of some of the mucous glands of the tongue. 142 THE ORGANS. The Pharyngeal Tonsils. — These are lymphatic structures which lie in the nasopharynx. They resemble the lingual tonsils. The tonsils make their first appearance toward the end of the fourth month of intra-uterine life. The earliest of the tonsillar lym- phoid cells are white blood cells which have migrated from the ves- sels of the stroma of the mucosa and have infiltrated the surrounding connective tissue. Further development of the tonsil is by prolif- eration of these cells. The crypts are at first solid ingrowths of sur- face epithelium. These later become hollowed out. The blood-vessels and nerves have a distribution similar to those of the lymph nodes, but enter the organ on its attached side and not at a definite hilum. Of the lymphatics of the tonsil little is known. TECHNIC. Normal human tonsils are so rare, owing to the frequency of inflammation of the organ, that it is best to make use of tonsils from one of the lower animals (dog, cat, or rabbit). Treat as in technic i, p. 134. care being taken that sections pass longitudinally through one of the crypts. The Spleen. The spleen is a lymphatic organ, the peculiar structure of which appears to depend largely upon the arrangement of its blood-vessels. The surface, except where the organ is attached, is covered by a serous membrane, the peritoneum (page 129). Beneath this is a cap- sule of fibrous tissue containing numerous elastic fibres and smooth muscle cells. From the capsule strong connective-tissue septa, simi- lar to the capsule in structure, extend into the interior of the organ. These branch and unite with one another to form a series of anasto- mosing chambers. The capsule and septa form, as in the lymph node, the connective-tissue framework of the organ (Fig. 82). The chambers formed by the connective-tissue framework are filled in with lymphatic- tissue, which occurs in two forms, diffuse and compact. The diffuse lymphatic tissue constitutes the substantia propria of the organ and is everywhere traversed by thin-walled vas- cular channels, the lymphatic tissue and vascular channels together constituting the splenic pulp (Fig. 83). Compact lymphatic tissue occurs as spherical, oval, or cylindrical aggregations of closely packed lymphoid cells. These are known as Malpighian bodies or splenic LYMPHATIC ORGANS. 143 corpuscles (Figs. 82 and 83) and are distributed throughout the splenic pulp. Each splenic corpuscle contains one or more small arteries. Fig. 82. — Section through Portion of Cat's Spleen, to show general topographv. X 15. (Technic 1, p. 146.) J, Capsule ; b, trabeculse containing blood-vessels ; c, germinal centres ; d, trabeculse; •?.•) ~J?-'~~i '--', &•; *j^, : Sl.:'. V *! 3 , ^ ,/ a Fig. 83.— Section of Human Spleen, including portion of Malpighian body with its artery and adjacent splenic pulp. X 300. (Technic 2, p. 146.) , group of fat cells ; c, blood-vessel ; d, trabecular of bone. large central cavity surrounded by spongy bone, which, however, passes over on its outer side into a layer of bone of great hardness and known as hard or compact bone. Spongy bone forms the ends 148 THE SKELETAL SYSTEM. 149 and lines the marrow cavities of the long bones, and forms the centre of the short and flat bones. Compact bone forms the bulk of the shafts of the long bones and the outer layers of the flat and short bones. In compact bone the layers or lamellae of bone tissue have a defi- nite arrangement into systems, the disposition of which is largely Fig. 87. -Longitudinal Section of Hard (Undecalcified Bone) Shaft of Human Ulna. X 90. (Szymonowicz.) Haversian canals, lacunas, and canaliculi in black. dependent upon the shape of the bone and upon the distribution of its blood-vessels. In spongy bone (Fig. S6) there is no arrangement of the bone tis- sue into systems. The trabecular consist wholly of bony tissue laid down in lamellae. These trabecular anastomose and enclose spaces which contain marrow and which serve for the passage of blood-ves- sels, lymphatics, and nerves. On examining a longitudinal section of compact bone (Fig. 87) .there are seen running through it irregular channels, the general direction of which is parallel to the long axis of the bone. These channels anastomose by means of lateral branches, and form a com- 150 THE ORGANS, plete system of intercommunicating tubes. They are known as Haversian canals, contain marrow elements, and serve for the trans- mission of blood-vessels, lymphatics, and nerves. They anastomose — .-4 tffe •>' > < ■'■'. *a*g FlG. 88 —Cross section of Hard ( Un'iecalcified ) Bone from Human Metatarsus. X 90. (Szy- monowicz.) Haversian canals, lacunas, and canaliculi ill black, a, Outer circumferential lamellae ; />, inner circumferential lamellae ; c, Haversian lamella; ; if, interstitial lamellaa. not only with one another, but are in communication with the surface of the bone and with the marrow cavity. Between the Haversian canals the lamellae are seen running parallel to the canals. In a cross section through the shaft of a long bone (Fig. 88), three distinct systems of lamella: are seen. These are known as Haversian lamella, interstitial lamella?, wad circumferential lamella. 0) Haversian Lamellae (Fig. 89). — These are arranged in a concentric manner around the Haversian canals. Between the lamellae, their long axes corresponding to the long axes of the Haver- THE SKELETAL SYSTEM. 151 sian canals, are the lacuna with their enclosed bone cells (page 83). The lacunae of adjacent lamellae are usually arranged alternately. In a section of ordinary thickness the lacunae are not nearly so numerous as the lamellae, and are seen only between some of the lamellae. The lacunae of a Haversian system communicate with one another and with their Haversian canal by means of the canaliculi. In Haversian systems the fibres of the matrix (see page S3) run in some lamellae parallel to the canal, in others concentrically. Adja- cent fibres thus frequently cross at right angles. The Haversian canal itself contains marrow, blood-vessels, lymphatics, and nerves. (2) Interstitial (Intermediate or Ground) Lamell.e (Figs. 88 and 89). — These are irregular short lamellae, which occupy the spaces left between adjacent Haversian systems. ( 3 ) Circumferential Lamellae (Fig. 8S). — These are parallel lamellae which run in the long axis of the bone, just beneath the peri- osteum and at the outer edge of the marrow cavity. Occa- sionally circumferential lam- ellae are absent, the Haversian systems abutting directly upon periosteum. Channels for the passage of blood-vessels from the periosteum to the Haversian canals pierce the circumfer- ential lamellae. They are known as Volkmann ' s canals, and are not surrounded by concentric lamellae as are the Haversian lamellae, but are mere channels through the bone. Similar canals pass from the inner Haversian canals into the marrow cavity. The Periosteum. — This is a fibrous connective-tissue membrane which covers the surfaces of bones except where they articulate. It Fig. 89. — Transverse Section of Compact Bone from Shaft of Humerus. X 150 and slightly re- duced. (Sharpey.) (Technic 1, p. S3. 1 Three Haversian canals with their concentric lamellae and lacunae ; canaliculi connecting lacunae with each other and with Haversian uanal. Between ihe Haversian systems of lamellae are seen the interstitial lamellae. 152 THE ORGANS. is firmly adherent to the superficial layers of the bone and consists of two layers. The outer layer is composed of coarse fibrillated fibres and contains the larger blood-vessels. The inner layer consists of fine white fibres and delicate elastic fibres which support the smaller blood-vessels. From the periosteum distinct bundles of white fibres, with often some elastic fibres, pierce the outer layers of the bone. These are known as the perforating fibres of Sharpey. When tendons and ligaments are attached to bone, their fibres are prolonged through the periosteum into the bone as perforating fibres. Bone Marrow. Bone marrow is a soft tissue which occupies the medullary and Haversian canals of the long bones and fills the spaces between the trabecules of spongy bone. Marrow occurs in two forms — red mar- row and yellow marrow. Red marrow is found in all bones of embryos and of young ani- mals, also in the vertebras, sternum, ribs, cranial bones, and epiphyses of long bones in the adult. In the diaphyses of adult long bones the marrow is of the yellow variety. The difference in color between red marrow and yellow marrow is clue to the much greater proportion of fat in the latter, yellow marrow being developed from the red by an almost complete replacement of its other elements by fat cells. Red marrow is of especial interest as a blood-forming tissue, being in the healthy adult the main if not the sole source of red blood cells, and one of the sources from which the leucocytes are derived. The blood-forming function of marrow must be borne in mind in studying the various forms of marrow cells. Red marrow (Fig. 90) consists of a delicate reticular connective tissue which supports the following varieties of cells : (1) Marrow Cells — Myelocytes. — These resemble the mononu- clear and some of the transitional forms of leucocytes. The nucleus is large and may be lobulated. It contains a comparatively small amount of chromatin and therefore stains faintly. The cytoplasm is finely granular and stains with neutrophile dyes. Myelocytes are not present in normal blood, but occur in large numbers in leukxmia. It is from the myelocytes that those leucocytes, which are of bone- marrow origin, are derived. THE SKELETAL SYSTEM. 153 (2) Nucleated Red Blood Cells. — These are divisible into erythro- blasts and normoblasts. The former represents an earlier, the latter a later stage in the evolution of the non-nucleated adult red blood cell. The erythroblast, the younger of the two, has a well-formed nu- cleus with a distinct intranuclear network. The protoplasm contains but little haemoglobin. In the normoblast the intranuclear network — b gp ..:-.--.-. d e f FIG. go.— Section of Red Bone Marrow from Rabbit's Femur. X 700. (Technic 4, p. 156.) a, Fat cells ; b, myeloplax ; c, fat space ; d, nucleated red blood cells ; e, myelocytes ; f, re- ticular connective tissue ; g, leucocytes. has disappeared and the protoplasm has become much richer in hae- moglobin. The normoblast is converted into the adult red blood cell either by extrusion of its nucleus or by the disintegration of the nucleus within the cell body. (3) Non-Nucleated Red Blood Cells. — These are the same as are found in the blood (page 85). (4) Multinuclear Cells — Myeloplaxes. — These are large cells with abundant protoplasm. Each cell may contain a single large spheri- cal nucleus or a much lobulated nucleus or several nuclei. Myelo- 154 THE ORGANS. plaxes are probably derived from leucocytes, and are closely related to, if not identical with, the osteoclasts of developing bone. (5) Eosinopkile cells are frequently found in marrow. They have the same structure as in blood (page 8y). (6) Mast ails may be present. They are usually not numerous. (For description see page 88.) (7) Fat Cells. — These are usually round and rather evenly distrib- uted throughout the marrow. Yellow marrow (Fig. 91) consists almost wholly of fat cells, which have gradually replaced the other marrow elements. Under PlG. 91, Yellow Marrow from Rabbit's Femur. X 560. (Technic 4, p. 156.) a. Nucleated red blood cells; b, myeloplax, c, fat celis ; , cartilage trabecula ; c, osteo- genetic tissue in primary marrow space ; d, blood-vessel ; e, cell spaces ; f, cartilage cells. 1 62 THE ORGANS. known as the calcification zone and everywhere precedes the forma- tion of true bone (Fig. 94). 3. Subperiosteal or subperichondrial development (Fig. 94) has already been largely described in connection with intramem- branous ossification, and differs in no important respect from the latter. It always accompanies one of the other forms of ossification. Bone appears beneath the perichondrium somewhat earlier than within the underlying cartilage. Beneath the perichondrium is a layer of rich cellular osteogenetic tissue. The cells of this tissue FIG. 97. — Intracartilaginous Bone Development. Same specimen as Fig. 94 (X 350), showing bone being deposited around one of the trabeculse of cartilage, a, Blood-vessel ; b. bone ; c, cartilage remains ; d. bone cell ; e, cartilage cell space; f, osteoblasts ; g; osteogenetic tissue ; /i, lamella of bone ; i, connective-tissue cells ; /, cartilage cell. nearest the cartilage become osteoblasts and arrange themselves in a single layer along its surface. Under their influence bone is laid down on the surface of the cartilage in the same manner as in intra- membranous ossification. Intracartilaginous and subperiosteal bone can be easily differen- tiated by the presence of cartilaginous remains in the former and their absence in the latter. All bone is at first of the spongy variety. When this is to be converted into compact bone, there is first absorption of bone by osteoclasts, with increase in size of the marrow spaces and reduction of their walls to thin plates. These spaces are now known as Haver- sian spaces. Within these new bone is deposited. This is done by osteoblasts which lay down layer within layer of bone until the Haversian space is reduced to a mere channel, the Haversian canal. THE SKELETAL SYSTEM. 163 In this way are formed the Haversian canals and the Haversian sys- tems of lame lice. Some of the interstitial lamellae are the remains of the original spongy bone not quite removed in the enlargement of the primary marrow spaces to form the Haversian spaces ; other in- terstitial lamellae appear to be early formed Haversian lamellae which have been more or less replaced by Haversian lamellae formed later. While these varieties of ossification have been described, we would emphasize the essential unity of the process. The likeness between intramembranous and subperiosteal ossification has been already noted. The differences observed in intracartilaginous ossi- fication are more apparent than real. In intracartilaginous ossifica- tion the bone is developed in cartilage but not from cartilage. As in intramembranous and in subperiosteal ossification, intracartilaginous bone is developed from osteogenetic tissue. This osteogenetic tissue is a differentiation of embryonal connective tissue, in this case car- ried into the cartilage from the periosteum in the periosteal buds. In intramembranous ossification the bone is developed within and directly from the embryonal connective tissue of which the mem- brane is composed. In intracartilaginous ossification there is the same embryonal connective-tissue membrane, but within this mem- brane the form of the bone is first laid down in embryonal cartilage. Surrounding the cartilage there remains the embryonal connective tissue of the membrane, now perichondrium. It is from tissue which grows into the cartilage from this membrane — embryonal connective tissue — that the bone, although developed in cartilage, is formed. Growth of Bone. The growth of intramembranous bone by the formation of succes- sive layers beneath the periosteum has been already described (page 159)- Intracartilaginous bones grow both in diameter and in length. Grozvth in diameter is accomplished by the constant deposition of new layers of bone beneath the periosteum. During this process, absorption of bone from within by means of osteoclasts leads to the formation of the marrow cavity. The hard bone of the shaft of a long bone is entirely of subperiosteal origin, the intracartilaginous bone being completely absorbed. 1 64 THE ORGANS. Growth in length takes place in the following manner: Some time after the beginning of ossification in the shaft or diaphysis, in- dependent ossification centres appear in the ends of the bone (epiph- yses). So long as bone is growing, the epiphyses and diaphysis remain distinct. Between them lies a zone of growing cartilage, the epiphyseal or intermediate cartilage. Increase in length of the bone takes place by a constant extension of ossification into this cartilage from the ossification centres of the epiphyses and diaphysis. After the bone ceases to grow in length, the epiphyses and diaphysis become firmly united. ECHNIC. (i) Developing Bone — Intramembranous. — Small pieces are removed from near the edge of the parietal bone of a new-born child or animal. These pieces should include the entire thickness of bone with the attached scalp and dura mater. Treat as in technic i, p. 156, except that the sections which are cut perpendicular to the surface of the bone should be stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 16) and mounted in balsam. (2) Developing Bone — Intracartilaginous and Subperiosteal. — Remove the forearms and legs of a human or animal embryo by cutting through the elbow and knee-joints. (Foetal pigs from five to six inches long are very satisfactory.) Treat as in technic (1). Block so that the two long bones will lie in such a plane that both will be cut at the same time. Cut thin longitudinal sections through the ossi- fication centres, stain with hamiatoxylin-picro-acid-fuchsin, and mount in balsam. Cut away the ends of one or two of the embedded bones, leaving only the ossifica- tion centres. Block so as to cut transverse sections through the ossification centre. Stain and mount as the preceding. In the picro-acid-fuchsin stained sections of developing bone the cartilage is stained blue: cells, including red blood cells, yellow; connective tissue from pale pink to red, according to density; bone a deep red. The Cartilages. The costal cartilages are hyaline. They are covered by a closely adherent connective-tissue membrane, the perichondrium. Where cartilage joins bone tbere is a firm union between the two tissues and the perichondrium becomes continuous with the periosteum. The articular cartilages are described below under articulations. The other skeletal cartilages, such as those of the larynx, trachea, bronchi, and of the organs of special sense, are more conveniently considered with the organs in which they occur. THE SKELETAL SYSTEM. 165 Articulations. Joints are immovable (synarthrosis) or movable (diarthrosis). In synarthrosis union maybe cartilaginous (synchondrosis), or by means of fibrous connective tissue (syndesmosis). Synchondrosis. — The cartilage is usually of the fibrous form except near the edge of the bone, where it is hyaline. The interver- tebral discs consist of a ring of fibro-cartilage surrounding a central gelatinous substance, the nucleus pulposus, the latter representing the remains of the notochord. Syndesmosis. — Union is by means of ligaments. These may consist wholly of fibrous tissue, the fibres and cells being arranged much as in tendon, or mainly of coarse elastic fibres separated by loose fibrous tissue. In such syndesmoses as the sutures of the cranial bones, the union is by means of short fibrous ligaments between the adjacent serrated edges. Diarthrosis. — In diarthrosis must be considered (a) the articular cartilages, (/;) the glenoid ligaments and interarticular cartilages, (c) the joint capsule. (a) Articular cartilages cover the ends of the bones. They are of the hyaline variety, 1 being the remains of the original cartilaginous matrix in which the bones are formed. Next to the bone is a narrow strip of cartilage in which the matrix is calcified. This is separated from the remaining uncalcified portion of the cartilage by a narrow so-called "striated" zone. The most superficial of the cartilage cells are arranged in rows parallel to the surface; in the mid-region the grouping of cells is largely in twos and fours as in ordinary hyaline cartilage (page 80) ; while in the deepest zone of the uncal- cified cartilage the cells are arranged in rows perpendicular to the surface. (b) The glenoid ligaments and interarticular cartilages conform more to the structure of dense fibrous tissue than to that of cartilage. (c) The joint capsule consists of two layers, an outer layer of dense fibrous tissue intimately blended with the ligamentous struc- tures of the joint and known as the stratum fibrosum, and an inner ! In the acromioclavicular, sternoclavicular, costo-vertebral, and maxillary articulations the cartilage is of the fibrous form. The same is true of the carti- lage covering the head of the ulna, while the surface of the radius, which enters into the wrist-joint, is covered not by cartilage, but by dense fibrous tissue. 1 66 THE ORGANS. layer, the stratum synoviale or synovial membrane, which forms the lining of the joint cavity. The outer part of the stratum synoviale consists of areolar tissue with its loosely arranged white and elastic fibres interlacing in all directions and scattered connective-tissue cells and fat cells. Nearer the free surface of' the membrane the fibres run parallel to the surface and the cellular elements are more abundant. The cells are scattered among the fibres and are stel- late branching cells like those usually found in fibrous connective tissue. On the free surface, however, the cells are closely packed and although in places often several layers deep, are probably of the nature of endothelium. From the free surfaces of synovial membranes processes {synovial villi — Haversian fringes) project into the joint cavity. Some of these are non-vascular and consist mainly of stellate cells similar to those of the synovial membrane. Others have a distinct core of fibrous tissue containing blood-vessels and covered with stellate con- nective-tissue cells. From the primary villi small secondary non- vascular villi are frequently given off. TECHNIC. (i) Joint Capsule and Articular Cartilage. — Remove one of the small joints — human or animal — cutting the bones through about one-half inch back from the joint. Treat as in technic i. p. 156, making longitudinal sections through the en- tire joint. (2) Synovial Villi. — Remove a piece of the capsular ligament from near the border of the patella and cut out a bit of the velvety tissue which lines its inner surface. Examine fresh in a drop of normal salt solution. Fix a second piece of the ligament in formalin-Miiller's fluid (technic 5, p. 5), make sections perpen- dicular to the surface, stain with ha.matoxylin-eosin (technic 1, p. 16), and mount in balsam. General References for Further Study. Kolliker: Handbuch der Gewebelehre, vol. i. Stohr: Text-book of Histology. Schafer: Histology and Microscopical Anatomy, in Quain's Elements of Anatomy. CHAPTER IV. THE MUSCULAR SYSTEM. 1 The voluntary muscular system consists of a number of organs — the muscles — and of certain accessory structures — the tendons, tendon sheaths, and bursa. A voluntary muscle consists of striated muscle fibres arranged in bundles or fascicles and supported by connective tissue. The entire muscle is enclosed by a rather firm connective-tissue sheath or capsule — the cpimysium (Fig. 98). This sends trabecular FlG. 98. — From a Transverse Section of a Small Human Muscle, showing relations of muscle fibres to connective tissue, a, Epimysium ; 6, perimysium; c, muscle fibres; d, arteries; e, endomysium. of somewhat more loosely arranged connective tissue into the sub- stance of the muscle. These divide the muscle fibres into fascicles. Around each fascicle the connective tissue forms a more or less 1 Definite arrangements of smooth muscle, such as are found in the stomach and intestines, also the muscle of the heart are properly a part of the muscular sys- tem. They are, however, best considered under tissues and in connection with the organs in which they occur. 167 i68 THE ORGANS. ;77, . ; '■ fowl >. V,''/ > l iA V ! iV.n i',vV>'.3 ' ' ' i ' ,',-, V,", ^ till definite envelope, the per {fascicular sheath or perimysium. From the latter delicate strands of connective tissue pass into the fascicles between the individual muscle fibres. This constitutes the intrafas- cicular connective tissue or endomysium, which everywhere completely separates the fibres from one another so that the sarcolemma of one fibre never comes in contact with the sarcolemma of any other fibre. It should be noted that these terms indicate merely location; epi-, peri-, and endomysium all being connective tissue grading from coarse to fine, as it passes from without inward. The structure of the muscle as an organ is thus seen to conform to the structure of other organs, in that it is surrounded by a con- nective-tissue capsule, which sends septa into the organ, divid- ing it into a number of com- partments and serving for the support of the essential tissue of the organ, the muscle fibres or parenchyma. The structure of tendon has been described (see page 67). Tendon slieatJis and burses are similar in structure, consist- ing of mixed white and elastic fibres. Their free surfaces are usually lined by flat cells, which are described by some as connec- tive-tissue cells, by others as endothelium. At the junction of muscle and tendon, the muscle fibre with its sarcolemma ends in a rounded or blunt extremity (Fig. 54, p. 96). Here the fibrils of the tendon fibres are in part cemented to the sar- colemma, and in part are continuous with the fibres of the endo- and perimysium. Along the line of union of muscle and tendon the muscle nuclei are more numerous than elsewhere (Fig. 99, b), and it has been suggested that there is here a zone of indifferent or forma- tive tissue which is capable of developing on the one hand into mus- cle, on the other into the connective tissue of tendon. ■ / rli / ( < 1 ■■> '/i 'luff , • « • 'i Y^^r^ri- -) 1 i it m ■■. ..; .; i t * Fig. 99. — From a Longitudinal Section through Junction of Muscleand Tendon. X150. (Bohm and Davidoff.) a, Tendon ; b, line of union showing increase in number of muscle nuclei; c, muscle. THE MUSCULAR SYSTEM. 169 Growth of muscle takes place mainly at the ends of the fibres where the nuclei are most numerous. In addition to the growth incident to increase in size of the individual or of the particular mus- cle, there is a constant wearing out of muscle fibres and their replacement by new fibres. This is accomplished as follows : The muscle fibre first breaks up into a number of segments (sarcostyles), some of which contain nuclei while others are non-nucleated. The sarcostyles next divide into smaller fragments, and finally completely disintegrate. This is followed by a process of absorption and complete disappearance of the fibre. From the free sarcoplasm new muscle fibres are formed. In the early stages of their development these are known as myoblasts. The latter develop into muscle fibres in the same manner as described under the histogenesis of muscle (page 99). Blood-vessels. — The larger arteries of muscle run in the perimy- sium, their general direction being parallel to the muscle bundles. From these, small branches are given off at right angles. These in turn give rise to an anastomosing capillary network with elongated meshes, which surrounds the individual muscle fibres on all sides. From these capillaries, veins arise which follow the arteries. Even the smallest branches of these veins are supplied with valves. In tendons blood-vessels are few. They run mainly in the con- nective tissue which surrounds the fibre bundles. Tendon sheaths and bursae, on the other hand, are well supplied with blood-vessels. The lymphatics of muscle are not numerous. They accompany the blood-vessels. In tendon definite lymph vessels are found only on the surface. Nerves. — The terminations of nerves in muscle and tendon are described under nerve endings (page 350). TECHNIC. (1) A Muscle.— Select a small muscle, human or animal, and, attaching a weight to the lower end to keep it stretched, fix in formalin-Midler's fluid (technic 5, p. 5), and harden in alcohol. Stain transverse sections with haematoxylin-picro-acid- fuchsin (technic 3, p. 16) and mount in balsam. (2) Junction of Muscle and Tendon. — Any muscle-tendon junction may be selected. Fix in formalin-Muller's fluid, keeping stretched by means of a weight attached to the lower end. Cut longitudinal sections through the muscle-tendon junction, stain with haematoxylin-picro-acid-fuchsin, and mount in balsam. The gastrocnemius of a frog is convenient on account of its small size, and because by bending the knee over and tying there, the muscle can be easily put on the stretch and kept in that condition during fixation. Place the entire preparation in the fixative, removing the muscle-tendon from the bone after fixation. CHAPTER V. GLANDS AND THE GENERAL STRUCTURE OF MUCOUS MEMBRANES. Glands — General Structure and Classification. Attention was called in describing the functional activities of cells (page 37) to the fact that certain cells possess the power of not only carrying on the nutritive functions necessary to the main- tenance of their own existence, but also of elaborating certain prod- ucts either necessary to the general body functions (secretions), or necessary that the body should eliminate as waste (excretions). Such cells are known as gland cells or glandular epithelium, and their association to form a definite structure for the purpose of carry- ing on secretion or excretion is known as a gland. A gland may consist of a single cell, as, e.g. , the mucous or gob- let cell on the free surface of a mucous membrane. Such a cell undergoes certain changes by which a portion of its protoplasm is transformed into or replaced by a substance which is to be used out- side the cell in which it is elaborated. When fully developed the cell surface ruptures and the secretion is poured out upon the free surface. Most glands are, however, composed of more than one cell, usu- ally of a large number of cells, and these cells, instead of lying directly upon the surface, line more or less extensive invaginations into which they pour their secretions. In the simplest form of glandular invagination all of the cells lining the lumen are secreting cells. In more highly developed glands only the deeper cells secrete, the remainder of the gland serving merely to carry the secretion to the surface. This latter part is then known as the excretory duct, in contradistinction to the deeper secreting portion. In both the duct portion and secretory portion of a gland the epithelium usually rests upon a more or less definite basement membrane or membrana propria (page 53). Be- neath the basement membrane, separating and supporting the glandu- 170 GLANDS AND MUCOUS MEMBRANES. i/i lar elements, is the connective tissue of the gland. This varies greatly in structure and quantity in different glands. When the secreting portion of the gland is a tubule the lumen of which is of fairly uniform diameter, the gland is known as a tubu- lar gland. When the lumen of the secreting portion is dilated in the form of a sac or alveolus, the gland is known as a saccular or alveolar gland. A gland may consist of a single tubule or saccule, or of a single system of ducts leading to terminal tubules or saccules — simple gland. A gland may consist of a number of more or less elaborate duct systems with their terminal tubules or saccules — compound gland. All compound glands are surrounded by connective tissue which forms a more or less definite capsule. From the capsule connective- tissue septa or trabecules extend into the gland. The broadest septa usually divide the gland into a number of macroscopic compartments or lobes. Smaller septa from the capsule and from the interlobular septa divide the lobes into smaller compartments usually microscopic in size — the lobules. A lobule is not only a definite portion of the gland separated from the rest of the gland by connective tissue, but represents a definite grouping of tubules or alveoli with reference to one or more terminal ducts. The glandular tissue is known as the parenchyma of the gland, in contradistinction to the connective or interstitial tissue. Glands may thus be classified according to their shape and ar- rangement as follows : 1. Tubular glands. f straight, i (a) Simple tubular -J coiled. [_ branched. (b) Compound tubular. 2. Saccular or alveolar. (a) Simple saccular. (b) Compound saccular or racemose. I. Tubular Glands. — (a) Simple tubular glands are simple tubules which open on the surface, their lining epithelium being con- tinuous with the surface epithelium. All of the cells may be secret- ing cells or only the more deeply situated. In the latter case the upper portion of the tubule serves merely as a duct. In the more v^ THE ORGANS. highly developed of the simple tubular glands we distinguish a month, opening upon the surface, a neck, usually somewhat con- stricted, and ■& fundus, or deep secreting portion of the gland. Simple tubular glands are divided according to the behavior of the fundus, into (i) straight, (2) coiled, and (3) branched. (1) A straight tubular gland is one in which the entire tubule runs a straight unbranched course, e.g., the glands of the large intes- tine (Fig. 100, /). (2) A coiled tubular gland \s> one in which the deeper portion of the tubule is coiled or convoluted, e.g. , the sudoriferous glands of the skin (Fig. 100, 2). (3) Forked or branched tubular glands are simple tubular glands in which the deeper portion of the tubule branches, the several 6 fa FIG. 100. — Diagram Illustrating Different Forms of Glands. Upper row, tubular glands ; /, 2, and j, simple tubular glands ; ./, compound tubular gland. Lower row, alveolar glands ; / a, 2<7, and ja, simple alveolar glands ; ./a, compound alveolar gland. For description of / u, sa, and j a, see simple alveolar glands in text. branches being lined with secreting cells and opening into a super- ficial portion, which serves as a duct. Examples of slightly forked glands are seen in the cardiac end of the stomach and in the uterus. Other tubular glands show much more extensive branching. The main duct gives rise to a number of secondary ducts, from which are given off the terminal tubules. The mucous glands of the mouth, GLANDS AND MUCOUS MEMBRANES. 173 oesophagus, trachea, and bronchi are examples of these more elabo- rate simple tubular glands. (/?) Compound tubular glands consist of a number, often of a large number, of distinct duct systems. These open into a common or main excretory duct. The smaller ducts end in terminal tubules. Many of the largest glands of the body are of this type, e.g. , the salivary glands, liver, kidney, testis. In certain compound tubular glands, as, e.g., the liver, extensive anastomoses occur between the terminal tubules. These are some- times called reticular glands. 2. Alveolar Glands. — (a) Simple Alveolar Glands. — The simplest form of alveolar gland consists of a single sac connected with the surface by a constricted portion, the neck, the whole being shaped like a flask (Fig. 100, Id). Such glands are found in the skin of certain amphibians ; they do not occur in man. Simple alveolar glands, in which there are several saccules (Fig. 100, 2a), are represented by the smaller sebaceous glands. Simple branched alveolar glands, in which a common duct gives rise to a number of saccules (Fig. ioo, Ja), are seen in the larger sebaceous glands and in the Meibomian glands. (/;) Compound Alveolar Glands. — These resemble the com- pound tubular glands in general structure, consisting of a large num- ber of duct systems, all emptying into a common excretory duct. The main duct of each system repeatedly branches, and the small terminal ducts, instead of ending in blind tubules, as in the compound tubular gland, end in sac-like dilatations, the alveoli or acini (Fig. ioo, , stroma ; c, epithelium ; d, secondary papilla. From the summit of each papilla are given off several secondary papilhe. The epithelium covering these papilla? is hornified and often extends from the surface as a long thread-like projection — hence the name, filiform. (2) Fungiform Papilltk (Fig. 102).- — Scattered irregularly over the entire dorsum among the filiform papillae, but fewer in number, are larger papillae of somewhat different structure known as fungiform papilla:-. Their apices are rounded instead of pointed and their bases are narrowed. Secondary papillae are given off not only from the summit, but from the sides of the papilla. The epithelial covering is comparatively thin and is not hornified. The connective-tissue core of these papillae contains but few elastic fibres. (3) The Cikcumvallate Papill/k (Fig. 103). — These are from nine to fifteen in number, and are grouped on the posterior surface of the dorsum of the tongue. They resemble the fungiform papillx, THE DIGESTIVE SYSTEM. 18 1 but are much larger. Each lies rather deep in the mucous membrane, surrounded by a groove or trench and wall (whence the name circum- vallate). The wall is somewhat lower than the papilla, thus allowing the latter to project slightly above the surface. Secondary papillae are confined to the upper surface of the papilla, the sides being free from secondary papillae. The surface of the papilla and the borders of the groove and wall are covered by stratified squamous epithelium. Lying in the epithelium of the side wall and sometimes of the oppo- site trench wall are oval bodies, the so-called taste buds, which serve as organs for the nerves of taste (see nervous system). Into the trench surrounding the circumvallate papilla open the ducts of serous glands (Ebner's glands). The lymph follicles of the tongue have been already described (page 141) under the head of the lingual tonsils. For glands of the tongue see page 178. The larger blood-vessels run in the connective-tissue septa. These give off smaller branches, which break up into capillary net- &■- •'. ■•-:.:;.:.•. ,. . •_ ..,.,.■ *;_■:■.■; _: .. ;.._._: Fig. 102.— Vertical Section through Fungiform Papilla of Human Tongue. X 45. (Szymono- wicz.) a, Secondary papilla ; b, epithelium ; c, muscle fibres. works surrounding the muscle fibres and forming a plexus just be- neath the epithelium. From the latter are given off capillaries to I 82 THE ORGANS. the papillae. The capillaries converge to form veins, which in gen- eral follow the course of the arteries. Fine lymph spaces occur in the papillae and open into a plexus of small lymph capillaries just beneath the papillae. These communi- cate with a deeper plexus of larger lymphatics, which increase in size Jui W- FlG. 103. — Vertical Section through a Cireumvallate Papilla of Human Tongue. X 37. (Szy- monowicz.) a. Secondary papilla ; b, wall ; c, trench ; d, epithelium of tongue ; <\ stroma ; /, submucosa ; g, Ebner's glands. and number as they pass backward and form an especially dense lym- phatic network at the root of the tongue in the region of the lingual tonsils. Nerves. — Sympathetic fibres pass mainly to the smooth muscle of the blood-vessels and to the glands. Medullated motor nerve fibres supply the lingual muscles. (For motor nerve endings see page 353.) Medullated sensory nerves include those of the special sense of taste as well as those of ordinary sensation. They end freely among the epithelial cells or in connection with special end-organs — the taste buds mainly in the cireumvallate papillae, and the end- bulbs of Krause in the fungiform papillae (see page 349). TECHNIC. Remove pieces of the dorsum of the tongue, selecting parts thai will include the different forms of papillae and cutting well into the underlying muscular tissue. THE DIGESTIVE SYSTEM. 183 Treat as in technic 2, p. 179, or sections may be stained with haematoxylin-picro- acid-fuchsin (technic 3, p. 16). In sections from the back part of the tongue good examples of mucous and serous glands are usually found. In small sections of the tongue the muscle fibres are seen arranged in bundles, surrounded by connective tissue and interlacing in all directions. For the study of the arrangement of the different planes of muscle, complete transverse sections should be made at intervals through the entire tongue. The muscle and connec- tive-tissue relations are best brought out by the haematoxylin-picro-acid-fuchsin stain. The Teeth. A tooth is a hard bone-like structure, part of which projects above the surface of the jaw as the crown, while the deeper portion, the root, is buried in a socket of the alveolar margin (Fig. 104). A tooth consists of a soft central core, the pulp cavity, surrounded by dentine (Figs. 104 and 105). The latter constitutes the main bulk of the tooth. The exposed portion of the dentine is covered by a thin layer of extremely hard substance, the enamel (Fig. 104, /), while the alveolar por- tion of the dentine is covered with cementum (Fig. 104, J). Of these the dentine and cementum are of connec- tive-tissue origin, the enamel of epi- thelial. The pulp cavity occupies the cen- tral axis of the tooth (Figs. 104 and 105). In the root it is known as the root canal. At the apex of the root it communicates with the underlying tissue by means of a minute open- ing, through which blood-vessels and nerves enter the pulp cavity. The dentinal pulp consists of loose connective tissue of an embryonal type, composed of fusiform and stel- late cells and delicate fibrils not joined to form bundles. This tissue supports the blood-vessels and nerves which are found onlv FIG. 104. — Vertical Section of Tooth in Situ. X 15. (Waldeyer.) ! **■* I •■*-** £- : **» V te 1 * * * k 1 *■ * . & \' >» 11./ ' />* FIG. 105.— Cross-section through Root of Human Canine Tooth (X 25) (Sabotta), showing re- lations of pulp cavity, dentine, and cement um. / J , Pulp cavity ; D, dentine ; C, cement uni ; A', 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, /)) resembles bone. It is peculiar in that it contains canaliculi, dental canals (Figs. 106 and 107, Dk), but no lacunas 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. 185 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 ; D, dentine ; A", Tomes' granular layer ; Dk, dental canals ; KH, lacunas of cementum. These are known as interglobular spaces (Fig. 107, Jg). They are filled with a substance resembling uncalcified dentine. In the outer part of the dentine of the root are similar spaces iS6 THE ORGANS. which are smaller and more closely placed. These form the so-called Tomes' granular layer (Fig. 106, A'). 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, 5) — 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 >Jg FIG. 107. — From Longitudinal Section of Crown of Human Premolar (X 200) (Sabotta), show- ing junction of enamel and dentine. S, Enamel; D, dentine; Sfl, enamel prisms; Dk, dental canals;/^-, 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. arc homogeneous ; in the embryo they show a longitudinal fibrillation. Rather indistinct parallel lines (the lines oi 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 cuticula dentis. The CEMENTUM (Fig. 1 06, 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 ha it nt.e 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. 187 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 clown 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 cf 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 Eferm of a vascular membrane, the dental sac. The attenuated strand 1 88 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 fretal existence the germs of all milk and of some permanent teeth. The enamel organ at this stage consists of three layers: (i) The outer enamel cells, somewhat flattened ; (2) the inner enamel cells, h- — b \ ■*■—■" ** Fig. 108.— Developing Tooth from Three-and-one-half-months' Human Embryo. X 65. (Szy- monowicz.) , 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 ; //, 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. 121, //, and 122, /). Other cells with dark- staining nuclei, " replacing cells,'' are found between the bases of the columnar cells (pages 56 and [97). In addition to the connective-tissue and lymphoid cells, which THE DIGESTIVE SYSTEM. 203 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, f; 122, cli) 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, e ; 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- cosae. These crypts are lined with an epithelium similar to and con- tinuous with that covering the villi. The cells are, however, lower, Fig. 120.— Vertical Section through Mucous Membrane of Human Jejunum. X 80. (Stohr.) j and b, Artifacts due to shrinkage; c, intestinal crypts (Lieberkuhn); d, oblique and transverse sections of crypts ; e, stroma ;f, epithelium ; g; tangentially cut villi ; //, mus- cularis mucosas ; i, 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 204 THE ORG Ays. 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. 121. -i gm ■y?-' : -'iiL-" <''-'-s-^ KlK m ©IP %3$c ■ — -- '-.^ys^m^: ■ -t# 3 WP z'A*) J^r"* -—-' -' .'^K i$ ^^ Fig. 122. FIG. i2i.— Longitudinal Section of Vilhis from Small Intestine of Dog. (Piersol.) ./, Colum- nar epithelium ; b< goblet cells ; A, leucocytes ; c, basement membrane : d, core of villus ; e, blood vessels ; f, lacteal. FIG. 122 — Cross-section of a Villus of Human Small Intestine. X 530. CKolliker.) The stroma of the villus has shrunken away from the epithelium. />, Goblet cell ; c, cuticula showing striations; e, columnar epithelial cell ; jr»i, basement membrane with nuclei ; /, leucocyte in epithelium ; /', leucocyte just beneath epithelium ; ;;/, 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 198). 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. 205 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 mucosae. 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 MUCOS/E (Figs. 120 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 mu- Fig. 123. - Longitudinal Section of n ... r Fundus of Cr}-pt of Lieberkuhn. X cosae and empty either into a crypt of Lieberkuhn or on the surface between the villi. Brunner's glands frequently occur in the pylorus, and it is not un- common for the pyloric glands to ex- tend downward somewhat into the duodenum. Meissners plexus of nerve fibres, mingled with groups of sympathetic ganglion cells, lies in the submucosa (see page 214). 3. The muscular coat (Figs. 119 and 125) consists of two well- 530. (Kolliker.) b, Goblet cell show- ing mitosis ; e, epithelial cell ; A, cell of Paneth ; /, leucocyte in epithe- lium ; m, mitosis in epithelial cell. Surrounding the crypt is seen the stroma of the mucous membrane. :o6 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.) J^v^/ "R.\-. FIG. 125.— From Vertical Longitudinal Section of Cat's Duodenum to show Brunner's Glands. (Larrabee.) cz, Villus; />, epithe- lium ; c, stroma ; if, crypts ; e, muscularis mucosas ; /, Brunner's glands ; g; submu- cosa ; //, circular muscle layer. 208 THE ORGANS. 4. The serous coat consists, as in the stomach and small intestine, of 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 Fig. 126. Fig. 127. PlG. 126.— From Vertical Longitudinal Section of Cat's Large Intestine. (LarrabeeO a Epithelium ; /<, stroma ; c, fundus of gland ; M£fi .0 Nerves of the Stomach and §mm§M Intestine. '|| The nerves which sup- ply the stomach and intes- tines are mainly non-med- n . j ,i .• j-i FIG. 131.— Section through Glands of Fundus of Hu- Ullated Sympathetic fibres man Stomach in Condition of Hunger. X 500. which reach the intestinal (Bohm and von Davidoff.") a, Stroma; />, parietal cell ; c, lumen ; d, chief cell. walls through the mesen- tery. In the connective tissue between the two layers of muscle these fibres are associated with groups of sympathetic ganglion cells to form the plexus myentericus or plexus of Auerbach. The dendrites of the ganglion cells interlace, forming a large part of the plexus. The axones are grouped together in small bundles of 214 THE ORGANS. non-medullated fibres, which pass into the muscular coats, where they form intricate plexuses, from which are given off terminals to the smooth muscle cells. From Auerbach's plexus fibres pass to the submucosa, where they form a similar but finer-meshed, more delicate plexus, also associated with groups of sympathetic gan- glion cells, the plexus of Meissner. Both fibres and cells are smaller than those of Auerbach's plexus. From Meissner's plexus delicate fibrils pass to their terminations in submucosa, muscularis mucosae, and mucous membrane. Secretion and the Absorption of Fat. The secretory activities of epithelial cells have already been mentioned (page 37). The epithelium of the gastro-intestinal tract must be considered as having two main functions: (1) The secretion of substances necessary to digestion ; and (2) the absorption of the products of digestion. (1) Secretion. — The production of mucus takes place in the mucous or goblet cell, which, as already mentioned, represents a Fig. 132.— Section through Glands of Fundus of Human Stomach during Digestion. X 500. 11 and von Davidoff.) it, Lumen ; /), stroma ; c, chief cell ; rf, parietal cell. differentiation of the ordinary columnar epithelial cell. The chief cells, " peptic cells," of the stomach glands arc large and clear dur- ing fasting, become granular and cloudy with the onset of digestion, THE DIGESTIVE SYSTEM. 215 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 iipil -^ ■ ■ -■■■■■ ■■■■ j'~zrr-" — ■*^P^?-«S~ ** — •!'■ © ®" 3 V© "•■"1 '<£>; ©lit ^ © -<5*\ A—- k © ft ° A © © I ■lv. a 85 © •> -'^ . • ■ W /yf • ti> FIG. 133. — Fat Absorption. Longitudinal section of villus of cat's small intestine, three hours after feeding-. X 350. Osmic acid, a, Fat droplets in epithelial cells ; />, fat droplets in leucocytes in stroma ; c, fat droplets in leucocytes within lacteal ; d, 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 Brun- ner's glands undergo changes during digestion, which are quite 216 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 204). (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, (/>) 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 haematoxylin-eosin (technic 1, p. 16), or will) haematoxylin-picro-acid-fuchsin (technic 3, p. 16), and mounted in balsam. (2) A vermiform appendix, as fresh as possible, should be cut transversely into small pieces, fixed- in formalin-Midler's fluid (technic 5, !»• 5), and hardened in THE DIGESTIVE SYSTEM. 217 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 20.) 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 j (/;) The sublingual. J (r) The submaxillary. 2. The pancreas. 3. The liver. 1. The Salivary Glands. The salivary glands are all compound tubular glands. In man the parotid is serous; the sublingual and submaxillary, mixed serous and mucous (page 177). Only the general structure of these glands is here described, the minute 'structure of mucous and serous glands having been described on page 177. 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 218 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- 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; d. terminal tubule. lobular connective tissue, are known as interlobular duets. 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 parenchyma of the gland in contradistinction to the connective or interstitial tissue. The parotid gland in man, dog, cat, and rabbit is a purely serous gland. Its duet 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. 219 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 177). 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 (Bartholini) sends off branches which are continuous with tubules, showing a few secretory mucous cells. These open directly into the terminal tubules (Fig. 134, B). 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 177). The crescents of Gianuzzi (page 178) 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 secretory 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 177). The crescents of the mucous tubules (page 178) 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- 220 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 s f e FIG. 135. —Section of Human Submaxillar}' Gland. X 252. (Stohr.) a, Mucous tubule ; /', se- rous tubule ; c, intermediate tubule ; d, "secretory" tubule ; e, demilune ; j\ 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 arc 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 DIGES TI VE S YS TEM. 2 2 I 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. (i) The salivary glands should be fixed in Flemming's fluid (technic 7, p. 6), or in formalin-Midler's fluid (technic 5, p. 5). Sections are cut as thin as possible, stained with haematoxylin-eosin (technic 1, p. 16), 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. 6). 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 due to the incompleteness of the connective-tissue septa, the human pancreas in this respect resembling the human liver. Rarely the human pancreas is distinctly lobulated. THE ORGANS. The gland has a main excretory duct, the pancreatic duct or duct of IVirsuug. 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. Fach 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. 1 39, ^4 and B). 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. 223 zones are of about equal size. During the early stages of activity (intestinal digestion) the granules largely disappear and the clear FIG. 137. — Section of Human Pancreas. X 112. (KSlliker.) ai\ 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 the lumen. It would thus seem probable that the zymo- gen granules are the intracell- ular representatives of the secretion of the gland. In sections of the gland there are seen within the lu- mina of many of the secreting tubules one or more small cells of which little but the nucleus can usually be made out. These cells lie in con- tact with the secreting cells, and resemble the flat cells which line the intermediate tubule. They are known as the centro-acinar {centro-tubular) cells of Langer- FlG. 138.— From Section of Human Pancreas. X 700. (Kolliker.) a, Gland cell ; 6, basement mem brane ; s, intermediate tubule; c, centroacinar cells ; sk, intracellular secretory tubule. 224 THE ORGANS. Iians i Fig. 138, c). Their significance is not definitely known. Langerhans considered them to be derived from the intermediate tub- ule, the epithelium of which, instead of directly joining that of the Flo. 139.— Sections of Alveoli from Rabbit's Pancreas. (Foster, after Kiihne and Lea.) A, Resting alveolus, the inner zone (a), containing zymogen granules, occupying a little more of the cell than the outer clear zone (b) ; c, indistinct lumen. B, Active alveolus, granules coarser, fewer, and confined to inner ends of the cell (a), the outer clear zone (/)) being much larger ; outlines of cells and of lumen much more distinct. terminal tubule as in the submaxillary gland, was continued over into the lumen of the terminal tubule (Fig. 138). This interpreta- tion has been quite generally accepted. Cells which differ from the secreting cells are frequently found A B FlO. 140 Sections through Alveoli of Human Pancreas Golgi Method (Dogiel), to show in- tracellular secretory tubules, a, Intermediate tubule giving off several terminal tubules, from which pass off minute intracellular secretory tubules ; b, gland cellsliuing terminal I ubules. wedged in between the latter. They extend from the lumen to the basement membrane and are probably sustentacular. Passing from the lumen of the terminal tubule, sometimes between the centro-tubular cells, directly into the cytoplasm of the THE DIGESTIVE SYSTEM. 225 secreting cells are minute intracellular secretory tubules. These are demonstrable only by special methods (Golgi) (Fig. 140). The pancreas also contains peculiar groups of cells, the cell-isl- ands of LangerhaiiSyhaMmg a diameter from 200 to 300 //. (Figs. 137, 141, and 142). The "island" cells differ quite markedly from those which line the terminal tubules (Fig. 141). They contain no zymo- gen granules. Their protoplasm is unstained by basic dyes, but stains homogeneously with acid dyes. Their nuclei vary greatly in Fig. 141. — Island of Langerhans and few surrounding Pancreatic Tubules. (Bohm and von Davidoff.) a, Capillary ; b, lumen of 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 15 226 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 1 1 ® ■ Fig. 142.— From Section of Pancreas, the blood-vessels of which had been injected (Kiihneand 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 embryologicai 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 are axoncs 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. '. 2 7 to plexuses among the secreting tubules, to which and to the walls of the vessels they send delicate terminal fibrils. TECHNIC. (i) The general technic for the pancreas is the same as for the salivary glands (page 221). (2) Zymogen granules may be demonstrated by fixation in formalin-Muller's fluid (technic 5, p. 5), and staining with picro-acid-fuchsin (technic 2, p. 16), or with Heidenhain's iron haematoxylin (technic 3. p. 14). (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 20). 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. FlG. 143. — Section of Lobule of Pig's Liver X 60 (technic 1, p. 234), showing lobule completely surrounded by connective tissue, a. Portal vein ; /', bile duct ; t', hepatic artery ; J, por- tal canal ; e, capillaries ; _/", central vein ; ^, cords of liver cells ; //, hepatic vein. The liver is surrounded by a connective-tissue capsule, the cap- sule of Glisson. At the hilum this capsule extends deep into the 22 8 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 3 P n - ''•;■:>• a M'^Wa K«] v -: v. : Wu H vm& ■mm PIG i 14. -Section of Human Liver. X 8o. (Hendrickson.) P, Portal vein; H, hepatic ar- . /'. bile duct. I\ //, 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. 229 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 ; V, portal vein ; />, bile duct. 230 THE ORGANS. Within the liver the duct divides and subdivides, giving off inter- lobar, and these in turn interlobular branches. 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 ?t*2 m '§-> m - IXtO, • IS s 9* ( ^'*r—.^ ^ ™ -■■■ "', 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 -o- 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 ^*lfP 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. 233 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 smooth 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 Heist erian 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 ruga, which anastomose and give the mucous surface a reticular appear- 234 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. 5). Cut sections near and parallel to the surface. Stain with hae- matoxylin-picro-acid-fuchsin (technic 3, p. 16) 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 1 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. 16 . (3) The secretory tubules and smaller bile ducts may be demonstrated by technic 4. p. 23. 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 ha-matoxylin-eosin (technic 1. p. 16). and mounted in balsam. ) 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 446). 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 How- 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. 239 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 349) 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 mucosas 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 Wrisburg 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- 240 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 trachea! cartilages. '■"-'■-''■ -.^ ' . 1 ' FIG. i;o. — From Longitudinal Section of Human Trachea, x 40. (Technic 3, p. 242.) a, Epi- thelium; /', stroma; c, cartilage; , stroma; c, muscularis mucosa?; d, lung tissue; e, fibrous coat ; /, 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 115. (Technic 2, p. 251.) n, Stroma ; />, epithelium ; c, muscularis mucosas ; 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 141). 244 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 ram. is reached, when they disappear. I 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 pulmonary 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. Flsewhere in the lung the lobules have an irregular arrangement. THE RESPIRATORY SYSTEM. 245 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 {F'\g. 154,^; Figs. 155 and 156, BR). From each FIG. 154. — From Lung of an Ape. The bronchi and their dependent ducts and alveoli have been filled with quicksilver. X 15. (Kolliker, after Schulze.j 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 infnndibula. 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.R., Respiratory bronchus ; D.A., alveolar duct ; A., atrium ; A.S., air sacs. directed away from the alveolar passage. From the sides of the alveolar passage and from the infundibula are given off the alveoli — air vesicles or air cells (Fig. 154, i; Figs. 155 and 156, AS). According to Miller a further subdivision of the alveolar passage 246 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, Fig. 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 ; ^..S"., air sacs. the vestibula. Each vestibulum is about 0.2 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- FiG. 157.— From Section of Cat's Lung Stained with Silver Nitrate. (Klein.) (Technic i, p. m.i Small bronchus surrounded by alveoli, in which are seen both flat cells (respira- tory epithelium), and cuboidal cells (foetal cells). sac passages from which open the air sacs. From the latter are given off on all sides, the air cells ox alveoli. Alveoli are not, however, THE RESPIRATORY SYSTEM. 247 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. 23) to show arrangement of elastic tissue, a, Alveolus cut through side walls only ; b, 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 "festal" cells (see Development, page 251). The respiratory cells (Fig. 157) are some of them large, flat, non- 248 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. 61). ^- Fig. T -g. — Parts of Four Alveoli from Section of Injected Human Lung. X 200. (Technics, p. 251J a, Wall of alveolus seen on flat; c. same, but only small part of alveolarwall in plane of section ; b, alveoli in which plane of section includes only side walls ; alveolar wall seen on edge. The "foetal" cells are granular, nucleated cells which are scat- tered among the respiratory cells. In the embryonic lung the air passages and alveoli contain only this type of cells. In the alveolar passage the basement membrane almost entirely disappears, the epithelium being supported by delicate elastic fibrils intermingled with a few white fibrils and connective-tissue cells. The alveolus is similar in structure to the alveolar passage, its walls consisting mainly of delicate elastic fibrils supporting respi- ratory and foetal cells. Around the opening of the alveolus the elas- tic fibres are more numerous, forming a more or less definite ring. The interalvcolar connective tissue, while extremely small in amount, serves to separate the alveoli from one another. Somewhat THE RESPIRATORY SYSTEM. 249 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 broudiial 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 bronchia/ 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- Blood. ""-- c Fig. 160. — Diagram of Tissues Interposed Between Blood and Air in Alveolus, a. Respira- tory epithelium ; &, basement membrane ; 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 : (1) 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 (see diagram, Fig. 160). The "foetal " cells appear to lie rather between the capillaries than upon the capillaries, as do the respiratory cells. 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. 250 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 respiratory 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 medullated 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 down 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 RES P IRA TOR Y SYS TEM. 2 5 1 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 fcetal 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. 242). 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, 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-Miil- ler's fluid (technic 5, p. 5), 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 hsematoxylin-eosin (technic 1, p. 16), 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. 61). (4) Elastic Tissue of the Lung (technic 3, p. 23). (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 20) 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 layer of cuboidal epithelial cells. Two types of cells are recognized, 252 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. 253 TECHNIC. The thyroid and parathyroid glands are best fixed in formalin-Midler's fluid. Sections may be stained with haematoxylin-eosin or hsmatoxylin-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. Kolliker: Handbuch der Gewebelehre des Menschen. CHAPTER VIII. '% e THE URINARY SYSTEM. The Kidney. The kidney is a compound tubular gland. It is enclosed by a 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 capsule into the gland, dividing it into a number of lobes or renculi. In some animals, e.g., the guinea-pig and rabbit, the entire kidney consists of a single renculus (Fig. 161). In the adult human kidney the division into renculi is not complete, the per- ipheral parts of the different ren- culi blending. Rarely the foetal division into renculi persists in adult life, such a kidney being known as a " lobulated kidney." On the mesially directed side of the kidney is a depression known as the hilum (Fig. 161). This serves as the point of en- trance of the renal artery and of exit for the renal vein and ureter. On section a division of the organ into two zones is apparent to the naked eye (Figs. 161 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 254 % FIG. 161. Longitudinal Section Through Kid- ney of Guinea-pig, including hilum and beginning of ureter. X 5. (Technic 1, p. 269). a, Pelvis; b, papilla; c, wall of pelvis; d, ureter; e, ducts of Bellini;/, cortical pyamids ; g; medullary rays; //, cortex ; /, medulla ; /, renal corpuscles. THE URINARY SYSTEM. 255 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 i Pelvis Fig. 162. Fig. 163. FIG. 162.— Longitudinal Section of Kidney Through Hilum. a, Cortical pyramid; b, medul- lary ray ; c, medulla ; d, cortex ; e, renal calyx ; f, hilum ; g, ureter ; h, renal artery ; i, obliquely cut tubules of medulla ; /and k, renal arches ; /, column of Bertini ; in, con- nective tissue and fat surrounding renal vessels ; //, medulla cut obliquely ; 0, papilla ; f, 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; 6', spiral tubule; D, descending arm of Henle's loop; L, Henle's loop ; A, ascending arm of Henle'sloop; /, 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. 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 lobules in the foetal kidney. The pyramidal seg- 256 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 raiis. 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. 1 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 body becomes constricted to form the neck (Figs. 163, N, and 164). It next broadens out into a greatly convoluted portion, the first ox 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 Henle's loop (Fig. 163, D) — turns sharply upon itself — /fculc's loop (Fig. 163, L) — and passes again toward the surface — ascending arm op Henle's loop (Fig. 163) A, — THE URINARY SYSTEM. 257 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 .«?.«. V ;,.* V m * «." ■ E " ^ ' V" FIG. 165. — Malpighian Body from Human Kidney. X 280. (Technic 2, p. 269). a, Bowman's capsule ; l>, 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 "-•"■•■ ■;.,:> . p. v.r; ^r\-j : ^ ^ J A Fig. 166.— Proximal Convoluted Tubules of Human Kidney. X 350. (Technic 2, p. 269.) 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 17 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 membra ua propria upon which rests a single layer of epithelial cells. The shape and structure of the epithelium differ in different portions of the tubule. r. The Malpighian 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 fl • ft m m FIG. 167.— Tubules of Human Kidney. X 560. From longitudinal section. (Technic 2, p. 269.) 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 capsule closely invests the tuft of capillaries. This 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. 259 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 or proximal convoluted tubule (Fig. 166) measures from 40 to 70 fj. 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 15// 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, longitudinal sections are apt to present a wavy appearance. 5. Henle ' 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 Henle' s loop (Fig. 167, 2) is broader than the descending, measuring from 20 to 30,7. 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. 260 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 MR Fig. 168.— Cross Section Through Cortex of Human Kidney. X 60. (Technic2, p. 269.) a. Con- voluted tubules of cortical pyramid ; d, interlobular artery ; c, medullary rays ; d, Mal- pighian bodies. tubules, and increases in diameter until in the ducts of Bellini (Fig. 167, r\ ,G '''II ^ v ft JS^^S /> FIG. 169.— Cross Section Through Medulla of Human Kidney, X 465. (Technic 2, p. 169.) , collecting tubule ; c, asce'nding arms of Henle's loops ; if, descending arms of Henle's loops. 170). These give off small twigs to the calyces and capsule, and then without further branching pass between the papillae through the medulla to the junction of medulla and cortex. Here they bend sharply at right angles and following the boundary line between 262 THE ORGANS. cortex and medulla, form a series of arches, the arterice arciformes or arcuate arteries. From the arcuate arteries two sets of vessels arise, one supplying the cortex, the other the medulla (Figs. 163 and 170). The arteries to the cortex spring from the outer convex sides of the arterial arches, and as the interlobular arteries pursue a quite A B c D PIG. 170 — Diagram to Illustrate (left) the Course of the Uriniferous Tubule ; (right) the Course of the Renal Vessels. (Szymonowicz.) A, B, C\ I) form the kidney lobules ; a, afferent vessel ; ,, efferent vessel of glomerulus ; i, Bowman's capsule ; 2, first convoluted tubule ; 3, descending arm of Henle'sloop; 4, ascending arm of Henle's loop; 5, second convoluted tubule; 6 and 7, collecting tubules; 3, duct of Bellini; /', interlobular artery; c, inter- lobular vein ; d, renal arch (arcuate artery above and arcuate vein below) ; /", interlobar vein ; .<', interlobar artery ; //, medulla ; /, medullary ray ; /, cortex. straight course through the cortical pyramids toward the surface, about midway between adjacent medullary rays. From each inter- lobular artery arc given off numerous short lateral branches, each one THE URINARY SYSTEM. 263 of which passes to a Malpighian body. Entering a Malpighian body as its afferent vessel, the artery breaks up into a number of small arterioles, which in turn give rise to the groups of capillaries which form the glomerulus. Each group of glomerular capillaries arising from a single arteriole is separated from its neighbors by a rather larger amount of connective tissue than that which separates the individual capillaries. This gives to the glomerulus its lobular ap- pearance. From the smaller glomerular capillaries the blood passes into somewhat larger capillaries, which unite to form the efferent vessel of the glomerulus. As afferent and efferent vessels lie side by side, the glomerulus has the appearance of being suspended from this point. The entire vascular system of the glomerulus is arterial. After leaving the glomerulus, the efferent vessel breaks up into a second system of capillaries, which form a dense network among the tubules of the cortical pyramids and of the medullary rays. The mesh corresponds to the shape of the tubules, being irregular in the pyramids, long and narrow in the rays. In these capillaries the blood gradually becomes venous and passes into the interlobular veins. These accompany the interlobular arteries to the boundary between cortex and medulla, where they enter the arcuate veins, which accom- pany the arcuate arteries (Fig. 170). In addition to the distribution just described, some of the inter- lobular arteries extend to the surface of the kidney, where they enter the capsule and form a network of capillaries which anastomose with capillaries of the suprarenal, recurrent, and phrenic arteries. A further collateral circulation is established by branches of the above- named arteries penetrating the kidney and forming capillary networks within the cortex, even supplying some of the more superficial glomeruli. The most superficial of the small veins which enter the interlobular are arranged in radial groups, having the interlobular veins as their centres. These lie just beneath the capsule, and are known as the stellate veins of Verheyn. In addition to capillary anastomoses, direct communication between arteries and veins of both cortex and medulla, by means of trunks of considerable size, has been described. The lymph vessels of the kidney are arranged in two systems, a superficial system which ramifies in the capsule, and a deep system which accompanies the arteries to the parenchyma of the organ. 264 THE ORGANS. Little is known of the relation of the lymphatics to the kidney tubules. Nerves. — These are derived from both cerebro-spinal and sym- pathetic systems. The medullated fibres appear to pass mainly to the walls of the blood-vessels which supply the kidney capsule. Plexuses of fine non-medullated fibres (sympathetic) accompany the arteries to the glomeruli. Delicate terminals have been described as passing from these plexuses, piercing the basement membrane and ending freely between the epithelial cells of the tubules. The Kidney-Pelvis and Ureter. The kidney-pelvis, with its subdivisions the calyces, and the ureter constitute the main excretory duct of the kidney. Their walls con- sist of three coats : an inner mucous, a middle muscular, and an outer fibrous. The mucosa is lined by epithelium of the transitional type. There are from four to eight layers of cells, the cell outlines are usually well defined, and the surface cells instead of being dis- tinctly squamous are only slightly flattened. Less commonly large flat plate-like cells, each containing several nuclei, are present. The cells rest upon a basement membrane, beneath which is a stroma of delicate fibro-elastic tissue rich in cells. Diffuse lymphatic tis- sue frequently occurs in the stroma, especially of the pelvis. Occa- sionally the lymphatic tissue takes the form of small nodules. Mucous glands in small numbers are found in the stroma of the pelvis and upper part of the ureter. There is no distinct submucosa, although the outer part of the stroma is sometimes referred to as such. The muscularis consists of an inner longitudinal and an outer circular layer. In the lower part of the ureter a discontinuous outer longitudinal layer is added. The fibrosa consists of loosely arranged connective tissue and contains many large blood- vessels. It is not sharply limited exter- nally, but blends with the connective tissue of surrounding structures, and serves to attach the ureter to the latter. The larger blood-vessels run in the fibrous coat. From these, branches pierce the muscular layer, give rise to a capillary network among the muscle cells, and then pass to the mucosa, in the stroma THE URINARY SYSTEM. 265 of which they break up into a rich network of capillaries. The veins follow the arteries. The lymphatics follow the blood-vessels, being especially numer- ous in the stroma of the mucosa. Nerves. — Plexuses of both medullated and non-medullated fibres occur in the walls of the ureter and pelvis. The non-medullated fibres pass mainly to the cells of the muscularis. Medullated fibres enter the mucosa where they lose their medullary sheaths. Termi- nals of these fibres have been traced to the lining epithelium. The Urinary Bladder. The walls of the bladder are similar in structure to those of the ureter. The mucous membrane is thrown up into folds or is comparatively smooth, according to the degree of distention of the organ. The epithelium is of the same general type — transitional epithelium (see page 57) — as that of the ureter. The number of layers of cells and the shapes of the cells depend largely upon whether the bladder is full or empty. In the collapsed organ the superficial cells are cu- boidal or even columnar, their under surfaces being marked by pit- like depressions caused by pressure of underlying cells. Beneath the superficial cells are several layers of polygonal cells, while upon the basement membrane is the usual single layer of small cuboidal cells. In the moderately distended bladder the superficial cells become flatter and the entire epithelium thinner (Fig. 171). In the distended organ there is still further flattening of the superficial cells and thinning of the entire epithelium. The stroma consists of fine loosely arranged connective tissue containing many lymphoid cells and sometimes small lymph nodules. It merges without distinct demarcation into the less cellular submucosa (Fig. 171, c). The three muscular layers of the lower part of the ureter are con- tinued on to the bladder, where the muscle bundles of the different layers interlace and anastomose, but can be still indistinctly differ- entiated into an inner longitudinal, a middle circular, and an outer longitudinal layer (Fig. 171, d, c,f). The fibrous layer is similar to that of the ureter, and attaches the organ to the surrounding structures. The blood- and lymph-vessels have a distribution similar to those of the ureter. 266 THE ORGANS. N erves . — Sensory medullated fibres pierce the muscularis, branch repeatedly in the stroma, lose their medullary sheaths, and terminate among the cells of the lining epithelium. Sympathetic fibres form •- ■"' ■ „i:^ ■'-'-■■ ; '' i&&d£* .•'i*...-j~fxf. ~"'~"~^r~. — — | ,. .--V Fir,. 171.— Vertical Section through Wall of moderately distended Human Bladder. X 60. (Technic 5, p. 269.) 1/, Epithelium, b, stroma, of mucous membrane; c, submucosa; d, inner muscle layer ; e, middle muscle layer ; /, outer muscle layer. plexuses in the fibrous coat, where they are interspersed with numer- ous small groups of ganglion cells. Axones of these sympathetic neurones penetrate the muscularis. Here they form plexuses, from which are given off terminals to the individual muscle cells. For development of urinary system see page 308. The Adrenal. The adrenal is a ductless gland situated on the upper and ante- rior surface of the kidney. It is surrounded by a capsule and con- sists of an outer /.one or cortex and a central portion or medulla. The CAPSULE (Fig. 172, A) is composed of fibrous connective tissue and smooth muscle. In the outer part of the capsule the con- nective tissue is loosely arranged and merges with the surrounding fatty areolar tissue. The inner layer of the capsule is more dense THE URINARY SYSTEM. 267 SPs 1 * 1-5 and forms a firm investment for the underlying glandular tissue. From the capsule trabecular extend into the organ forming its frame- work and outlining compartments, which contain the glandular epi- thelium. This connective tissue is reticular in character. The cortex (Fig. 172. B) is sub- divided into three layers or zones : (a) A narrow, superficial layer, the glomern lar zone ; (b) a broad middle layer, the fascicular zone ; and (r) a narrow deep layer, the reticular zone. The names of the layers are indica- tive of the shape of the connective- tissue-enclosed compartments and of the contained groups of gland cells. In the glomerular zone (Fig. 172, a) the high, irregularly columnar epi- thelium is arranged in spherical or oval groups. The protoplasm of the cells is granular, and their nuclei are rich in chromatin. In the fascicular zone (Fig. 172, b) polyhedral cells are arranged in long columns or fascicles. The cytoplasm is granular and usu- ally contains some fat droplets. The nuclei are poor in chromatin. In the reticular zone (Fig. 172, c) similar though somewhat more darkly stain- ing cells form a coarse reticulum of irregular anastomosing cords. The medulla (Fig. 172, C) con- sists Of Spherical and Oval groups and Fig. lya.-Vertical Section of Adrenal. COrds Of polygonal Cells. After ako- ^kel-Iienle.) .4, Capsule ; B, cortex-, 1 J ° C, medulla ; a, glomerular zone ; b, fas- hol or formalin fixation these cells take a paler stain than those of the cortex. After fixation in solutions containing chromic acid or chrome salts the cells of the medulla assume a peculiar deep brown color, which cannot be removed by washing in water and which is quite characteristic of these ceils. YC cicular zone ; c, reticular zone ; v, vein in medulla. 268 THE ORGANS. Blood-vessels — The arteries supplying the adrenal first form a poorly defined plexus in the capsule. From this are given off three sets of vessels — one to the capsule, one to the cortex, and one to the medulla. The first set breaks up into a network of capillaries, which supply the capsule. The vessels to the cortex break up into capil- lary networks, the shape of the mesh corresponding to the arrange- ment of the connective tissue in the different zones. The vessels to the medulla pass directly through the cortex without branching and form dense capillary networks among the groups of medullary cells. The relations of the capillaries to these gland cells are ex- tremely intimate, especially in the reticular zone and medulla, where the cells in many cases immediately surround the capillaries in much the same manner as the glandular cells of a tubular gland surround their lumina. From the capillaries of both cortex and medulla small veins arise. These unite to form larger veins which empty into one or two main veins situated in the centre of the medulla. Lymphatics — These follow in general the course of the blood- vessels. The exact distribution of the adrenal lymph system has not been as yet satisfactorily determined. Nerves. — The nerve supply of the adrenal is so rich and the nerve elements of the gland are so abundant as to have led to its classification by some among the organs of the nervous system. Both medullated and non-medullated fibres- — but chiefly the latter — form plexuses in the capsule, where they are associated with groups of sympathetic ganglion cells. From the capsular plexuses fine fibres pass into the cortex, where they form networks around the groups of cortical cells. The nerve terminals of the cortex apparently do not penetrate the groups of cells. Bundles of nerve fibres, larger and more numerous than those to the cortex, pass through the cortex to the medulla. Here they form unusually dense plexuses of fibres, which not only surround the groups of cells, but penetrate the groups and surround the individual cells. Associated with the plexuses of the medulla, less commonly of the cortex, are numerous conspicuous groups of sympathetic ganglion cells. Development. — The cortex of the adrenal develops from meso- blast. As to the origin of the medulla two views are held. Accord- ing to one, the medullary cells are also derived from mesoderm and represent a further differentiation of the cortical cells. According to others, the medulla has an entirely independent origin, being THE URINARY SYSTEM. 269 derived from ectoderm, as part of the peripheral sympathetic nervous system. Flint describes the adrenal of a 3.5 cm. -long pig embryo as consisting wholly of cortical substance, surrounded by a capsule, which is closely associated with a plexus of the sympathetic. Cells of the type of medullary cells first appear just beneath the capsule, whence they later migrate to the centre of the organ. This migra- tion accounts for the frequency with which medullary cells are found in the cortex and cortical cells in the medulla. TECHNIC. (1) Fix the simple kidney of a rabbit or guinea-pig in formalin-Muller's fluid (technic 5, p. 5). Make sections through the entire organ including the papilla and pelvis, stain with haematoxylin-eosin (technic 1, p. 16), and mount in balsam. This section is for the study of the general topography of the kidney. (2) Fix small pieces from the different parts of a human kidney in formalin- Miiller's fluid or in Zenker's fluid. Thin sections should be made, some cutting the tubules longitudinally, others transversely, stained with hsematoxylin-eosin and mounted in balsam. (3) Blood-vessels. — For the purpose of demonstrating blood-vessels of the kidney the method of double injection is useful (page 22). (4) Ureter. —Cut transversely into short segments, fix in formalin-J\Iiiller*s fluid (technic 5, p. 5), and stain transverse sections with hsematoxylin-eosin (tech- nic 1, p. 16), or with hsematoxylin-picro-acid-fuchsin (technic 3, p. 16). Mount in balsam. (5) Bladder (technic 1, p. 199, or technic 2, p. 200). By the latter method any desired degree of distention may be obtained. (6) Adrenal. Technic same as (2) above. Thin vertical sections should in- clude both cortex and medulla. General References for Further Study. Kolliker: Handbuch der Gewebelehre, vol. iii. Gegenbauer: Lehrbuch der Anatomie des Menschen, vol. ii. Henle : Handbuch der Anatomie des Menschen, vol. ii. Johnston : A Reconstruction of a Glomerulus of the Human Kidney. Johns Hopkins Hosp. Bui., vol. xi., 1900. Miiller: Ueber die Ausscheidung des Methylenblau durch die Nieren. Deutsches Archiv f. klin. Med., Bd. 63, 1S99. Flint: The Blood-vessels, Angiogenesis, Organogenesis, Reticulum and His- tology of the Adrenal. Contributions to the Science of Medicine, Johns Hopkins Press, 1900. Pfaundler : Zur Anatomie der Nebenniere. Anzeiger Akad. Wien. 29, 1892. Nagel : Ueber die Entwickelung des Urogenitalsystem des Menschen. Arch. f. Mik. Anat., Bd. xxxiv. CHAPTER IX. THE REPRODUCTIVE SYSTEM. I. MALE ORGANS. The Testis. The testes are compound tubular glands. Each testis is enclosed in a dense connective-tissue capsule, the tunica albuginea (Fig. 173, a). Outside the latter is a closed serous sac, the tunica vaginalis, the visceral layer of which is attached to the tunica albuginea, while the par- ietal layer lines the inner surface of the scrotum. Posteriorly the serous sac is wanting, the testis really lying behind and outside of the tunica vaginalis. As the latter is derived from the peritoneum, being brought down with and invaginated by the testes in their descent to the scrotum, it is lined by mesothelial cells. To the inner side of the tunica albuginea is a layer of loose connective tissue rich in blood-vessels, the tunica vas- culosa. Posteriorly the tunica albu- ginea is greatly thickened to form the corpus Highmori, or mediastinum testis, from which strong connective- tissue septa radiate (Figs. 173, m and 174, b). These septa pass complete- ly through the organ and blend with the tunica albuginea at various points. In this way the interior of the testis is subdivided into a number of pyramidal chambers or lobules, with bases directed toward the periphery and apices at the mediastinum (Figs. 173 and 174). Behind the testis and outside of its tunica albuginea is an elon- 270 PlG. 173. -Diagram illustrating the Course and Relations of the Seminif- erous Tubules and their Kxcretory Duets. (Piersol.) a, Tunica albuginea ; f>, connective-tissue septum between lobules ; ;;/, mediastinum ; /, convo- luted portion of seminiferous tubule ; s, straight tubule; r, rete testis; e, vasa efferentia; c, tubules of head of epididymis; lc\ vas epididymis; vd, vas deferens; va, vas aberrans ; />, paradidymis. THE REPRODUCTIVE SYSTEM 271 gated body — the epididymis (Figs. 173, c and 1 74, r), consisting of convoluted tubules continuous with those of the mediastinum. The epididymis is divided into e a three parts : an expanded \ ^^_\ f^^~~ upper extremity, the head or globus major (Figs. 173 and 174,6'); a middle piece, the body (Fig. 174, d); and a slightly expanded lower extremity, the tail or globus minor. From the last named passes off the main excre- tory duct of the testis, the vas deferens (Fig. 173, vd). All of the tubules of the epididymis are continuous on the one hand with the tubules of the testicle, and on the other with the vas deferens. They thus con- stitute a portion of the com- plex system of excretory ducts of the testicle. The seminiferous tubule may be divided with reference to structure and location into three parts. (1) A much convoluted part, the convoluted tubule, which begins at the base and occupies the greater portion of a lobule of the testis. As they approach the apex of a lobule several of these convoluted tubules unite to form (2) the straiglit tubule. This passes through the apex of the lobule to the mediastinum, where it unites with other straight tubules to form (3) the irregular network of tubules of the medias- tinum, the rete testis (Fig. 177, c). 1. The Convoluted Tubule. — This, which may be considered the most important secreting portion of the lobule, since it is here that the spermatozoa are formed, has a diameter of from 1 50 to 250,". The tubules begin, some blindly, others by anastomoses with neigh- boring tubules, near the periphery of the lobule, and pursue a tortuous course toward its apex (Fig. 177, a). Fig. 174. —Longitudinal Section through Human Testis and Epididymis. X 2. (Bohm and von Davidoff.) The light strands are connective-tissue septa. <.i, Tunica albuginea ; b, mediastinum and rete testis ; c, head of epididymis ; d, body of epididymis ; e, lob- ule ; 5, straight tubules : /, vas epididymis. 2/2 THE ORGANS. The wall of the convoluted tubule (Fig. 175) consists of three layers : (a) An outer layer composed of several rows of flattened connective-tissue cells which closely invest the tubule; (/>) a thin Fig. 175. — Cross Section of Convoluted Portion of Human Seminiferous Tubule. X 480. (Kolli- ker.j M, Basement membrane ; z, its inner homogeneous layer, fs, its outer fibrous layer ; 5, nucleus of Sertoli cell ; sf>, spermatogone ; sc, spermatocyte ; sc', spermatocyte showing mitosis ; sf, nearly mature spermatozoon ; sf, spermatozoon free in lumen of tubule ; d, degenerating nucleus in lumen : /", 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 sustentacula,!' cells and the glandular cells proper, the spermatogenic cells. The sustcntacular 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. 1 76, 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- matogonia, but that, instead of developing into spermatozoa, they THE REPRODUCTIVE SYSTEM. 273 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 f \ / mm®/ // s 6 \. ..... , . \ 1 till!!! 1! '. - ! ■ I '.■'''.. £ ^.'kWZ ■ / >sp stcj: ■J •h sf Fig. 176. — Parts of Transverse Section of three Seminiferous Tubules from Testis of White Mouse. X 600. (Szymonowicz.) .?, Sertoli cell with nucleus; sp, spermatogone, resting state; sp'. spermatogone in mitosis ; sc, spermatocj-te ; st, spermatid ; sf, spermatid de- veloping into spermatozoon ; //, head of spermatozoon ; /, tails of developing spermato- zoa ; b, blood-vessel ; c, interstitial cell ; ;;/, basal membrane ;_/", 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. 1 75 and 1 76, sf>). — These are small cuboi- dal cells which lie against the basement membrane. Their nuclei are iS '4 THE ORGAXS. 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, se). — 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 ~ ... i0—* Fig. 177. Fig. 178. Fig. 177.— Passage of Convoluted Part of Seminiferous Tubules into Straight Tubules and of these into the Rete Testis. (Milhalkowicz.) , sperma- togones; sc, spermatocyte ; s/ : and s/%, spermatids lying along the surface of the Sertoli cell, s' and sl 3 ; at s/ s 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, si) are small round cells which line the lumen of the seminiferous tubule. They arc the direct progenitors of the spermatozoa. (For details of spermatogen- esis see page 281.) THE REPRODUCTIVE SYSTEM. 275 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 1 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 ■ -dff M.'f ;', , o v * ■ J "'T. , .■: lf ^ , j:'>'V'"*' 'V .cS-"„ s --v." '' < * v 7* ■ ; " ~^'.v\ F 'v . r^T " v .< " V •;.■■"// ■'/■-■• ■: '■ x •■',• ■■■Ki% ;•. _V ; *^' S *'W--~^ U ^..<*.-4^V'' /•■*.•.< r<-s=-..^, ' "' ' J£ FlG. 179.— From Section through Human Mediastinum and Rete Testis. X 96. (Kolliker.) ^4, Ar- tery ; F, 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 ; Si, 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, e). 2. The Straight Tubule. — With the termination of the con- voluted portion, the spermatogenic tissue of the gland ends, the 76 THE ORGANS. ^ FIG. iJSo.— 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 p. 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 efferentia, 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 intraepithelial 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. 277 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 m Fig. 181.— From Cross Section through Head of Epididymis. X 35. (Kolliker.) />, 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 i •w^'v 2;8 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, '// an d situated between the inner ; "^^-^=^^_^ ^^^ ^.w^rr — '' circular layer and the submucosa. The, fibrosa consists of fibrous tissue containing many elastic fibres. 1 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 action through Wail of ■ however, frequently markedly Tubules of Epididymis. X 700. (kolhker.) x J ■> , Con- pigmented and the milCOSa COll- nective-tissue and smooth muscle cells ; e, . . . . . , . . basal layer of epithelial ceils;/, high coi- tarns branched tubular glands. umnar epithelial cells; /. pigment gran- f^e Seminal VesideS and ules in columnar cells ; c. cuticula ; //, 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 ureter or into the ureter 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 foetal structures. These are: (1) The paradidymis ; or organ of Giraldes, situated between the THE REPRODUCTIVE SYSTEM. 279 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 cpididymidis, a vascular structure, not always present, lying near the appendix testis. It resembles the latter in structure. --/ ■ r V - -- - - • ; f jry.. . ■» : ■ ■-- I ','? ^SBflBHs^ - . . '^-"° : , •■ "Js FIG. 183. — Cross Section of Human Vas Deferens. X 37. (Szymonowicz.) a, Epithelium ; h, stroma ; c, submucosa ; d, inner circular muscle layer ; e, outer longitudinal muscle layer ; y, fibrotis layer ; g, 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 Mliller. Blood-vessels. — Branches of the spermatic artery ramify in the mediastinum and in the tunica vasculosa. These send branches into 2 'So 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 >). 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 fibrillated central core, the axial thread, surrounded by a protoplasmic capsule. Just behind the head the axial thread pre- sents a bulbous thickening, the terminal nodule 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 fibrillated 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. 281 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 tisually be differentiated, except by the use of special methods and very high-poiver 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. 176, sf) 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. 178, 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 tJie species (page 42). 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- FlG. ■Human Spermatozoa. (.Af- ter Retzius.) /, Head seen on flat ; 2, head seen on edge ; k, head ; w. body ; /. tail ; e, end piece. 2S2 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-M tiller's fluid (technic 5, p. 5). Antero-posterior longitudinal sections through the entire organ and in- cluding the epididymis should be stained with hamiatoxylin-picro-acid-fuchsin itechnic 3. p. 16) or with hrematoxylin-eosin (technic 1, p. 16) 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-Midler's or in Zenker's fluid (technic 9, p. 6). 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. 16) and mount in balsam. (3) For the study of spermatogenesis fix a mouse's testis in chrome-acetic- osmic mixture (technic 7, p. 6). 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 trabecule of the same structure as the capsule pass into the gland. The amount of connective tissue is large. It is less in tin- 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. 283 cuboidal 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- .vv "S ' ■!//.'. • *&& . .'^'v si y „/■ f '•■*'■■/' \ ■'. ; '£ „>'"'-•■- ■*"'.■' v- '■■■ : f '"'.'•■■■' '-.;•-"* -■-'" Fig. 185. —Section of Human Prostate. X 150. (Technic i, p. 284.) a, Epithelium of tubule: S, interstitial connective tissue ; c, corpora amj-lacea. centrically laminated bodies, crescentic corpuscles, or corpora amylacea, 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 {iitricidus prostaticus — uterus masculinus). It represents the remains of a foe- tal structure, the Mullerian duct (see page 310) 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- becular. 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. 284 THE ORGANS. The lymphatics begin as blind clefts in the trabecule 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 trabeculae and of the walls of the blood-vessels. Their mode of termination is not known. Timofeew describes afferent niedullated 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-Miiller's fluid (teclinic 5, p. 5). Stain sections with haematoxylin-eosin (technic 1. p. (6) 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 albuginca, 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. 285 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. In sinuses become filled with venous blood. erection these FIG. 186.— Transverse Section through Human Penis, a, Skin; 6, subcutaneous tissue; c, fibrous tunic ; d, dorsal vein ; e, corpora cav- ernosa ; f, corpus spongiosum ; ,?, urethra. 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 . . . ■ ■ : .&• ' '» Fig. 187.— Erectile Tissue of Corpus Spongiosum of Human Penis. X 60. a, Trabeculae 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. 2 86 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 albugineaand 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 trabecule 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 (a 7 ) Meissner's corpuscles in the papillae. (For details see pages 348, 349, antl 350. 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 (corona). 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 '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 urethra] opening. Glands of Littre' are present, but are fewer than in the male. THE REPRODUCTIVE SYSTEM. 287 to stratified squamous. The epithelium rests upon a basement mem- brane, beneath which is a thin stroma rich in elastic fibres and hav- ing papillae 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 ' p# g; character of erectile tissue (Fig. 189). ' j % :: ff\ - 'v '"■ , : ^ -. 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- r h & - d Fig. 18S. 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 ; «, epithelium; b, stroma; c, cavernous veins; cf, con- nective tissue of submucosa. posed smooth muscle cells are found in the outer part of the sub- mucosa of the penile urethra. Throughout the mucosa of the entire urethra, but most numerous in the penile portion, are simple branched tubular mucous glands, the glands of Littre. They are lined with columnar epithelium and the longer extend into the submucosa. 2 88 THE ORGANS. TECHNIC. I 1) For the study of the general topography of the penis, remove the skin from the organ and cut into transverse slices about 0.5 cm. in thickness. Fix in forma- lin-Midler's fluid (technjc 5, p. 5). cut rather thick sections across the entire penis, stain with haematoxylin-picro-acid-fuchsin (technic 3. p. 16) or with haema- toxylin-eosin (technic 1. p. 16) and mount in balsam. (2) For the study of the structure of the penile portion of the urethra and of the erectile tissue of the corpus spongiosum, cut away the corpora cavernosa, leav- ing only the corpus spongiosum and contained urethra, and treat as above. Sec- tions should be thin and stained with haematoxylin-eosin. (3) The same technic is to be used for the membranous and prostatic portions of the urethra. II. FEMALE ORGANS. The Ovary. The ovary is classed as one of the ductless glands. Its specific secretion is the ovum. The ovary has no duct system which is di- rectly continuous with its structure. In place of this it is provided with what may be considered to be a highly specialized disconnected excretory duct — the oviduct or Fallopian tube — which serves for the transmission of its secretion to the uterus. On one side the ovary is attached by a broad base, the Jiilum, 10 the broad ligament. Elsewhere the surface of the ovary is covered by a modified peritoneum. At the hilum the tissues of the broad ligament pass into the ovary and spread out there to form the ovarian stroma. This consists of fibrous connective tissue rich in elastic fibres and containing many smooth muscle cells. In the deeper central portion of the organ stroma alone is found. Here it contains man) r large blood-vessels, and constitutes the medulla or zona vas- culosa of the ovary (Fig. 190, 2). From the medulla the stroma radiates toward the surface of the ovary and becomes interspersed with glandular elements forming the ovarian cortex (Fig. 190, J, 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 mesothelium 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. 289 The structure of the Graafian follicle can be best appreciated by studying its development. The follicles originate from the germinal Fig. i9o.--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 ; />, germ hill; c. ovum; d, nucleus (ger- minal vesicle) ; e, nucleolus (germinal spot). epithelium during foetal life. At this time the germinal epithelium is proliferating, and certain of its cells differentiate into larger Fig. iqi. — Semidiagrammatic Drawing to show Development of Ovum from Germinal Epithelium of Ovary. (Duval. 1 spherical cells — primitive ova. The primitive ova pass down into the stroma accompanied by a considerable number of the undifferen- 19 290 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 Pflfigcr 's egg tubes or cords (Fig. 191, A, B, C). Each cord usually contains several ova. In some cases the differentiation Fig. 192.— Vertical Section through Cortex of Ovary of Young Girl. X 190. (Bohm and von Davidoff.) a, Germinal epithelium; l\ tunica albuginea; c, follicular epithelium; d, ovum ; e, primitive Graafian follicles in ovarian cortex; /', 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, If). This central ovum surrounded by a single layer of epithe- THE REPRODUCTIVE SYSTEM. 291 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 j. 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 germ hill {discus proligerus — cumu- lus obpkorus). The cells of the germ hill nearest the ovum become - £T—- 2 ■■ .. ■ arqej. FIG. 193.— From Section through Cortex of Ape's Ovary. X 150. (Szymonowicz.) .?, Prim- itive follicle ; b, ovum, with nucleus and nucleolus; c, zona pellucida ; if, follicular epithelium ; e, follicular cavity ; f, ovarian stroma ; £\ blood-vessel in stroma. columnar and arranged in a regular single layer around the ovum — the corona radiata (Fig. 193). The ovarian stroma immediately sur- rounding the Graafian follicle becomes somewhat modified to form a sheath for the follicle— the theca folliculi (Fig. 194). This consists of two layers, an outer more dense fibrous layer, the tunica fibrosa, 292 THE ORGANS. and an inner more cellular and vascular, the tunica vasadosa. Be- tween the thecafolliculi and the stratum granulosum is an apparently structureless basement membrane. While these changes are taking place in the follicle, the ovum is also undergoing development. The ovum of the primitive follicle is a spherical cell, having a diameter of from 40 to 70 ;>■ and the struc- ture of a typical cell. The nucleus or germinal vesicle (so called on account of the part it takes in reproduction) is about half the diameter of the cell, and is spherical and centrally placed (Fig. 193). It is -g , ,;;c^r^ — e ■:!> d FlG. 194.— Section through Graafian Follicle of Ape's Ovary. X 90. (Szymonowicz.) Later stage of development than Fig. 193. a, Germ hill ; />, ovum with clear zona pellucida, germinal vesicle, and germinal spot; d, follicular epithelium (membrana 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. 293 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 hcemorrhagicum, which represents the earliest stage in the de- velopment of the corpus lutcuni. 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 294 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, FlG. 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 Macmilkm 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. Hy 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 In tea (corpora THE REPRODUCTIVE SYSTEM. 295 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 ^HP it b-- 3 Fig. 196.— Formation of the Corpus Luteum according 1 to Sabotta. 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. Z>, Central cavity replaced by connective tissue resembling mucous tissue, columns broken up by anastomosis of con- nective-tissue strands, it. Follicular epithelium ; £, vascular bud ; c. theca folliculi ; if, germinal epthelium ; e, leucocytes. corpus luteum is a periodically rejuvenated ovarian gland, which gives to the uterus a cyclic nutritional impulse, which prepares it for the implantation of the ovum or favors menstruation whenever the ovum is not fertilized. 296 THE ORGANS. Of the large number of ova— estimated at seventy-two thousand — in the human ovaries only comparatively few, according to Henle about four hundred, reach maturity. The majority undergo, together with their follicles, retrogressive changes known as atresia of the follicle. The nucleus of the ovum, as well as the nuclei of the fol- licular cells, passes through a series of chromatolytic changes, or in some cases apparently simply atrophies. The cell bodies undergo fatty or albuminous degeneration and the cell becomes reduced to a homogeneous mass, which is finally absorbed, leaving in its place a connective-tissue scar, probably the remains of the theca folliculi. Blood-vessels. — The arteries, branches of the ovarian and uterine, enter the ovary at the hilum and ramify in the medulla. From these are given off branches which pass to the cortex and end in a capil- lary network in the tunica albuginea. In the outer layer of the theca folliculi the capillaries form a wide-meshed network, which gives rise to a fine-meshed network of capillaries in the inner layer of the theca. From the capillaries veins arise which form a plexus in the medulla and leave the ovary at the hilum. Lymphatics. — These begin as small lymph spaces in the cortex, which communicate with more definite lymph vessels in the medulla, the latter leaving the organ at the hilum. Nerves. — Medullated and non-medullated fibres enter the ovary at the hilum and follow the course taken by the blood-vessels. Many of the fibres end in the vessel walls ; others form plexuses around the follicle and end in the theca folliculi. Some describe fibres as passing through the theca and ending in the follicular epithelium. Others claim that nerve fibres do not enter the follicle proper. Groups of sympathetic ganglion cells occur in the medulla near the hilum. As is the case with the testicle, certain rudimentary organs, the remains of foetal structures, are found connected with the ovary. The paroophoron consists of a number of cords or tubules of epi- thelial cells, sometimes ciliated, sometimes non-ciliated. It is found in the medulla, or, more commonly, in the connective tissue of the hilum. The epoophoron is a similar structure found in the folds of the broad ligament. Its tubules open into a duct known as Gartner's duct. In man this duct ends blindly. In some of the lower animals THE REPRODUCTIVE SYSTEM. 297 it opens into the vagina. Both paroophoron and epoophoron are remains of the embryonal mesonephros, the former of its posterior segment, the latter of its middle segment. The Oviduct. The oviduct or Fallopian tube is the excretory duct of the ovary, serving for the transmission of the discharged ovum from ovary to uterus. Although there is no sharp demarcation between them, it is convenient to divide the tube into three segments: (1) The isthmus, beginning at the uterus and extending about one-third the length of FIG. 197.— Cross Section of Oviduct near Uterine End. a, Mucous membrane ; i, circular mus- cle coat ; c, longitudinal muscle coat ; d, connective tissue of serous coat. (Orthmann.) the tube ; (2) the ampulla, about twice the diameter of the isthmus, and occupying somewhat more than the middle third ; and (3) the fimbriated or ovarian extremity. The walls of the oviduct consist of three coats: (1) Mucous, (2) muscular, and (3) serous (Figs. 197 and 198). The mucous membrane presents numerous longitudinal foldings. In the embryo four of these folds can usually be distinguished, and these are known as primary folds. In the adult many secondary- folds have developed upon the primary, especially in the ampulla and fimbriated extremity where the folds are high and complicated (Fig. 198). The epithelium lining the tube is of the simple columnar 298 THE ORGANS. ciliated type, and completely covers the foldings of the mucous mem- brane. The ciliary motion is toward the uterus. The stroma con- sists of a cellular connective tissue, quite compact in structure in the isthmus, where the folds are low, more loosely arranged in the high folds of the ampulla and fimbriated extremity. The muscular coat consists of an inner circular and an outer longitudinal layer. The latter is a comparatively thin layer in the Ife**, m M FIG. 198.— Cross Section of Oviduct near Fimbriated Extremity, showing complicated fold- ings of mucous membrane. (Orthmann.) isthmus, consists of discontinuous groups of muscle cells in the am- pulla, and in the fimbriated extremity is frequently absent. The serous coat has the usual structure of peritoneum. The larger blood-vessels run in the stroma along the bases of the folds. They send off branches which give rise to a dense capillary network in the stroma. Of the lymphatics of the tube little is known. The nerves form a rich plexus in the stroma, from which branches pass to the blood-vessels and muscular tissue of the walls of the tube and internally as far as the epithelial lining. TECHNIC. (1) Child's Ovary. — Remove the ovary of a new-born child, being careful not to touch the surface epithelium, fix in Zenker's fluid (technic 9, ]»• c >)< and harden in alcohol. Cut sections of the entire organ through the hilum. Stain with haematoxylin eosin (technic 1. p. 16) and mount in balsam. THE REPRODUCTIVE SYSTEM. 299 (2) For the purpose of studying the Graafian follicle in the different stages of its development remove an ovary from an adult cat or dog and treat as above. Technic (1). These sections also as a rule are satisfactory for the study of the cor- pus luteum. (3) The human adult ovary is little used for histological purposes on account of the few follicles it usually contains and its proneness to pathological changes. Its study is, however, so extremely important, especially with reference to the pathology of the ovary, that if possible a normal human ovary should be obtained from a young subject for purposes of comparison with the above. Technic same (1). (4) For studying the egg tubes of Pfliiger and their relation to the germ epi- thelium, ovaries of the human foetus, and of very young cats, dogs, and rabbits are satisfactory. Technic (1). (5) Sections of the fimbriated end of the oviduct are usually found in the sec- tions of ovary. For the study of other parts of the tube, cut out thin pieces from different regions, fix in formalin-M tiller's fluid, stain transverse sections with has- matoxylin-eosin, and mount in balsam. The Uterus. The wall of the uterus consists of three coats which from without inward are serous, muscular, and mucous. The serous coat is a reflection of the peritoneum, and has the usual structure of a serous membrane. The muscu/an's consists of bundles of smooth muscle cells sepa- rated by connective tissue. The muscle has a general arrangement into three layers, an inner, a middle, and an outer, which are distinct in the cervix, but not well defined in the body and fundus. The inner layer — stratum submucosum — is mainly longitudinal, although some obliquely running bundles are usually present. The middle layer — called from the large venous channels which it contains, the stratum vasculare — is the thickest of the three layers forming the main bulk of the muscular wall. It consists mainly of circularly disposed muscle bundles. The outer layer — stratum supravasculare — is thin and consists partly of circular bundles, partly of longitudinal. The latter pre- dominate and form a fairly distinct layer just beneath the serosa. The muscle cells of the uterus are long spindle-shaped elements, some having pointed, others blunt, branched, or frayed ends. In the virgin uterus they have a length of from 40 to 60 , a. During pregnancy the muscular tissue of the uterus is greatly increased. This is clue partly to increase in the number, partly to increase in the size of the muscle cells. At term the muscle cells frequently have a length of from 250 to 600,". ;oo THE ORGANS. The mucous membrane. As the mucosa presents marked varia- tion in structure, dependent upon the functional condition of the organ, it is necessary to describe : i. The mucosa of the resting uterus. 2. The mucosa of the menstruating uterus. 3. The mucosa of the pregnant uterus. wt tOlr.S. i. The Mucosa of the Resting Uterus. This is from 1 to 2 mm. thick, and consists of a stroma, glands, and a lining epithelium (Fig. 199). The stroma resembles embryonal connective tissue, consisting of fine fibrils and long, irregular branch- ing cells which form a sort b of network, the meshes of which are filled in with lym- phoid cells and leucocytes. The epithelium is ' of the c simple high columnar ciliated variety, the ciliary motion being toward the cervix. A basement membrane separates the epithelium from the under- lying stroma. The glands are simple forked tubules lined by a single layer of columnar ciliated cells resting upon a basement membrane and con- tinuous with the surface cells. The glands extend completely through the stroma. Near the surface they run a compara- tively straight course. Deeper in the stroma their course is more tortuous, while the fundus is frequently turned at right angles to the rest of the tubule. In the cervix the stroma is firmer and less cellular, and the rau- cous membrane is thicker and presents numerous folds — the plicce palmalce. The epithelium is higher than in the body of the organ. In addition to glands like those found in the body of the uterus, the Pi Prom Uterusof Young Woman. (From Hohm and von Davidoff; preparation by Dr. J. Amann.) X 34. a. Mucous membrane ; J, surface epithelium ; c, gland ; e, muscle. THE REPRODUCTIVE SYSTEM. 301 cervical mucosa contains peculiar short, sac-like invaginations lined with a continuation of the surface epithelium, which secrete a glairy mucus. Closure of the mouths of some of these sacs fre- quently occurs, leading to the . .. ": -^\ formation of retention cysts, the so-called ovula Nabothi. At fa about the junction of middle a .. ■-■ .— • ..:... .--■ and lower thirds of the cervical /, ....... -i ^r- :; ^ d canal a change takes place in .. : ' \ the epithelium. Here the e '" \X' // " '"• - simple columnar ciliated epi- ... >r ' '^ — ---.:' *'"' . -^ ■--. thelium of the upper part of f~ """"---// the cervix gradually passes over m Fig. 200. — From Section of Dog's Cervix. X 4. into a Stratified Squamous epi- (Technics' 2, p. 310.) «, Cervical canal; b, mu- thelium. Near the external os cosa ' c \ folds °/ mucosa W™**}™^ * muscle la\-ers of cervix; e, epithelium of va- papilla? appear, the vaginal SUr- ginaand vaginal surface of cervix;/, vaginal . . epithelium ; g, vaginal mucosa ; //, submucosa face Of the CerVIX being COVered and muscularis of vagina ; /, blood-vessels. with a stratified squamous epi- thelium with underlying papillae similar to and continuous with that of the vagina. Near the external os the epithelium changes over into the strati- fied squamous epithelium with underlying papillae, similar to that of the external surface of the cervix. 2. The Mucosa of the Menstruating Uterus. This consists of the same structural elements as the mucosa of the resting uterus : stroma, glands, and lining epithelium. These, however, undergo certain changes which maybe conveniently divided into three stages : (a) The stage of preparation. (/>) The stage of menstruation proper. (c) The stage of reparation. (a) The Stage of Preparation. — This begins several days before the actual flow of blood, and is marked by an intense hyperae- mia determining a swelling and growth of the entire mucosa. The blood-vessels, especially the capillaries and veins, become greatly distended, thus contributing largely to the increase in thickness of the mucosa. There are also proliferation of the connective-tissue ;o2 THE ORGAXS. lllllll; cells, an increase in the number of leucocytes, and a growth of the uterine glands. The surface at the same time becomes irregular, the glands opening into deep pits or depressions, and the glands them- selves become more tortuous and their lumina more widely open. The mucous membrane has now reached a thickness of about 6 mm., and is known as the decidua menstrualis (Fig. 201). (&) The Stage of Men- struation Proper. — This is marked by the escape of blood from the engorged vessels and the appearance of the external phenomena of menstruation. The blood escapes partly by rupture of the vessel walls, partly by diapedesis. The hem- orrhage is at first subepithelial, but the epithelium soon gives way and the blood escapes into the cavity of the uterus. Much difference of opinion exists as to the amount of epithelial de- struction during menstruation, some claiming that the entire epithelium is destroyed with each menstrual period, others that the epithelium remains almost in- tact. Complete destruction of the epithelium is hardly compatible with the restoration of the epithe- lium which always follows menstruation. While there is undoubtedly destruction of most or all of the surface epithelium and of the glands to some considerable depth, the deeper portions of the glands always remain to take part in the succeeding regenerative phenomena. (c) The Stage of Reparation. — After from three to five days the bleeding from the uterine mucosa ceases and the return to the resting condition begins. This is marked by disappearance of the congestion, by decrease in thickness of the mucosa and in the size of the glands, and by restoration of the surface epithelium. U~i^(iWf-;''', cavernous layer of clecidua serotina ; C, muscular is. resting upon a layer of embryonal connective tissue which attaches it to the chorion. The chorion consists of (a) a compact layer — the membrana chorii — composed at first of embryonal, later of fibrous, connective tissue, and containing the main branches of the umbilical I 01 E igs. 202 and 203, also for many facts as t<> die structure of the placenta, the writer is indebted to the excellent chapter on the subject added by Prof, Alfred Si haper to the fifth edition of StShr's " Textbook of 1 1 istology." THE REPRODUCTIVE SYSTEM. 305 vessels and an inner villous layer, which gives rise to finger-like projections which extend down from the foetal into the maternal pla- centa and serve to connect the two. The chorionic villi first appear as short projections composed entirely of epithelium. Each of these primary villi branches di- chotomously, giving rise to a num- ber of secondary villi. As they develop, the central portion of the original solid epithelial structure is replaced by connective tissue. Septa of connective tissue from the ma- ternal placenta pass down among the villi and separate them into MMT^i groups or cotyledons. The main or primary villi run a quite straight course from the chorion into the maternal placental tissue, appar- ently serving to secure firm union between the two. They are thus known as roots of attachment or fastening villi (Fig. 202,/). The secondary villi are given off later- ally from the primary villi, end freely in the spaces between the latter — intervillous spaces (Fig. 202, d) floating villi (Fig. 202, e). The chorionic villus thus consists of a central core of connective tissue covered by a layer of epithelium. The connective tissue is of the mucous type and serves for the transmission of numerous blood- vessels. In the villi of early pregnancy the epithelium consists of an inner layer of distinctly outlined cells and an outer layer of fused cell bodies — a syncytium (Fig. 203, A, a) — containing small scattered nuclei. The villi of the later months of pregnancy have no definite epithelial covering, but are surrounded by a delicate homogeneous membrane, probably the remains of the syncytium. At various points on the surface of the villus are groups of nuclei. These stain intensely, are surrounded by a homogeneous protoplasm, and form knob-like projections above the general surface of the villus. They are known as cell patches, or more properly as unclear groups (Fig. Fig. 203. — Cross Sections of Human Chori- onic Villi at End of Pregnancy. X 250. (Schaper.) .4, Small villus; B, larger villus, a. Protoplasmic coat (syncytium); b, epithelial nucleus ; c, nuclear groups ; d, small artery ; e, small vein ; f, capil- laries" -and are known as free or 306 THE ORGANS. 203, c), and represent remains of the nuclei of the epithelium of the younger villus. Between the nuclear groups the villus is covered only by a thin homogeneous membrane. Small villi usually resem- ble more closely in structure the younger villus, being frequently covered by a nucleated syncytium. Portions of the syncytium, espe- cially of older villi, sometimes become changed into a peculiar hya- line substance containing numerous channels. This is known as canalized fibrin, and may form dense layers upon the surface of the chorion. The Placenta Uterina. — This develops from the decidua sero- tina. The latter becomes much thinner than the rest of the decidua (decidua vera), but still shows a division into a deeper spongy portion containing gland tubules, and a superficial compact portion in which are large numbers of decidual cells. From the superficial portion connective-tissue septa — placental septa — grow into the fcetal pla- centa, as described above, separating its villi into cotyledons. Near the margins of the placenta these septa pass to the chorionic mem- brane and form beneath it a thin membrane, the snbcliorionic placental decidua. At the edge of the placenta, where decidua serotina passes over into the thicker decidua vera, there is a close attachment of the chorion to the former. As the placenta serves as the place of interchange of materials be- tween the maternal and the fcetal circulations, the arrangement of the placental blood-vessels is of especial importance. Arterial branches from vessels of the uterine muscularis enter the serotina. In the very tortuous course which these vessels take through the serotina (Fig. 202, g) their walls lose their muscular and connective-tissue ele- ments and become reduced to epithelial tubes. These branch in the placental septa and finally open into the intervillous spaces along the edges of the cotyledons. The veins take origin from these spaces near the centres of the cotyledons. The maternal blood thus passes through the intervillous spaces from periphery to centre, and in its course comes into direct contact with the freely terminating chorionic villi. It is to be noted that the blood-vessel systems of the mother and of the foetus are both closed systems, and that consequently there is no direct admixture of maternal and fcetal blood. Interchange of materials must therefore always take place through the capillary walls and through the walls of the chorionic villi. Blood-vessels. — The arteries enter the uterus from the broad liga- THE REPRODUCTIVE SYSTEM. 307 raent and pass to the stratum vasculare of the muscularis, where they undergo extensive ramification. From the arteries of the stratum vasculare branches pass to the mucosa and give rise to capillary net- works, which surround the glands and are especially dense just be- neath the surface epithelium. From these capillaries the blood passes into a plexus of veins in the deeper portion of the mucosa, and these in turn empty into the venous plexuses of the stratum vascu- lare. Thence the veins accompany the arteries, leaving the uterus through the broad ligament. Lymphatics. — These begin as minute spaces in the stroma and empty into the more definite lymph channels of the muscularis, which are especially well developed in the stratum vasculare. These in turn communicate with the larger lymph vessels in the subserous connective tissue. Nerves. — Both medullated and non-medullated nerve fibres occur in the uterus. The latter are associated with minute sympathetic ganglia and supply the muscular tissue. The medullated fibres form plexuses in the mucosa, from which are given off fine fibres which terminate freely between the cells of the surface epithelium and of the uterine glands. The Vagina. The wall of the vagina consists of four coats, which from without inward are fibrous, muscular, submucous, and mucous. The fibrous coat consists of dense connective tissue with many coarse elastic fibres. It serves to connect the vagina with the sur- rounding structures. The muscular coat is indistinctly divided into an outer longitu- dinal and an inner circular layer. The latter is usually not well developed and may be absent. The submucosa is a layer of loose connective tissue, especially rich in elastic fibres and blood-vessels. Numerous large venous channels give to the submucosa the character of erectile tissue. The mucous membrane consists of a papillated connective-tissue stroma of mixed fibrous and elastic tissue. The stroma usually con- tains diffuse lymphoid tissue and more rarely solitary nodules. Covering the stroma is a stratified squamous epithelium, the surface cells of which are extremely thin. The surface of the mucosa is not smooth, but is folded transversely, forming the so-called rugce. Most 3°S THE ORGANS. authorities agree that glands are wanting in the vagina, the mucus found there being derived from the glands of the cervix. Blood-vessels. — The larger blood-vessels run in the submucosa, giving off branches which break up into capillary networks in the submucosa, muscularis, and stroma. The vascular networks have a genera] direction parallel to the surface. The capillaries empty into veins which form a plexus of broad venous channels in the muscu- laris. The Lymphatics. — These follow in general the distribution of the blood-vessels. Nerves. — Nerve fibres from both cerebro-spinal and sympathetic systems are found in the vagina. Medullated (sensory) fibres, the dendrites of spinal ganglion cells, form plexuses in the mucosa, from which are given off delicate non-medullated terminals to the epithe- lial cells. Non-medullated sympathetic fibres supply the muscularis and the muscle of the vessel walls. Along these nerves are small sympathetic ganglia. In the vestibule the epithelium gradually takes on the structure of epidermis. Here are located small mucous glands — glandultc vestibulares minores — especially numerous around the clitoris and opening of the urethra. Larger mucous glands — glandules vestibu- lares ma/ores, or glands of Bartholin — analogous to Cowper's glands in the male, are also found in the walls of the vestibule. The clitoris consists mainly of erectile tissue similar to that of the corpora cavernosa of the penis. It is covered with a thin epithe- lium with underlying papillae, and is richly supplied with nerves hav- ing highly specialized terminations. Development of the Urinary and Reproductive Systems. The urinary organs are peculiar in that three consecutive organs arc concerned in their development, although only the last of these actually gives rise to functionating adult organs, the other two being represented in the adult only by rudimentary structures. These three organs, in the order of their appearance, are the pronephros, the mesonephros, and the metanephros. All of these bodies develop from the mesoderm, first appearing as symmetrically placed ridges, which project into the primitive body cavity as the Wolffian ridges. THE REPRODUCTIVE SYSTEM. 309 The pronephritic or Wolffian ducts and the pronephros are the earliest of the urinary structures to appear. The former consist at first of solid, elongated groups of cells, situated in the Wolffian ridge. These later acquire lumina and become tubules. The pronephros consists of two evaginations of the epithelium of the primitive body cavity into the tissues of the Wolffian ridges, near the anterior end of the Wolffian duct. Only one of these develops into tubules, and both disappear early in embryonic life. In the female the Wolffian duct degenerates ; in the male it remains to form the vas deferens and tail of the epididymis. When the human embryo has reached a length of from 3 to 4 mm., a number of cords of cells, the origin of which is still doubtful, appear in the Wolffian ridge. These acquire lumina, which at one end communicate with the Wolffian duct, while at the other glomeruli develop, which contain blood-vessels derived from the aorta. This development of tubules and glomeruli results in a large increase in the size of the Wolffian ridges, which are now known as the Wolffian bodies or mesonephros. The latter reach their greatest development between the sixth and eighth embryonic week, after which the tubules and glomeruli undergo retrogressive changes. In the male the anterior tubules remain to form the head of the epidid- ymis, while the posterior tubules are represented only by the rudimen- tary paradidymis or organ of Giraldes. In the female the Wolffian body remains only as two rudimentary structures — the parovarium and the paroophoron. During the retrogressive changes in the mesonephros a new tubular structure appears as an outgrowth from the dorsum of the Wolffian duct. This tubule is known as the metanephros, and from it are developed the ureter and kidney. The end of the tubule at which the kidney is to develop next divides into a number of branches, which end iu expansions, the primary renal vesicles. From the lat- ter the uriniferous tubules develop. During the development of these tubules septa grow in from the capsule, which now surrounds the primitive kidney in such a manner as to separate the groups of tubules which develop from each primary vesicle. In this way the kidney becomes lobulated, the lobulation, however, disappearing after birth. The renal corpuscles or Malpighian bodies are formed, as already described (page 258), by invaginations of the developing tubules by branches of the renal artery. 3iO THE ORGANS. At about the height of development of the Wolffian body there appears along the inner side of each Wolffian ridge a thickening of the mesodermic cells, which thus form a distinct projection, the genital ridge. This is the earliest trace of a sexual gland, and is at first identical in the two sexes. By differentiation of these meso- dermic cells are formed, according to sex, the ovaries or testes. Into the genital ridge there extends an invagination of the peri- toneum to form the Mullerian duct. In the male this degenerates, its anterior part being represented in the adult by the stalked hy- datid or hydatid of Morgagni, its posterior part by the uterus mascu- linus. In the female the Mullerian ducts unite below to form the uterus, while above they remain separate, forming the Fallopian tubes. TECHNIC. (i) A human uterus — if possible from a young adult — or, if this cannot be ob- tained, the uterus of a cat or dog, is cut transversely into slices about i cm. thick and fixed in Zenker's fluid (technic 9, p. 6) or in formalin-Midler's fluid (technic 5. p. 5). For topography these slices are cut in half through the middle of the uterine cavity and sections made through the entire half organ. These are«stained with naematoxylin-picro-acid-fuchsin (technic 3, p. 16) and mounted in balsam. For details of the mucous membrane cut away most of the muscle from around the half slice, being careful not to touch the mucous surface ; make thin sections, stain with haematoxylin-eosin (technic 1, p. 16), and mount in balsam. (2) Sections of the cervix may be prepared in the same manner as the preceding. (3) Placental tissue may be cut into small cubes and treated with the same technic (1). (4) If a human or animal uterus with the placenta in situ is obtainable it should be cut into thin slices and fixed in formalin-Midler's fluid. The blocks of tissue should be so arranged that sections include the utero-placental junction. They may be stained with haematoxylin-eosin or with haematoxylin-picro-acid-fuch- sin (see above). (5) Treat pieces of the human vagina according to technic (t, p. 199). General References for Further Study. Kolliker: Handbuch der Gewebelehre des Menschen. Nagel: Das menschliche Ei. Arch. mik. Anat., Bd. xxxi., [SSS. Ruckert: Zur Eireifung derCopepoden. Anat. Hefte, I. Abth., Bd. iv., 1894. Sobotta: Ueber die Bildung des Corpus luteum bei dvr Mans. Arch. mik. Anat.. Bd. xlvii.. 1896.— Ueber die Bildung des Corpus luteum beim Kaninchen. Anat. Hefte, 1. Abth., Bd. viii., [897. Hertwig : Lehrbuch der Entwickelungsgeschichtedes Menschen und der Wir- ere, Jena. 1896, Schaper: Chapter on the Placenta in Stohr's Text-book of Histology, 5th ed. Ballowitz : Weitere Beobachtungen Liber den feineren Bau , duct of sweat gland ; c, stratum lucidum; d, stratum germinativum ; ^papilla of derma;/, derma ; g, blood-vessel : //, sweat gland ; /, fat lobule ; /, sweat pore. Externally this layer is marked by minute folds which are visible to> the naked eye, and can be seen intersecting one another and enclos- ing small irregular areas of skin. In the thick skin of the palms and soles these furrows are close together and parallel, while between them are long corresponding ridges. In addition to the furrows and ridges the entire surface of the corium is beset with minute papillae. These vary in structure, some ending in a single point — simple pa- pilla — others in several points — compound papilla ; some containing blood-vessels — vascular papilla; others containing special nerve terminations — nerve papilla' (Fig. 206). Smooth muscle cells occur in the corium in connection with the sweat glands. In the skin of the scrotum — tunica dartos — and of the nipple, the smooth muscle cells are arranged in a network parallel to the surface. In the face and neck striated muscle fibres penetrate the corium. Beneath the corium is the subcutaneous tissue. This consists of THE SKIN AND ITS APPENDAGES. 313 vertically disposed bands of connective tissue — the retinaculce cutis — which serve to unite the corium to the underlying structures and enclose fat lobules. In some parts of the body this subcutaneous fat forms a thick layer — the pannicuhis adiposus. The Epidermis.— This is composed of stratified squamous epi- thelium. In the comparatively thin skin of the general body surface the epidermis is divided into two sub-layers: (1) One lying just above the papillary layer of the derma, and known as the stratum germinativum (stratum mucosum — stratum Malpighii) ; (2) the other constituting the superficial layer of the skin — the horny layer or V < >**— _.v' FIG. 206— Froai Vertical Section through Skin of Human Finger Tip. X 200. (Schafer.) a, Stratum corneum ; 3, stratum lucidum ; r, stratum granulosum ; d, stratum germinati- vum. To the left a vascular papilla ; to the right a nerve papilla containing tactile cor- puscle. stratum corneum. In the thick skin of the palms and soles two additional layers are developed ; (3) the stratum granulosum ; and (4) the stratum lucidum (Fig. 205). (i) The stratum germinativum consists of several layers of cells. The deepest cells are columnar and form a single layer (stratum cylindricum), which rests upon a basement membrane separating it 3H THE ORG ASS. : W..-. <3s '/§fv from the derma. The membrane and cells follow the elevations and depressions caused by the papillae. The rest of the stratum germi- nativum consists of large polygonal cells. These cells have well-developed intercellular bridges, which appear as spines projecting from the surfaces of the cells. For this reason the cells are sometimes called "prickle" cells, and the layer, the " stratum spinosum. " The spines cross minute spaces between the cells, which are believed to communicate with the lymph spaces of the derma (Fig. 207, c). (2) The stratum granulosum is well developed only where the skin is thick. It consists of from one to three layers of flattened polygonal cells. The pro- toplasm of these cells contains deeply staining granules — keratohyaline gran- ules — which probably represent a stage in the formation of the horny substance — keratin — of the corneum cells. The nuclei of these cells always show de- generative changes, and there is reason for believing that this karyolysis is closely associated with the formation of KrlS ® ' ^) '. \ the keratohyaline granules (Fig. 207, />). (3) The stratum lucidum is also best developed where the skin is thickest. It consists of two or three layers of flat clear cells, the outlines of which are frequently so indistinct that the layer appears homogeneous. The trans- parency of the cells is due to the pres- ence of a substance known as clcid'ni, and derived from the kerato- hyaline granules of the stratum granulosum < Fig. 207, a). (4) The stratum corneum varies greatly in thickness, reaching its greatest development in the skin of the palms and soles. The cells are flattened and horny, especially near their surfaces. Some appear -■;- "§ Wm m ■'& &£ (&-:/&: •'■ few V-V m w ■ um >; <, stratum granulosum ; C, stratum germinativum, showing intercel- lular bridges. THE SKIN AND ITS APPENDAGES. 315 homogeneous, others have a lamellated appearance. They contain pareleidin, a derivative of the eleidin of the stratum lucidum. Nu- clei are lost, but in many of the cells can be seen the spaces which the nuclei once occupied. Constant desquamation of these cells goes on, cells from the deeper layers taking their place. The cells of the stratum germinativum are usually in a state of active mitosis. The color of the skin in the white races is due to pigmentation of the deeper layers of the epidermis. In certain parts of the body pigmentation of the connective-tissue cells of the derma also occurs. In the dark races all cells of the epidermis are pigmented, although there is less pigment in the surface cells than in the cells more deep- ly situated. Two kinds of glands occur in the skin — sebaceous glands and sweat glands. Sebaceous Glands. — These are usually associated with the hair follicles, and will be described in that connection. Sebaceous glands unconnected with hair occur along the margin of the lips, in the glans and prepuce of the penis, and in the labia minora. Sweat Glands {glandules su do rip a res). — These are found through- out the entire skin with the exception of the margin of the lips, the inner" surface of the prepuce, and the glans penis. They are simple coiled tubular glands. The coiled portion of the gland usually lies in the submucosa, although it may lie wholly or partly in the deeper portion of the pars reticularis. The excretory duct runs a quite straight course through the derma, and enters the epidermis in one of the depressions between the papillae. In the epidermis the duct takes a spiral course to the surface, where it opens into a minute depression, just visible to the naked eye — the sweat pore. The coiled portion of the gland is lined with' a simple cuboidal epithe- lium, having a granular protoplasm. In the smaller glands the epi- thelium rests directly upon the basement membrane. In the larger glands a longitudinal layer of smooth muscle cells separates the glandular epithelium from the basement membrane. The walls of the ducts consist of two or three layers of cuboidal epithelial cells, resting upon a delicate basement membrane, outside of which are longitudinally disposed connective-tissue fibres. On reaching the horny layer the epithelial wall of the duct ceases, the duct consisting of a mere channel through the epithelium. i6 THE ORGANS. TECHNIC. (i) Fix the volar half of a finger-tip in formalin-M tiller's fluid (technic 5, p. 5) or in absolute alcohol. Curling may be prevented by pinning to pieces of cork. Sections are cut transversely to the ridges, stained with haematoxylin-picro- acid-fuchsin (technic 3. p. 16), and mounted in balsam. Thick sections should be cut for the study of the coil glands with their ducts; thin sections for cellular de- tails of the layers. (2) Prepare m the same manner and for contrast with the preceding, sections of thin skin from almost any part of the body. (3) Prepare a piece of negro skin in the same manner and note the position of the pigment. The Nails. The nails are modified epidermis. Each nail consists of: (a) a body, the attached uncovered portion of the nail; (/;) &free edge, the anterior unattached extension of the body ; (c) the nail root, the pos- terior part of the nail which lies under the skin (Fig. 208). The nail lies upon a specially modified portion of the corium, the nail bed, which beneath the nail root and somewhat forward of the e d "-., V,.. ■' (S I ■-■■ - Fig. :■■>',. Longitudinal Section through Root of Human Nail and Nail lied. < 10. (Schaper.) , free edge; c, root of nail ; , derma; c, arrector pili muscle; d, se- baceous gland; e, outer root sheath; f, inner root sheath ; g, connective-tis- sue follicle ; //, vitreous membrane ; i, hair bulb;y', papilla ; s, epidermis. THE ORGANS, (i) The root sJicatJi 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). Henles 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). (/;) 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,^"). (b) 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. 350. (Kolliker.) n. Medulla; b. cortex; r, cuticle. THE SKIN AND ITS APPENDAGES. 321 As its name would indicate, this layer is especially rich in blood- vessels (Figs. 213, 214, and 215, i). (c) The outer layer consists of rather coarse, loosely woven bun- dles of white fibres, which run mainly in a longitudinal direction. Among: these are elastic fibres and a few connective-tissue cells. FlG. 213. — Longitudinal Section of Lower End of Root of Hair, including- Papilla. (Kolliker.) a, Root of hair ; b, cuticle of hair ; c, cuticle of root sheath ; • -^ a FIG. 214.— Transverse Section through Root of Hair and Hair Follicle. (K£ Hair; /;, hair cuticle; C, cuticle of root sheath; d, Huxley's layer; e, Henle' outer root sheath ; /, connective-tissue follicle. Hiker.) a, s layer ; /", 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. 323 . - ' " 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.— Prom Logitudinal Section through Hair and Hair Follicle. En- larged to Soo diameters. (Schafer.) A, Hair, a, Cortex of hair; l\ cu- ticle of hair, fi, Inner sheath, c, Cuticle of root sheath ; d, Huxley's layer ; e, Henle's layer ; f, outer root sheath ; £-, vitreous membrane; i, connective-tissue follicle ; m, fat cells. 3-4 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 pass over the papilla and turn upward to form the medulla of the hair; the stratum spinosum of the outer root sheath becomes continuous with the cortex 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 surface, 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 (technics, p. 5). 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 hsematoxylin-picro-acid- fuchsin (technic 3, p. 16) 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. 325 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) In tlic 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) In the derma ; Tactile corpuscles of Meissner and Wagner. These are found in the papillae, especially 326 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) /;/ 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. 20) 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 down 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. 327 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 328 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- Fig. 216. Prom Section of Human Inactive Mammary Gland, x 25. (Technic 1, p. 331.) 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. 329 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 r S^MJS?*^!?*? }•■■■■-■■ '■ ■■y:-:,i. i - : ■ : :--: -•- -' - - - " - ■&i£ ; V A IIP! yfWBBS^ FlG. 217.— From Section of Human Mammary Gland during' Lactation. X 50. CStohr.) Branch of excretory duct ; 6, 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 330 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.) ii, Basement membrane; />, 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. 331 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. 5). Stain sections with haematoxylin-eosin (technic 1, p. 16), 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 Menschen. Ranvier: Traite Technique d'Histologie. Schafer: Essentials of Histology. Spalteholz : Die Vertheilung der Blutgefasse in der Haut. Arch. Anat. u. Phys., Anat. Abth., 1S93. McMurrick : Development of the Human Body. CHAPTER XL 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 forcbrain (anterior cerebral vesicle — prosencephalon), the midbrain (middle cerebral vesicle- — mesencephalon), and the Iiiud- brain (posterior cerebral vesicle — rhombencephalon). From the an- terior cerebral vesicle are developed the cerebral hemispheres, the 332 THE NERVOUS SYSTEM. 333 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 epcndymal 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 the ancestors of the 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- 334 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 (/?) 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. 335 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-Muller's fluid (technic 5, p. 5) and stain sections with haematoxylin-eosin (technic 1, p. 16). 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.) .7, Ventral nerve root; fr, dorsal nerve root ; c, mixed spinal nerve; d, groups of ganglion cells; e, nerve fibres ; f, perineurium ; g, fat ; //, blood-vessel. capsule connective-tissue trabecules extend into the ganglion, forming a connective-tissue framework. Within the ganglion the nerve cells 336 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 ' ' • ' • i.- : :\ M h : ' : -pSS t-~p, wf * 3r-y FIG. 220.— Large Spinal Ganglion Cell from Human Spinal Ganglion showing Connective- tissue Capsule. (From Barker, after von Lenhossek.) s, Capsule; />, peripheral zone of 1 [ear cytoplasm ; />', axone hill ; «, axone ; //, 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 mcdullated fibres. These remain within the ganglion and end in dense felt works around THE NERVOUS SYSTEM. 337 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 ; F, communicating branch to sympathetic ; a, spinal gaDglion 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 ; o, spinal ganglion cell of second type, the axone of which (u) ends in a pericellular network around the bodies of cells of the first type ; .f, sympathetic fibres ending in plexuses around the bodies of cells of the second type. 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 351. The Sympathetic Ganglia. — The larger ganglia resemble the spinal ganglia in having a connective-tissue capsule and framework. 338 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-AIiiller's fluid (technic 5, p. 5). Stain sections with haematoxylin-eosin (technic 1, p. 16), or with lnematoxylin- picro-acid fuchsin (technic 3, p. 16). (2) Fix spinal and sympathetic ganglia in absolute alcohol or in ten-per-cent formalin, and stain sections by Nissl's method (technic, p. 28). (3) See also technic 1, p. 356. 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. 353, 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 dendrites of cell bodies situated in the spinal ganglia (see p. 347 and Figs. 227 and 236). These leave the ganglion and join with the fibres of the motor root to form the mixed spinal nerve (Fig. 227,/). The connection 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). 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. 339 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 shcatJi or perineurium. From the latter, delicate strands of connective tissue pass into the fascicle, separating the individual nerve fibres. This constitutes the intrafascicular con- FlG. 222.— From Transverse Section of Human Nerve Trunk. (Osmicacid fixation.) (Ouain.) ep, Nerve sheath or epineurium surrounding the entire nerve and containing blood-ves- sels (z>) 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. ncctivc 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. 108). For description of medullated and non-medullated nerve fibres see pages 107 and 108. For sensory nerve terminations see page 348 ; for motor nerve terminations see page 353. 14-0 THE ORGAXS. 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. 5). Stain transverse sections in hasmatoxylin-picro-acid fuchsin (technic 3, p. 16) 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 eulaigc- tnents. 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 31 spinal nerves; 8 cervical, 12 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. (i) 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 flatten 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, wasli in water to which a little formalin has been added, harden in THE NERVOUS SYSTEM. 341 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 haematoxylin-picro-acid fuchsin (technic 3, p. 16) and another section through the same level in Weigert"s hematoxylin (technic p. 25J. 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 tt % / Fig. 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 ; d, lateral column ; e, anterior column ; /, cell groups of anterior horn ; g, poste- rior horn ; //, posterior root fibres; /, Clarke's column and fibres entering it ; J, 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 J, 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. 342 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 column and 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. 343 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 -act 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 sJieath (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 Lissaucr. 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 the plexus 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. 340, remove small seg- ments from each of the following levels : (1) the twelfth dorsal, (2) the mid-dorsal. 344 THE ORGANS. and (3) the cervical enlargement. The segments are embedded in celloidin, sec- tions cut 15 to 20" thick, stained by Weigert's method (page 25), 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 ; />, 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 arc 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 arc even more slender THE NERVOUS SYSTEM. 345 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; 6, Clarke's column. Section through the Cervical Enlargement (Fig. 226). — Note the marked increase in size of the cord, which affects both gray 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; b, Clarke's column: c, septum between column of Goll and column of Burdach. 346 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 Burdach. Origin of the Fibres which Make u 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 J ganglion cells). cord. (Extrinsic cells.) \ (2) Cells in other parts of the central nervous sys- [ tern (the brain). ( (3) Root cells, such as those of the anterior horn, whose axones form the ventral root. (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 cctodcrmic 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. 347 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 FlG. 227. — Transverse Section through Spinal Cord and Posterior Root Ganglia of an Embrvo 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 (6); 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/) out of the cord as the ventral root fibres (e) which join the peripheral processes of the spinal gan- glion cells to form the mixed spinal nerve (/); col, collateral from axone of ventral horn cell. Dendrites of the anterior horn cells are seen crossing the median line in the ante- rior commissure. About the centre of the cord is seen the central canal ; dorsal and ven- tral to the latter some ependymal cells stretching from the canal to the periphery of the cord. development the two processes of the bipolar cell approach each other and in the adult are connected with the cell body by a single process. The adult spinal ganglion cell is thus apparently a unipolar cell, its single process dividing and sending one arm toward the per- iphery, the other toward the spinal cord. Entirely analogous to the spinal ganglia are the ganglia of the sensory cranial nerves, an exception to the unipolarity of the ganglion cell being found in the acoustic ganglia, where in man, and in mam- mals generally, the bipolar condition remains throughout life. The PERIPHERAL ARMS OF THE SPINAL GANGLION CELLS make up the sensory or afferent portions of the spinal nerves. The modes 343 THE ORGAXS. of termination of these peripheral processes are extremely varied and complicated. These peripheral terminations are always free, in the sense that, while possibly sometimes penetrating cells, they probably never become directly continuous with their protoplasm. In the skin, and in those mucous membranes which are covered with squamous epithelium, the nerve fibres lose their medullary sheaths in the subepithelial tissue, and, penetrating the epithelial layer, split up into minute fibrils which pass in between the cells and terminate there, often in little knob-like swellings (Fig. 228). In addition to such comparatively simple nerve endings, there are also found in the skin and mucous membranes, especially where sensation is most acute, much more elaborate terminations. These may be classified as (i) tactile cells, (2) tactile corpuscles, and (3) end bulbs. A simple tactile cell is a single epithelial cell, the centrally di- rected end of which is in contact with a leaf-like expansion of the Fig. 228.— Free Endings of Afferent Nerve Fibres in Epithelium of Rabbit's Bladder, (ket- zius.) o, Surface epithelium of bladder; bg, subepithelial connective tissue; n, nerve fibre entering epithelium where it breaks up into numerous terminals amony the epithe- lial cells. nerve terminal, the tactile meniscus. In the corpuscles of Grandry, found in the skin of birds, and in Merkel's corpuscles, which occur in mammalian skin, several epithelial cells are grouped together to receive the nerve terminations. These are known as compound tac- THE NERVOUS SYSTEM. 349 tile cells, the axis cylinder ending in a flat tactile disc or discs be- tween the cells. Of the tactile corpuscles (Fig. 229) those of Meissner, which occur in the skin of the fingers and toes, are the best examples. These corpuscles lie in the papillae of the derma. They are oval bodies, Fig. 229. Fig. 230. Fig. 229.— Tactile Corpuscle of Meissner, tactile cell and free nerve ending. (Merkel-Henle.) ti, Corpuscle proper, outside of which is seen the connective-tissue capsule ; b, fibre end- ing on tactile cell ; c, fibre ending freely among epithelial cells. FIG. 230.— Taste Bud from circumvallate papilla of tongue. (Merkel-Henle.) a, Taste pore ; b, nerve fibres entering taste bud and ending upon neuro-epithelial cells. On either side fibres ending freely among epithelial cells. (See also page 448.) surrounded by a connective-tissue capsule and composed of flattened cells. From one to four medullated nerve fibres are distributed to each corpuscle. As a fibre approaches a corpuscle, its neurilemma becomes continuous with the fibrous capsule, the medullary sheath disappears, and the fibrillar pass in a spiral manner in and out among the epithelial cells. Of the so-called end bulbs, the simplest, which are found in the mucous membrane of the mouth and conjunctiva, consist of a central core formed by the usually more or less expanded end of the axis cylinder, surrounded by a mass of finely granular, nucleated proto- plasm — the inner bulb — the whole enclosed in a capsule of flattened connective-tissue cells. More complicated are the Pacinian bodies found in the subepithelial tissues of the skin and in many other organs of mammalia. The Pacinian bodies (Fig. 231) are laminated, THE ORGANS. elliptical structures which differ from the more simple end bulbs already described, mainly in the greater development of the capsule. The capsule is formed by a large number of concentric lamellae, each lamella consisting of connective-tissue fibres lined by a single layer of flat connective-tissue cells. The lamellae are separated from one another by a clear fluid or semi-fluid substance. As in the simpler end bulbs there is a cylindrical mass of protoplasm within the capsule known as the inner bulb. Extending lengthwise through the centre of the inner bulb, and often ending in a knob-like extremity, is the axis cylinder (Fig. 231). In voluntary muscle afferent nerves terminate in Pacinian corpuscles, in end bulbs, and in complicated end organs called muscle spindles, or neuromuscular bundles. The' muscle spindle (Fig. 232) is an elongated, cylindrical structure within which are muscle fibres, connec- tive tissue, blood-vessels, and medullated nerves. The whole is enclosed in a con- nective-tissue sheath which is pierced at various points by nerve fibres. A single spindle contains several muscle fibres and nerves. According to Ruffini, there are three modes of ultimate terminations of the nerve fibrils within the spindles : one in which the end fibrils form a series of rings which encircle the in- dividual muscle fibre, he calls annular terminations; a second in which the nerve fibrils wrap around the muscle fibres in a spiral man- ner — spiral terminations ; a third in which the terminations take the form of delicate expansions on the muscle fibre — arborescent termina- tions. At the junction of muscle and tendon are found the elaborate afferent terminal structures known as the muscle-tendon organs of Golgi. In heart muscle (Fig. 233) and in smooth muscle (Fig. 234) the nerves of the sympathetic system end in fine feltworks of fibres, Fig. 231. — Pacinian Body from Mesentery of Cat. (Ranvier.) c. Lamina of capsule; d, epithe- lioid cells lying between lamina of capsule; «, nerve fibre, con- sisting of axis cylinder sur- rounded by Henle's sheath, leaving Pacinian body : f, peri- neural sheath; m, inner bulb; «, terminal fibre which breaks up at a into irregular bulbous terminal arborizations. THE NERVOUS SYSTEM. 351 which are in relation with the muscle cells. Satisfactory differen- tiation between efferent terminals and afferent terminals in heart and in smooth muscle has not yet been made. In organs whose parenchyma is made up of so-called glandular epithelium, the sympathetic nerves terminate mainly in free endings F A A FIG. 232. — Middle Third of Muscle Spindle from Striated Voluntary Muscle Fibre of Cat. (From Barker, after Ruffini.) A, rings; S, spirals ; F, dendritic branchings. which lie in the cement substance between the cells, thus coming in contact with, though not penetrating, the epithelial cells. It is important to bear constantly in mind the fact that these nerve terminals, however complicated, are in no sense nerve centres like the ganglion cells, but merely more or less elaborate end arborizations for the purpose of receiving impulses. Because of the fact that it transmits the impulse toward its cell of origin, as well as be- cause of certain other facts, Van Gehuchten considers this peripheral arm of the spinal ganglion cell of the nature of a protoplasmic process. The Centrally Directed' Arm of the Spinal Ganglion Cell. — According to Van Gehuchten this represents the true axone. It enters the spinal cord as one of the fibres of the posterior root, the entire bundle of posterior root fibres of a single spinal nerve consisting of all the central axones of the corresponding spinal ganglion (Fig. 227). Having entered the cord, the axone divides in the posterior columns into an ascending arm and a descending arm, [G. 233. — Nerve Endings on Heart Muscle Cells. (From Barker, after Hu- ber and De Witt.) 352 THE ORGANS. and these ascending and descending arms of the central processes of the cells of the spinal ganglia constitute the great majority of the fibres of the posterior columns. The descending arm is usually short, sends off branches known as collaterals into the gray matter of the cord, and itself terminates there at no great distance below its point of entrance into the cord. The ascending arm may behave in A. a I - "Ctai-,.,, Fig. 234.— Nerve Endings on Smooth Muscle Cells. (From Barker, after Huber and De Witt.) (/, Axrs cylinder ; b, its termination ; //, nucleus of muscle cell. a similar manner, passing up the cord but a short distance, where, after sending collaterals into the gray matter, it also terminates in the gray matter of the cord (Fig. 237). Instead of being short it may be of considerable length, passing some distance up the cord before finally terminating in the gray matter. It may, as one of the long fibres of the posterior columns, continue into the medulla to end there in one of the posterior column nuclei. (2) Cells Situated in Other Parts of the Central Nervous System which Contribute Axones to the White Columns of the Cord. The most important of these are the cells of the motor area of the cerebral cortex. The axones of these cells pass down the cord, form- ing the direct and crossed pyramidal tracts (page 362). (3) Root Cells — Motor Cells of the Anterior Horn. These are large multipolar cells found at all levels of the cord and having analogues in the motor nuclei of the cranial nerves. They are most numerous in the cervical and lumbar enlargements. In cross sections of the cord, especially through the enlargements, a more or less definite grouping of these cells is evident (Figs. 223 THE NERVOUS SYSTEM. 353 and 226). These groups extend for varying distances up and down the cord, forming nuclei, each one of which corresponds to the inner- vation of a particular muscle or group of muscles. Two columns of nuclei are quite constant throughout the entire length of the cord. They are known, from the positions which they occupy, as the medial column and the intermedio-lateral column, and are related to the muscles of the trunk. At certain levels these columns may be divided into secondary columns. In the cervical and lumbar enlarge- ments other groups of nerve cells appear which are concerned in the innervation of the muscles of the extremities. They are known re- spectively as the cell column of the upper extremity and the cell col- umn of the lower extremity. The cell columns are best seen in the sections from different levels of the cord described on pages 341 to 345. The dendrites of these cells ramify in the gray matter, where they intermingle with the terminal ramifications and collaterals of sensory fibres and of fibres of the direct and crossed pyramidal tracts. Their axones pass out of the ventral horn, across the ventro-lateral column, and leave the cord as the anterior, motor, or efferent roots of the spinal nerves (Fig. 227, e). The fibres of this root pass by the spinal ganglion without entering it, and beyond join the fibres from the ganglion to form the mixed spinal nerve. On their way to the muscles the motor axones may bifurcate several times, thus allowing one neurone to innervate more than one muscle fibre. In the peri- mysium the nerve fibres undergo further branching, after which the fibres lose their medullary sheaths and pass to the individual muscle fibres. Here each fibre breaks up into several club-like terminals which constitute the motor end plate. The location of the end plate, whether within or without the sarcolemma, has not been determined. As a rule each muscle fibre is supplied with a single end plate, though in large fibres there may be several. These neurones whose bodies are situated in the anterior horn and whose axones are the motor fibres of the spinal nerves, together with the analogous neurones of the cranial nerves (see page 368), con- stitute the peripheral motor or efferent neurone system. (4) Column Cells. These are cells which lie in the gray matter of the cord and send their axones into the white matter where they form columns of nerve 23 354 THE ORGANS. fibres. Some of the cells send their axones into the white matter of the same side of the cord. These are known as tautomeric cells (Fig. 235, E). Others send their axones as fibres of the anterior commis- sure to the white matter of the opposite side of the cord — heteromeric cells. In still others the axone divides, one branch going to the white matter of the same side, the other to the white matter of the opposite side — hecatcromeric cells (Fig. 235, A, B, C). The axones of many of these cells are short, constituting the short fibre tracts (fundamental columns — ground bundles) of the cord (see dorsal FlG. 235.— Cross Section through .Spinal Cord of Embryo Chick of Eight Days' Incubation. (Kdlliker, after Raym6n y Cajalj A, Hecateromeric cell with axone sending off side fibril to gray matter and then dividing, one branch passing to the white matter of the same side, d, the other through the ventral commissure to the white matter of the oppo- site side, a and d. B and C", Hecateromeric cells of the dorsal gray matter; the axones divided, one branch, a, passing to the dorsal while columns of the same side, the other, c, through the anterior commissure to the opposite side of the cord. /), Tautomeric cell, tn'' axones branching, but all brandies passing to the gray matter or white matter of the same side of the cord. /T, Tautomeric cell of the ventral horn with axone dividing into two branches, a and d, in the white matter of the same side. page 364); others are long (Gowers 1 tract and the direct cerebellar tract), passing up through the cord and medulla to higher centres (see page 361). From these axones, terminals and collateral branches are constantly re-entering the gray matter to end in arborizations around the nerve cells (Fig. 237). THE NERVOUS SYSTEM. 355 = Black. = Violet, o' Bniuiuuiimii = Blue. e = Brown f - = Green. Flr>. 236. — Scheme of the Xeurone Relations of the Spinal Cord ; Nerve Cells shown in Right Half of Cord ; Spinal Ganglion, Nerve Fibres, and Collaterals shown in Left Half of Cord. (From Barker, after von Lenhossek). The color scheme of the original is indicated by variations in shading (see explanation below and to the left of cut). Rig/it Side of Cord. — Black- two motor cells whose axones after giving off short side fibrils within the gray matter leave the cord as fibres of the ventral root, R. v. AV, cells of ihe posterior horn ; ) to the nucleus dorsalis or col- umn of Clarke ; (c) to the gray matter of the ventral horns, where they end around the motor cells ; (d) through the posterior commis- sure to the gray matter of the opposite side (Fig. 236). The neurones above described whose cell bodies lie in the spinal (and cranial — see medulla) ganglia, whose peripheral processes with their end organs constitute the receptive apparatus, and whose cen- tral processes terminate in the gray matter of the cord and medulla, constitute the peripheral sensory or afferent neurone system. II. The Direct Cerebellar Tract {Dorso- lateral Ascending Tract — Dorso- lateral Spino- cerebellar Fasciculus- — Tract oj Flcchsig). — This tract lies along the dorso-lateral periphery of the cord, being bounded internally by the crossed pyramidal tract (Fig. 236, 4, and Fig. 238, o). The fibres of the direct cerebellar tract are the axones of the cells of Clarke's column (p. 355, Fig. 236). These axones cross the intervening gray matter and white matter of the same side (tautomeric cells) (Fig. 236) and turn upward as the direct, cerebellar tract. In the medulla they pass into the restiform body or inferior cerebellar peduncle and thence to the cerebellum. Here they enter the gray matter of the vermis of the same or opposite side, ending in ramifications among the nerve cells. Some fibres either end in, or send off collaterals to, the cerebellar nuclei. The tract first appears in the upper lumbar cord, and increases in size until the upper limit of Clarke's column has been reached (page 344). As already noted above, some fibres of the posterior root (cen- tral processes of spinal ganglion cells), or their collaterals, end in the column of Clarke. The neurones whose cell bodies form Clarke's column, and whose axones constitute the direct cerebellar tract, are therefore a second neurone system in the sensory conduction path. III. Gowers' Tract {Antero- lateral Ascending Tract — Fascicu- lus Ventro-lateralis Superficial is). — This tract lies along the per- iphery of the cord, extending from the anterior limit of the direct cerebellar to the exit of the ventral roots (Fig. 236, 3, and Fig. 238, 10). It is formed by axones of neurones whose cell bodies are scattered through the central gray matter without any distinct group- ing (Fig. 22,6). Some fibres come from tautomeric, others from 362 THE ORGANS. heteromeric cells. The tract first appears in the upper lumbar cord and naturally increases in size as it passes upward. It appears to be formed partly of spinal association fibres, partly of fibres which pass to higher centres. The exact paths which these fibres take after leaving the cord and their terminations are not positively determined. Some of them end in the cerebellum, others have been described as ending in the corpora quadrigemina, in the thalamus, in the substan- tia nigra, and in the nucleus lentiformis. It seems probable that these varying results of investigation are due to the fact that the tract of Gowers does not represent a single physiologically distinct system, but is composed of fibres having several different functions and destinations. Descending Fibre Tracts of the Cord. I. The Pyramidal Tracts. — (i) The Crossed Pyramidal Tract. — This is a large tract of fibres lying in the dorsal part of the lateral column (Fig. 236,5/ Fig. 238, /). It extends to the lowermost part of the cord. In the cervical and dorsal regions it is separated from the surface of the cord by the direct cerebellar tract. In the lumbar region the latter tract is no longer present and the crossed pyramidal comes to the surface. (2) The direct pyramidal tract, or tract of Tiirck, occupies a small oval area adjacent to the anterior median fissure (Fig. 236, /; Fig. 238, 2). It decreases in size as the lower levels of the cord are reached, to disappear entirely in the middle or lower dorsal region. The pyramidal tracts vary greatly in size in different individuals and are apt to be asymmetrical, this being due to the lack of uni- formity as to the number of fibres which cross over in the pyramidal decussation (see page 363). These two tracts constitute the main motor or efferent fibre- system of the cord. The cell bodies of the neurones whose axones make up this system are situated in the cerebral cortex near the fis- sure of Rolando. Their axones converge and pass downward through the internal capsule, crura cerebri, pons, and medulla, sending off fibres to the motor nuclei of the cranial nerves. In the medulla the tracts come to the surface as the anterior pyramids. At the junction of medulla and cord occurs what is known as the pyramidal decus- THE NERVOUS SYSTEM. 363 sation (Fig. 241). Here most of the fibres of each tract cross to the opposite dorso- lateral region of the cord and continue downward as the crossed pyramidal tract. The minority of the fibres, instead of de- cussating, remain on the same side to pass down the cord along the anterior median fissure as the direct pyramidal tract. As these tracts descend they decrease in size from loss of fibres which con- tinually leave them to terminate in the ventral horns. The fibres of the crossed tract terminate mainly in the horn of the same side, while most of the fibres of the direct tract cross through the an- terior commissure to the opposite side of the cord. These tracts are thus mainly crossed tracts, as the great majority of their fibres cross to the opposite side of the cord. The tracts are apt to differ in size on the two sides of the cord, owing to the fact that the propor- tion of fibres which decussate is not constant. The axones termi- nate in arborizations around the motor cells of the ventral horns, thus constituting the corticospinal motor neurone system. It will be remembered that the neurones whose cell bodies are situated in the ventral horns constitute the spino-pcripheral motor neurone sys- tem. The two systems taken together form the cortico-spino-per- ipheral motor conduction path. II. The Antero-lateral Descending Tract. {Anterior Marginal Bundle of Locivcnthal.) — 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 the exact paths which these fibres follow in passing from the cerebellum to the cord. III. Von Monakow's Tract. {Rubro-spinal 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, p). IV. Helweg's tract is a small triangular bundle of fibres lying along the ventro-lateral margin of the cord, and is traceable upward as far as the olives (Fig. 238, J a). The origin and destination of its fibres are not definitely known. V. The Septo-marginal Tract. {Oval Bundle of Flechsig.)— This is a small bundle of fibres lying next the posterior septum (Fig. 238, sm). It is probably composed of descending axones of cells in the cord. 364 THE ORGANS. VI. 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). 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. 236 and 238). 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 short-fibre systems of the cord. 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 fibres as axones of cells of the gray matter, and the manner in which they re-enter the gray matter as terminals and collaterals have been considered (page 354.) From the neurones thus far studied and the tracts which their axones follow, we may determine the following general impulse path- wax' s in the cord : (1) The Direct Reflex rath (Fig. 239). — (a) The peripheral sen- sory neurone ; its peripheral process and end organ, the spinal gan- glion cell, its centra] process with collaterals terminating around motor cells of anterior horn ; {/>) the peripheral motor neurone ; motor cell of anterior horn with axone passing to muscles, etc. This is a two- neurone reflex path, chiefly uncrossed, and in most cases involving only closely adjacent segments. THE NERVOUS SYSTEM. 365 (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- 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. (3) Direct Ascending- Paths to Higher Centres. — The peripheral sensory neurone as in the direct reflex, but with central process pass- ing as fibre of Goll or Burdach to the nucleus of one of these columns in medulla (Fig. 242). (4) Indirect Ascending Paths to Higher Centres. — (a) Peripheral sensory neurone as in direct reflex, but communicating in cord with FlG. 239. — Diagram Illustrating Path followed by and Neurones involved in a Simple Direct Reflex. (Van Gehuchlen ) 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. column cells of direct cerebellar and of Gowers' tracts. (/>) 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- 3 66 THE ORGANS. 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 FIG. 240. — Diagram Illustrating Pathway of Compound Keflex (Van Gehuchteti) 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. From 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. horn of the same side, those of the direct tract in the horn of the op- posite side. (/>) 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 363, by which an impulse may pass from higher centres to the cord. TECHNIC. ii) A human cord from a case in which death has occurred some lime after fracture of the vertebrae with resulting crushing of the cord, furnishes valuable but oi 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 Weigerl (page 25). Thepicturein (he cord isdependent upon the fact that axones 1 hi rill from their cells of origin degenerate and are finally replaced by connective tissue. After a complete transverse lesion of the cord, therefore, all ascending tra< is are found degenerated above the lesion, all descending tracts below the le- 1 Marchi's solution consists of two parts M tiller fluid and one pari one-per- cent aqueous solution osmic acid. /Alter hardening for from seven to ten days in Muller's fluid, thin slices of tissue are transferred to the Marchi solution, where "they remain lor alinut the same length of time. Sections are usually mounted without further staining:, in balsam. THE NERVOUS SYSTEM. 367 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 norma! 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 26). (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 spinai 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 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 anterolateral 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 cr 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. o 68 THE ORGANS. 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 mainly in the fillet and restiform body; the direct cerebellar tract passes into the res tiform body, while the tract of Gowers continues as such through the medulla. Of the descending tracts, the most important, the direct and crossed pyramidal tracts, are represented in the medulla by the ante- rior pyramids. Of the spinal gray matter, there are still remnants 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 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 THE NERVOUS SYSTEM. 369 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 does 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 neu- rones 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 340). Trans- verse sections should be cut through the following typical levels, stained by Wei- gert's method (page 25), 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. q. Through the exit of the eighth cranial nerve. 6. Through the exits of the sixth and seventh cranial nerves, 24 6/' THE ORGANS. 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 345 1 and note the following structures studied in the cord sections : 1. The posterior column: (a) The column of Goll (funiculus gracilis), and (/;) the column of Burdach (funiculus cuneatus) remain as in the cervical cord. 2. The lateral column : (a) The crossed pyramidal tract is smaller owing to the fact that fewer fibres have crossed to it from the ante- 4 , 1 rior pyramid (see 8, p. 371); (b) the tract of Gowers, (c) the direct cerebellar tract, and ((/) von Monakow's bundle occupy about the same positions as in the cervical cord (Fig. 238). (While the general locations of these lateral column tracts should be noted, they cannot be differentiated in the normal adult human medulla.) 3. The anterior column; in- creased in size. This is due to the fact that fewer fibres have left it to decussate and enter the crossed pyramidal tracts (see 8, p. 371). Lateral to the pyra- midal tract are: (a) The tract of Melweg, (/;) the sulco-marginal tract, and (r) the anterior ground bundles, occupying about the same positions as in the cervical cord. (These subdivisions of the anterior column cannot usually be dis- tinguished 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 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. FIG. 241.— Transverse Section of the Medulla at the Level of the Pyramidal Decussation. (Dejerine.) /, Posterior column; /a, col- umn of Goll; id, column of Burdach; 2, lateral column ; j, anterior column ; 4, pos- terior horn ; 5, anterior horn ; 7, reticular formation ; S, decussation of the pyramids ; g, dorsal root of first cervical nerve; /o, gelatinous substance of Rolando ; x, neck of posterior horn. THE NERVOUS SYSTEM. 371 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 lateral gray matter by longitudinal fibres- — mainly continuations into the medulla of the lateral fundamental column fibres of the cord. 8. Decussation of the pyramids. This is the most important fea- ture 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 cerebral 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. 362, and Fig. 236,5; Fig- 238, /). A few remain in their original anterior po- sition to continue down the cord as the direct pyramidal tract (p. 362, and Fig. 236, /; Fig. 238, 2). The bundles of fibres do not cross in a transverse plane, but take a downward direction at the same time. For this reason transverse sections show these fibres cut rather obliquely. Because of the fact that the fibres cross in alternate bundles, the number of decussating fibres seen in any one section is greater on one side than on the other (Fig. 241). 9. 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. The posterior column. Both the column of Goll ( 2 4-) These ^ IG- 2 44- — Diagram of Origin of Cranial Nerves X and XII. (Schafer.) pyr. Pyramid; o, olivary nucleus; ;-, 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 accessor}- ; 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. ?i, cell of nucleus of solitary fasciculus sending axone as internal arcuate fibre to opposite side of cord (secondary vagus and glosso-pharyn- geal tract). 376 THE ORGANS. 3. Transverse Section of the Medulla through the Lower Part of the Olivary Nucleus (Fig. 245). Note the following already mentioned structures : 1. The posterior column, which has almost disappeared, its fibres having passed into the posterior column nuclei. 2. The lateral column. This still contains Gowers' tract, the direct cerebellar tract, and von Monakow's bundle. jrxn: FlG. 245.-Transver.se Section of the Medulla through the Lower Part of the Olivary Nucleus. (Dejerine.) /, Posterior column ; 2, lateral column; 3, pyramid ; SgR, gelatinous sub- stance of Rolando and remains of posterior horn ; j, remains of anterior horn ; A'C'/, nu- cleus of the posterior [column ; /?, internal arcuate fibres; /j, sensory decussation; 74, fillet ; /j, spinal root of fifth cranial nerve ; 77, accessory olivary nuclei ; /y a, dorsal acces- sory olivary nucleus; /,?, arcuate nucleus; jq, solitary fasciculus; JVxfi, nucleus of twelfth cranial nerve; A'//, root fibres of twelfth cranial nerve; posterior longitudinal fasciculus just ventral to Xxii but not distinguishable from the fillet; 22, external arcu- ate fibres; x, cerebello-olivary fibres ; 23, restiform body ; .?,/, olivary nucleus ; sj, fourth ventricle; 26, dorsal nucleus of ninth ami tenth nerves; 27, nucleus ambiguus; fio, oli- vary fibres. 3. The anterior column (anterior pyramid) ; now consists almost wholly of pyramidal tract fibres. 4. The posterior horn ; somewhat diminished in size. 5. The anterior horn. This is now largely lost in the reticular formation, part of the gray matter of which is its upward continu- ation. 6. The centra] canal; now opening into the fourth ventricle, THE NERVOUS SYSTEM. 377 the gelatinous substance and the nuclei of the floor of the ventricle constituting the central gray matter. 7. The reticular formation ; occupying a much larger area than in the preceding section (between SgR and median line). 11. The nuclei of the posterior column {NCp) ; diminished in size and not clearly defined. 12. The internal arcuate fibres ; more numerous. 13. The sensory decussation or decussation of the fillet; now more extended dorso-ventrally, forming the median raphe. 14. The fillet or median lemniscus; larger, more of the decus- sating fibres having now joined it. 15. The spinal root of the fifth cranial nerve (trigeminus); larger, as fewer fibres have left it to terminate in the gray matter. 17. The accessory olivary nucleus ; smaller than in the preceding section. 18. The arciform nucleus. 19. The solitary fasciculus (see p. 374, 19). 20. The nucleus of origin (Alrii) of the twelfth cranial nerve (hypoglossal) (see p. 374, 20) and the root fibres of the nerve (XII) ; passing along the lateral margin of the fillet and thence to the sur- face between the olivary nucleus and the anterior pyramid (Fig. 245). 21. The posterior longitudinal fasciculus; now more dorsal and not easily differentiated at this level from the fillet. 22. The external arcuate fibres; more numerous than in the pre- ceding section. Some of the more dorsal of these fibres are fibres of the direct cerebellar tract passing to the restiform body. 23. The restiform body; larger and not extending as far ven- trally. (For details see p. 380, 23.) 24. The olivary nucleus. This is now an irregularly convoluted lamina of gray matter, dorso-lateral to the anterior pyramid. Note the fibres which pass as internal arcuate fibres from each olive through the median raphe to the opposite restiform body (Fig. 245, .1) and thence to the cerebellum (cerebello-olivary fibres). Some of these latter are probably ascending axones from cells in the olivary nucleus ; others are probably descending axones from cells in the cerebellum. Fibre tracts also connect the olives with the cord, passing through the ventral and lateral ground bundles. It is uncertain whether these are descending or ascending fibres, or both (see Fig. 264, Neurone No. 8). 378 THE ORGANS. Note the following new structures : i j a. Dorsal accessory olivary nucleus. 25. The fourth ventricle, or cavity of the medulla into which the central canal has now opened. 26. The dorsal nucleus of the ninth 1 glossopharyngeal) and tenth (vagus) cranial nerves; a group of cells lying just to the outer side of the nucleus of the twelfth nerve. The dorsal part of the nucleus belongs to the ninth, the ventral to the tenth nerve (Fig. 245). 27. The nucleus ambiguus, motor nucleus of the ninth and tenth cranial nerves; often difficult to distinguish, lies in the lateral part of the reticular formation. From the cells of this nucleus fibres pass dorsally to just below the sensory nuclei of their nerves, where they turn sharply ventro-laterally and join the sensory root fibres (Fig. 244). 28. The root fibres of the tenth cranial nerve (vagus) (Fig. 245, X). 4. Transverse Section of the Medulla through the Middle of the Olivary Nucleus (Ninth and Tenth Nerves) (Fig. 246). The following structures present in the last section have now disappeared : 1. The posterior column. 5. The anterior horn. 6. The central canal. 1 1. The nuclei of the posterior column. 13. The sensory decussation. Note the following structures seen in last section : 2. The lateral column. The direct cerebellar tract is now a part of the restiform body. (lowers' tract and von Monakow's bundle oc- cupy about the same positions as in the preceding section. 3. The anterior pyramid ; remains the same. 4. The posterior horn ; smaller and more vague. 7. The reticular formation (SRg)\ increased in extent, reaching its maximum in this and the next succeeding level. 12. The internal arcuate fibres ; no longer derived mainly from the posterior column nuclei; being now largely decussating fibres from sensory cranial nerve nuclei and from other nuclei in the reticu- THE NERVOUS SYSTEM. 379 lar formation. Many internal arcuate fibres also represent cerebello- olivary fibres connecting the restiform body with the opposite olive. 14. The fillet, or median lemniscus (SRa); now completely formed and much extended dorso-ventrally. While the fillet must be regarded as the main continuation brainward of the spinal sen- sory conduction path, other fibres enter into its formation. Thus we find in the fillet axones of cells situated in the reticular formation ITrair- FlG. 246.— Transverse Section of the Medulla through the Middle of the Olivary Nucleus. (Dejerine.) 2, Lateral column ; 3, pyramid; 4, gelatinous substance of Rolando and re- mains of posterior horn; Sftg, reticular formation; J2, internal arcuate fibres ; S/?j, fillet; jj, spinal root of fifth cranial nerve; jy, accessory olivary nucleus; iS, arciform nucleus; jq, solitary fasciculus; Nxii, nucleus of origin of twelfth cranial nerve; from the nucleus fibres can be seen passing ventrally just to the mesial side of the olivary nu- cleus ; posterior longitudinal fasciculus just ventral to jo but not distinguishable from the fillet ; 22, external arcuate fibres ; 2j, restiform body : 25, fourth ventricle ; 26, dorsal nucleus of ninth and tenth cranial nerves ; 2j, nucleus ambiguus ; A", root fibres of tenth nerve; Nviiir, spinal root of vestibular division of eighth nerve ; jo, nucleus of funiculus teres ; Cj, gray matter adjacent to the restiform body, sometimes called the corpus j'uxta- restiforme ; Nr, nucleus of the reticular formation; .r, nucleus of the restiform body; c, choroid plexus. of the medulla and of the pons, also axones from the nuclei of termi- nation of the sensory cranial nerves. The termination of the fillet is also very complex. Though the majority of its fibres terminate in the nuclei of the thalamus, some may pass directly to the cerebral cortex, while still others end in the gray matter of the medulla (espe- cially of the olives), pons, midbrain, and hypothalamic region. 380 THE ORGANS. 15. The spinal root of the fifth nerve; larger, for the same reason as in the last section. 17. The accessory olive; may be present or absent. There may be a dorsal accessory olive just above the inner end of the main olivary nucleus. iS. The arciform nucleus; usually present. 19. The solitary fasciculus; now larger and more distinct. In some sections, some of the sensory root fibres of the ninth and tenth nerves can hi seen passing into the solitary fasciculus (see also p. 374, 19)- 20. The nucleus of origin of the twelfth cranial nerve (hypoglos- sal) {Xxii) ; about the same size as in the preceding section. From it are seen passing out the root-fibres of the twelfth nerve {XII). 21. The posterior longitudinal fasciculus; dorsal to the fillet and not distinguishable from the latter at this level. 22. External arcuate fibres. These may be seen running par- allel to the surface of the medulla just under the pia mater. 23. The restiform body; much larger than in the last section. Note along its lateral margin a narrow strip of gray matter, the nu- cleus of the restiform body (Fig. 246, .1). The restiform body or inferior cerebellar peduncle now contains fibres from the nuclei of the posterior columns of both the same and opposite sides (internal and external arcuate fibres) and from the col- umns of Goll and Burdach direct (p. 375, 22, p. 373, 1 1) ; fibres con- necting the olivary nucleus with the cerebellum (p. 377, 24) ; fibres which represent the continuation upward of the direct cerebellar tract (see diagram, Fig. 264). 24. The olivary nucleus ; larger than in the preceding section. (For details see p. 377, 24.) 25. The fourth ventricle; more widely open. Note its roof now formed by the choroid plexus. 26. The dorsal nucleus of the ninth and tenth nerves; about the same size, but nearer the ventricle (see p. 378, 26). 27. The nucleus ambiguus ; about the same as in the preceding section. 2.S. (.V ) Root fibres of the ninth and tenth cranial nerves. Note the following structures not present in the preceding section : 29. The descending or spinal root of the vestibular portion of the THE NERVOUS SYSTEM. 38 1 eighth cranial nerve (auditory) (Nviiir); in the lateral wall of the fourth ventricle. (For details see p. 383, Figs. 247 and 248.) 30. The nucleus of the funiculus teres. 5. Transverse Section of the Medulla through the Exit of the Eighth Nerve (Fig. 247). The following structures present in the preceding section have now disappeared : 17. The accessory olives; although a small dorsal or internal ac- cessory olive may be present. 19. The solitary fasciculus. 20. The nucleus of origin of the twelfth cranial nerve. 26. The dorsal nucleus of the ninth and tenth nerves. 27. The nucleus ambiguus. 28. The root fibres of the ninth and tenth nerves. 29. The spinal root of the vestibular portion of the eighth nerve. 30. The nucleus of the funiculus teres. Note the following structures seen in the preceding section : 2. The remains of the lateral column (containing Gowers' tract and von Monakow's bundle). 3. The anterior pyramid. 4. The remains of the posterior horn. 7. The reticular formation {SR). 12. The internal arcuate fibres. 14. The fillet. 15. The spinal root of the fifth nerve; increased in size. 18. The arciform nucleus (Narc). 21. The posterior longitudinal fasciculus. 22. The external arcuate fibres. 23. The restiform body; much larger, being now almost com- pletely formed. If the roof of the fourth ventricle and part of the cerebellum be included in the section, the restiform body can be seen passing into the cerebellum as its inferior peduncle. 24. The olivary nucleus; much reduced in size. 25. The fourth ventricle with the choroid plexus in its roof. The following new structures are to be noted : 31. {VIII) The root fibres of the eighth cranial nerve (audi- 382 THE ORGANS. tory) and its nuclei (see also Fig. 248). The auditory nerve is divided into two parts : the cochlear nerve and the vestibular nerve. The fibres of the cochlear root (l^IIIc) enter at a lower level than those of the vestibular. Some of them enter a nucleus ventral to the restiform body (ventral cochlear nucleus) (Nviii, c) ; the remainder 12 Tvfra. 3 4 Time FIG. 247. — Transverse Section of tiie Medulla through the Upper Fart of the Olivary Nucleus and Exit of the Eighth Cranial Nerve. CDejerine.) 2, Remains of lateral column; 3, pyramid ; 4. remains of posterior horn serving as terminal nucleus for spinal root of fifth nerve ; S/\. reticular formation ; 13, internal arcuate fibres ; /•,-, spinal root of fifth nerve ; .Wire, annate nucleus; .?/, posterior longitudinal fasciculus; 22, external arcuate fibres, mainly cerebello-olivary fibres; 2j, restiform body ; 24, olivary nucleus; 23, fourth ven- tricle ; VJlIc, cochlear root of the eighth cranial nerve ; VJIlv, vestibular root of eighth al nerve; Nviiic, ventral cochlear nucleus; XI), Deiter's nucleus; Xviiiv, median or principal vestibular nucleus ; /'//. root fibres of seventh cranial nerve; Nvii, nucleus of origin of seventh cranial nerve; /'/, root fibres of sixth cranial nerve; .,-y, acoustic striae; jy, transverse pontile fibres ; 37, central tegmental tract ; Nci, nucleus of the retic- ular I": mat ion ; Xr. nucleus of the median raphe ; Tr, trapezoid body. pass dorsal ward to a nucleus external to the restiform body (dorsal - cochlear nucleus, or nucleus of the aroustic tubercle) (seen only in lower sections of this level). THE NERVOUS SYSTEM. 333 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 Fig. 248.— Diagram of Origin of Eighth Cranial Nerve and some of its more Important Central Connections. (Obersteiner.) Cbfl, 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 ; VJI root fibres of seventh nerve ; Rl, cochlear root of eighth nerve (axones of cells in spinal ganglion or ganglion of Corti) passing to their terminations in the ventral cochlear nucleus, Xacc, and iu 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, XD, and in the median vestibular nu- cleus, Ntr (the nucleus of von Bechterew and the spinal vestibular nucleus are not seen at this level); Strm, stria? acustica? ; Tgm. tegmentum ; Os. superior olivary nucleus; Ost, fibres from superior olivary nucleus to nucleus of sixth nerve; Nt, trapezoid nucleus; dr. trapezoid body; Ltnl, lateral lemniscus or lateral fillet; Qa, anterior corpus quad- rigeminum ; Qp, posterior corpus quadrigeminum. {Xviii, v) ; and (J) the spinal vestibular nucleus which accompa- nies the descending fibres of the vestibular root (spinal eighth, see p. 380, 29). The fibres of the cochlear nerve are axones of bipolar cells in the spiral ganglion, or ganglion of Corti (see p. 439, Fig. 283). The 384 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. 387, 36). Some of the cochlear fibres pass both ventral and dorsal nuclei to end in the superior olivary and trapezoid nuclei. Axones from the cells 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. 386, 32.) 33. The root fibres of the sixth (abducens) cranial nerve (Fig. 247, 17) and its nucleus of origin. ( For details see p. 386, 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. 381, 31). $j. Transverse fibres of the pons Varolii ; crossing ventral to the pyramids (see p. 386, 35). 37. The central tegmental tract (see 37, p. 387). 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 : 2. The lateral columns as such ; Govvers' and von Monakow's tracts here lying in the ventral part of the tegmentum between the fillet and the root of the seventh nerve. 18. The arciform nucleus. THE NERVOUS SYSTEM. 385 22. The external arcuate fibres; unless the superficial transverse pons fibres be classed as arcuate fibres. 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 striae. Vllg VELt w^ 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 ; R//i, fillet ; /j, spinal root of fifth nerve ; 2/, posterior longitudinal fasciculus ; 23, restiform body or inferior cerebellar peduncle; 25, fourth ventricle; VIIIv, vestibular root of eighth cranial nerve; Nvii, nucleus of origin of seventh cranial nerve; VII7, root fibres of seventh nerve passing from nucleus of origin to floor of fourth ventricle ; VUg, transversely cut bundle of root fibres of seventh nerve ascending in floor of fourth ventricle ; IV/, root fibres of seventh nerve leaving medulla ; Nvi, nucleus of origin of sixth cranial nerve; IV, root fibres of sixth cranial nerve ; 33 bi, superficial transverse fibres of the pons ; 33 b2, deep transverse fibres of pons; j5, lateral lemniscus ; Pec, central tegmental tract ; Xci, nucleus of the reticular formation ; A r p, pontile nuclei ; TV, trapezoid body ; r, median raphe ; 38, supe- rior olivarv nucleus. The following structures are still present : ?. The tracts of Gowers and von Monakow. 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). 12. The internal arcuate fibres; rather indefinite. 14. The fillet (A?///), now called the median lemniscus to distin- 2 5 ;86 THE ORGANS. 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. 31. 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 '7). 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 (171). (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), 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 to the surface. 35. The pons Varolii. This occupies the ventral part of the sec- r///> y/' FIG. 250.— Diagram of Origin of Sixth and Seventh Cranial Nerves. (Schafer.) pyr, Pyramid ; cr, restiform body ; dV, spinal root of fifth nerve; Ventr.IV, fourth ventricle; VIII v, vestibular root of eighth nerve; n. VI, chief nucleus of sixth nerve ; n' VI, ac- cessory nucleus of sixth nerve; /'/, sixth nerve ; ;/. 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, aVII, and ascend to the "genu," K, where they turn ami pass ventro-laterally to the surface as the seventh nerve, VII. THE NERVOUS SYSTEM. 387 tion. It consists of longitudinal fibres, transverse fibres, and gray matter (pontile nuclei). (a) The pontile nuclei (Np) are masses of gray matter lying among the fibres of the pons. They are nuclei of origin of the transverse pontile fibres. {b) The transverse pontile fibres, or midlde peduncle of the cere- bellum, connect the pontile nuclei with the opposite cerebellar hemi- sphere. They are divided by the longitudinal fibres into (In), superficial transverse fibres and (#2) deep transverse fibres. (c) 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. Its fibres are mainly a sec- ondary cochlear tract, the axones of cells in the terminal nuclei of the cochlear nerve (see p. 381, 31). The fibres of the lateral lem- niscus terminate mainly in the gray matter of the anterior and pos- terior corpora quadrigemina, most of them in the corpora quadri- gemina of the same side, a few in the corpora quadrigemina of the opposite side. Some fibres of the lateral lemniscus probably pass both anterior and posterior corpora quadrigemina, to end in the cor- tex 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, semi- lunar nucleus, and trapezoid nucleus) are terminal nuclei for the sec- ondary cochlear fibres of the trapezium (7>). Some of the trans- verse fibres passing through the ventral part of the tegmentum are the decussating fibres of the secondary acoustic tract (see page 384) to the lateral fillet. 388 THE ORGANS. 7. Transverse Section Through the Exit of the Root Fibres of 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 cerebellum Nrt FlG. 251. — Transverse Section of the Medulla through the Exit of the Fifth Cranial Nerve. (Dejerine.) 3, Pyramidal fibres and longitudinal pontile fibres; 67?, reticular formation ; Rm, fillet; Tr, trapezoid body; sj, spinal root of fifth nerve; 21, posterior longitudinal fasciculus ; 2j, fourth ventricle ; jjl>, transverse pontile fibres ; jj bi, superficial transverse pontile fibres ; j; l>2, deep transverse pontile fibres ; Fee, central tegmental tract; Nrt, nucleus of the reticular formation ; Np, pontile nuclei ; jS, superior olivary nucleus ; /', 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;^/', transverse pontile fibres forming middle cerebellar peduncle ; Lip, middle lobe of cere- bellum ; -V, fibres passing to superior cerebellar peduncle. the lower sections of this level a small remnant of the posterior horn may be present. 3 1 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. 3 §9 2. The tracts of Gowers and von Monakow; 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 (AW). 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 reticu- lar formation and dorsal to the deepest of the transverse pontile fibres. 15. The spinal root of the fifth nerve (see p. 374, 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. 386, 35-) 36. The lateral lemniscus. (For details see p. 3S7, 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. {a) 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. (//) The fibres of the motor root leave the medulla just internal to those of the sensorv 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 39° 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 the 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 ; f.s.ii. V, principal sen- sory nucleus; d.s.n.V, descending sensory or spinal nucleus; d.s.l', 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. 391 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. 380, 23.) The middle peduncle (35 b) has been described in connec- tion with the transverse fibres of the pons (see p. 387, 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. 393, 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. 39 2 THE ORGANS. 3/ '. The central tegmental tract; no longer distinguishable. 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 crusta : 3. The pyramid ( VP). The numerous bundles of pyramidal fibres seen in the last section among the transverse pontile fibres _NR1 Fig. 253. — Transverse Section of the .Midbrain through the Exit of the Fourth Cranial Nerve. (Oejerine.) V ' P, Pyramid; 7, reticular formation; Rm, fillet; 21, posterior longitudinal fasciculus; 2y, iter; jb, lateral lemniscus; NRl, nucleus of lateral lemniscus; I'd, de- scending or mesencephalic root of fifth nerve ; 40, superior cerebellar peduncle ; 40 a, dor- sal decussation of superior peduncles ; 40 /», ventral decussation of superior peduncles ; AV, lateral nucleus, a band of gray matter lying between the superior peduncles and the fillet ; L)i, 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), including fibres to the motor nuclei of the cranial nerves; the mesial fifth, mainly by axoncs 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. 393 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. Govvers' tract and von Monakow's bundle. A part of the former has passed into the cerebellum. The part to the thalamus lies in the ventral part of the lateral lemniscus. Von Monakow's bundle lies just dorsal to the fillet. 7. The reticular formation ; much diminished in size and con- taining among other fibres ascending axones from cells of the fifth nerve nuclei (secondary trigeminal tract). 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. 36. The lateral lemniscus; occupying with its nucleus (XRI) the extreme dorso-lateral part of the tegmentum. 39 b. ( Vd) The descending motor or mesencephalic root of the fifth nerve. (For details see p. 390.) 40. The superior cerebellar peduncles or brachia conjunctiva. These occupy the greater part of the tegmentum and can be seen decussating in the median line. They are composed mainly of ax- ones from cells in the dentate nucleus and probably in other cere- bellar nuclei. Crossing to the opposite side in the decussation, these axones terminate mainly in the red nucleus. There are probably also axones in the superior peduncles, which come from cells situated in higher centres and which terminate in the cere- bellum. 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 {xIV). 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 394 THE ORGANS. medullary velum they turn dorso-mesially to form the above-men- tioned decussation, after which they pass to the surface. 42. The substantia nigra (see general description, p. 391). Transverse Section of the Midbrain through the Exit of the Third Cranial Nerve (Oculomotor) (Fig. 294). The following structures seen in the preceding section have dis- appeared : 39 b. The descending motor (mesencephalic) root of the fifth nerve. 41. The fourth cranial nerve. The following structures are still present : 2. Gowers' tract near lateral surface; those fibres which pass to the anterior corpus quadrigeminum. FlG. 254.— Transverse Section of the Midbrain through the Exit of the Third Cranial Nerve. (Dejerine.) j, Pyramid; 7, reticular formation of tegmentum; /y, fillet, /•"//, posterior longitudinal fasciculus ; 2,-, iter ; j\ lateral lemniscus : not marked, but lying just dorsal to most dorsal fillet fibres, //,• 40, superior cerebellar peduncle; Ln, substantia nigra; a, a, anterior corpora quadrigemina ; l>, brachium of anterior corpus quadrigeminum; Cgi, internal geniculate body ; ./_?, red nucleus; x, central gray matter ; /-'cop, lateral gray matter of tegmentum (superior lateral nucleus of Flechsig); xF, ventral tegmental de- cussation or decussation of Forel ; xAI, dorsal tegmental decussation or decussation of Meynert ; .\7//, nucleus o£ 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. 395 7. The reticular formation; less extensive. 14. The fillet; just dorsal to the substantia nigra. 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 (Lu), between the crusta and tegmentum. The following new structures are to be noted : 43. The anterior corpora quadrigemina (a, a). (For description see p. 396.) 44. The geniculate bodies, only the medial of which {Cgi) can be seen in the section; two masses of gray matter, lying, the mesial just dorsal, the lateral, dorso-lateral to the crusta. The lateral genic- ulate bodies are connected with the optic tracts (see Optic Nerve, p. 426). 45. The red nucleus; a large mass of gray matter lying between the substantia nigra and the posterior longitudinal fasciculus just to the outer side of the root fibres of the third nerve. The relation of this nucleus to the superior peduncles of the cerebellum was de- scribed in connection with the preceding section (p. 393, 40). From cells in this nucleus axones pass upward to higher centres and down- ward (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 {III). 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 terminate in the gray matter of the posterior corpus quadrigemi- num come mainly from the lateral lemniscus (for fibres which this 396 THE ORGANS. contains see p. 3S7, 36). From the cells of the gray matter of the posterior corpus quadrigeminum some axones descend in the lateral lemniscus; other axones ascend, joining the fibres of that part of the lateral lemniscus which passes by the posterior corpus quadri- geminum. These together form the inferior brachium quadrigemi- num 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 also serves as a terminal nucleus for some of the axones of the lateral lemniscus — i.e., for the secondary acoustic tract. The neu- rones whose cell bodies are situated in the anterior corpora quad- rigemina send their axones mainly downward. Their destinations are not fully known. Some appear to cross through Meynert's de- cussation (Fig. 254) to the opposite side, where they continue spinal- ward, giving off collaterals and terminals to the nuclei of the third, fourth, and sixth cranial nerves. Other axones pass downward on the same side, mingling with the fibres of the fillet and probably end in the pontile nuclei, thus bringing the corpora quadrigemina into connection with the opposite cerebellar hemisphere. Still other axones from the anterior corpora quadrigemina probably pass upward in the tegmentum to the thalamus. 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. 392, 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. 397 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 seiies 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, (/?) an external or molecular layer, and between these (c) a layer composed of a single row of very large cells, the layer of Purkinje cells. (i) The white 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 ccrebelli, may be subdivided into: (a) an internal, granular or nuclear layer; (//) 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 ; 9 8 THE ORGANS. of protoplasm (Fig. 255). Intermingled with these cells are meclul- 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. ( Boh m and von Davidoff.) /, Hlood-vessel ; 2, dendrite of Purkinje cell ramifying in mo- lecular layer ; _?, body of Purkinje cell at junction of molecular and granular layers; ./ 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, ', gray matter of cortex ; a, superficial tangential fibres ; d, deep tangential fibres ; b and c, intermediate bands of tangential fibres, b sometimes known as the outer line of Baillarger, c, as the inner line of Baillarger ; e, radiation fibres 'association, commissural, and projection fibres); j\ association fibres between the two adjacent convolutions. The cortex cerebri may be divided into three fairly distinct layers: (a) an outer, barren, or molecular layer, or layer of few THE NERVOUS SYSTEM. 403 nerve cells, (b) a middle layer, or layer of pyramidal cells, and (c) an inner layer, or layer of polymorplious cells. (a) The Barren or Molecular Layer (Fig. 261, A). — The nerve cells of this layer are known as the cells of Cafal. They are fusiform, triangular, or irregular in shape, and both their dendrites and axones ram- ify in this outer layer, the axones passing mainly in a direction parallel to the surface. This layer also con- tains the terminations of the apical dendrites of the pyramidal cells (Fig. 261, a), some medullated nerve fibres running parallel to the surface and known as the superficial tangential fibres (Fig. 260, a), and a rich plexus of neuroglia. (b) The Layer of Pyramidal Cells (Fig. 261, B and C). — This is often described as two separate layers, an outer layer of small pyramidal cells (B) and a deeper layer of large pyramidal cells (C). It seems bet- ter to describe it as a single layer composed mainly of small pyramidal cells, in the deeper portion of which the larger pyramidal cells are found. Each pyramidal cell has passing off from its outwardly directed angle a large apical or main dendrite (Fig. 261, d). This dendrite sends off small lateral twigs and terminates in numerous branches in the molecular layer. Smaller dendritic processes pass off from the sides and base of the cell. The axone (Fig. 261, e) originates from the base of the cell and enters the white matter of the corona radiata. During its passage FTG. 261. — From Vertical Transverse Section of Cerebral Cortex of a Mouse. Golgi method. (Ramon y Cajal.) A, Barren or molecular layer ; B, layer of small pyramidal cells ; C, layer of large pyramidal cells ; D, layer of polymorphous cells ; jS 1 , white matter; a, dendritic ramifi- cations of p3Tamidal cells showing gemmules; b, small superficial py- ramidal cell ; c, axone of small pyram- idal cells; (not marked with letter on cut), heteromeric cell of olivary nucleus (spinal connections uncertain) ; restiform bodv to cerebellum. From Cerebellum to Cerebral Cortex. Neurone No. 3. — Cells of the cerebellar cortex, of the dentate nucleus and of other cerebellar nuclei ; superior cerebellar peduncles to opposite red nucleus and thalamus. Neurone No. 4. - Cells of red nucleus and of thalamus to cerebral cortex. From Cerebral Cortex to Cerebellum. Neurone No. j.—ja. Cell of frontal cerebral cortex ; mesial part of crusta to pontile nuclei (fronto-pontile fibres) ; jb, cell in temporal cerebral cortex ; lateral part of crusta to pontile nuclei (temporo-pontile fibres). Neurone No. 6. — Cells of pontile nuclei; middle peduncle of cerebellum (tranverse pontile fibres) to opposite cerebellar hemisphere. From Cerebellum to Periphery. Neurone No. 7.— Cells in cerebellum ; tract of Loewenthal to ventral horn of cord. Neurone No. A— Motor cells of ventral horn through mixed spinal nerve to muscle ; or Neurone No. 7.— Ceils in cerebellar cortex : restiform body to opposite olivary nucleus (cerebello-olivary fibres). Neurone No. 8.— Cells in olivary nucleus to ventral horn of cord (tract of Helweg ?). Neurone No. 9.— Motor cell of ventral horn through mixed spinal nerve to muscle. Ass'n, Association neurone of cerebellum. 410 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 trabeculse 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 THE NERVOUS SYSTEM. 411 may contain colloid. (3) An inner layer of nerve cells and neuroglia cells. These react to the Golgi stain, the nerve cells having axones and dendrites. Most of the axones appeared to pass in the direction of the infundibulum, but could not be traced into the latter. The posterior lobe is of ectodermic origin, developing as a diverticulum from the floor of the third ventricle. The remains of the diverticulum consti- tute the infundibulum. The Pineal Body. The pineal body originates as a fold of the wall of the primary brain vescicle. It lies at first upon the dorsal surface of the brain, and in some lower animals continues to occupy this position. Its ventral position in the higher animals and in man is due to the great development of the cerebral hemispheres. The pineal body is ap- parently of the nature of a rudimentary sense organ, being some- times referred to as the median or pineal eye. In man it is sur- rounded by a firm connective-tissue capsule, which is a continuation of the pia mater. This sends trabecular into the organ, which anas- tomose and divide it into many small chambers. The latter contain tubules or alveoli lined with cuboidal epithelium. This may be simple or stratified, and frequently almost completely fills the tu- bules. Within the tubules are often found calcareous deposits known as " brain sand." TECHNIC. The general structure of the pituitary body and of the pineal body can be studied by fixing material in formalin-M filler's fluid (technic 5, p. 5) and staining sections with haematoxylin-eosin (technic 1, p. 16). General References for Further Study. Barker : The Nervous System and its Constituent Neurones, New York, 1899. Dejerine : Anatomie des centres nerveux, Paris, 1895. Van Gehuchten: Anatomie du systeme nerveux de l'homme, Louvaine, 1900. Golgi : Untersuchungen fiber den feineren Bau des centralen und peripheri- schen Nervensystems, Jena, 1894. Kolliker : Handbuch der Gewebelehre des Menschen. Leipsic. 1896. Ramon y Cajal: Beitrage zum Studieren der Medulla Oblongata. Leipsic. 1896.— Les nouvelles idees sur la structure du systeme nerveux chez l'homme et chez les vertebres. Paris, 1894. Von Lenhossek : Der feinere Bau des Nervensystems im Lichte neuester For- schungen. Berlin, 1895. Obersteiner : Anleitung beim Studieren des Baues der nervosen Centralorgane, Leipsic, 1806. Marburg : Atlas des menschlichen Centralnervensystems, Leipzig und Wien, 1904. CHAPTER XII. THE ORGANS OF SPECIAL SENSE. The Organ of Vision. The eyeball and optic nerve constitute the organ of vision. To be described in connection with them are the eyelid and the lacrymal apparatus. The Eyeball or Bulbus Oculi.— This is almost spherical, although slightly flattened antero-posteriorly. It consists of a wall enclosing a cavity filled with fluid. The wall of the eyeball consists of three coats : (a) An external fibrous coat — the sclera and cornea ; (p) a middle vascular — the cho- roid ; and (c) an. internal nervous — the retina (Fig. 265). The Sclera (Figs. 265 and 266). — This consists of dense fibrous tissue with some elastic fibres. The fibres run both meridionally and equatorially, the tendons of the straight muscles of the eyeball being continuous with the meridional fibres, those of the oblique muscles with the equatorial fibres. The few cells of the sclera lie in distinct, very irregular cell spaces, and frequently contain pigment granules. Pigmented cells in considerable numbers are regularly present near the corneal junction, at the entrance of the optic nerve, and on the inner surface of the sclera. Where the optic nerve pierces the sclera, the continuity of the latter is broken by the enter- ing nerve fibres, forming the lamina cribrosa (Fig. 274). The pig- mented layer of the sclera next the choroid is known as the lamina fusca, and is lined internally by a single layer of flat non-pigmented endothelium. Anteriorly a loose connective tissue attaches the sclera to the scleral conjunctiva. The Cornea (Figs. 267 and 270). — This is the anterior con- tinuation of the sclera so modified as readily to allow the light to pass through it. It is about 1 mm. thick and consists of five layers, which from before backward are as follows (Fig. 267): (i) Anterior epithelium. 412 THE ORGANS OF SPECIAL SENSE. 413 (2) Anterior elastic membrane or membrane of Bowman. (3) Substantia propria corneae. (4) Posterior elastic membrane or membrane of Descemet. (5) Posterior endothelium or endothelium of Descemet. (i) 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 j k Fig. 265.— Diagram of Eyeball showing Coats. (Merkel-Henle.) a, Sclera ; b, choroid ; c, retina ; d, cornea ; e, lens ; f, iris ; £■, conjunctiva ; //, ciliary body ; t, sclero-corneal junc- tion and canal of Sehlemm ; J, fovea centralis ; k, 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 clastic 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- 4H THE ORGAXS. 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, Choroid, and Pigment Layer of Retina. (Merkel- Henle.) A, Sclera; B, choroid ; C, pigmenr layer of retina; d, lamina suprachoroidea ; e, Haller's layer of straight vessels ; _/", choriocapillaris ; j?, 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 lamella? 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 the cornea by means of canal iculi. 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 cornea/ 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 Dcsccmct (Fig. 267, a\) 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. 2 ^7> 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 OF SPECIAL SENSE. 41 5 The Choroid. — This is made up of four layers which from with- out inward are as follows (Fig. 266) : (1) The lamina suprachoroidea. (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 suprachoroidea (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 suprachoroidea 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 — vencs vorticoscc — one in each quadrant of the eyeball. FlG . z67 . -vertical Section of Cornea. A narrow boundary ZOne, rich in (Merkel-Henle.) i, Anterior epithe- ■* Hum; 2, anterior elastic membrane : elastic fibres and free from pigment, 3, substantia propria corneas ; 4, ,. . i-i • , n t, • , posterior elastic membrane ; j. pos- limits this layer internally. It is much terior endothelium . 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 fibrosum, while in the carnivora its structure — several layers of flat cells — gives it the name of the tapetum cellulosum. (3) The choriocapillaris (Fig. 266, /) consists of connective tissue supporting a dense network of capillaries, which is most dense 416 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 - ■v^.'y. JIP m : : : -¥v- ■■' •■■■■•■ ■■■■.'.'..;'■ '•■;»•'■ ■/.{;>■' . .;•'•' '■■•■:■ ; -'- i r ; r%i.;v;-- ; :f Fig. 268. — Section of Human Cornea cut Tangential to Surface— X 350 (technic y, p. 71)— showing corneal cell spaces (lacunas) 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. •"./' y i'S / , --■■/. \ \ .'■'•■ s ) ■" f ' / '"•■- 1 ..-..A. ,' PlG. 269, — Section of Human Cornea cut Tangential to Surface — X 350 (technic 8, p. 71) — showing corneal cells and their anastomosing processes. The CILIARY Body.— This is the anterior extension of the cho- roid and consists of the ciliary processes and the ciliary muscle (Fig. THE ORGANS OF SPECIAL SENSE. 417 270). It extends from the ora scrrata (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 choroid 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., iiilPL 111 Will Cornea — BfilibpliF Anterior chamber Canal of Schlemm Iris Pars iridica retinas Ciliary process of Fontana — sri f ■ • Jr,- ~ ■""■Vs^"-' 7 '-'■'. »|s fe ~ Ji| Circular fibres ' ' ' 1 '■ ■ "Kf^^r^^S^ ii °f ciliary muscle Conjunctiva" 1 "^Wi'ii '"'-'' ' ■! • <• (i^ rtaUial fibres of ! I ! 1 1 HTOSSJsSttif \ivi:s» ■.: 1 : i ciliary muscle Par3 ciliaris retinae Perichorioidal lymph space. 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 retime (Fig. 270). This consists of two layers of columnar epithelial cells, the outer layer being pigmented, the inner non-pigmented. 27 4i8 THE ORGANS. 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; [/>) an outer meridional group lying next to the sclera and known as the tensor choroideae, 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 ligamentum pectinatum (Fig. 270), a continuation of the pos- terior elastic lamina of the cornea. Within the ligament are spaces (spaces of Fontanel) 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 choroid. 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 (i ) The anterior endothelium. ss^ b PIG. 271. —Vertical Section through Iris. (Merkel-Henle.) a, Anterior endothelium ; 6, stroma or substantia propria; c, vitreous membrane; d, pigment layer ; v, blood-vessel. THE ORGANS OF SPECIAL SENSE. 419 (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. — A, Scheme of retina as shown by the Golgi method. />', Vertical section of retina to show layers as demonstrated by the ha^matoxylin-eosin stain. (Merkel-Henle.) B.—i, 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; ', j, 10) are parts of the sustentacular apparatus of the retina, being connected with the cells or fibres of Miillcr (Fig. 272, A, /p 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. 42 3 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, 10). 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, f), 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, j), 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 lutea presents certain peculiarities. Its name is derived from the yellow pigment which is distrib- 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 Fig. 273. -Two Muller's Fibres from Retina of Ox showing Relation to Layers of Retina. (Ramon y Cajal.) _?, Outer limiting layer ; 4, outer nu- clear layer \s, outer molecular layer; 6, inner nuclear layer; 7, inner molecular layer ; S, layer of nerve cells ; q, layer of nerve fibres ; 10, inner limiting layer; a, nucleus; b, cup-like depression caused by pressure from surrounding cells. 424 THE ORGANS. non-nervous retinal extension over the ciliary body {pars ciliaris re- time) and over the iris {pars iridica retince) 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, b) 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- roid, separate the connec- tive-tissue bundles so that they form a lattice-work, the already mentioned lamina cribrosa (Fig. 274, //). 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 commu- FlO. 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 ; d, optic nerve ; //, third neurone system of optic tract (excepting Rd) connecting thalamus, lateral genicu- late body, and anterior corpus quadrigem- inum with the cortex, Co. 426 THE ORGANS. Neurone System No. II. — This has been already partly de- scribed in connection with the axone terminations of neurone system No. I. The cell bodies of the second neurone system (Fig. 272, A, S, p) are in the layer of nerve cells and are, as above noted, associ- ated either directly or by means of their dendrites with the axones of the first neurone system. Their axones pass into the layer of nerve fibres and ultimately become fibres of the optic nerve (Fig. 272, A, 10). The optic nerves (Fig. 275, No) unite at the base of the brain to form the optic decussation or chiasma (Fig. 275, CM). Here the axones from the mesial part of the retina cross to the optic tract of the opposite side, while those of the lateral part of the retina remain in the optic tract of the same side. The axones of the optic tract (Fig. 275, Tro) terminate in the thalamus, in the lateral geniculate body, and in the anterior corpus quadrigeminum (Fig. 275). Neurone System No. III. — The neurones of this system have their cell bodies in the thalamus, lateral geniculate body, and ante- rior corpus quadrigeminum (Fig. 275). Their axones terminate in the cortical visual centres in the occipital lobe (Fig. 275, Co). Some few axones of retinal neurones may pass the above nuclei to terminate directly in the cortex (Fig. 275, Rd). The Lens. — The lens is composed of lens fibres which are laid down in layers (Fig. 276, a). The lens fibre is a long hexagonal, flattened prism with serrated edges. Most of the lens fibres are nucleated, the nucleus lying at about the centre of the fibre near the axis of the lens. The most central of the lens fibres are usually non-nucleated. The fibres extend meridionally from before back- ward through the entire thickness of the lens. They are united by a small amount of cement substance. The lens is surrounded by the lens capsule (Fig. 276, /;), a clear homogeneous membrane which is about 12 //. thick over the anterior surface of the lens, about half as thick over the posterior surface. Between the capsule and the anterior and lateral surfaces of the lens is a single layer of cuboidal epithelial cells (Fig. 276, c), the lens epithelium. Attached to the capsule of the lens anteriorly and posteriorly are membrane-like structures which constitute the suspensory ligament of the lens. These pass outward and unite to form a delicate membrane, the zonula ciliaris or zonule of Zinn (Fig, 270). This bridges over the THE ORGANS OF SPECIAL SENSE. 427 inequalities of the ciliary processes and, continuing as the hyaloid membrane, forms a lining for the vitreous cavity of the eye. The triangular space between the two layers of the suspensory ligament and the lens is known as the canal of Petit. The vitreous body is, a semifluid substance containing fibres which run in all directions and a small number of connective-tissue cells and leucocytes. Traversing the vitreous in an antero-posterior direc- tion is the so-called hyaloid or h r a ~ Cloquef s canal, the remains of the embryonic hyaloid artery (page 428). Blood-vessels. — The blood-ves- sels of the eyeball are divisible Fig. 276. Fig. 277. Fig. 276.-From Longitudinal Section through Margin of Crystalline Lens, showing longitud- inal sections of lens fibres and transition from epithelium of capsule into lens fibres. (Merkel-Henle.) a, Lens fibres; b, capsule; c, epithelium. Fig. 277. — From Cross Section of Crystalline Lens, showing transverse sections of lens fibres and surface epithelium. (Merkel-Henle.) a, Lens fibres ; fr, epithelium. into two groups, one group being branches of the central artery of the retina, the other being branches of the ciliary artery. The central artery of the retina enters the eyeball through the centre of the optic nerve. Within the eyeball it divides into two branches, a superior and an inferior. These pass anteriorly in the nerve-fibre layer, giving off branches, which in turn give rise to cap- illaries which supply the retina, passing outward as far as the neuro- epithelial layer and anteriorly as far as the ora serrata. The smaller 42 S THE ORGANS. branches of the retinal arteries do not anastomose. In the embryo a third vessel exists, the hyaloid artery. This is a branch of the cen- tral retinal artery and traverses the vitreous to the posterior surface of the lens, supplying these structures. The hyaloid canal, or canal of Cloquet, of the adult vitreous, represents the remains of the degen- erate hyaloid artery (page 427). The veins of the retina accompany the arteries. The ciliary arteries are divisible into long ciliary arteries, short ciliary arteries, and anterior ciliary arteries. The long ciliary arte- ries are two in number and pass one on each side between the cho- roid and sclera to the ciliary body, where each divides into two branches, which diverge and run along the ciliary margin of the iris. Here the anastomosis of the two long ciliary arteries forms the greater arterial circle of the iris. This gives rise to small branches which pass inward supplying the surrounding tissues and unite near the margin of the pupil to form the lesser arterial circle of the iris. The branches of the short ciliary arteries pierce the sclera near the optic nerve entrance, supply the posterior part of the sclera, and ter- minate in the choriocapillaris of the choroid. The anterior ciliary arteries enter the sclera near the corneal margin and communicate with the choriocapillaris and with the greater arterial circle of the iris. The anterior ciliary arteries also supply the ciliary and recti muscles and partly supply the sclera and conjunctiva. Small veins accompany the ciliary arteries; the larger veins of this area are peculiar, however, in that they do not accompany the arteries, but as venae vorticosae converge toward four centres, one in each quad- rant of the eyeball. At the sclero-corneal junction is a venous channel, the canal of Schlemm, which completely encircles the cornea (Fig. 270). Lymphatics. — The eyeball has no distinct lymph-vessel system. The lymph, however, follows certain definite directions which have been designated by Schwalbe "lymph paths." He divides them into anterior lymph paths and posterior lymph paths. The anterior lymph paths comprise (a) the anterior chamber which communicates by means of a narrow cleft between iris and lens with the posterior chamber; (p) the posterior chamber; (c) the lymph canaliculi of the sclera and cornea and the canal of Petit. The posterior lymph paths include (a) the hyaloid canal (see above) ; (/>) the subdural and in- trapial spaces, including the capsule of Tenon; (c) the perichoroidal THE ORGANS OF SPECIAL SENSE. 429 space, and id) the perivascular and pericellular lymph spaces of the retina. Nerves. — The nerves which supply the eyeball pass through the sclera with the optic nerve and around the eyeball in the supracho- roid layer. From these nerves, branches are given off as follows : (1) To the choroid, where they are intermingled with ganglion cells. (2) To the ciliary body, where they are mingled with ganglion cells to form the ciliary plexus. The latter gives off branches to the ciliary body itself, to the iris, and to the cornea. Those to the cor- nea first form a plexus in the sclera — the plexus annularis — which encircles the cornea. From this, branches pierce the substantia pro- pria of the cornea, where they form four corneal plexuses, one in the posterior part of the substantia propria, a second just beneath the anterior elastic membrane, a third sub-epithelial, and a fourth intra- epithelial. The fibres of the last named are extremely delicate and terminate freely between the epithelial cells. Krause describes end- bulbs as occurring in the substantia propria near the margin of the sclera, while according to Dogiel some of the fibres are connected with end-plates. The Lacrymal Apparatus. The lacrymal apparatus of each eye consists of the gland, its ex- cretory ducts, the lacrymal canal, the lacrymal sac, and the nasal duct. The lacrymal gland 'is a compound tubular gland consisting of two main lobes. Its structure corresponds in general to that of a serous gland. The excretory ducts are lined with a two-layered col- umnar epithelium which becomes simple columnar in the smaller ducts. The alveoli are lined with irregularly cuboidal serous cells, which rest upon a basement membrane beneath which is a richly elastic interstitial tissue. The lacrymal canals have a stratified squamous epithelial lining. This rests upon a basement membrane beneath which is the stroma containing many elastic fibres. External to the connective tissue are some longitudinal muscle fibres. The lacrymal sac is lined with a two-layered stratified or pseudo- stratified columnar epithelium resting upon a basement membrane. The stroma contains much diffuse lymphatic tissue. 430 THE ORGANS. The nasal duct has walls similar in structure to those of the lac- rymal sac. In the case of both sac and duct the walls abut against periosteum, a dense vascular plexus being interposed. The blood-vessels, lymphatics, and nerves of the lacrymal gland have a distribution similar to those of other serous Hands. The Eyelid. The eyelid consists of an outer skin layer, an inner conjunctival layer, and a middle connective tissue layer. The epidermis is thin and the papillae of the derma are low. Small sebaceous glands, sweat glands, and fine hairs are present. The conjunctiva (Fig. 278, a 7 ) is a mucous membrane consisting of a lining epithelium and a stroma. The epithelium is stratified columnar consisting of two or three layers of cells. Among these cells are cells resembling goblet cells. Although not always upon the surface, they are believed to be mucous cells, probably analogous to the so-called Leydig's cells found in the larvae of amphibians and fishes. Diffuse lymphoid tissue is regularly present in the stroma, while lymph nodules are of rare occurrence. Small glands, similar to the lacrymal glands in structure, are usually present (Fig. 278, /'). At the margin of the eyelid where skin joins mucous membrane are several rows of large hairs, the eyelashes (Fig. 278, h). Con- nected with their follicles are the usual sebaceous glands (Fig. 27S, g) and the glands of Mall, the latter probably representing modified sweat glands. The middle layer contains the tarsus (Fig. 278, e) and the mus- cular structure of the eyelid (Fig. 278, /;). The tarsus is a plate of dense fibrous tissue which lies just beneath the conjunctiva and ex- tends about two-thirds the height of the lid. It contains the tarsal or Meibomian glands (Fig. 278, c). These are from thirty to forty in number, each consisting of a long duct which opens externally on the margin of the lid behind the lashes (Fig. 278, f), and internally into a number of branched tubules. The duct is lined with stratified squamous epithelium. The tubules resemble those of the sebaceous glands. Between the tarsus and the skin are the muscular structures of the eyelid in which both smooth and striated muscle are found. Blood-vessels. — Two main arteries pass to the eyelid, one at each angle and unite to form an arch, the tarsal arch, along the margin of THE ORGANS OF SPECIAL SENSE. 431 the lid. A second arch, the external tarsal arch, is formed along the upper margin of the tarsus. From these arches are given off capil- lary networks which supply the .<«., I mm structures of the lid. Lymphatics. — These form two anastomosing plexuses, one anterior, the other pos- terior to the tarsus. Nerves. — The nerves form plexuses in the substance of the lid. From these, terminal fibrils pass to the various struct- ures of the lid. Many of the fibres end freely in fine net- works around the tarsal glands, upon the blood-vessels, and in the epithelium of the conjunc- tiva. Other fibres terminate in end-bulbs which are espe- cially numerous at the margin of the lid. Development of the Eye. The eyes begin their de- velopment very early in em- bryonic life. As optic depres- sions they are visible even before the closure of the med- ullary groove. As a result of the closure of this groove, the optic depressions are trans- formed into the optic vesicles. The connection between ves- icle and brain now becomes narrowed so that the two are connected only by the thin optic stalk. The surface of the optic vesicle be- comes firmly adherent to the epidermis and as a result of prolif- eration of ectodermic cells at this point is pushed inward (invag- inated), forming the optic cup. The invagination of the optic FIG. 278.— Vertical Section through Upper Eyelid. (Waldeyer.) a, Skin ; fi, orbicularis muscle; fi\ ciliary bundle of muscle; c, involuntary muscle of eyelid ; d, conjunctiva ; e, tarsus containing Meibomian glands;/", duct of Meibomian gland ; £■, sebaceous gland with duct lying near eye- lashes ; //, eyelashes ; i, small hairs in outer skin ; J, sweat glands ; fc, posterior tarsal glands. 43? THE ORGANS. vesicle extends also to the stalk, the sulcus in the latter being known as the choroid fissure. The latter serves for the introduction of mesenchyme and the development of the hyaloid retinal artery. Three distinct parts may now be distinguished in the developing eye, which at this stage is known as the secondary optic vesicle : {a) The proliferating epidermis which is to form the lens; (/;) the more su- perficial of the invaginated layers which is to become the retina; and (c) the surrounding mesodermic tissue from which the outer coats of the eye are to develop. TECHNIC. (i) For the study of the general structures of the eyeball the eye of some large animal, such as an ox, is most suitable. Fix the eye for about a week in ten-per- cent formalin. Then wash in water and bisect the eye in such a manner that the knife passes through the optic-nerve entrance and the centre of the cornea. The half eye should now be placed in a dish of water and the structures shown in Fig. 265 identified with the naked eye or dissecting lens. On removing the vitreous and retina, the pigmented epithelium of the latter usually remains attached to the choroid from which it may be scraped and examined in water or mounted in gly- cerin. In removing the lens note the lens capsule and the suspensory ligament. The lens may be picked to pieces with the forceps, and a small piece, after further teasing with needles, examined in water or mounted in glycerin or eosin-glycerin. The retinal surface of the choroid shows the iridescent membrane of Bruch. By placing a piece of the choroid, membrane-of-Bruch-side down, over the tip of the finger and gently scraping with a knife in the direction of the larger vessels, the latter may be distinctly seen. By now staining the piece lightly with hematoxylin and strongly with eosin, clearing in oil of origanum and mounting in balsam, the choriocapillaris and the layer of straight vessels become distinctly visible with the low-power lens. In removing the choroid note the close attachment of the latter to the sclera, this being due to the intimate association of the fibres of the lamina suprachoroidea and of the lamina fusca. If the brown shreds attached to the inner side of the sclera be examined, the pigmented connective-tissue cells of the sclera can be seen. (2) For the study of the liner structure of the coats of the eye, a human eye if it is possible to obtain one, if not, an eye from one of the lower animals, should be fixed in formalin-Midler's fluid (technic 5, p. 5) and hardened in alcohol. (A few drops of strong formalin injected by means of a hypodermic needle directly into the vitreous often improves the fixation.) The eye should next be divided into quadrants by first carrying the knife through the middle of the cornea and of the optic-nerve entrance and then dividing each half into an anterior and a poste- rior half. Block in celloidin, cut the following sections, and stain with ha?matoxy- lineosin (technic i, p. 16). (ei) Section through the sclero-corneal junction, including the ora serrata, ciliary body, iris, and lens. Before attempting to cut this section almost all of the lens should be picked out of the block, leaving only a thin anterior and lateral rim attached to the capsule and suspensory ligament. The block should then be so clamped to the microtome that the lens is the last part of the block to be cut. The above precautions are necessary on account of the density of the lens, making it difficult to cut. THE ORGANS OF SPECIAL SENSE. 433 (b) Section through the postero-lateral portion of the eyeball to show struct- ure of sclera, choroid, and retina. This section should be as thin as possible and perpendicular to the surface. (c) Section through the entrance of the optic nerve. Haematoxylin-picro acid- fuchsin also makes a good stain for this section. It is instructive in cutting the eye to cut a small segment from the optic nerve and to block it with the optic- nerve entrance material in such a manner that it is cut transversely. In this way both longitudinal and transverse sections of the optic nerve appear in the same section. (d) For the study of the neurone relations of the retina material must be treated by one of the Golgi methods (page 27). 1 4) The connective-tissue cells and cell spaces of the cornea may be demon- strated by means of technics 8 and 9, page 71. (5) The different parts of the lacrymal apparatus may be studied by fixing material in formalin-Muller's fluid and staining sections in hasmatoxylin-eosin. (6) The Eyelid. An upper eyelid, human if possible, should be carefully pinned out on cork, skin side down, and fixed in formalin-Muller's fluid. Vertical sections should be stained with hasmatoxylin-eosin or with haematoxylin-picro-acid- fuchsin. The Organ of Hearing. The organ of hearing comprises the external ear, the middle ear, and the internal ear. The External Ear. The external ear consists of the pinna or auricle, the external auditory canal, and the outer surface of the tympanic membrane. The pinna consists of a framework of elastic cartilage embedded in connective tissue and covered by skin. The latter is thin and contains hairs, sebaceous glands, and sweat glands. The external auditory canal consists of an outer cartilaginous por- tion and an inner bony portion. Both are lined with skin continuous with that of the surface of the pinna. In the cartilaginous portion of the canal the skin is thick and the papillae are small. Hair, sebaceous glands, and large coiled glands {ceruminous glands) are present. The last named resemble the glands of Mall (page 430) and are probably modified sweat glands. Their cells contain numerous fat droplets and pigment granules. They have long narrow ducts lined with a two-layered epithelium. In children these ducts open into the hair follicles ; in the adult they open on the surface near the hair follicles. The secretion of these glands plus desquamated epithelium consti- tutes the ear -wax. In the bony portion of the canal the skin is thin, free from glands and hair, and firmly adherent to the perios- teum. 28 434 THE ORGANS. The tympanic membrane (ear drum) separates the external ear from the middle ear. It consists of three layers : a middle layer or substantia propria, an outer layer continuous with the skin of the external ear, and an inner layer continuous with the mucous mem- brane of the middle ear. The substantia propria consists of closely woven connective-tis- sue fibres, the outer fibres having a radial direction from the head of the malleus, the inner fibres having a concentric arrangement and being best developed near the periphery. The outer layer of the tympanic membrane is skin, consisting of epidermis and of a thin non-pap i Hated corium, excepting over the manubrium of the malleus, where the skin is thicker and papillated. The inner layer is mucous membrane and consists of a stroma of fibro-elastic tissue covered with a single layer of low epithelial cells. Blood-vessels. — Blood is supplied to the tympanic membrane by two sets of vessels, an external set derived from the vessels of the external auditory meatus and an internal set from the vessels of the middle ear. These give rise to capillary networks in the skin and mucous membrane respectively and anastomose by means of perfo- rating branches at the periphery of the membrane. From the capil- laries the blood passes into two sets of small veins, one extending around the periphery of the membrane, the other following the handle of the malleus. Lymphatics. — These follow in general the course of the blood- vessels. They are most numerous in the outer layer. Nerves. — The larger nerves run in the substantia propria. From these, branches pass to the skin and mucous membrane, beneath the surfaces of which they form plexuses of fine fibres. The Middle Ear. The middle ear or tympanum is a small chamber separated from the external ear by the tympanic membrane and communicating with the pharynx by means of the Eustachian tube. Its walls are formed by the surrounding bony structures covered by periosteum. It is lined with mucous membrane and contains the ear ossicles and their ligamentous and muscular attachments. The epithelium is of the simple low cuboidal type. In places it may be ciliated and not in- frequently assumes a pseudostratified character with two layers of THE ORGANS OF SPECIAL SENSE. 435 nuclei. Beneath the epithelium is a thin stroma which contains some diffuse lymphoid tissue and blends with the dense underlying periosteum. Small tubular glands are usually present, especially near the opening of the Eustachian tube. The fenestra rotunda is covered by the secondary tympanic mem- brane. This consists of a central lamina of connective tissue covered on its tympanic side by part of the mucous membrane of that cham- ber, on the opposite side by a single layer of endothelium. The ossicles are composed of bone tissue arranged in the usual systems of lamellae. The stapes alone contains a marrow cavity. Over their articular surfaces the ossicles are covered by hyaline car- tilage. The Eustachian Tube. — This is a partly bony, partly cartilaginous canal lined with mucous membrane. The epithelium of the latter is of the stratified columnar ciliated variety consisting of two layers of cells. In the bony portion of the tube the stroma is small in amount and intimately connected with the periosteum. In the cartilaginous portion the stroma is thicker and contains, especially near the pharyn- geal opening, lymphoid tissue and simple tubular mucous glands. The Internal Ear. Amjpvllaif- The internal ear consists of a complex series of connected bony walled chambers and passages containing a similar-shaped series of membranous sacs and tubules. These are known respectively as the osseous labyrinth and the membranous la by r i n t h. Be- tween the two is a lymph space, which contains the so-called perilymph^ while within the membranous labyrinth is a sim- ilar fluid, the endolymph. The bony labyrinth consists of a central chamber, the ves- tibule, from which are given Off FIG. -jg.-The Bony Labyrinth. the three semicircular canals and the cochlea. The vestibule is separated from the middle ear by a plate of bone in which are two openings, the fenestra ovalis and the Ampulla (Heitz- 436 THE ORGANS. fenestra rotunda. Just after leaving the vestibule each canal pre- sents a dilatation, the ampulla. As each canal has a return opening into the vestibule and as the anterior and posterior canals have a common return opening (the canalis communis), there are five open- FlG. 280. — Diagram of the Perilymphatic and Endolymphatic Spaces of the Inner Ear. (Tes- tut.) Endolymphatic spaces in gray ; perilymphatic spaces in black. /, Utricle; 2, sac- cule ; j, semicircular canals ; 4, cochlear canal ; j, endolymphatic duct ; 6, subdural endo- lymphatic sac ; 7, canalis reuniens; S, scala tympani ; 9, scala vestibuli ; jo, their union at the helicotrema; //, aqueduct of the vestibule; 12. aqueduct of the cochlea; /^perios- teum : 14, dura mater ; jj, stapes in fenestra ovalis ; ib, fenestra rotunda and secondary tympanic membrane. ings from the vestibule into the semicircular canals (Fig. 279). The bony labyrinth is lined with periosteum, covered by a single layer of endothelial cells. The Vestibule and the Semicircular Canals. — In the vestibule the membranous labyrinth is subdivided into two chambers, the sac- cule and the utricle, which are connected by the utriculosaccular duct. From the latter is given off the endolymphatic duct which communicates through the aqueduct of the vestibule, with a subdural lymph space, the endolymphatic sac. The saccule opens by means of the ductus reuniens into the cochlea, while the utricle opens into the ampullseof the semicircular canals. The saccule and utricle only partly fill the vestibule, the remaining space, crossed by fibrous bands and lined with endothelium, constituting the perilymphatic space. THE ORGANS OF SPECIAL SENSE. 437 Saccule and Utricle. — The walls of the saccule and of the utricle consist of fine fibro-elastic tissue supporting a thin basement membrane, upon which rests a single layer of low epithelial cells. In the wall of each chamber is an area of special nerve distribution, the macula acustica. Here the epithelium changes to high columnar and consists of two kinds of cells, sustentacular and neuro-epithelial. The sustentacular cells are long, irregular, nucleated cylinders, narrow in the middle, widened at each end, the outer end being frequently split and resting upon the basement membrane. The neuro-epithelial cells or "hair cells" are short cylinders which extend only about half- way through the epithelium. The basal end of the cell is the larger and contains the oval nucleus. The surface of the cell is provided with a cuticular margin from which project several long hair- like processes, the auditory hairs. Small crystals of calcium carbonate are found on the surfaces of the hair cells. These are known as otoliths and are embedded in a soft substance, the otolithic membrane. The hair cells are the neuro-epithelial end-organs of the vestibular division of the auditory nerve and are, therefore, closely associated with the nerve fibres. The latter on piercing the basement mem- FlG. 281.— Diagram of the Right Membranous Labyrinth. (Testut.) /, Utricle; 2, superior semicircular canal ; 3, posterior semicircular canal ; 4, external semicircular canal ; j, sac- cule ; 6, endolymphatic duct ; 7 and 7', canals connecting utricle and saccule respectively with the endolymphatic duct ; S, endolymphatic sac; g. cochlear duct; ). 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 347) 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 383). 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. 445 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. (i) For the study of the general structure of the pinna and walls of the exter- nal auditory meatus, material may be fixed in formalin-Midler's fluid (technic 5. p. 5) and sections stained with haematoxylin-eosin (technic 1. p. 16). 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. 6). 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 (page 7). The bone is next decalcified, either by one of the methods men- tioned on page 8 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 ha?matoxvlin-eosin and mount in balsam. 446 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 27). 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. The Olfactory Mucosa. — This has been described (page 237). 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. (Ramon y Cajal.) be, Bipolar cells of olfactory mucosa; sm, submucosal etlim, cribriform plate of ethmoid; a, layer of olfactory fibres; off, olfactory glomeruli; me, 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. 447 plate of the ethmoid (Fig. 285, etJiui) 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; (I?) the layer of glomeruli; (c) the molecular layer; (d) the layer of mitral cells ; (f) 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 106) 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 axones of the mitral and brush cells. These fibres 44§ 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. 6). Stain thin vertical sections with haematoxy- lin-eosin (technic 1. p. 16) 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 182) and under sensory end-organs (page 349)- The taste buds are found in the side walls of the circumvallate papillae (page 181), 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 KIG. 286. —Taste-bud from Side Wall of Circumvallate Papilla. • Merkel-Menle.) a, Taste-pore ; /', 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. 449 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. 182). (2) For the study of the nerve terminals the method of (iolgi should be used {page 27). General References for Further Study. Schwalbe : Lelirbuch der Ana torn ie der Sinnesorgane, 18S7. Kolliker : Handbuch der Gewebelehre des Menschen. Ramon y Cajal : La retine des vertebres. La Cellule, ix., 1893. McMurrich : The Development of the Human Body. 29 IN DEX. Absorption, 214 of fat, 216 Accessory olivary nucleus, 374, 377, 380 Achromatic spindle, 40 Acid aniline dyes, 15 Acidophile granules, 87 Acini, 173 Acoustic stria?, 384 Adrenal, 266 blood-vessels of. 26S development of, 268 nerves of. 268 structure of, 266 technic of, 269 Adventitia of arteries, 121 of lymph vessels. 129 of veins, 124 Afferent peripheral nerves, 338 Agminated follicles, 204 Air-cells, 246 Air-passages, 245 Air-sacs, 246 Air-vesicles, 245 Alcohol, as a fixative, 4 dilute as a fixative, 5 -ether celloidin, 9 for hardening, 7 graded, 7 Ranvier's, 4 strong, as fixative, 5 Alimentary canal, 175 development of, 234 endgut. 206 foregut. 191 headgut, 176 midgut. 200 Altmann's granule theory of protoplas- mic structure. 34 Alum-carmine, 15 for staining in bulk. 17 Alveolar "lands. 17^; Amacrine cells, 422 Amitosis, 39 Amoeboid movement, 3S Amphophile granules, Sy Ampullae, 436 of Thoma, 146 Anaphase. 41 Aniline dyes, acid, 15 basic, 15 Anistrophic line, 93 Annular terminations, 350 Annuli fibrosi, 126 Anterior horns, 342 root or motor cells of. 352 median fissure. 341 pyramids. 362, 376. 37S. 3S1. 3S5, 3% 392 Antero-lateral ascending tract. 361 Antrum, 291 Appendix epididymidis, 279 testis, 279 vermiform is. 20S Arachnoid membrane, 334 of optic nerve. 424 Arbor vita?. 397 Arborescent terminations. 350 Archoplasm, 37 Arciform nucleus, 374. 377. 3S0. 3S1 Arcuate fibres, external. 375. 377. 3S0, 3' s < internal. 374. 377, 37S. 3S1. 385. 3 S 9 Arrector pili muscle. 322 Arterias arciformes. 262 Arteries. 1 r8 adventitia of. 121 aorta and other large. 121 arcuate. 262 arteriole. 1 19 coats of. 1 iS development of, 128 45' 452 INDEX. Arteries, elastic tissue of. 121 greater arterial circle 01 iris. 428 hepatic. 229 intima of. 120 large, like the aorta. 121 lesser arterial circle of iris. 428 media of. 12 1 medium-sized. 119 phrenic. 263 precapillary artery, 119 recurrent. 263 small. 119 suprarenal. 263 technic of. 124 vasa vasorum of, 124 Arteriole. 1 19 Articular cartilages, 165 Articulations. 165 diarthrosis, 165 synchondrosis, 165 syndesmosis, 165 technic of. 166 Association fibres, 405 Atresia of follicle, 296 Atria. 246 Attraction sphere, 36 Auditory canal. 433 hairs. 437 pit. 444 Auerbach's plexus. 202, 213 Auriculo-ventricular ring. 126 Axis cylinder. 108, 350 Axolemma and neurilemma, relation of, 109 Axone. 106 development of, 333 hill. ro6 medullated, 109 non-medullated, 107 BAILLARGER, inner line of. 402 outer line 0!. 402 Balsam, Canada, for mounting. 18 Bartholin, .udm'ds ( ,f, 308 Basal granule, 59 Basic aniline dyes. 15 Basket cells. 178 Basophile granules, 87 Bertini, < olumns of, 256 Bethe, concerning continuity of axo- lemma and neurilemma. 109 Betz, cells of. 404 Bipolar nerve cells. 102 Blastoderm, 46 Blastomeres, 45 Blocking, 9 Blood. 85 corpuscles, SS crenation of red cell, 86 development of, 88 erythrocytes of. 85 haemoglobin of, 85 Jenner's stain for, 24 leucocytes of, S6 platelets, 88 red cells of, 85 smears, technic of, 90 stroma of, 85 technic of. 89 vascular unit, 249 white cells of, 86 Blood-islands, 88, 128 Blood-vessel system, 115 arteries, 1 18 capillaries, 1 17 heart. 125 lining of, 1 17 technic of, 124, 127 veins, 122 Blood-vessels. 1 16 lymph channels of, 124 nerves of. 124 technic of, 124 Body cavity, 129 Bone breakers. 159 decalcification of, 8 formers. 157 Bone marrow. 152 technic of, 156 red. 152 cells of. 152 eosinophile cells of, 154 fat cells of, 151 mast cells of, 154 multinuclear cells of, 133 myelocytes ol, 152 myeloplaxes ol. 153 non nucleated red blood cells ol. ,53 nucleated red blood cells of, '53 yellow. 154 INDEX. A r 1 4 DO Bone marrow, yellow, gelatinous, 154 Bone tissue, 82 cells of, S3 cementum, 1S6 lacunas and canaliculi of, S3 lamellae of, S3 technic of, S3 Bones, 14S blood-vessels of. 155 cancellous or spongy, 14S circumferential lamellae of, 151 development ofr 156 growth of, 163 hard or compact, 14S Haversian canals of. 150 lamellae of, 150 interstitial lamellae of, 151 lymphatics of. 155 nerves of. 155 perforating fibres, 152 fibres of Sharpey, 152 periosteum of. 151 technic of. 156 developing" bone. 164 Volkmann's canals. 151 Bony spiral lamina. 43S Borax-carmine, alcoholic solution. 17 Bowman, capsule of. 256 membrane of. 413 Brachia conjunctiva. 393 Brain, see Cerebrum membranes of. 334 arachnoid. 334 dura mater. 334 pia mater. 334 relation to optic nerve. 424 sand. 41 1 Bronchi. 242 development of, 250 primary. 242 respiratory. 245 structure of walls of. 242 technic of. 251 terminal. 245 Bruch, membrane of. 416 Brunner"s glands. 205 Bulbus oculi, see Eyeball Burdach. column of. 346. 3C Bursas. 16S Biitschli's theory of protoplasm struct- ure. 34 Cajal, cells of. 403 Cajeput oil for clearing sections, iS Calcification centre, 157 Calcification zone, 162 Canaliculi of bone, S3 Canalis communis, 436 Canalized fibrin, 306 Cancellous bone, 158 Capillaries, 117 chyle, 213 development of, 128 technic of, 124 Capillary network. 11S Capsule of Glisson, 227 Carbol-xylol for clearing specimens, 18 Cardiac glands, 196 Carmine, alum, 15 borax, 7 gelatin, 20 neutral, 15 picro-. 16 Carotid gland. 130 Cartilage, 79 cells. 79 development of, 82 elastic. Si fibrous. Si hyaline. So perichondrium, 82 technic of. S2 Cartilages, the. 164 articular. 164 costal, 164 skeletal, 164 technic of. 166 Caryochromes. 104 Cell. the. ^1 body of. 33 centrosome of. ^ function of. 37 irritability of. 37 membrane of. ^ metabolism of. yi motion of. 38 nucleolus of. 36 nucleus of. 33 primary germ layers of. 43 reproduction of, 39 structure of. 33 technic of. 46 vital properties of. 37 454 INDEX. Cell-division, direct. 39 indirect. 39 Cell islands of Langerhans, 225 Celloidin, alcohol-ether. 9 clove-oil. 10 embedding", 9 Cells, acid. 215 adelomorphous, 195 air. 245 amacrine. 422 basket. 178. 399 blood. 85 bone. S3 brush. 447 centro-acini. of Langerhans, centro-tubular. 223 chief, 195. 410 chromophile. 410 colloid. 252 compound tactile, 34S decidual, 303 Deiter's, 441 delomorphous, 195 eosinophile. 154 epithelial. 53 extrinsic, 346 foetal, 248 goblet, 201 Golgi, Type I.. 106 Golgi, Type II., 106, 404 granule, 398 gustatory. 448 hair, 437. 441 hecateromeric, 354 Hensen"s, 441 . heteromeric, 354 intrinsic, 346 Kupffer's, 233 Leydig's, 430 lutein, 293 marrow, 152 mast. 65. 88. 154 mesamceboid cells. 52 mitral, 447 nerve. 346 neuroepithelial, 437 of Claudius. 442 oxyntic, 195 Paneth's, 203 parietal, 196 peptic. 195 Cells, pillar, 441 plasma, 64 prickle, 314 Purkinje, 397, 399 replacing, 56 respiratory, 247 Sertoli, 272 simple tactile, 348 spermatids, 274 spermatocytes, 274 spermatogenic, 272 spermatogones, 273 sustentacular, 224. 437 tautomeric. 354 wandering", 04, 202 Cementing glycerin mounts. 18 Cementum, 186 Central canal, 342 Central gelatinous substance, 342, 343 nervous system, see Nervous sys- tem {cerebro-spinal) tegmental tract, 384, 387, 389 Centro-acinar cells of Langerhans, 223 Centrosome, 33, 36 Centrosphere, 36 Cerebellar peduncles, 391 Cerebello-olivary fibres, 377, 379 Cerebellum, 397 arbor vita?, 397 basket cells of, 399 cortex of, 397 dentate nucleus of, 401 general histology of, 397 gray matter of, 397 lamin;e of, 397 peduncles of, 391, 402 Purkinje cells of, 397 teclinic of, 407 Cerebral convolution, 402 peduncles, 396 Cerebro-spinal ganglia, 336 teclmic ol, 338 Cerebro-spinal nervous system, sec Nervous system {cerebrospinal) Cerebrum, 402; see also Cortex cerebri convolutions of, 402 cortex of, 402 histology of, 402 technic of. 407 Ceruminous glands, 433 Cervix, 300 INDEX. 455 Cervix, technic of, 310 Chiasma, optic, 426 Chloride of gold for staining connective- tissue cells, 23 Choriocapillaris, 415 Chorion, 304 Chorionic villi, 305 Choroid, the, 415 choriocapillaris of, 415 fissure, 432 Haller's layer of, 415 lamina citrea, 416 suprachoroidea, 415 perichoroidal lymph spaces of, plexus, 380, 38 1 tape turn cellulosum of, 415 fibrosum of, 415 venae vorticosa? of, 415 vitreous membrane of, 416 Chromatin, 36 Chrome-silver method of Golgi, 23 Chromophilic bodies, 104 Chromosomes, 40 Chyle vessels, 213 Ciliary artery, 427 movement, 38 plexus, 429 processes, 417 Ciliary body, the, 416 blood-vessels of, 428 canal of Schlemm, 41S ligamentum pectinatum, 418 muscles of, 41S pars ciliaris retinae, 417 spaces of Fon tana, 418 Circulatory system, 115 blood-vessel system, 115 development of, 128 lymph-vessel system, 128 Circumferential lamellae, 151 Circumvallate papillae. 180 Clarke's columns, 344 Claudius, cells of, 442 Clearing specimens before mounting, 18 Clefts of Schmidt Lantermann, 109 Climbing fibres, 400 Clitoris. 30S Clouet*s canal, 427 Clove-oil celloidin. 10 Coccygeal glands, 130 Cochlea, 438 bony spiral lamina of, 43S cupola of, 438 hamulus of, 43S helicotrema, 439 membranous spiral ligament of, 43S modiolus of, 438 scala tympani, 439 vestibuli, 439 spiral ligament of, 438 Cochlear duct, 439 basilar membrane of, 440 crista basillaris, 440 external spiral sulcus. 440 membrane of Reissner, 440 organ of Corti, 441 spiral prominence of, 440 stria vascularis, 440 zona pectinata, 441 tecta, 441 Coelum, 129 Cohnheim's field, 94 Collaterals, 106 Colloid, 251, 410 Colostrum corpuscles, 330 Columnar rectales, 210 Columns of Bertini, 256 of spinal cord, 342, 344, 346, 352, 353 Commissural fibres. 405 Conduction path, 358 Cone association neurones. 425 Cone fibres. 421 Cone-visual cell, 421 Cones, layer of rods and, 421 Conjunctiva. 430 end -bulbs of, 349 Connective tissue. 63 adipose or fat, 75 areolar, 67 bone, 82 cartilage, 79 cells, 64 characteristics of, 63 chloride of gold method for demon- strating cells of, 23 classification of, 63 elastic, 68 embryonal, 71 fibrillar, 64 formed, 67 histogenesis of. 63 45 6 INDEX. Connective tissue, interalveolar, 248 intercellular substance of, 65 intrafascicular, 16S. 339 lymphatic, 75 loose, 67 Mallory's stain for. 24 mucous, 71 neuroglia. 84 periglandular, 327 reticular, j^ retinaculas cutis, 313 staining cells of, 23 technic for, 70, 73, 75, 79 theories of development of fibres of. 68 Constrictions of Ranvier, 109 Corium. see Derma Cornea, the, 412 anterior elastic membrane of, 413 corneal corpuscles of, 414 endothelium of Uescemet of, 414 epithelium of, 412 layers of, 412 membrane of Bowman of, 414 of Descemet of, 414 perforating or arcuate fibres of. 414 posterior elastic membrane of, 414 substantia propria of, 414 Corneal corpuscles, 414 Cornua. 342 Corona radiata, 396, 405 Corpora amylacea, 283 cavernosa, 284 quailrigemina, 395 anterior, 391, 395, 396 posterior, 391, 395 Corpus albicans, 294 callosum, 405 haemorrhagicum, 293 Highmori, or mediastinum testis, 270 luteum, 293 theory of. 29 spongiosum. 2S4 Corpuscles, blood. .S5 colostrum, 330 crescentic, 383 of Grandry, 348 Meissner, 349 Merkel, 348 Pacinian, 350 Corpuscles, Ruffini, 325 Cortex cerebelli, 397 ; see also Cere- bellum Cortex cerebri, 402 ; see also Cerebrum association fibres of, 405 barren or molecular layer of, 403 cells of Betz, 404 of Golgi, Type II., 404 of Martinotti, 404 commissural fibres, 405 corona radiata of, 405 deep tangential fibres of. 404 layer of polymorphous cells. 403, 405 of pyramidal cells of, 403 projection fibres, 405 superficial tangential fibres of, 403 Cortical pyramids, 256; see also Kid- ney Corti's arches, 442 organ, 441 ; see also Organ of Corti tunnel, 442 Cotyledons, 305 Cowper's glands, 284 Cox-Golgi method of staining. 28 Cranial nerves, 368 ; see also Nerves, cranial Crenation, 86 Crescentic corpuscles, 283 Crescents of Gianuzzi, 178 Crista acustica, 438 basillaris, 440 Crura cerebri, 396 Crusta, 35, 3m Crypt of Lieberkiihn, 203 Cumulus oophorus, 291 Cupola, 438 Cupula, 438 Cuticle, see Epidermis Cuticula. 35 dentis, 186 Cystic duct, 233 Cytoplasm, 34, 103 Decalcifying, 8 fluids, 8 Decidua basalis. 303 capsularis, 303 graviditatus, 303 menstrualis, 302 placentalis subchorialis. 306 INDEX. 457 Decidua basalis, reflexa, 303 serotina, 303 vera, 303 Decolorizing fluid for Weigert's hema- toxylin, 26 Decussation of fillet, 374, 377 optic, 426 of pyramids, 371 sensory, 374, 377 Dehiscent glands, 173 Deiter's cells, 441 nucleus, 383 Delafield's haematoxylin, 14 Demilunes of Heidenhain. 178 Dendrites, the. 106 Dental periosteum, 187 Dental sheath, Neumann's, 185 Dentate nucleus, 401 Dentinal pulp, 183 Dentine, 185 Derma, or corium, 311 corpuscles of Meissner, 349 pars papillaris, 312 reticularis, 311 Descemet, endothelium of. 414 membrane of, 414 Deutoplasm, 35 Development of teeth, 187 common dental germ, 187 cuticular membrane, 189 dental papilla. 187 ridge, 1S7 enamel organ, 1S7 special dental germ, 187 technic of, 190 Tomes' process, 189 Diapedesis, 88 Diarthrosis, 165 Diaster, 42 Digestive system. 175 alimentary tract of, 175 development of, 234 endgut, 206 foregut. 191 headgut, 176 larger glands of, 217 midgut, 200 pancreas. 221 the gall-bladder. 233 the liver. 227 Direct cerebe'lar tract. 361 Discus proligerus, 291 Dissociation of tissue elements. 4 Dogiel's theory of structure of spinal ganglion, 336 Dorsal accessory olivary nucleus, 378. 380 Dorso-lateral ascending tract, 361 spino-cerebellar fasciculus. 361 Duct systems of glands, 173 Ducts, aberrans Halleri. 279 Bartholini's, 219 Bellini's, 257 cochlear, 439 common, 233 excretory, of glands, 173 cystic, 233 ejaculatory, 278 Gartner's, 296 hepatic, 229 Alullenan, 310 nasal. 430 oviduct, 297 pancreatic, 222 pronephritic, 309 reuniens, 436 Santorini's, 222 secondary pancreatic. 222 seminal, 276 utriculo-saccular, 436 Wharton's, 219 Wirsung's, 222 Wolffian, 309 Dura mater, 334 blood-vessels of. 335 technic of, 335 Dyes, aniline, 15 nuclear, 13 plasma, 15 Ear, external, 433 auricle, 433 blood-vessels of, 434 ceruminous glands of. 433 ear drum , 434 external auditory canal. 433 lymphatics of. 434 nerves of. 434 pinna, 433 tympanic membrane. 434 internal, 435 ampulla, 436 458 INDEX. Ear. internal, blood-vessels of. 443 canalis communis. 436 cochlea. 438 duct. 439 endolymph of. 435 fenestra ovalis. 435 lymphatics of. 443 membrana tectoria. 443 membranous labyrinth. 435. 436 nerves of, 443 organ of Corti. 441 osseous labyrinth of, 435 perilymph of. 435 saccule. 437 semicircular canals, 436 utricle. 437 vestibule. 435 middle, or tympanum. 434 fenestra rotunda of, 435 ossicles of, 435 Ear drum. 434 wax. 433 Ebner's glands, 1S2 hydrochloric-salt solution, 8 Ectoderm. 45 tissue, derivations from, 51 Efferent peripheral nerves, 338 Egg nest. 290 Ejaculatory ducts. 278 Elastic cartilage. 81 tissue. 68 Weigert's stain for, 23 Eleidin. 314 Ellipsoid of Krause. 421 Ellipsoids of spleen, 144 Embedding. 9 celloidin. 9 paraffin, 1 1 Embryonal tissue, 7; Enamel. [86 fibres. 186 organ, 187 prisms. 186 lines of Retzius of. 186 End-bulbs, 349 of Krause. 182. 326 Endgut, 206 large intestine. 206 rectum. 210 \ ermiform appendix, 208 Endocardium, 126 Endochondral ossification, 159 Endolymph, 433 Endolymphatic sac, 436 Endomysium. 16S Endoneurium, 339 Endoplasm, 35 Endothelial tube, 12S Endothelium. 60 Entoderm, 46 tissue derivations from, 51 Eosin, 15 Eosinophile granules, 87 Ependymal cells, 333 Epiblast. 45 Epicardium, 127 Epicranium, 158 Epidermis (or cuticle), 313 stratum corneum of, 314 cylindricum of, 313 germinativum of, 313 granulosum of, 314 lucidum of, 314 Malpighii of, 313 mucosum of. 313 spinosum of, 314 Epididymis, 271 Epidural space. 334 Epimysium, 167 Epineurium. 339 Epiphyseal cartilage, 164 Epithelium, 53 cells of, 53 ciliated, 59 classification of, 54 cuboidal, 55 general characteristics of, 53 germinal, 288 glandular, 60 histogenesis of, 53 intercellular bridges of. 53 lens, 426 membrana propria of, 53 neuro-, 60 pigmented, 60 pseudo-stratified, 56 respiratory, 247 simple, 54 columnar, 54 pseudo-stratified, 56 squamous, 54 stratified. 56 INDEX. 459 Epithelium, stratified, columnar. 58 squamous. 56 transitional, 57 surface, of mucous membranes, 174 syncytium. 305 technic of, 61 transitional, 57 Eponychium, 318 Epoophoron, 296 Erectile tissue, 285 Erythroblasts, 153 Erythrocytes, 85 Erythrosin. 15 Eustachian tube, 435 Exoplasm, 35 External arcuate fibres. 375, 377, 380, 381 External ear. see Ear, external Eyeball (or bulbus oculi), 412 blood-vessels of, 427 choroid of. 415 ciliary body of, 416 cornea of, 412 iris of. 418 lymphatics of, 428 nerves of. 429 retina of, 420 sclera of. 412 technic of, 432 Eyelid, the. 430 blood-vessels of. 430 conjunctiva of, 430 epidermis of. 430 glands of, 430 of Mall, 430 lymphatics of, 431 Meibomian glands, 430 muscles of, 430 nerves of, 431 tarsus of, 430 technic of. 433 Fallopian tube, see Oviduct Fascicles of muscle, 167 of nerves. 339 Fasciculus, posterior longitudinal, 375, 377. 3S0. 381. 386 solitarius,.374. 377. 3S0 ventrolateralis superhcialis, 361 Fat, absorption of, 214 technic of. 217 Fat. osmic-acid stain for, 24 secretion, 214 subcutaneous, 313 tissue, 75 Female genital organs, 288 Fenestra ovalis, 435 rotunda, 435 Fertilization of the ovum, 42 Fibre baskets, 423 systems, 358 short, 364 tracts of spinal cord, 358 of spinal cord, technic of, 366 Fibre tracts of spinal cord (ascending), 360 anterolateral ascending tract. 3 61 direct cerebellar tract. 361 dorsolateral ascending tract, 36i dorso-lateral spino cerebellar fasciculus, 361 fasciculus ventrolateralis su- perficialis, 361 Gowers' tract, 361 posterior columns, 360 tract of Flechsig, 361 (descending), 362 anterior marginal bundle of Loewenthal, 363 antero-lateral descending", 363 comma tract of Schultze, 364 crossed pyramidal tract. 362 direct pyramidal tract, 362 Hehveg's, 363 oval bundle of Flechsig. 363 pyramidal tracts, 362 rubro-spinal, 363 septo-marginal, 363 tract of Tiirck, 362 Von Monakow's tract. 363 Fibres, 449 calcined. 157 cone. 421 dentinal. 184 development of connective-tissue, 6S enamel. 186 genioglossal, 179 intergeminal. 449 intrageminal. 448 460 INDEX. Fibres, lens, 426 Mallory's method of staining con- nective tissue, 24 mantle. 40 Miiller's. 422 nerve, see also Nerve fibres medulla ted. 10S non-medulla ted. 107 neuroglia. 1 12 of areolar tissue. 67 of bone. S3 of developing muscle, 99 of formed connective tissue, 67 of Remak, 108 of Sharpey, 152 olfactory, layer of. 447 perforating or arcuate, of cornea. 4H of Sharpey. 152 picro-acid-fuchsin for staining con- nective-tissue, 16 rod, 421 tendon, 67, 168 tunnel. 444 voluntary muscle. 92. 95 Weigert's method for staining elas- tic. 23 method for staining nerve. 25 white or fibrillated, 65 yellow or elastic. 65. 68 Fibrillar connective tissue. 64 theory of protoplasm structure, 34 Fibroblasts, 68 Fibrous cartilage, 81 Filiform papilla 1 . 1S0 Fillet, the. 374. 377, 379, 381, 3S5, 389, 393 Filum term in ale, 340 Fissure, anterior median. 41 choroid. 432 Fixation, 4 by injection, 5 in to to, 5 i ixatives, 5 Flechsig, oral bundle of, 363 tract of. 361 1- "le. nming's fluid, 6 . oam theory of protoplasm structure, 34 1 oiiate papilla?, 448 Follicle, ( Graafian, 289 Follicular cavity or antrum. 291 Folliculi linguales, see Tonsils Foil tana, spaces of. 418 Foramina nervosa. 444 Forebrain. 332 Foregut, the. 191 general structure of walls of the gastro- intestinal canal. 192 oesophagus. 191 stomach. 194 Forel. decussation of, 394 Formalin, as a fixative, 5 for macerating. 4 Formalin-M idler's fluid, 5 Fossa navicularis. 287 Fovea centralis. 423 Fuchsin. 1 5 Function of cells, 37 Fundamental columns of spinal cord, 3 6 4 Fungiform papilla;, 1S0 Funiculus teres, nucleus of, 3S1 Gage's hematoxylin, 13 Gall-bladder, 233 Ganglia, 335 Gasserian, 389 spinal. 336 spiral, 444 sympathetic, 337 Ganglion cells, 336 Gasserian ganglion, 389 Gastric glands. 194 Gastro-ihtestinal canal, general struct- ure of the walls of, 192 Gelatin, carmine for injecting. 20 Prussian -blue, for injecting. 20 Gelatinous marrow. 154 substance of Rolando, 342, 343 Genioglossal fibres, 179 ( renital ridge. 310 Gentian violet, 15 Genu or bend, 396 (ierm hill, 291 Germinal spot, 292 vesicle. 42 Gianuzzi, crescents of, 17S, 240 < riraldes, organ ol . 278 Gland cells, 170 Glands, 170 acini of, 173 INDEX. 461 Glands, adrenal, 266 alveolar. 173 compound, 173 simple. 173 alveoli of, 173 axillary. 330 Bartholini's, 30S Brunner's, 205 cardiac. 196 carotid. 130 cells of. 170 ceruminous, 433 classification of, 171 coccygeal. 130 compound. 17 1 Co\vper*s, 2S4 epithelium of. 1 70 excretory ducts of, 17.-- dehiscent, 173 ductless, 173 Ebner's, 1S2 gastric, 195 haemolymph. 135 internal secreting. 173 intraepithelial, 276 lacrymal. 429 Lieberkuhn's, 203 lingual. 17S Littre s. 2S7 liver. 227 lobes of, 171 lymph, 131 Mall's. 430. 433 mammary. 327 Meibomian. 430 mixed. 177 mucous. 177 of the oral mucosa, 177 pancreas. 221 parathyroids, 252 parenchyma of, 171 parotid. 218 peptic. 195 pineal. 41 1 prostate. 2S2 pyloric. 197 reticular. 173 saccular, 171 salivary, 2 17 sebaceous. 315. 323 secreting portions of. 170 Glands, serous, 177 simple. 171 spleen, 142 sublingual. 219 submaxillary, 219 sweat. 315 tarsal. 430 thymus, 138 thyroid, 251 tonsils, 140 tubular, 171 compound. 173 simple branched. 172 simple coiled. 172 simple straight, 172 Tyson's, 286 Glandulae sudoriparae. 315 vestibulares majores. 308 minous, 308 Glandular epithelium. 170 Glans penis, 2S6 Glenoid ligaments, 165 Glisson. capsule of, 227 Globus major, 271 minor, 271 Glomerulis. of kidney, 256 olfactory, 447 Glycerin for mounting specimens, 17 Glycogen granules. 231 Golgi bichlorid method for nerve tissue, 2 7 cell. Type I.. 106 cell, Type II., 106, 404 method, bichloride, 27 Cox modification. 28 formalin bichromate. 27 mixed, 27 muscle-tendon organs of, 350 net, 1 1 1 rapid, 27 silver. 27 silver, for nerve tissue. 27 Golgi's chrome-silver method of stain- ing, 23 Golgi-Mazzoni corpuscles. 325 Goll, column of, 346, 360 Gowers' tract, 361. 370. 376. 378, 381, 3S4, 3S9. 393. 394 Graafian follicles. 289 antrum of, 291 germ hill of. 291 462 IXBEX. Graafian follicles, liquor folliculi, 291 ovum of. 292 rupture of, 293 technic of, 299 theca folliculae of, 291 Graded alcohols. 7 Grandry. corpuscles of. 34S Greek letter granules of Ehrlich. 87 Ground bundles of spinal cord, 364 Gustatory canal. 44S H.EMALU.M, Mayer's. 14 Haematin. 14 Haematoidin, crystals of, 294 Ha-matoxylin, 13 and eosin, for staining double, 16 and picro-acid fuchsin, 16 Delafield"s, 14 Gage's. 13 Heidenhain's, 14 Mallory's stain, 24 Weigert*s. 2? Haemoglobin, S5 Hamolymph nodes. 135 blood sinuses ot. 135 blood-vessels of. 137 cells of, 136 marrow-lymph, 136 splenolymph. 136 technic of. 137 Hair. 318 arrector pili muscle of the, 322 bulb, 318 cells ol the. 319. 437 cortex of. 319 cortical fibres of the. 319 cuticle of. 319. 320 cuticle, Henle's layer of. 320 1 1 uxley's layer of. 320 root sheath of, 320 the prickle cells of. 320 development of the. y<> eyelashes, 430 follicle. 319 germ, 324 growth of the. 324 lanugo, the. 319 layers of the. 319, 320 medulla of. 319 papilla of. 318 Hair, root of the. 321 sebaceous glands ol' the. 322 sebum of the, 323 shaft of, 31S shedding of the. 2> 2 3 technic of the. 324 Haller's layer, 415 Hamulus, 438 Hardening. 7 celloidin-embedded specimens. 9 clove-oil celloidin-embedded speci- mens, to HassaTs corpuscles, 139 Haversian canals, 150 fringes. 166 lamellae, 150 spaces, 162 Headgut, 176 the mouth. 176 the pharynx. 190 the teeth, 183 the tongue, 179 Hearing, organ of, 433 Heart, 125 annuli fibrosi. 126 auriculo-ventricular tint;' of. 126 blood-vessels of. 127 development of, 128 endocardium of. 126 epicardium of. 126 lymphatics of. 127 muscle, 96 ; see Involuntary stri- ated muscle myocardium of, 126 nerves of. 127, 350 technic of, 127 valves of. 127 Hecateromeres. 354 I leidenhain. demilunes of, 178 Heidenhain's heematoxylin, 4 I leisterian valve. 233 1 lelicotrema, 439 1 lelweg. tract of, }<>;-, I lenle's layer, 320 loop, 256 I [enle, sheath of, 339 I [ensen's cells, 44 1 line, 93 1 [epatic artery. 229 cells, 230 cords, 232 INDEX. 46; Hepatic duct. 229, 233 Heteromeres, 354 Hindbrain, 332 His, marginal veil of, 333 spongioblasts of, 333 Howship's lacunae, 159 Huxley's layer. 320 Hyaline cartilage, 80 Hyaloid canal. 427 membrane, 427 Hyaloplasm. 34 Hydatid of Morgagni. 310 Hydrochloric acid for decalcifying, 8 Hyoglossal fibres. 179 Hypoblast, 46 Hyponychium. 318 Hypophysis cerebri. 410 ; see also Pitu- itary body Implantation cone. 106 Incisures of Schmidt-Lanterman. 109 Inferior brachium quadrigeminum. 396 cerebellar peduncle, see Restiform body Injecting, 20 apparatus, 21 double. 22 separate organs. 21 whole animals, 21 Inner bulb, 350 Innervation of muscles. 353 Intercellular bridges of epithelium. 53 substance. 12 of connective tissue, fibres of, 65 silver-nitrate method of stain- ing. 23 Intermediate lamella?, 151 Internal arcuate fibres, 374, 377. 37S, 381, 385, 3S9 Internode, 109 Interstitial lamellae. 151 Intestine, see Small intestine and Large intestine Intestines, development of. 235 Intima, 120 of arteries. 120 of lymph vessels. 129 of veins. 123 Intracartilaginous ossification. 159 Intrafascicular connective tissue. 168, 339 Intramembranous ossification. 157 Intranuclear network of typical cell. 36 Involuntary striated muscle (heart mus- cle). 96 Cohnheim's field. 97 McCallum's views, 96 membrane of Krause. 97 muscle columns of Kolliker. 97 nerves of, 350 sarcoplasm of. 97 technic of. 100 smooth muscle. 91 intercellular bridges of. 92 Iodine to remove mercury. 7 Iris, the, 418 greater arterial circle. 42S layers of the. 418 lesser arterial circle. 428 muscles of the. 419 Irritability of cells. 37 Islands, blood. 8S. 128 of Langerhans. 225 Isolated smooth muscle cells. 91 technic of. 99 Isotrophic line. 93 Jexxer's blood staix. 24 Joint capsule. 165 Joints, 165 Karyokixesis. 39 Karyolysis. 314 Karyoplasm. 36 Karyosomes. 36 Keratin, 314 Keratohyaline granules. 314 Kidney, the. 254 blood-vessels of. 261 Bowman's capsule. 256 columns of Bertini. 256 convoluted tubules of. 259 cortex of. 254 cortical pvramids or labyrinths of. duct of Bellini. 257 glomerulus of. 256 Henle's loop. 259 hilum of. 254 lobulated. 254 lymphatics of. 264 main excretory duct of. 264 46 posterior corpora, 391, 395 Ranvier's alcohol, as macerating fluid, 4 Raphe, 438 Rectum, the, 210 columna.' rectales, 210 technic of, 2 16 INDEX. 47$ Reissner, membrane of. 440 Remak, fibres of, 10S Renal corpuscle, development of, 256 Renculus, 254 Replacing cells. 56 Reproduction of cells. 39 Reproductive system, 270 development of, 308 female organs, 288 ovary. 288 urethra, 2S6 uterus. 299 vagina. 307 male organs. 279 Co\vper*s glands. 2S4 penis, 284 prostate gland. 282 testis, 270 urethra, 286 Respiratory system, development of, 250 the bronchi. 242 the larynx. 239 the lungs. 244 the nares. 237 the trachea. 239 Restiform body, 375. 377, 3S0, 381 Rete testis, tubules of. 276 vasa efferentia, 276 Reticular formation, 371. 377, 378, 381, 3 8 5- 3 8 9- 393 glands. 173 process. 342 tissue. 73 Retina, the. 420 blood-vessels of, 428 cells of. 420, 421. 422. 423 ellipsoid of Krause, 421 fibre baskets of, 423 fovea centralis, 423 ganglionic layer. 420 inner limiting membrane, 422 molecular layer, 421 nuclear layer. 421 layer of nerve cells. 422 of nerve fibres. 422 of neuro-epithelium. 420 of pigmented epithelium. 420 of rods and cones. 421 macula lutea, 423 Miiller's cells and fibres. 422 ora serrata. 420 Retina, outer limiting membrane. 420 molecular layer of. 421 nuclear layer of. 421 pars ciliaris retinae. 420 iridica retinae. 42c optica retinae. 420 . relation to optic nerve. 424 rod and cone cells of. 421 visual purple of. 421 Retzius, lines of, 186 Rhombencephalon. 332 Ribboning, paraffin sections. 13 Rod association neurones, 425 Rod fibres. 421 Rod-visual cells. 421 Rods, layer of rods and cones. 421 Rolando, gelatinous substance of, 342, 343 Rollett's theory of striated voluntary muscle, 94 Ruffini, corpuscles of, 325 Rugae, 194 Saccule, 437 and utricle, 437 auditory hairs of. 437 macula acustica, 437 neuro-epithelial or hair cells of, 437 otolithic membrane of. 437 otoliths of. 437 sustentacula! - cells of. 437 Safronin, 15 Salivary corpuscles. 141 glands, 217 blood-vessels of. 219 development of, 235 ducts of. 21S lymphatics of. 220 nerves of. 220 parotid, the. 218 structure of, 21S sublingual, 219 submaxillary. 219 technic of. 221 tubules of. 21S Santorini. duct of. 235 Sarcolemma. 92 Sarcostyles, 169 Scala media. 439 tympani. 439 4/6 IXDEX. Scala vestibuli. 439 Scarpa's ganglion. 3S4 Scheme of neurone relations of the spi- nal cord, y^ Schlemm. canal of. 41S Schmidt- Lantermann segments. 109 Schultze. comma tract of. 364 Schwalbe. lymph paths of. 428 Schwann, sheath of. 109 Sclera, the. 412 lamina cribrosa of. 412 fusca of. 412 Scrotum, skin of. 312 Sebaceous glands, 315 development of, 327 Sebum. 323 Secondary cochlear tract. 3S4 trigeminal tract. 393 vestibular tract. 384 Secretion. 214 Secretory capillaries. Golgi method of demonstrating, 23 Section cutting. 12 celloidin specimens. 12 paraffin specimens. 12 staining. 16 Segmentation of ovum. 44 Semen. 280 Semicircular canals, 436. 438 crista acustica of, 438 cupula of, 438 raphe of, 438 semilunar fold of. 438 Seminal ducts. 276 epididymis. 276 vesicles. 278 Seminiferous tubule. 271 cells of. 272 convoluted portion of, 271 spermatids, 274 spermatocytes. 274 spermatogones, 273 straight portion of, 275 tubules of the rete testis. 276 Sensory decussation, 374, 377 peripheral nerves. 338 Septa renis, see Kidney Septum lingua-, 179 Serial sections. 1 3 Sertoli, cells of. 272 Sharpey's fibres. 152 Sheath of Henle, 339 of Schwann, 109 Silver-nitrate method of staining inter- cellular substance, 23 Skeletal system, articulations, 165 bone marrow, 152 bones, 14S cartilages. 164 Skin, 31 1 blood-vessels of, 324 color of, 315 corpuscles of Meissner, 349 of Rufhni. 325 derma of, 31 1 development of the, 326 epidermis of. 313 Golgi-Mazzoni corpuscles of. y^ hair follicle of. 319 Krause's end- bulbs of, 326 lymphatics of, 325 Merkel's corpuscles of, 348 mitosis of cells of. 315 nerves of, 325, 348 of scrotum. 312 Pacinian bodies of, 349 panniculus adiposus of. 313 peripheral nerve terminations in. 34 s prickle cells of, y\ sebaceous glands of. 315 subcutaneous tissue of. 312 sweat glands of (glanduhe sudori- para?). 315 sweat pores of. 315 tactile cells of, 348 corpuscles of, 326, 349 technic of, 316 for blood-vessels of. 326 Vater-Pacinian corpuscles of, 325 Skin and its appendages, 31 1 development of. 326 hair, 318 mammary gland, 327 the nails. 316 Small intestines, 200 Auerbach's plexus, 207 blood-vessels of, 211 lirunner's glands, 205 cells of, 201 chyle capillaries of. 213 coats of, 201 INDEX. 477 Small intestines, crypt of Lieberkuhn, -°3 lymphatics of, 213 Meissner's plexus, 205 muscle of, 205 nerves of, 213 Peyer's patches, 204 te clinic of. 216 valvule? conniventes of, 200 villi of, 200 Smooth muscle ; see Involuntary muscle Sodium hydrate, as a macerating fluid, 4 Solitary fasciculus, 374, 377, 3S0 follicles. 19S Somatochromes, 104 Spermatids. 274 Spermatocytes. 274 Spermatogenesis, 281 Spermatogones. 273 Spermatozoa, 42, 2S0 development of, 281 technic of. 2S2 Spermatozoon, 42 Spinal cord, the. 340 anterior column of. 342 horns of, 342 median fissure, 341 nerve roots of, 342 white commissure of, 343 antero-lateral column of, 342 arachnoid membrane of, 334 cell column of the lower extremity, 353 column of the upper extremity, 353 central canal of, 342 gelatinous substance of, 342, 343 cervical enlargement of, 340 Clarke's column of, 344 column of Burdach, 346. 360 of Goll. 346, 360 cornua of, 342 crossed pyramidal tract, 352, 362 descending paths from higher cen- tres, 365 direct ascending paths to higher centres. 365 pyramidal tract, 352. 362 reflex path of, 364 dorsal gray commissure of, 3J.2 dura mater of, 334 Spinal cord, efferent fibre systems of, 362 fibre tracts of, 358 filum terminale of, 340 fundamental columns of, 364 gelatinous substance of Rolando of, 342, 343 gray matter of, 340. 342 ground bundles of, 364 indirect ascending paths to higher centres, 365 reflex paths of, 365 intermedio-lateral column. 353 lateral column of, 342 longitudinal section of six days' chick embryo, 357 lumbar enlargement of, 340 main motor fibre systems of. 362 medial column of. 353 medullated fibres of, 343. 344 membranes of, 334 multipolar ganglion cells of. 343 neuroglia cells of. 343 tissue of. 342 origin of fibres of white matter. 346 of posterior columns of, 346 peripheral motor or efferent neu- rone system, 353 pia mater spinalis of, 334. 341 posterior column of, 342 horns of, 342 median septum. 341 root fibres of, 342. 343 pyramidal tracts. 352. 362 reticular process of, 342 section through cervical enlarge- ment of. 345 through lumbar enlargement. 34i through mid-dorsal region of, 344 through the twelfth dorsal seg- ment. 344 segments of, 340 short fibre systems of. 364 spinal ganglion cell. 346 technic of. 340. 343. 356. 366 transverse section of six days' chick embryo, 356 ventral gray commissure of. 342 white commissure of. 343 4/8 IXDEX. Spinal cord, white matter of. 340, 342 zone of Lissauer of. 343 Spinal ganglia. 336 development of. 334 technic of. 33S Spinal ganglion cells, ascending arms of central processes of. 352 centrally directed arm of. 351 collaterals, 352 descending arms of central proc- esses. 352 development of. 347 ectodermic origin of, 346 peripheral arms of, 347 relation to dorsal roots, 351 technic of, 356 Spiral ganglion, 444 ligament, 43S prominence, 440 terminations, 350 Spireme, 39 Spleen, the. 142 blood vessels of, 144 cells of, 145 connective tissue of, 142 corpuscles of, 143 lymphatics of, 146 Mall's theory of vascular channels of pulp. 145 Malpighian bodies of, 142 cords of, 144 nerves of. 146 pulp of. 144 spindles of, 144 technic of, 146 Splenic corpuscles, 143 pulp, 144 Spongioblasts of His, 333 Spongioplasm, 34 Staining, 13 double with ha.-matoxylin-eosin, 16 in bulk. 17 methods, special neurological, 25 paraffin sections, 19 sections, t6 triple, wilh hematoxylin picro- acid fuchsin, \(> special methods of. 23 Stains, nuclear dyes. 13 plasma dyes. 13 Stalked hydatid. 310 Stapes. 435 Stomach, the, 194 Auerbach's plexus, 207 blood-vessels of, 211 cardiac glands of, 196 chief cells of. 196 development of, 235 epithelium of, 194 gastric crypts of, 194 glands of, 195 pits of, 194 lymphatics of, 213 mucous membrane of, 194 muscular coat of, 199 nerves of, 213 parietal cells of, 196 peptic glands of, 195 rugae of, 194 secretion, 214 solitary follicles of, 198 technic of, 199 Stomata, 129 Stratum fibrosum, 165 synovial, 166 Stria vascularis, 440 Stroma of mucous membranes, 174 Styloglossal fibres, 179 Sublingual gland. 219 duct of Bartholin of, 219 Submaxillary gland, 219 Wharton's duct of, 219 Submucosa of mucous membranes, 174 Subperichondral ossification, 162 Subperiosteal ossification, 162 Substantia nigra, 391, 393 propria cornea;', 414 Sulcus, external spiral, 440 Superior cerebellar peduncles, 393 olive, 387, 389 Suspensory ligament, 426 Sustentacular cells, 224 Sweat glands, 315 development of, 327 muscle tissue of, 327 Sympathetic ganglia, 337 development of, 334 technic of, 338 Sympathetic nervous system, see Ner- vous system (sympathetic) Synarthrosis, 165 Synchondrosis, 165 INDEX. 479 Syncytial tissue, 98 Syncytium, 305 Syndesmosis, 165 Synovial membrane, 166 villi, 166 Tactile cells, 348 corpuscles, 326 of Meissner, 349 meniscus, 348 Tapetum cellulosum, 415 fibrosum, 415 Tarsal glands, 430 Tarsus, 430 Taste buds, 182, 349, 448 Tautomeres, 354 Teasing, 4 Teeth, the, 138 blood-vessels of, 187 cementum of, 1S6 crown of, 183 dental periosteum. 187 dentinal pulp of, 183 dentine of, 183 development of, 187 ; see also under Development of teelh enamel of, 186 lymphatics of, 187 nerves of, 187 Neumann's dental sheath, 185 odontoblasts of, 184 peridental membrane, 187 pulp cavity of, 183 root of, 183 technic of, 189 Tegmentum, 391 Telophase, 42 Tendon, structure of, 67 sheaths, 168 Tenon, capsule of, 428 Tensor choroideae, 418 Terminal nucleus, 359 Testis, 270 corpus Highmori or mediastinum testis, 270 ducts of, 276 epididymis of, 271 mediastinum, 270 secretion of. 280 seminiferous tubule of. 271 spermatozoa, 271 Testis, technic of. 282 tunica albuginea of, 270 Theca follicular, 289 Thionin, 15 Thoma, ampullae of, 146 Thrombocytes, 88 Thymus, the, 138 blood-vessels of, 139 development of, 138 Hassal's corpuscles, 139 lymphatics of, 139 nerves of, 139 structure of, 138 technic of, 139 Thyroid, 251 blood supply of, 252 colloid of, 251 development of, 252 isthmus of, 251 lymphatics of, 252 nerves of, 252 structure of, 251 technic of, 253 Tissue elements, dissociation of, 4 Tissues, 49 adipose, 75 blood, 85 bone, 82 cartilage, 79 classification of, 51 connective, 63 epithelial, 53 erectile, 285 examination of fresh, 4 histogenesis of, 51 lymphatic, 75 muscle, 91 nerve, 101 osteogenetic. 157 subcutaneous, 312 Toluidin blue, 15 Tomes' process, 189 Tongue, the, 179 blood-vessels of, 181 circumvallate papilla-, 1S0 connective tissue of. 179 Ebner's glands, 1S2 end-bulbs of Krause. 1S2 filiform papilla?, 180 fungiform papillae, 1S0 glands of. 1S1 480 INDEX. Tongue, longitudinal fibres of. 179 lymph follicles of, 141, 1S1 spaces, 1S2 muscles of, 179 nerves of, 1S2 septum linguae. 179 taste buds. 1S2 technic of. 1S2 transverse fibres of, 179 vertical fibres of. 179 Tonsils, the, 140 blood-vessels of, 142 crypts of, 140 development of. 142 lingual: follicuii linguales, 141 lymphatics of, 142 nerves of, 142 palatine or true, 140 pharyngeal tonsils, 142 salivary corpuscles of, 141 structure of. 140 technic of. 142 Trachea, 239 cartilages of the, 240 structure of the, 239 technic of the, 242 Tract of Flechsig, 361 Transitional leucocytes, 87 Trapezium, 387 Tunica albuginea, 284 dartos. 312 propria, of mucous membranes, 174 vaginalis. 270 Tympanic membrane, 434 Tympanum. 434; see also Ear, middle Tyson, glands of, 286 Ultimate fihkilLjE, 93 I 'nipolar nerve cells, 102 Ureter, 264 technic of, 269 Urethra, male, 286 blood-vessels of, 287 fossa navicularis, 287 glands of, 287 structure of, 286 technic of, 288 Urinary bladder, 265 epithelium of, 265 Urinary system, 254 adrenal, 266 Urinary system, development of, 308 kidney, 254 kidney-pelvis, 264 ureter, 264 urinary bladder, 265 Uriniferous tubule, 256 arched tubule of, 260 ascending arm of Henle's loop, 259 descending arm of Henle's loop, 259 epithelium of, 260 first on proximal convoluted tubule of, 259 Henle's loop of, 259 Malpighian body, 25S neck of, 259 second or distal convoluted tubule, 259 straight or collecting tubule of, 260 Uterus, blood-vessels of, 306 decidual cells of, 303 development of, 308; see also De- velop meat of reproductive system lymphatics of, 307 masculinus, 283 muscle tissue of, 299 nerves of, 307 placenta, 304 mucosa of menstruating, 301 of pregnant, 303 of resting, 300 stage of menstruation proper, 302 of preparation, 301 of reparation, 302 structure of, 299 technic of, 310 with placenta in situ, technic of, 310 Utricle, 437 ; see also Saccule and Utricle Utriculosaccular duct, 436 Utriculus prostaticus, 283 Vagina, 307 blood-vessels of, 308 nerves of, 308 technic of, 310 Valvulae conniventes, 200 Vas deferens, 277 ampulla of, 278 technic of, 282 Vas epididymis, 277 INDEX. 48 1 Vasa efferentia, 277 vasorum, 124 Vascular papillae, 312 Vascular system, see Circulatory system Vater-Pacinian corpuscles. 325 Veins, 122 adventitia of, 124 central, 229 coats of, 122 development of, 128 intima of, 123 media of, 123 portal, 229 stellate, of Verheyn, 263 technic of, 124 vasa vasorum, 124 venae vorticosae, 415 Venae vorticosae, 415 Ventricle, fourth, 378, 380, 386, 389 Verheyn, stellate veins of. 263 Vermiform appendix. 208 lymph nodules of. 210 technic of. 216 Vesicle, anterior cerebral. 332 middle cerebral. 332 optic. 431 otic, 444 posterior cerebral. 332 Vesicula prostatica, 283 Vestibular ganglion, 384 Vestibule, 308, 436 ductus reuniens of. 436 endolymphatic duct. 436 saccule of, 436. 437 utricle of. 436, 437 utriculo saccular duct of, 436 Vieussens, valve of. 391 Villi, 201 Visual purple, 421 Vitreous body, 427 Cloquet's canal, 427 hyaloid canal of, 427 Vitreous membrane of choroid, 416 Vocal cords, the, 239 Volkmanivs canal. 151 Voluntary striated muscle, 92 Cohnheim*s held. 94 end-bulbs of. 350 Hensen's line. 93 Krause's line, 93 muscle columns of Kolliker, 94 discs, 93 spindles of. 350 nerves of, 350 Pacinian corpuscles of, 350 Rollett's theory, 94 sarcolemma, 92 technic of, 100 white and red fibres, 95 Von Bechterew's nucleus, 383 Von Monakow, tract or bundle of, 363, 37°. 376, 378. 3 s * • 3841 389. 393 Weigert's elastic-tissue stain, 23 method of staining medullated nerve fibres, 26 Weigert Pal method, 26 Wirsung, duct of. 222 Wolffian bodies. 309 ridge. 309 Xylol, and cajeput oil for clearing, 18 -paraffin for embedding. 11 Zenker's fluid, for decalcifying, 8 for fixation, 6 Zinn, zonule of, 426 Zona pectinata, 441 pellucida, 292 tecta, 441 Zone of Lissauer, 343 of oval nuclei, 238 of round nuclei, 238 Zonula ciliaris, 426 Zonule of Zinn, 426 Zymogen granules, 222 8 1 s" a n s-s-( 8 b' COLUMBIA UNIVERSITY LIBRARIES 0041079361