Columtiia ®nibersJitp ^cfjool of Bental anb 0val ^urgetrp J^eference %ihxaxp ^ A TEXT-BOOK OF HISTOLOGY BY FREDERICK R. BAILEY, A. M., M. D. FOURTH REVISED EDITION PROFUSELY ILLUSTRATED NEW YORK WILLIAM WOOD AND COMPANY M D C C C C X 1 1 1 Copyright, 1913 By WILLIAM WOOD AND COMPANY THE . MAPLE . PRESS- YORK. PA PREFACE TO THE FOURTH EDITION The very gratifying approval which the previous editions of the Text-book received has made it seem unwise to attempt any change in the general plan and scope of the work as outlined in the preface to the first edition. The text has been thoroughly revised, some parts of it rewritten. Some figures have been replaced by new ones and a considerable number of new figures, have been added. For these the writer wishes to acknowledge his obliga- tions. To Prof. H. von W. Schulte and to Mr. A. M. Miller the writer is indebted for many valuable criticisms and suggestions. The chapter on the nervous system which was rewritten by Dr. Oliver S. Strong for the third edition, has been revised by him for the present edition. For Dr. Strong's careful and painstaking work on this chapter, for his thoroughly original treatment of his subject, and for the original drawings and photographs in this chapter, the author wishes again to express his most grateful appreciation. Dr. Strong wishes to acknowledge his indebtedness to Dr. Adolf Meyer for many ideas and terms found valuable in the preparation of the chapter on the nervous system. Digitized by the Internet Archive in 2010 with funding from Open Knowledge Commons (for the Medical Heritage Library project) http://www.archive.org/details/textbookofhistol1913bail PREFACE TO THE FIRST EDITION The primary aim of the writer in the preparation of these pages has been to give to the student of medicine a text-book of histology for use in connection with practical laboratory instruction, and espe- cially to furnish to the instructor of histology a satisfactory manual for classroom teaching. With these objects in view, the text has been made as concise as possible consistent with clearness, and the writer has attempted to make the more essential elements stand out somewhat from the necessarily accompanying details. It has been impossible to accomplish this without some sacrifice of uniformity of treatment and of logical sequence. This is especially 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 is 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 tech- nic 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. For all of these the author wishes to express his appreciation and obli- gation. 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. Free- born, his predecessor as Instructor of Histology at the College of Phy- sicians and Surgeons, for many valuable suggestions; and to Dr. T. Mitchell Prudflcn for his careful and critical review of the author's copy. CONTENTS PART I.-HISTOLOGICAL TECHNIC CHAPTER I General Technic. Page. General Considerations, 3 Examination of Fresh Tissues, 3 Dissociation of Tissue Elements, 4 Teasing, 4 INIaceration, 4 Preparation of Sections, S Fixation, S Hardening, ° Preserving, 9 Decalcifying, ^° Embedding, ^^ Celloidin Embedding, ^'^ Paraffin Embedding, ^3 Section Cutting, ^4 Celloidin Sections, ^4 Paraffin Sections, ^5 Frozen Sections, ^5 Staining, ^7 Nuclear Dyes, ^7 Plasma Dyes, ^9 • Staining Sections, ^° Staining in Bulk, ^i Mounting, ^^ Staining and Mounting l*araQin Sections, 23 Injection, ^S CHAPTER II Special Staining Methods. Silver Nitrate Method of Staining the Intercellular Substance, 28 Chlorid of Gold for Demonstrating Connective-tissue Cells, 28 Weigert's Elastic Tissue Stain, 28 Verhoeff's Differential Elastic Tissue Slain, 28 Golgi's (!hromc-silver for staining Secretory 'I'ubiilcs 29 Mallory's Phosphomolybdic Acid Ha;matoxylin Stain for Connective Tissue, 29 Mallory's Phosphotungstic Acid Haematoxylin Stain for Connective Tissue, . 30 vii viii CONTENTS Page Mallory's Aniline Blue Stain for Connective Tissue 3° Maresh's Modification of Bielschowsky's Stain for the Finer Connective Tissue Fibrils, ^i Osmic Acid Stain for Fat, 3i Jenner's Blood Stain, 3 1 CHAPTER III Special Neurological Staining Methods. Weigert's Method of Staining Medullated Nerve Fibres, 32 Weigert-Pal Method, - 33 Marchi's Method for Staining Degenerating Nerves, 34 Golgi Methods of Staining Nerve Tissue, 35 Slow Method, 35 Rapid Method, 35 Mixed Method, 35 Formalin Bichromate Method, ■, 3^ Bichloride Method, 36 Golgi-Cox Method, 36 Cajal's Method, 37 Nissl's Method, • • ■ 38 General References on Technic, 39 PART II.-THE CELL CHAPTER I The Cell, . 43 General Structure, 43 Structiure of a Typical Cell, . 43 The Cell Body, 44 The Cell Membrane, 47 The Nucleus, 47 The Centrosome, 49 Vital Properties of Cells, . 50 Metabolism, 50 Function, 51 Irritability, 51 Motion, 51 Amoeboid, 52 Protoplasmic, 52 Ciliary, 52 Reproduction, 52 Direct Cell-division, 53 Indurect Cell-division, 53 Fertilization of the Ovum, . ■ 58 Technic, ■ 63 References for further study, 64 CONTENTS IX PART III.-THE TISSUES CHAPTER I Page Histogenesis — Classification, 67 Tissues Derived from Ectoderm, 67 Tissues Derived from Entoderm, 67 Tissues Derived from Mesoderm, 68 CHAPTER II Epithelium (Including Mesothelium and Endothelium), 69 Histogenesis, 69 General Characteristics, 69 Classification, 7° Simple Epithelium, 7^ Simple Squamous, 7^ Simple Columnar, 7^ Pseudostratified, 73 Stratified Epithelium, 73 Stratified Squamous, 73 Stratified Columnar, 74 Transitional, 74 Modified Forms of Epithelium, 75 Ciliated Epithelium, ■, ■ • 75 Pigmented Epithelium, 76 Glandular EpitheUum, 77 Neuro-epithelium, 77 Mesothelium and Endothelium, 77 Technic, 7^ CHAPTER III The Connectrtl Tissues, So General Characteristics, 80 Classification, 81 Development, 81 Connective Tissue, Proper 82 Embryonal Connective Tissue, 82 Fibrillar Connective Tissue, 82 Connective-tissue Cells, 83 Intercellular Substance, 86 Areolar or Loose Connective Tissue, 87 Fat Tissue, 87 Formed Connective Ti.ssue, 91 Tendons and Ligaments, 91 Elastic Tissue, 92 Reticular Tissue, 94 Technic, 95 Cartilage, 97 Hyaline, 98 x: CONTENTS Page Elastic, 99 Fibrous, 99 Technic, 100 Bone Tissue, 100 Technic, 102 CHAPTER IV The Blood, 103 Red Blood Cells, 103 White Blood Cells, 105 Blood Platelets, 108 Blood Dust, . 108 Development, 108 Technic, no CHAPTER V Muscle Tissxje, in Involuntary Smooth Muscle, in Voluntary Striated Muscle, in Involuntary Striated Muscle (Heart Muscle), 119 Development of Muscle Tissue, 122 Technic, 124 CHAPTER VI Nerve Tissue, 126 The Neurone, 126 General Structure, 126 The Cell Body, 126 The Nucleus, 127 The Cytoplasm, 128 Neurofibrils, 128 Perifibrillar Substance, 128 Chromophilic Bodies, 129 The Protoplasmic Processes ot Dendrites, 131 The Axone, 132 Non-meduUated Axones (Non-medullated Nerve Fibres), . 132 Medullated Axones (Medullated Nerve Fibres), 133 Theories as to Physiology of the Neurone, 137 Significance of Degenerative Changes in the Neurone, 139 Neuroglia, 142 Technic, 144 General References, 145 PART IV.— THE ORGANS CHAPTER I The Circulatory System, 151 : , The Blood-vessel System, - • • iSi CONTENTS ^^ Page 151 General Structure, ^^^ Capillaries, j^^ Arteries, .... 158 Veins, 160 Technic, . . 161 The Heart, ^^^ Technic, ^5, Development of the Circulatory System ' . . 164 The Lymph-vessel System, • •■ ' ^5^ Lymph Capillaries, . . 165 Lymph Spaces, j5g Development of Lymph-vessel System, ' . 166 Technic, • ■ ■ ^^^ General References on Circulatory System, CHAPTER II 167 LYMPH.A.TIC Organs, ^^^ The Lymph Nodes, . . 171 Development, Technic, ^-^ Hsmolj^mph Nodes, . . 17S Technic, ^^^ The Thymus, ^^^ Development, j. g Technic, j«g The Tonsils, . . 178 The Palatine Tonsils, ^g^ The Lingual Tonsils, ^g^ The Pharj-ngcal Tonsils, . 180 Development, ^g^ Technic, .181 The Spleen, 187 Technic, jg^ General References, CHAPTER HI .... 188 The Skeletal System ^gg The Bones, ^^2 Bone Marrow, ^ Red Marrow, Yellow Marrow, . Technic, ^^^ Development of Bone, Intramcmbranous Development Intracartilaginous Development, ^^^ Subperiosteal, Growth of Bone, 203 Technic, 204 The Cartilages, xil CONTENTS Page Articulations, 204 Technic, 205 General References, 206 CHAPTER IV The Muscular System. A Voluntary Muscle, 207 Tendon Sheaths and Bursse, 208 Growth of Muscle, ' 209 Technic, 211 CHAPTER V Glands and the General Structure of Mucous Membranes, 212 Glands — General Structure and Classification, 212 Duct Glands, 215 Tubular Glands 215 Aveolar Glands, 217 Ductless Glands, 217 General Structure of Mucous Membranes, 218 CHAPTER VI The Digestive System, 220 Anatomical Divisions, 220 The Headgut, 221 The Mouth, 221 The Mucous Membrane of the Mouth, 221 Glands of the Oral Mucosa, 221 Technic, 223 The Tongue, 223 Technic, ^ 227 The Teeth, * 227 Development of the Teeth, 238 Technic, 242 The Pharynx, 242 Technic, 243 The Foregut, ' 243 The Oesophagus, 243 Technic, 245 General Structure of the WaUs of the Gastro-intestinal Canal, .... 245 The Stomach, 247 Technic, 254 The Midgut, ■ 255 The Small Intestine, 255 Peyer's Patches, 260 The Endgut, 263 The Large Intestine, 263 The Vermiform Appendix, 265 The Rectum, 266 CONTENTS X"l Page The Peritoneum, Mesentery, and Omentum, 267 Blood-vessels of the Stomach and Intestine, 268 Lymphatics of the Stomach and Intestine, 270 Nerves of the Stomach and Intestine, 270 Secretion and the Absorption, ^7i Technic, '^ The Larger Glands of the Digestive System, 275 The SaUvary Glands, ^^0 The Parotid, ^' -^ The Subungual, ^77 The Submaxillary, ^^8 Technic, The Pancreas, Technic, The Liver, ^^j Excretory Ducts of the Liver, -^9 The Gall-bladder, ^^5 Technic, ^^^ Development of the Digestive System, 29 General References, CHAPTER VII The Respiratory System, "99 TheNares, "^^ The Lar>'nx, ^ The Trachea, ^°^ Technic, ^ ^ The Bronchi, 304 The Lungs, ^ Development of the Respiratory System, 3^5 Technic, ^ ' General References, ■3^' CHAPTER VIII The Urinary System, 3i8 The Kidney, 3i8 The Kidney— Pelvis and Ureter, 329 The Urinary Bladder, 33© Technic, ^■^^ General References, 332 CHAPTER IX The Reproductive System, 333 Male Organs, 333 The Testis, ^^^ The Seminal Ducts, 339 The Ei)ididymis, 339 The Vas Deferens, 34° xiv CONTENTS Pack The Seminal Vesicles and Ejaculatory Ducts, 341 Rudimentary Structures Connected with the Development of the Genital System, 342 The Spermatozoon, 343 Development of the Spermatozoon, 344 Technic, 346. The Prostate Gland, 347 Cowper's Glands, 348 Technic, 349 The Penis, , 349 The Urethra, ■ 351 Technic, 352 Female Organs, 352 The Ovary, 352 The Graafian Follicle, 354 The Corpus Luteum, 359 Rudimentary organs connected with the Ovary, 363 The Oviduct, 364 Technic, 365 The Uterus, 366 The Mucosa of the Resting Uterus, 367 The Mucosa of the Menstruating Uterus, . . '. 368 The Mucosa of the Pregnant Uterus, 370 The Placenta, 371 The Vagina, 375 Development of the Urinary and Reproductive Systems, 376 Technic, 379 General References, 379 CHAPTER X The Skin and its Appendages, 380 The Skin, 380 Technic, 384 The Nails, 385 Technic, 387 The Hair, ■ 387 Technic, 393 Blood-vessels of Skin, 393 Technic, 395 Development of Skin, Nails, and Hair, 39S The Mammary Gland, 395 Technic, 401 General References, 401 CHAPTER XI The Thyreoid and Parathyreoid, the Pituitary Body, the Paraganglia and THE Adrenal. The Thyreoid, 402 Development, 403 CONTENTS XV Page The Parathvreoids, 404 Technic 4o6 The Pituitary Bocb', 4°; Paraganglia, 409 Carotid Glands, 4io Coccygeal Gland, 4" Adrenal Gland, 4i2 General References, 4iS CHAPTER XII The Nervous System, 4^6 Histological Development and General Structure, 4i6 Membranes of the Brain and Cord, 422 Technic, 424 The Peripheral Nerves, 424 Technic, 426 The Afferent Peripheral Neurones, 426 The Cerebro-spinal GangUa, 426 The Peripheral Processes of the Cerebro-spinal GangUon Cells, . 429 The Central Processes of the Cerebro-spinal Ganglion Cells, ... 436 The Sympathetic GangUa, 436 Technic. 44i The Efferent Peripheral Cerebro-spinal Neurones, 44i The Spinal Cord, 442 Origin of the Fibres which make up the White Matter of the Cord, ... 443 (i) The Spinal Ganglion Cell and the Origin of the Posterior Col- umns, 443 (2) Cells Situated in Other Parts of the Central Nervous System which Contribute Axones to the White Columns of the Cord, . 444 (3) Root Cells— Motor Cells of the Anterior Horn, 444 (4) Column Cells, 444 (5) Cells of Golgi Type II, 445 Technic, 445 Practical Study, 447 General Topography of the Cord, Cell Groupings, Arrangement of Fibres and Finer Structure, 44° Practical Study of Sections through Lumbar Enlargement, .... 448 General Topography, 44° Cell Groupings, . . . . 45° Arrangement of Fibres, 452 Finer Structure, 453 Blood-vessels, 454 Variations in Structure at Different Levels, 455 I'ractical Study, 455 Section through the Twelfth Thoracic Segment, 455 Section through the Mid-thoracic Region, 455 Section through the Cervical Enlargement, 455 Fibre Tracts of the Cord, 459 Ascending Tracts, 46 1 I. Long Ascending Arms of Dorsal Root Fibres, 461 xvi CONTENTS Page II. Spino-thalamic Tract, - ... 462 III. Dorsal Spino-cerebellar Tract, 463 IV. Ventral Spino-cerebellar Tract, 463 Descending Tracts, * 464 I. The Pyramidal Tracts, 464 II. The Colliculo-spinal Tract, 465 III. The Tract from the Nucleus of the Posterior or Medial Longitudinal Fasciculus, 465 IV. The Rubro-spinal Tract, 466 V. The Deitero-spinal Tract, 466 VI. The Fasciculus of Thomas, 467 VII. Helweg's Tract, 467 VIII. The Septo-marginal Tract, 467 IX. The Comma Tract of Schultze, 467 Fundamental Columns or Ground Bundles, 468 A Two-neurone Spinal Reflex Arc, .• . . . 469 A Three-neurone Spinal Reflex Arc, 469 A Cerebellar Arc, 470 A Cerebral or Pallial Arc, 471 Technic, 47i The Brain, 473 General Structure, 473 Segmental Brain and Nerves, 474 Suprasegmental Structures, '. 478 The Hindbrain or Rhombencephalon, 479 The Medulla Oblongata or Bulb, '. 479 The Pons, 482' The Cerebellum (also p. 507), 482 Technic, . 482 Practical Study, 483 1. Transverse Section of the Medulla through the Decussation of the Pyramidal Tracts (Motor Decussation), 483 2. Transverse Section of the Medulla through the Decussation of the Fillet or Lemniscus (Sensory Decussation), .... 485 3. Transverse Section of the Medulla through the Lower Part of the Inferior Olivary Nucleus, 487 4. Transverse Section of the Medulla through the Middle of the Olivary Nucleus, 401 5. Transverse Section of the Medulla through the Entrance of the Cochlear Root of Nerve VIII, 491 6. Section through the Hindbrain at Level of Junction of Pons, and Cerebellum and Entrance of Vestibular Nerve, . . . 499 7. Transverse Section of the Hindbrain through the Roots of Nerves VI (Abducens) and VII (Facial), 502 •8. Transverse Section of the Hindbrain through the Roots of Nerve V (Trigeminus), 504 The Cerebellum, So7 The Cerebellar Cortex, S09 The Isthmus, 5iS Practical Study, 5^5 CONTENTS xvii Page 9. Transverse Section through the Isthmus at the Exit of Nerve IV (Trochlearis) , 515 The Midbrain or Mesencephalon, 517 Practical Study, Si7 10. Transverse Section through Midbrain at Level of Anterior Corpora Quadrigemina and Exit of Nerve III (Oculomotor), 517 The Forebrain or Prosencephalon, 522 The Interbrain (Diencephalon or Thalamencephalon), 522 Practical Stud}', 524 11. Transverse Section through the Junction of Midbrain and Thalamus, 524 12. Section through the Interbrain at the Level of the Optic Chiasma, 526 The Endbrain or Telencephalon, 532 The Rhinencephalon, 532 The Corpus Striatum, 532 The Pallium 532 Practical Study, 53S 13. Transverse Section through the Cerebral Hemispheres, Corpora Striata and Thalamus, 535 The Cerebral Cortex, 536 Technic, 543 The Pineal Body, 544 Technic, 547 Table of Cranial and Spinal Nerves, 545 General Reference for Further Study, 547 ^"^^ilk. CHAPTER XIII The Organs of Special Sense, 548 The Organ of Vision, 548 The Eyeball, 548 The Cornea, 548 The Chorioid, 550 The Ciliary Body, 552 The Iris, 554 The Retina, 555 The Optic Nerve, 559 The Relations of Optic Nerve to Retina and Brain, 560 The Lens, 564 The Lacrymal Apparatus, 5^7 The Eyelid, 567 Development of the Eye, 569 Technic, 57° The Organ of Hearing, 572 The External Ear, 572 The Middle Ear, 573 The Internal Ear, 574 The Vestibule and Semicircular Canals, 575 The Saccule and Utricle, 575 xvm CONTENTS The Semicircular Canals, . . Page The Cochlea, ... 575 Development of the Ear, . ' 576 Technic, ' -g The Organ of Smell, ...'.'.' .584 Technic, eg The Organ of Taste, . . . .' ' ■ • 586 Technic, ^g„ General References, rg? '^""^' ■■'■'.'.'.'.'.. • • 588 589 PART I HISTOLOGICAL TECHNIC CHAPTER I GENERAL TECHNIC Certain body fluids, e.g., blood, urine, etc., may be examined by simply placing them on a slide under a cover-glass. A few tissues, e.g., thin membranes, such as the omentum and the mesentery, may be examined fresh in some such inert medium as blood serum or normal salt solution (0.75-per-cent. aqueous solution sodium chlorid). For such examination the tissue is immersed in the salt solution on a slide and covered with a cover-glass. Most tissues and organs, how- ever, require much more elaborate preparation to render them suitable for microscopic examination. Tissues too dense and thick to be readily seen through with the microscope must be so treated as to make them transparent. This is accomplished either by pulling the tissue apart into fine shreds, teasing, or by cutting it into thin slices, section cutting. Some tissues admit of teasing in a fresh condition; others can be satisfactorily teased only after they have been subjected to the action of a chemical which breaks down the substance holding the tissue elements together, maceration. Fresh tissue can rarely be cut into sections sufficiently thin for microscopic examination. It must first be treated in such a manner as to preserve as nearly as possible the living tissue relations, fixation. If too soft for section cutting it must next be put through a process known as hardening. If, however, as in the case of bone, the tissue is too hard, it must be softened by dissolving out the mineral salts, decalcification. If very thin sections are to be cut, it is further necessary to impregnate the tissue with some fluid substance which will harden in the tissue and give to the mass a firm, even consistency. This is known as embedding. Furthermore, most tissue elements have refractive indices which are so similar that their differentiation under the microscope 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 of intensity. This is known as differential or selective staining. 3 4 HISTOLOGICAL TECHNIC The final step in the process is the mounting of the specimen, after which it is ready for microscopic study. Only the more common procedures used in the preparation of tissues for microscopic study are described in this section. At the end of each section are given the technical methods most satisfactory for the demonstration of the tissues described in that section. For other methods the student is referred to special works upon micro- scopic technic. Dissociation of Tissue Elements This method of preparing tissues for microscopic study has only a limited application, most specimens being preferably fixed, and cut into thin sections. Certain of the structural features of such tissues as nerves, muscle, and epithelium, which have but little intercellular substance may be well demonstrated by dissociation. This is accomplished by (i) teasing, or (2) maceration, or both. (i) Teasing. — This consists in pulHng apart fresh or preserved tissues by means of teasing needles. Instructive specimens of such tissues as muscle and nerve may be obtained in this way. (2) Maceration. — This is the subjecting of a tissue to the action of some chemical which breaks down the substance uniting the tissue elements, thus allowing them either to fall apart or to be more easily dissociated by teasing. The most commonly used macerating fluids are: (a) Ranvier's Alcohol (33-per-cent., made by adding 35 c.c. of 96-per-cent. alcohol to 65 c.c. of water). — Bits of fresh tissue are placed in this fluid for from twenty-four to forty-eight hours. The cells may then be easily separated by shaking or by teasing. Ran- vier's alcohol is an especially satisfactory macerating fluid for epithelia. (b) Formaldehyde, in very dilute solutions (0.2- to 0.4-per-cent. commercial formalin^) .- — ^Tissues should remain in the formaldehyde solution from twenty-four to forty-eight hours. This is also espe- ^cially useful for dissociating epithelial cells. f j-^j (c) Sodium or Potassium Hydrate (30- to 3 5-per-cent. aqueous solu- tion).— From twenty minutes to an hour is usually sufficient to cause the tissue elements to fall apart or to be readily pulled apart with teas- ing needles. If it is at any time desirable to stop the action of the ., ^Commercial formalin is a 40-per-cent. solution of formaldehyde gas in water. GENERAL TECHNIC 5 caustic alkali, this may be accomplished by neutralizing with glacial acetic acid or by replacing the alkali with a 6o-per-cent. aqueous solution of potassium acetate. The specimens 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. {d) Nitric acid (10- to 20-per-cent. aqueous solution). — This is especially useful for dissociating involuntary and voluntary muscle. After any of the above procedures, the macerating fluid containing the tissue elements should be placed in a long tube, allowed to stand for a time and the fluid decanted. Water is then poured into the tube, the tissues allowed to settle and the water poured off, this being repeated until all trace of macerating fluid is removed. The tissue elements may then be preserved or mounted in glycerin or in glycerin jelly. It is frequently advisable to stain the tissues. For this purpose alum-carmin (p. 19) is especially satisfactory. (For details see technic i, p. 124 and technic 2, p. 124), After staining and wash- ing, the tissues may be preserved or mounted in glycerin, eosin glyc- erin, or glycerin jelly. It frequently happens that on examining dissociated tissue elements after mounting, the bits of tissue are still too large. This may be remedied by gently tapping on the cover-glass with a lead pencil. PREPARATION OF SECTIONS I. Fixation Even tissues, the structure of which admits of their being exam- ined in the fresh condition, as described on p. 3, soon undergo post-mortem changes if placed in an inert medium such as normal salt solution or blood serum. As a result they soon lose their char- acteristic appearance and disintegrate. A proper killing or fixation is therefore the first step in the preparation of most tissues for micro- scopic study, the object being to so preserve the tissues that they retain as nearly as possiljlc, and more or less permanently, the same structure and relation which they had during life. Fixed tissue may be placed in water, salt soluticm, alcohol, etc., in fact may be subjected to many and varied manipulations which would destroy fresh tissues, without disturbing the relation of the tissue elements. Fixation of such a thin film of tissue as a blood smear may be accomplished by 6 HISTOLOGICAL TECHNIC heating. A blood smear or even small pieces of tissue may be fixed by exposure to certain chemical vapors as, e.g., the vapor of formalin or of osmic acid. Fixation is, however, usually accomplished by means of chemicals in solution, the solution being known as a fixing agent or fixative. The tissue is immersed in the fixative and allowed to remain there until fixation is complete. The pieces of tissue should be small, and large quantities of the fixative should be used. It may be necessary to change the fluid a number of times, in order to keep it up to the proper strength. The length of time required depends upon the character of the tissue and upon the fixative used. In general it should be only long enough to bring about the desired result, as prolonged immersion in the fixative may make the tissue brittle or may interfere with the subsequent treatment of the tissue, especially staining. Organs and even bodies may be fixed in toto by injecting the fixa- tive through an artery and allowing it to escape through the veins. After the injection, the whole specimen should be placed in a large quantity of the same fixative. This method fills the entire vascular system including the capillaries with the fixative, thus bringing the latter into very prompt and close contact with the tissue elements. The result is a very rapid and accurate fixation, which is especially valuable where it is necessary to preserve the topographic relations of various parts of an organ or a body. A mercuric chlorid solution (p. 8) followed immediately by strong alcohol makes a very good injection fixative. Satisfactory fixation is largely dependent upon the freshness of the tissue when placed in the fixative. The following are the fixatives in most common use: (i) Strong Alcohol (96-per-cent.). — This is a rapid fixative and should be used on small pieces of tissue. The time required is from six to twenty-four hours, though tissues may remain longer without injury. The alcohol should be changed after two or three hours. This fixative should not be used where fine histological detail is desired, since it causes some shrinkage. One advantage in its use is the fact that tissues are hardened and ready for embedding at the end of fixation. (2) Dilute Alcohol (30-per-cent. to 80-per-cent.) . — This, as a rule, gives unsatisfactory results, causing much shrinkage of the tissue elements. (3) Formalin (2-per-cent. to lo-per-cent. aqueous solution). — Formalin is rapid in its action and probably has better penetrating GENERAL TECHNIC 7 qualities than any other fixative. For general purposes a lo-per-cent, solution (i part commercial formalin to 9 parts water) should be used. This fixes in from six to twenty-four hours. The results after formalin are not always good, owing to the fact that it has Uttle hardening power, and the subsequent action of alcohol is Hkely to cause some distortion of the tissues. It acts better when combined with other fixatives than when used alone. (See Orth's fluid.) (4) Midler's Fluid. Potassium bichromate, 2 . 5 gm. Sodium sulphate, i . o gm. Water, 100. o c.c. This fluid gives very good results, but is extremely slow in its action, requiring from a week to several months. Fairly large pieces of tissue may be fixed, but in all cases large quantities of the fixative should be used and frequently renewed. (5) Orth's Fluid. Miiller's fluid (double strength), [ _, , Formalin, 8-per-cent., J This is one of the best general fixatives. Its action is similar to that of Miiller's fluid but much more rapid, fixation being accomplished in from twenty-four to forty-eight hours, though specimens may remain in the fluid several days without disadvantage. Fairly large pieces of tissue may be fixed with good results. The fixative should be changed after a few hours. Fixation with Orth's fluid gives an excellent basis for a hsematoxylin-eosin stain (see (i), p. 20.) The fixative should always be freshly prepared. It is convenient to keep the 8-per-cent. formalin solution and the double-strength Miiller's fluid in stock. Orth's fluid is then prepared by simply tak- ing equal parts of each. (6) Osmic Acid. — This, in a i-per-cent. aqueous solution, is a quick and excellent fixative of poor penetrating power. Very small pieces of tissue must therefore be used. They should remain in the fluid from twelve to twenty-four hours. Osmic acid stains fat and myelin black and is consequently useful in demonstrating their presence in tissues. Fixation should take place in the dark. (7) Flemming's Fluid. Chromic acid, i-per-cent. aqueous solution, 25 c.c. Osmic acid, i-per-cent. aqueous solution, 10 c.c. Glacial acetic acid, i-pcr-ccnt. aqueous solution, 10 c.c. Water, 55 c.c. 8 HISTOLOGICAL TECHNIC Flemming's fluid is one of the best fixatives for nuclear structures, and is of especial value in demonstrating mitotic figures. Very- small pieces of tissue should be placed in the fixative, where they remain for from twenty-four hours to three days. The solution should be freshly made as required, or a stock solution without the osmic acid may be kept, and the latter added at the time of using. (8) Mercuric Chlorid. — This may be used either in saturated aqueous solution or in saturated solution in 0.75-per-cent. salt solu- tion. Fixation is complete in from twelve to twenty-four hours, and is usually very satisfactory. A saturated solution of mercuric chlorid in 5-per-cent. aqueous solution of glacial acetic acid also gives good results. (9) Zenker's Fluid. Potassium bichromate, 2 . 5 gm. Sodium sulphate, i . o gm. Mercuric chlorid, 5.0 gm. Glacial acetic acid, S . o c.c. Water, 100. o c.c. This fluid should be freshly made, or the salts may be kept in solution and the acetic acid added at time of using. Zenker's fluid is a good general fij^ative, but usually causes some shrinkage of the tissue elements. Fixation requires from six to twenty-four hours. The most serious drawback to Zenker's fluid is the fact that the mercuric chlorid sometimes produces dark, irregular precipitates in the tissues. This may be remedied, however, by the use of iodine and iodid of potassium in the hardening process (see Hardening, p. 9). (10) Picric acid is an excellent fixative for cytoplasm. It may be used in: (a) Saturated aqueous solution; (b) saturated solution of picric acid in i-per-cent. aqueous solution of acetic acid; (c) saturated solution of picric acid in 2-per-cent. aqueous solution of sulphuric acid. n. Hardening Most fixatives are also hardening agents if their action is pro- longed. This is, however, often detrimental. It is, therefore, customary, after fixation is complete, to carry the specimens, with or without washing, through successively stronger grades of alcohol for the purpose of hardening the tissues. For general histological GENER.^ TECHNIC 9 purposes the specimens may be transferred directly to 70-per-cent. or 80-per-cent. alcohol, which should be changed once or twice. In the case of delicate tissues the first grade of alcohol should be 40-per-cent. or 50-per-cent., the second 70-per-cent., and the third 80-per-cent. The specimens should remain in each grade from twelve to twenty-four hours. Washing the tissues after fixation is not a matter of indifference. In some cases water should be used, while in other cases water is liable to undo the action of the fLxative, in which cases alcohol must be used for washing. After fixation in alcohol no washing, of course, is necessary. Specimens fixed in strong alcohol are embedded immediately (see Embedding, p. 11), or preserved (see Preserving, p. 9). After fixation in dilute alcohol the specimens are passed through the graded alcohols up to 80-per-cent. After fixation in formalin solutions the specimens are passed directly through the graded alcohols without washing in water. Specimens fixed in any solution containing picric acid should not be washed in water, but passed directly through the alcohols; and it is usually necessary to change each grade in order to wash out the picric acid. Specimens fixed in osmic acid or any solution containing osmic acid should be washed in running water before being passed through the graded alcohols. After solutions containing potassium bichro- mate the specimens should be washed in water sufficiently to remove the excess of bichromate, though too prolonged washing seems to be detrimental. A precipitate forms in the alcohols, but this apparently does no harm. After mercuric chlorid or Zenker fixation the washing may be done either in water or in alcohol. To avoid precipitates in the tis- sues add a small quantity of an iodin solution (equal parts tincture iodin and lo-per-cent. aqueous solution potassium iodid) to any of the grades of alcohol. As the alcohol becomes clear more of the solu- tion is added until the alcohol remains slightly tinged. III. Preserving Hardened tissues are usually preserved in 80-pcr-ccnt. alcohol. Formalin in aqueous solutions of i-per-ccnt. to lo-per-ccnt. is also used as a preservative. In either case, when it is necessary to pre- 10 HISTOLOGICAL TECHNIC serve the specimens for a considerable length of time (several months or longer), the tissues are hkely to lose their staining qualities to a certain extent. Preserving the specimens in equal parts of strong alcohol, glycerin, and distilled water is successful as a partial remedy for this. IV. Decalcifying Tissues containing lime salts, like bones and teeth, must have the lime salts dissolved out before sections can be cut. This process is known as decalcification. Tissues to be decalcified must be firSt fixed and hardened. Fix- ation in Orth's fluid and hardening in graded alcohols give good re- sults. 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 scalpel. The time required varies with the size and hardness of the specimen and the decalcifying fluid used. 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. (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, i part. Water, 2 parts. Hydrochloric acid, sufficient to make a from 2-per-cent. to 5- per-cent. solution. The addition of the salt prevents swelling of the tissue. This fluid is slow in acting and should be changed every day. (2) Nitric Acid. — This is less used than the preceding. The strength should be from i-per-cent to lo-per-cent. aqueous solution. Weak solutions (i-per-cent. to 2-per-cent.) will decalcify small foetal bones in from three to twelve days. For larger bones stronger solutions and longer time are required. Transferring from the nitric solution to a 5-per-cent. alum solution for twenty-four hours before washing will prevent swelling of the tissue. Phloroglucin is some.- GENER.\L TECHNIC 11 times added to the nitric acid solution for the purpose of protecting delicate tissues or of allowing stronger solutions of the acid to be used. One gram of phloroglucin is dissolved in lo c.c. of nitric acid. To this are added loo c.c. of lo-per-cent. aqueous solution of nitric acid. Small pieces of tissue decalcify in a few hours. If less rapid action is desired, reduce the amount of nitric acid without changing the percentage of phloroglucin. (3) Small bones may be satisfactorily decalcified in Zenker's fluid (see Fixatives, page 8), or in the following: Picric acid. I part. Chromic acid, I part. Glacial acetic acid, V. Embedding S parts, Most hardened tissues are still not firm enough to be cut into the thin sections suitable for microscopic study. In order to support the tissue elements and render them more firm for section cutting, re- course 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 tissues. 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. — Two solutions should be made. Solution No. 2. Thick celloidin — a 5-per-cent. solution of cel- loidin in equal parts 96-per-cent. alcohol and ether. Solution No. i. Thin celloidin — made by diluting solution No. 2 with an equal volume of equal parts of alcohol and ether. The hardened tissues are placed successively in: Strong alcohol (96-per-cent.) twelve to twenty-four hours, to dehydrate. Equal parts alcohol and ether, twelve to twenty-four hours. Thin celloidin, twenty-four hours to several days. Thick celloidin, twenty-four hours or longer. The exact time tissues should remain in the different grades of celloidin depends upon the character of the tissue, the size of the specimen, and the thinness of section desired. Many tissues may be advantageously left for weeks in thin celloidin. 12 HISTOLOGICAL TECHNIC The specimen must now be "blocked" and the celloidin hardened. By blocking is meant fastening the specimen impregnated with cel- loidin to a block of wood or other suitable material which may be clam.ped in the microtome (see Section Cutting, p. 14). The specimen may be taken from the thick celloidin (considerable of the latter adhering to the specimen), quickly pressed upon a block of wood or vulcanized fibre, allowed to harden five to ten minutes in air, and then immersed in 80-per-cent. alcohol. The alcohol gives an even hard- ening 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), sUghtly larger than the specimen, and covering with thick celloidin. The celloidin should dry slowly under a bell-jar for from two to twelve hours, according to the amount of celloidin, after which it should be immersed in 80-per-cent. alcohol and the paper pulled off. Such a block may be cut into any desired shape. It is attached to the wooden or vulcanized block by dipping for a moment in thick celloidin, and then pressing firmly down upon the block. After five to ten minutes' drying in air it is transferred to 80-per-cent. alcohol. Old, hard, celloidin-embedded specimens are sometimes difficult to attach to blocks. This usually may be accompKshed by first thoroughly drying the specimen and then dipping it in equal parts alcohol and ether for a few minutes. This softens the surface of the celloidin, after which the specimen is dipped in thick celloidin and blocked. Embedded or blocked specimens can be kept in 80-per- cent. alcohol. After several months, however, the celloidin is likely to become too soft for good section cutting. In that case the speci- mens can be readily re-embedded by dissolving out the old celloidin with alcohol and ether and putting them again through the regular embedding process. (2) Clove-oil Celloidin. — A more rapid impregnation of the tissue may be obtained by means of what is known as clove-cil cel- loidin. Celloidin, 30 gm. Clove oil, 100 c.c. Ether, 400 c.c. Alcohol, absolute, 20 c.c. The celloidin is first placed in a jar and the clove oil and ether added. From two to four days are required for solution of the celloidin. GENERAL TECHNIC 13 During this time the jar should be shaken several times. After the celloidin is dissolved the absolute alcohol is added and the solution is ready for use. The specimen must be thoroughly dehydrated, placed in alcohol and ether or pure ether for a few hours, and then transferred to the clove-oil celloidin. From six to twelve hours is sufficient to impreg- nate small pieces of tissue. The tissue is now taken from the cel- loidin, placed directly upon a wooden or vulcanized block, and im- mersed in chloroform. The celloidin hardens in about an hour, and is then ready for sectioning. The specimen is very firm, and very thin sections can be cut. A disadvantage in clove-oil celloidin is that neither the blocks nor the sections can be kept permanently in alcohol, as can those embedded in alcohol-ether celloidin. They may, however, be kept for several weeks in pure chloroform. Paratfin Embedding For paraffin embedding a thermostat or parafiSn oven is necessary in order that a constant temperature may be maintained. The tem- perature should be about 56° C. Pure parafi&n, the melting-point of which is from 50° to 55° C, is used. In very warm weather it may be necessary to add to this a little paraffin, the melting-point of which is 62° C. The hardened tissue is first put in 96-per-cent. alcohol for from twelve to twenty-four hours, and then completely dehydrated by put- ting in absolute alcohol for the same length of time, or less for small specimens. It is then transferred to some solvent of paraffin. Some of the solvents used are xylol, oil of cedarwood, chloroform, and toluol. Of these the best are perhaps xylol and oil of cedarwood. The tissue should remain in either of these for several hours, or until the tissue becomes more or less transparent. It is then placed in melted paraffin, in the paraffin oven, for from one to three hours, according to the size and density of the specimen. This allows the tissues to become impregnated with the melted parafiin. The paraffin should be changed twice. In case of very delicate tissues it is well to transfer them from the absolute alcohol to a mixture of equal parts absolute alcohol and xylol for a short time before putting them into the pure xylol. In the same way a mixture of equal parts xylol and paraffin may be used before putting the tissues into pure paraffin. 14 HISTOLOGICAL TECHNIC For hardening the parafhn in and around the tissue a very con- venient apparatus consists of a plate of glass and several L-shaped pieces of iron or lead. Two of these are placed on the glass plate in such a manner as to enclose a space of the desired size. Into this are placed the specimen and sufficient melted paraffin to cover it. Both glass and irons should be smeared with glycerin to prevent the par- affin from adhering, and should be as cold as possible, so that the par- affin may harden quickly. The same paper boxes described under celloidin embedding may also be used for paraffin. Another good method for small pieces of tissue is to place the specimen in paraffin in an ordinary watch-glass which has been coated with glycerin. Both paper-box and watch-glass specimens are immersed in cold water as soon as the surface of the paraffin has become hard. After the paraffin has hardened any excess may be cut away with a knife. Paraffin-embedded specimens may be kept indefinitely in air. For section cutting, the block of paraffin is attached to a block of wood or of vulcanite or to the metallic block-holder of the microtome. This is done by heating the block-holder, pressing the paraffin block firmly upon it, and then dipping the whole into cold water. VI. 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 lo/x, although GENER.\L TECHNIC 15 under favorable conditions sections 5//^ or even 3/-« in thickness may be obtained. In cutting paraffin sections the knife is kept dry and is passed not obliquely but straight through the specimen, only a small part of the cutting edge being used. An exception is made in the case of very large parafhn sections, where an oblique knife is used. Sections are removed from the knife by a dry or slightly moistened brush. If not desired for immediate use the sections may be conveniently kept for a short time on a piece of smooth paper. If sections curl they may be flattened by floating on warm 30-per-cent. alcohol or on warm water. Paraffin sections may be so cut that the edges of the sections ad- here. Long series or "ribbons" of sections may thus be secured. This is of decided advantage when serial sections are desired. Fail- ure of the secticns to cut evenly or to adhere in ribbons is usually due to the paraffin being too hard and brittle, which of course is due to its low temperature. If much section cutting is to be done, the operator will find himself amply repaid by having the room-temperature fairly high. In case paraffin of a melting-point of 50° to 55° C. is used, a room temperature of 73° to 75° F. will greatly facihtate the work. Where it is not possible to have a high room temperature, recourse may be had to coating the surface of the block with a paraffin of lower melting-point than that used for the embedding. A similar effect may be obtained by holding a heated metal plate or bar near the block until the paraffin is slightly softened. This process may be re- peated as often as necessary. In addition to the fact that ribbon series can be cut in paraffin, this embedding substance also has the advantage over celloidin that thinner sections can be obtained. On the other hand, celloidin em- bedding produces less shrinkage of the tissues than paraffin embedding with the accompanying heat. Frozen Sections. — This method, while more used in pathology where rapid diagnosis is more often important, is still sometimes extremely useful to the histologist, especially as an easy and quick method of testing material. Perfectly fresh tissues may be used or tissues which have been previously fixed. Fixed tissues must have the fixative thoroughly removed before freezing. Of fixatives 10- per-cent. formalin is probably the best and the tissue should remain in * H =micromiUimetcr or micron = , ,}(,<, of a milli meter = microscopic unit of measure ■« about j.'jioo ^^f '^^ inch. 16 HISTOLOGICAL TECHNIC the fixative from one to two hours. If time is important the tissue may be boiled for two or three minutes in the formalin solution. Ether, rhigolene and carbon dioxid are the most common freezing agents. A special freezing microtome and knife may be used, or a freezing attachment may be used with the ordinary microtome. In ether freezing the ether is vaporized by means of compressed air and the vapor carried to the under side of a metalHc plate upon which the block of tissue is placed. In carbon dioxid freezing, which has now largely superseded ether freezing on account of convenience and econ- omy, the commercial cylinder of gas such as is used for charging soda fountains is used. As the liquid carbon dioxid and not the gas is required for freezing, the cylinder should be hung valve end down, with the valve about the level of the microtome. The carbon dioxid is carried to the metallic disc of the microtome by means of a stout rubber tube and it is advisable to screw a cap with a small hole in it over the valve and to attach a longer handle to the valve lever in order to more perfectly control the flow. The piece of tissue to be frozen should not be over 5 mm- thick and the freezing should be done slowly. A proper hardness of the tissue is important and is of course determined by the amount of carbon dioxid admitted. As the frozen section is not supported by any embedding mass, special handling is required, the section being attached to a slide or cover-glass before staining. For this purpose either egg albumen or celloidin may be used. Egg albumen, 50 c.c. Distilled water, 150 c.c. Saturated solution sodium salicylate (made slightly alkaline with lithium carbonate) 50 c.c. (or sufficient to completely dissolve the albumen). Sections . from material previously fixed are transferred directly from the microtome to the albumen solution. Sections from fresh material are placed for three to five minutes in 5- to lo-per-cent. formalin solution before being transferred to the albumen solution. From the albumen solution sections are floated on a slide or cover-glass, the excess of solution drained off and the section firmly blotted with several thicknesses of washed cheese-cloth. The slide or cover-glass is next immersed in alcohol or in equal parts alcohol and ether to coagulate the albumen, thus fixing the section to the glass. The section may now be stained and mounted in the usual way GENERAL TECHNIC 17 In the celloidin method the section is transferred from the micro- tome to water, floated upon a sHde, blotted with filter paper, flooded with absolute alcohol and drained. Before the alcohol dries the slide is flooded with a very thin celloidin solution which is immediately drained off. The celloidin is then allowed to harden a moment and the slide immersed in water. The section is now fixed to the slide and may be stained and mounted as usual. VII. Staining This is for the purpose of more readily distinguishing the different tissue elements from one another by their reactions to certain dyes. Based upon their action upon the different tissue elements, stains may be classified as (i) nuclear dyes, which stain nuclear structures; and (2) plasma dyes, which stain the cell body or cytoplasm. Plasma dyes, also, as a rule, stain the intercellular tissue elements, and are therefore known as diffuse stains. The dyes most frequently used for staining tissues are : I. Nuclear dyes: (a) Haematoxylin and its active principle, hasmatein; (b) carmine and its active principle, carminic acid; (c) basic aniUne dyes. II. Plasma dyes: (a) Eosin; (b) neutral carmine, (c) picric acid; (d) acid aniline dyes. I. Nuclear Dyes. — (a) Hematoxylin. 1. Gage's Hcematoxylin. Ammonia or potash alum, 7 . 5 gm. Distilled water, 200.0 c.c. Boil for 10 minutes to sterilize; cool and add the following solution: Haematoxylin crystals, o.i gm. Alcohol 9S-pcr-cent., 10. o c.c. Four grams chloral hydrate are then added to the mixture to prevent growth of germs. This dye may be used in full strength or diluted with alum water. It stains in from two to five minutes. 2. Delafield's IIcBmatoxylin. Ifa;matoxylin crystals, I gm. Alcohol, 6 c.c. Ammonia alum, saturated aqueous solution, 2 100 c.c. 18 HISTOLOGICAL TECHNIC The hsematoxylin should be first dissolved in the alcohol and then added to the alum solution. The mixture should next be allowed to stand in the light for from seven to ten days to ripen. It is then filtered, and to the filtrate are added: Glycerin, 25 c.c. Wood naphtha, 25 c.c. The mixture is again allowed to stand for from two to four days and filtered. It may be used full strength or diluted with equal parts of water. It stains in from two to five minutes. 3. Heidenhain's Hcematoxylin. Hcematoxylin crystals, i gm. Water, 100 c.c. Sections are first placed for from four to eight hours in a 2.5-per-cent. aqueous solution of ammonium sulphate of iron. They are then washed in water and transferred to the haematoxylin solution until they are intensely blue or black (usually several hours). The sec- tions are now washed in water and differentiated by again placing in the iron solution till they have a light grayish color. After this they are thoroughly washed in water. 4. Mayer's Hmmalum. Hsematein, i gm. Alcohol, 50 c.c. Ammonia alum, 5-per-cent. aqueous solution, 1,000 c.c. The h^ematein is first dissolved in the alcohol, after which the alum is added. This dye does not require any ripening, and is thus avail- able for immediate use. It is a rapid nuclear dye usually requiring not more than from three to five minutes. 5. A combination of Gage's and Mayer's formulee makes a very satisfactory nuclear dye. Haematein, 5 gm. Alcohol, 50 c.c. Chloral hydrate, 20 gm. Ammonia alum, 5-per-cent. aqueous solution (steril- ized), 1,000 c.c. The haematein is first dissolved in the alcohol and then added with the chloral hydrate to the alum solution. This solution is used in full strength and stains in from three to five minutes. GEXER.\L TECHNIC 19 6. Weigerfs HcBmatoxylin. Two stock solutions should be made up as follows: A. i-per-cent. hjematoxylin, in Q6-per-cent. alcohol. B. Hydrochloric acid (sp. gr. 1.126), 10 c.c. Ferric chloride, 30-per-cent., 40 c.c. Distilled water, 950 c.c. For use, mix equal parts of A and B. The mixture will keep two or three days. This is a rapid stain usually requiring not mere than a minute. A more brilliant nuclear stain may be obtained by over- staining and then decolorizing. After the stain wash the sections in water and then decolorize to the pioper degree in weakly acidu- lated water. To step the action of the acid the sections should be dipped in water made slightly alkaline with ammonia. This is an excellent stain and gives brilliant results. It is especially good in cases where the material has become old and lost its affinity for ordi- nary haematoxylin stains. {h) Alum-carmine. Carmine, 2 gifi. x\lum, 5 gm. Carbolic acid, . . 2 gm. Water, 100 c.c. The alum is first dissolved in the 100 c.c. of warm distilled water, after which the carmine is added. This mixture is then boiled for twenty minutes, allowed to cool, and filtered. The carbolic acid is then added. This is a slow-acting pure nuclear dye. (c) Basic Aniline Dyes — gentian violet, methyl violet, methyl green, methyl blue, toluidin blue, fuchsin, thionin, safranin, etc. These are best kept in stock in saturated alcoholic solutions. WTien desired to use, a few drops of the alcoholic solution are added to distilled water. No rule as to exact proportions can be given, as these depend upon the material, the fixation, and the intensity of stain desired. II. Plasma Dyes. — (a) Eosin. This is prepared as follows: Water-soluble eosin is dissolved in water to saturation. It is then precipitated by hydrochloric acid and the precipitate washed with water upon a lilter until the filtrate is tinged with eosin. After drying, the precipitate is dissolved in strong alcohol, i gm. of eosin to 1,500 c.c. of alcohcl, or water- soluble eosin may be kept in saturated aqueous solution containing 20 HISTOLOGICAL TECHNIC a trace of thymol as a preservative. This may be diluted to any desired strength at time of using. Eosin is a rapid plasma stain. (b) Neutral Carmine. Carmine, i 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. (c) Picric Acid — used mainly as the plasma-staining element of such a staining mixture as picro-acid-fuchsin. (d) 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). Erythrosin is of especial value for sections which take the eosin stain poorly. Staining Sections It is often of advantage to stain the different tissue elements dif- ferent colors. This may be accomphshed 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- toxyhn (solutions i, 2, 4, 5, or 6, pp. 17-19) from one to five minutes. After being thoroughly washed in water, they are dehydrated in strong alcohol and transferred to the alcoholic eosin solution (page 19). Most sections stain in from two to five minutes. By this method nuclei are stained blue or purple, cell bodies and intercellular substances red. Very often a more brilliant staining may be accomplished as follows: Overs tain in haematoxylin and wash thoroughly in water; decolorize in water slightly acidulated (8 or 10 drops of hydrochloric acid to 100 c.c. of water) until only the nuclei retain the stain; wash in water which has been made slightly alkaline with ammonium hy- drate; then stain with eosin as usual. (2) Staining with Picro-acid-fuchsin. Acid-fuchsin, i-per-cent. aqueous solution, 5 c.c. Picric acid, saturated aqueous solution, 100 c.c. GENERAL TECHNIC 21 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- tissue fibres red by the fuchsin. After staining, the sections are washed in distilled water. (3) Triple Staining with Hematoxylin and Picro-acid- FUCHSiN. — This is the same as the preceding except that before stain- ing with picro-acid-fuchsm, the sections are overstained in heema- toxylin (solutions i, 2, 4, 5, or 6, pp. 17-19). The usual purple of haematoxyhn-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 haematoxylin may be com- pletely removed. After staining, sections are washed in distilled water and transferred to 96-per-cent. alcohol. 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 96-per-cent. alcohol should be tinged with picric acid. (4) Staining with Picro-carmine. Ammonium carminate, i gm. Distilled water, 35 c.c. Picric acid, saturated aqueous solution, 15 c.c. 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 l)ulk stains. (i) Alum-carmine. Carmine, 0.5 to i gm. Ammonia alum, 4-pcr-ccnt. aqueous solution, 100 c.c. 22 HISTOLOGICAL TECHNIC 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 speci- men is dehydrated and embedded in the usual way. (2) Borax-carmine, Alcoholic Solution. Carmine, 3 gm. Borax, 4 gm. 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. Vm. Mounting It is usually desirable to make permanent preparations or "mounts" of the stained specimens. The most satisfactory media for mounting specimens are glycerin and Canada balsam. (i) Glycerin. — Sections may be transferred to glycerin from either water or alcohol. In the case of double-stained specimens — hsematoxylin-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 hsematoxyHn-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 put on. Glycerin mounts, as a general rule, are unsatisfactory. Further- more, they must be cemented to exclude the air. This can be done by painting a ring of gold-size, or a thick solution of gum shellac in alcohol to which a Httle castor oil has been added, around and over the edge of the cover-glass. Both cover-glass and slide must be cleaned free from glycerin before the cement is appHed. A cam.ers- hair brush moistened with alcohol is the best means of removing the excess of glycerin. Glycerin jelly is a more satisfactory mounting medium than pure glycerin. It can be purchased from firms dealing in microscopical GENERAL TECHNIC 23 supplies and needs merely the application of heat to make it fluid. Specimens can then be mounted in it in the usual manner, and after being allowed to cool, do not require cementing. (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, espe- cially 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, carboKc-acid crystals, 22 gm.), followed by pure xylol. (3) Xylol and cajeput oil, equal parts, followed by pure xylol. After clearing, the section is transferred by means of a section- lifter to a glass mounting slide. In case oil of origanum is used, 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 completely removed. The specimen should also be blotted firmly, giving the oil time to soak into the paper. These two precautions are necessary to prevent the section adhering to the paper instead of to the shde. After blotting, a drop of balsam is placed upon the centre of the specimen and a cover-glass put on. When xylol is used, blotting is not necessary. Drain off the excess of oil, put a droj) of balsam on the specimen and put on a cover-g'ass. 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 cellodin is transparent and may remain perma- nently in the specimen, paraffin is opaque and must be dissolved out before the section is fit for microscopic study. 24 HISTOLOGICAL TECHNIC Bulk staining with carmine (page 21) is frequently used for speci- mens which are to be embedded in parafi&n. Sections may be coun- terstained 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 ends of the ribbons to prevent them floating off. Stand slides on end a few hours to allow water to evaporate. 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 gc-per-cent. alcohol. Transfer to 80-per-cent. alcohol. Transfer to "50-per-cent. alcohol. (May be omitted.) Transfer to water. Stain with an aqueous stain. Wash in water. Transfer to 50-per-cent. alcohol. (May be omitted.) Transfer to 80-per-cent. alcohol. Transfer to 90-per-cent. alcohol. Transfer to absolute alcohol. Transfer to xylol. Transfer to fresh 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 is omitted. If it is desired to stain double with eosin-hsematoxylin (page 20) use the above technic in staining with haematoxyhn; then the alcoholic eosin stain before final transfer to absolute alcohol. GENERAL TECHNIC 25 IX. 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 colored gelatin. The gelatin solution is prepared by soaking i part gelatin in from 5 to ID 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 aque- ous solution Prussian blue to the gelatin solution, the proportions depending upon the depth of color desired. Both solutions should be at a temperature of 60° C. After thoroughly mixing, the blue gelatin is filtered through cloth. Carmine gelatin is prepared by first dissolving i gm. carmine in 30 c.c. distilled water. To this is added ammonia until the mixture becomes a dark cherry red. A lo-per-cent. aqueous solution of acetic acid is next added, drop by drop, with constant stirring until the mix- ture 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 alkahne, 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 lo-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 carmine in the shape of coarser granules is of advantage when it is desired to have an in- jection 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 connecting with the cannula may be attached to a faucet in the vessel or to 26 HISTOLOGICAL TECHNIC 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 necessary. Whole animals or separate organs may be injected. For injecting 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 sahne 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 usually cut rather thick, that the vessels may be traced the greater distance. Better results are frequently obtained by injecting separate organs. This is accomphshed 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. GENERAL TECHNIC 27 Double injection is accomplished by first washing out the vessels with normal saHne and then connecting the artery with the red gelatin, the vein with the blue gelatin, and injecting both at the same time, the pressure driving the saline out of the vessels into the tissues. The difficulty is that either the arterial injection carries over into the veins, or the venous injection carries over into the arteries. 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 arter- ies and capillaries in the usual manner. This method is especially useful in demonstrating the vessels of the kidney, Hver, and gastro- intestinal canal. CHAPTER II SPECIAL STAINING METHODS Or these only the more common will be described. (i) Silver-nitrate Method op Staining Intercellular Sub- stance.— ^After first washing, the tissue, e.g., omentum or cornea, is placed in a from 0.2- to i-per-cent. solution of silver nitrate, or better, protargol, where it is kept in the dark for a half-hour or more according to the thickness and density of the tissue. The specimen is then washed in water, transferred to 40-per-cent. alcohol and placed in the direct sunlight until it assumes a hght brown color. It is then placed in fresh 80-per-cent. alcohol for preservation. (2) Chlorid of gold in i-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 gm. Resorcin, 4 gm. 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 Hquid has become cool. After drying, 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) Verhoefe's Differential Elastic Tissue Stain. — Haematoxylin crystals (Grubler), 0.15 gm. Absolute alcohol, 25.00 c.c. Dissolve by heating, then add 5-per-cent. aqueous solution ammonium hydrate, i drop. Allow to stand 5 minutes or longer, then add Lugol's solution (iodin 2 parts, potassium, iodid 4 parts), 22 . 00 c.c. Filter, let stand in corked bottle 24 hours. 28 SPECL-^L STAINING METHODS 29 In using this stain add to each cubic centimeter required of the above solution i drop of a y-per-cent. solution of ferric chlorid in absolute alcohol. Sections are carried from alcohol into the staining fluid where they remain one to three hours. The stain should be kept covered to prevent evaporation. Sections are next washed in water one or two minutes and examined under the microscope. If further differentia- tion is desirable, place a few seconds in i-per-cent. aqueous solution ferric chlorid. Wash in water. Counterstain in 0.2-per-cent. eosin, dehydrate in alcohol, clear and mount in balsam. Elastic tissue is stained black, nuclei take haematoxylin, while other connective tissues, myelin and neuroglia stain with eosin. Fixation by Zenker's fluid gives perhaps the best results although the stain may be used after other fixations. Water in the staining fluid causes precipitates. It is therefore important to see that all dishes used are perfectly dry or washed with alcohol before using. Too much ferric chlorid gives precipitates and overstaining, too Httle results in failure of the elastic fibres to stain. (5) GoLGi's Chrome-silver Method for Demonstrating Se- cretory Tubules.- — ^Small pieces of perfectly fresh tissue, e.g., hver, are placed in the following: Potassium bichromate, 4-per-cent. aqueous solution, 4 vols. Osmic acid, i-per-cent. aqueous solution, i vol. After three days they are transferred without washing to a 0.75-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 days, after which they are rapidly dehydrated, embedded in celloidin, and cut into rather thick sections. (6) Mallory's Phosphomolybdic Acid H. Fig. 28. — Mucous Connective Tissue from Umbilical Cord of Eight-inch Foetal Pig. X600. At this stage the ground substance shows some fibrillat:':)n. Connective -tissue Cells. — (a) The ordinary fixed connective- tissue cell is often the only connective-tissue cell seen in ordinary sections. It is an irregular shaped cell, often quite flattened with clear or slightly granular cytoplasm, and an oval nucleus. (Figs. 29 and 30.) In loosely arranged tissue when the cells are well separated, the cell is usually stellate with many branches, which anastomose with branches of neighboring cells. In a dense tissue such as the cornea, these cells apparently lie in little cell spaces or lacunar from which minute channels {canaliculi) extend in all directions to unite with canaliculi from adjoining spaces (Fig. 31). Delicate cell processes extend into the canaliculi and there anastomose with processes from other cells thus forming a sort of syncytium (Fig. 32). Owing to the extreme sensitiveness of the 84 THE TISSUES h , /' -/ -' t-7 Fig. 29. — Areolar Connective Tissue (Rauber-Kopsch). a, White fibre, b, Elastic fibre, c, Fixed connective tissue cell, d, Clasmocyte. e, Leucocyte (wandering cell). /, Mast cell. Fig. 30. — Fibrillar Connective Tissue (Areolar Type) from Subcutaneous Tissue of Rabbit (technic 2, p. 96). X500. a, Fixed connective-tissue cell; b, fibrillated fibres; c elastic fibre with curled broken end; d, elastic fibres showing Y-shaped branching. THE CONNECTIVE TISSUES 85 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. Somewhat similar cells with more coarsely- granular or vacuolated cytoplasm have been designated "clas- mocytes" (Ranvier). By some these are believed to be of leucocyte- origin, by others to be an earlier stage in the development of the fixed connective-tissue cell. {h) Plasma Cells. — These cells occur mainly near the smaller blood-vessels. Their protoplasm is finely granular and stains with Fig. 31. — Section ot Human Cornea cut Tangential to Surface. X350. (Technic 9, p. 97.) Connective-tissue Cell Spaces (Lacunae) and Anastomosing Canaliculi, white; whole Intercellular Substance (Ground Substance and Fibres), dark. basic aniline dyes. They frequently contain vacuoles. Their nuclei are small, spherical and usually excentric. Small plasma cells are about the size of leucocytes, which they closely resemble. Large plasma cells are larger than leucocytes and richer in protoplasm. Some consider them as derived from leucocytes, others as a modified form of the ordinary fixed connective-tissue cell. (c) Mast cells are spherical or irregular-shaped cells, found like the preceding in the neighborhood of the blood-vessels. Their proto- plasm contains coarse granules which stain intensely with basic aniline dyes (Fig. 29). They are bcHeved by some investigators to be connected with the formation of fat; by others to represent a stage in the development of the fixed connective-tissue cell. (d) Clasmalocyles. — These are large, mostly spindle shaped cells with granular protoplasm. ob THE TISSUES Connective-tissue cells may be pigmented (Fig. 33). In such cells the cytoplasm is more or less filled with brown or black pigment granules. In man pigmented connective-tissue cells occur in the skin, chorioid and iris. The so-called wandering cells (Fig. 29) are not properly a part of ■i y , _^ -1 t , ,. i '"•'■^' : ■''.■■■?■-"■ ;'■ — Jj, ^•f A / T , //t / i ,? V //'"■■, \ \ ^' Z:^, -Ts \ ^ \\ X < . . / Fig. 32. — Section of Human Cornea cut Tangential to Surface. X350. (Technic 8, p. 97.) Connective-tissue Cells with Anastomosing Processes, stained; Intercellular Substance. (Ground Substance and Fibres), unstained. connective tissue, being merely amoeboid white blood cells (see page 107) which have passed out from the vessel into the tissues. They are not pecuHar to connective tissue, being found in other tissues, e.g., in epithelium. The Intercellular Substance. — (a) Fibres. White or fihrillated •?«•'■:••!••:- - — fibres are bundles of ex- tremely fine fibrillae (o . 5// in diameter) (Fig. 30). The fibrillae He parallel to one another and are united by a small amount of cement substance. The fibrillae do not branch. The fibre bundles, on the other hand, branch dichotomously and anas- tomose. White fibres, on boihng, yield gelatin. Yellow or elastic fibres are apparently homogeneous, highly re- fractive fibres, varying in diameter from i to 10// (Fig. 30). They branch and anastomose, forming networks. The smaller fibres are ''■'SS^>*'rI- Fig. 2)3- — Pigmented Connective-tissue Cells from Chorioid Coat of Human Eye. X350. (Technic 7, p. 97.) THE CONNECTIVE TISSUES 87 round on cross section, the larger flattened or hexagonal (Figs. 43 and 44) . Their elasticity is easily demonstrated in teased specimens by curKng of the broken ends of the fibres (Fig. 30). On boiling they }ield elastin. Although, when subjected to the usual technic, elastic fibres appear homogeneous, they are probably composed of a thin sheath or membrane, enclosing the more granular elastin. The latter stains intensely with magenta, the sheath remaining unstained. In addition to the white fibres and elastic fibres above described, so-called "reticular" fibres are frequently present in fibrillated connective tissue. (See p. 82.) {h) Basement or ground substance occurs in extremely minute amounts between the individual fibrillce of the white fibres, where it acts as a cement substance. The same material also forms the basement or ground substance in which the connective- tissue cells and fibres He (Fig. 31). Difliculty in seeing this ground substance is due to its transparency. It may be demonstrated by staining with silver nitrate. (See technic 9, p. 97.) Much variation exists in regard to the proportions of the different elements. This gives rise to variations in the physical characteristics of the tissue. When fibres predominate over cells and ground sub- stance, the tissue is dense and hard and is known as dense fibrous tissue. The terms fine connective tissue and coarse connective tissue desig- nate the character of the fibres. When many cells are present, the tissue is softer and is known as cellular connective tissue. According to the arrangement of the white fibres, fibrous connec- tive tissue is subdivided into areola or loose connective tissue and formed connective tissue. Areolar or Loose Connective Tissue In this the fibres are irregular, running in all directions and in- terlacing, leaving between them meshes or areolce (Fig. 30). Subcutaneous connective tissue is a typical example of areolar tissue. Both white and elastic fibres are present, although the former predominate. Areolar tissue varies greatly as regards the relative number of cells and fibres and the closeness with which the different elements are packed. It thus varies greatly in density. Fat Tissue. — Adipose tissue or fat tissue is a form of areolar tissue in which some 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 and determine the THE TISSUES character of the tissue. The adult fat cell is surrounded by a distinct cell membrane, and almost the entire cell is occupied by a single droplet of fat (Figs. 35 and 36). The nucleus, flattened and sur- rounded by a small amount of cytoplasm, is usually found pressed against the cell wall (Fig. 36). This appearance of a distinct cell membrane enclosing the spherical fat droplet, with the nucleus and cytoplasm pressed into a cresent-shaped mass at one side, has given rise to the term "signet-ring cell." Fat cells which occur singly, Fig. 34. — Fat Tissue from Human Subcutaneous Tissue (Child) to show Lobulation. X25. (Technic i, p. 95.) or in small groups, or in the developing fat of young animals are spherical (Fig. 35). In large masses of adult fat, the closely packed cells are subjected to pressure and are polyhedral (Fig. 36). Fat cells are usually arranged in groups or lobules, each lobule being separated from its neighbors by fibrillar connective tissue (Fig. 34). 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 connective-tissue cells which are to become fat cells gather in groups in the meshes of the capillary network which marks the ending of a small artery. Each group is destined to become an adult fat lobule (Fig. 37)- THE CONNECTIVE TISSUES a b 89 Fig. 35- — Young Fat from Human Subcutaneous Tissue. (Child.) X175. (Technic I, p. 95.) a, Interlobular connective tissue; b, fixed connective-tissue cell; c,_fat cells; d, artery; e, nucleus of fat cell and remains of cytoplasm ("signet ring")- Fig. 36. — Adult Fat Tissue from Human Subcutaneous Tissue. Xi75- (Technic r, p. 95.; a, Fat cells; b, intcrloljular connective tissue; c, nucleus of fat cell and remains of cytoplasm f" signet ring"); d, artery. 90 THE TISSUES Fat first appears as minute droplets in the cytoplasm of the em- bryonic connective- tissue cell (Fig. 38). These small droplets in- Fig. 37. — Developing Fat Tissue from Subcutaneous Tissue of Five-inch Foetal Pig. X75. (Technic 2, p. 96.) a, Arteriole breaking up into capillary network; h, embryonal connective tissue; c, embryonal fat lobule developing around blood-vessels. '*W Fig. 38. — Developing Fat Tissue from Subcutaneous Tissue of Five-inch Foetal Pig. (Technic 2, p. 96.) a, Arteriole breaking up into capillary network; b, embryonal connective tissue, embryonal cells from which fat cells are developing; c, capillaries. Fat droplets stained black. At the right are five individual cells showing stages of development from an embryonal cell to an adtdt fat cell. crease in number and finally coalesce to form a single larger droplet. This increases in size and ultimately almost wholly replaces the cyto- THE CONNECTIVE TISSUES 91 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. 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, 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, embry- onal connective-tissue cells which are not destined to become fat cells, but which develop into cells and fibres of ordinary fibrous connective tissue. A few of these remain among the fat cells to become the deUcate intralobular connective tissue seen in adult fat. The majority are, however, pushed to one side by the developing lobules, where they form the inter- lobular septa. Fig. 39. Fig. 40. Fig. 39. — Longitudinal Section of Tendon from Frog's Gastrocnemius. X 250. The nuclei of the flattened cells are seen lying in rows between the connective-tissue fibres. Fig. 40. — Teased tendon fibres with cells lying on their surface. X400. (Ranvier.) Formed Connective Tissue. — In formed connective tissue the tissue elements instead of being disposed irregularly as in areolar tissue, are arranged with some regularity or order, thus giving the tissue more or less definite form. Tendons and ligaments are examples of formed connective tissue in which the fibres all run in approximately the same direction (Fig. 39). Elastic fibres are absent or present in very small numbers. The predominance of the white fibres and their parallel arrange- ment result in great strength with almost no extensibility. While 92 THE TISSUES the individual fibrils do not branch, groups of fibrils pass from one bundle to another. There is little ground substance and cells are comparatively few. The latter are, however, so characteristic as to have received the name of tendon cells (Fig. 41). They are irregu- larly rectangular cells, have rather more breadth than length and are arranged in characteristic rows between the fibre bundles. The cell margins are contiguous and the usually excentric nuclei tend to lie in adjacent sides of two cells, thus giving the cells the appearance of being arranged in pairs (Fig. 40). Thin plate-like extensions of the ''^teasris*.^^ ' Fig. 41. Fig. 42. Fig. 41. — -Tendon Cells from the Tail of a Rat. Stained in methylene-blue {intra vitam). (Bohm-Davidoff.) Fig. 42. — Pavement Endothelium of Tendon of Rat. A , intercellular substance im- pregnated with silver nitrate; B, tendon fibres. Xir5. (Branca.) cell (Fig. 41) pass into the ground substance between the fibre bundles and when the cell is seen on flat, the greater thickness of the cell through the extensions gives the optical effect of dark lines in the cell body. Aponeuroses. — In thin aponeurotic tissue the fibres are disposed in two planes, the fibres of one plane running approximately at right angles to those of the other plane. The cells resemble tendon cells and he upon the fibre bundles. In thicker aponeuroses the fibres are arranged in planes but their disposition is more irregular. Elastic Tissue Elastic fibres occurring in fibrous connective tissue have been described. When the elastic fibres predominate the tissue is known as elastic tissue. Almost pure elastic tissue is found in the liga- mentum nuchse of quadrupeds. In man it occurs mainly in the ligamenta subflava, in some of the laryngeal ligaments, in the walls THE COXXECTIVE TISSUES 93 of the trachea and bronchi and of arteries. In the ligamentum nuchas the fibres are coarse and arranged in bundles separated from one another by white fibrous tissue containino; connective- FiG. 43. — Coarse Elastic Fibres from Ligamentum Xuchie. Xsoo. Teased specimen. (Technic 10, p. 97. j tissue cells. This white fibrous tissue also penetrates the bundles and separates the individual elastic fibres. A few white fibres and connective- tissue cells are also present (Figs. 43 and 44). ( .,- h V>- Fig. 44. — Cross Section of Coarse Elastic Fibres from Ligamentum Nuchae. Xsoo. CTechnic 10, p. 97.) a, Elastic fibres; b, white fibrous tissue and cement substance. The nuclei arc the nuclei of fixed connective-tissue cells. Elastic tissue may be arranged as thin membranes, as e.g., in the walls of blood-vessels. These membranes are usually described as composed of a dense mass of flat, ribbon-like elastic fibres, which 94 THE TISSUES interlace in such a manner as to leave openings in the membrane. Hence the term "fenestrated membrane." They have been re- cently described as consisting of a central layer composed of elastin, staining with magenta, and on either side a thin, transparent sheath unstained by magenta. This is seen to correspond to Mall's de- scription of the structure of the elastic fibre. Only the middle of theselayers is fenestrated. Reticular Tissue Reticular connective tissue is a form of fibrillar connective tissue. It consists of extremely delicate fibrils with no ground Fig. 45. — Reticular Tissue from a Human Lymph Node. (Technic, below.) a, Reticular connective tissue, in the meshes of which are suspended b, leucocytes, and c, lymphocytes. The reticular connective tissue is present also in the more dense lymphatic tissue seen in the lower part of the figure, but is not visible on account of the closely packed cells. substance. The fibrils are grouped in larger or smaller bundles which form a network or feltwork enclosing spaces, thus constitut- THE CONNECTIVE TISSUES 95 ing a reticulum. The fibrils present much the same microscopic appearance as the white fibres of areolar tissue. Also in certain organs, e.g., the lymph nodes, the direct continuity of the fibres of the coarser fibrous tissue of the trabeculaae with the fibrils of the reticular tissue can be easily demonstrated. In some locations, e.g., in the lymph nodes (Fig. 45) the cells of the reticular tissue lie upon the surface of the fibrils and so completely invest them that the fibrillar character of the tissue cannot be seen until the overlying cells have been removed. This led to the description of the reticulum of lymphatic tissue as composed wholly of anastomosing cells. Reticular tissue has been described as yielding on boiling, a sub- stance called by some elastin by others reticulin instead of gelatin which results from boihng fibrous tissue. Other recent studies upon the chemistry of reticular tissue are not however in accord with this view, reticular tissue bring found to yield gelatin on boil- ing. Both tissues resist pancreatic digestion, and the question as to the structural and chemical relations of the two tissues remains at present unsettled. Reticular connective tissue forms the framework of adenoid tissue and of bone-marrow. It is also present in large amounts in the spleen and in the mucous membrane of the gastro-intestinal tract, lung, liver, kidney, and other organs where it forms the finer part of the framework, supporting the capillaries, and often apparently ser\dng as a basement membrane for the gland cells. TECHNIC I. Areolar Tissue, to show White and Elastic Fibres. — Remove a bit of the subcutaneous tissue, as free from fat as possible, from a recently killed animal. Place it upon a mounting slide and with teasing needles quickly spread it out in a thin layer. During this manipulation the specimen should be kept moist by breathing on it. Put a drop of sodium chlorid solution upon the specimen and cover. As the specimen is unstained, a small diaphragm should be used for the micro- scopic examination. The white fibres are straight or wavy, are crossed in all directions, and are longitudinally striated. The elastic fibres have been stretched and show as sharp lines with curled ends where the fibres are broken. Place a drop of hydric acetate, i-per-cent. aqueous solution, at one side of the cover and a bit of filter paper at the other side. The filter paper absorbs the salt solution, which is replaced by the hydric acetate. The latter causes the white fibres to swell and become indistinct while the elastic fibres show more plainly. 96 THE TISSUES 2. Areolar Tissue, to show Cells and Elastic Fibres. — Prepare second speci- men of areolar tissue in the same manner as the preceding. Instead of mounting in salt solution, allow it to become perfectly dry, then stain in the following solution: Gentian violet, saturated aqueous solution, 40 c.c. Water, 60 c.c. Wash thoroughly, dry, and mount in balsam. The nuclei of the fixed connective-tissue cells are stained violet. Their deli- cate cell bodies show as an irregular haze around the nuclei. Both nuclei and cell bodies appear cut in all directions by the stretched elastic fibres. Wander- ing cells (leucocytes) may usually be seen. Plasma cells are frequently not demonstrable, and mast cells are only occasionally present. The elastic fibres are stained violet. The white fibres are almost unstained. While these methods are most satisfactory for bringing out the different con- nective-tissue elements, they are misleading to the student in that they show a picture of connective tissue after special preparation, rather than as it usually appears in sections. For contrast the student should study carefully the con- nective tissue as it appears in sections through the skin, the mucous membranes and other organs. 3. Formed Connective Tissue. — ^Fibrous tissue arranged in the form of a net- work may be seen in the specimen of omentum (technic 7, p. 79). 4. Densely formed connective tissue may be studied in tendon. Cut through the skin of the tail of a recently kiUed mouse about half an inch from the tip and break the tail at this point. By pulling on the end of the tail this portion may now be separated from the rest of the tail, carrying with it long delicate tendon fibrils, which have been pulled out of their sheaths. These should be immedi- ately examined in salt solution, using the high power and a small diaphragm. The fibrils are seen arranged in parallel bundles. 5. Place a drop of hydric acetate (2-per-cent. aqueous solution) at one side of the cover-glass, absorbing the salt solution from the opposite side by means of filter paper. The fibres swell and become almost invisible, while rows of connective-tissue cells (tendon cells) can now be seen. The cells may be stained by allowing a drop of haematoxylin or of carmine solution to run under the cover. After the cells are sufficiently stained, the excess of stain is removed by washing, and the specimen mounted in glycerin. 6. Fix a small piece of any good-sized tendon in formalin-Miiller's fluid (page 7). After a week, harden in alcohol, embed in celloidin, and make longitudinal and transverse sections. Stain strongly with haematoxylin, followed by picro- acid-fuchsin (page 20) . Mount in balsam. 7. Pigmented connective-tissue cells are most conveniently obtained from the chorioid coat of the eye. Fix an eye in formalin-Miiller's fluid (see page 7), cut in half, remove chorioid and retina and pick off the dark shreds which cling to the outer surface of the chorioid and inner surface of the sclera. These may be transferred directly to glycerin, in which they are mounted, or the bits of tissue may be first stained with haematoxylin (page 17). In addition to the pig- THE CONNECTIVE TISSUES 97 mented 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 28). Sections are made tangential to the convex surface and are mounted in glycerin. 9. Connective-tissue cell spaces (lacunae) and their anastomosing canaliculi may be demonstrated by staining a cornea with sUver nitrate (see page 28). 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. 31 and 32). 10. Coarse elastic fibres may be obtained from the ligamentum nuchas, which consists almost whoUy 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 transverse sections. These stained with picro-acid-fuchsin show well the rela- tion of the coarse elastic fibres (yellow) to the more delicate fibrous tissues (red). 11. Fat Tissue. — Human subcutaneous fat as fresh as possible is fixed in formalin-Miiller's fluid (technic 5, p. 7), hardened in alcohol and embedded in ceUoidin. Sections are stained with haematoxylin and picro-acid-fuchsin (technic 3, p. 21). The alcohol and ether of the celloidin remove the fat from the fat cells, leaving only the cell membranes. The fat gives the ceUoidin a milky appearance. Such ceUoidin does not cut weU. The ceUoidin should, there- fore, be changed untU it ceases to turn white. The sections are cleared in oU of origanum or carbol-xylol, and mounted in balsam. The fibriUar tissue is stained red by the fuchsin, and the protoplasm of the fat ceU yeUow by the picric acid. 12. Developing Fat Tissue. — Remove bits of tissue from the axUla 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 10, p. 31), wash thoroughly and mount in glycerin. A part of the tissue mounted should be thoroughly teased, the rest gently pulled apart. The teased portion wiU show the fat ceUs in various stages of development. The unteased part wiU usuaUy show brownish blood-vessels and the grouping of fat ceUs around them, to form embryonic 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. 13. Reticular Tissue. — Fix a lymph node in formalin-MuUer's fluid (technic 5, p. 7), and stain very thin sections with haematoxylin and picro-acid-fuchsin (technic 3, p. 21). In the lymph sinuses of the medulla the reticulum can usually be plainly seen. 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 98 THE TISSUES tissue. On boiling it yields chondrin. Cartilage cells are differen- tiated 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 im.mediately surrounding a lacuna is frequently arranged concentri- cally, forming a sort of capsule. Fine canaliculi connecting the lacunae are present in some of the lower animals and have' been de- scribed in human cartilage. They 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. ■^1 of''' 0m 4i>N ri9 (0) •fs) Fig. 46. — Hyaline Cartilage from Head of Frog's Femur. X350. (Technic 100.) Groups of cartilage cells in apparently homogeneous matrix. Cartilage is subdivided according to the character of its inter- cellular substance into three varieties: (i) Hyaline, (2) elastic, (3) fibrous. I . Hyaline Cartilage (Fig. 46) . — The cells occur singly or in groups of two or multiples of two. An entire group of cells frequently lies in one lucuna surrounded by a single capsule. Such a group of cells has developed within its capsule from a single parent cell. In other cases delicate hyaline partitions separate the cells of a group. The cells are spherical or oval, with flattening of adjacent sides. The nucleus is centrally placed, and has a distinct intranuclear network and membrane. The cytoplasm is finely granular, and may contain drop- THE CONNECTIVE TISSUES 99 lets of fat, of glycogen, or of both. Toward the perichondrium the arrangement of the cells in groups is less distinct. Here the cells are fusiform and parallel to the surface. The intercellular matrix, when subjected to the usual technic, appears homogeneous. By the use of special methods, such, e.g., as artificial digestion, this apparently structureless matrix has been shown to be made up of bundles of fibres, quite similar to those found in fibrous connective tissue. Hyaline cartilage forms the articular cartilages of joints, the costal cartilages, and the cartilages of the nose, trachea, and bronchi. In the embryo a young type of ^'1^% 4W Fig. 47. — Elastic Cartilage from Dog's Ear. X350. (Technic 2, p. 100.) Groups of cartilage cells in fibro-elastic matrix. hyaline cartilage, known as em- bryonal cartilage, forms the matrix in which most of the bones are developed. 2. Elastic cartilage (Fig. 47) resembles hyaline, but differs from the latter in that its hya- line matrix contains a large number of elastic fibres. These vary in size, many being ex- tremely fine. The elastic fibres branch and run in all directions, forming a dense network of inter- lacing and anastomosing fibres. Elastic cartilage occurs in the external ear, the Eustachian tube, the epiglottis, and in some of the laryngeal cartilages. 3. Fibrous cartilage (Fig. 48) is composed mainly of fibrillar con- nective tissue. The fibres may have a parallel arrangement, or may run in all directions. Cells are few, and are usually arranged in rows of from two to six, lying in elongated cell spaces between the fibre bundles. Fibrous cartilage occurs in the inferior maxillary and sternoclavic- ular articulations, in the symphysis pubis, and in the intervertebral discs. Cartilage, excej)t where it forms articular surfaces, is covered by a membrane, the perichondrium. This is composed of fibrillar con- nective tissue, and blends without distinct demarcation with the superficial layers of the cartilage. Like the other connective tissues, cartilage develops from meso- 100 THE TISSUES derm. It is at first wholly cellular. Each cell forms a capsule around itself, and by blending of these capsules are formed the first elements of the intercellular matrix. This increases in quantity and assumes the structural characteristics of one of the forms of cartilage. The white fibres of fibro-cartilage and the yellow fibres of elastic cartilage develop in the same manner as in fibrillar and elastic tissue. Fig. 48. — -Fibrous Cartilage from Dog's Intervertebral Disc. X3S0. (Technic3, p. 100.) Groups of cartilage cells in matrix of fibrillar connective tissue. TECHNIC (i) Hyaline Cartilage. — Remove a frog's femur and immediately immerse the head in saturated aqueous solution of picric acid. Cut sections tangential to the rounded head, keeping knife and bone wet with the picric acid solution. As bone must be cut, a special razor kept for the purpose should be used. Cut sections as thin as possible. The first sections consist whoUy of cartilage. As bone is reached, the cartilage is confined to a ring around the bone. Mount in the picric-acid solution, cementing the cover-glass immediately. (2) Elastic Cartilage. — Remove a piece of cartUage from the ear and fiji in formalin-Miiller's fluid (technic 5, p. 7). Stain sections strongly with haema- toxylin, followed by picro-acid-fuchsin (technic 3, p. 21). Clear in carbol-xylol and mount in balsam. The capsules around the cartilage cells are thick and, as they usually retain some hsematoxylin, can be readily seen. Note also the flattened cartilage cells near the surface, and the perichondrium. (3) Fibro-cartilage. — Fix pieces of an intervertebral disc in formalin-Miiller's fluid. Sections are stained either with hasmatoxylin-eosin or with hsematoxy- lin-picro-acid-fuchsin and mounted in balsam. Bone tissue Bone is a form of connective tissue in which the matrix is ren- dered hard by the deposition in it of inorganic matter, chiefly the THE CONNECTIVE TISSUES 101 Fig. 49.— Bone Tissue showing Lacunae and Canaliculi. X700. (Technic i, p. 102.) of cells and intercellular phosphate and the carbonate of calcium. These salts are not merely deposited in the matrix, but are intimately associated and coinbined with its histological structure. The intimacy of this association of the organic and inorganic constituents of bone is shown by the fact that, though the salts com- pose two-thirds of bone by weight, it is impossible to distinguish them by the highest magnification. Fur- thermore, if either the lime salts are dissolved out by means of acids (decalcifica- tion) or the organic matter removed by heating (calcina- tion), the histological struc- ture of the bone still remains. Like the other connective tissues, bone consists morphologically suhstance. Bone cells or hone corpuscles lie in distinct cell spaces or lacuncB. From the lacunae pass off in all directions minute canals — canaliculi — ■ which anastomose with canaliculi of neighboring lacunae (Fig. 49). At the surface of bone these canaliculi open into the periosteal lymph- atics. A complete system of canals is thus formed, which traverse the bone and serve for the passage of nutritive fluids. The bone cells themselves (Fig. 50) are flat, ovoid, nucleated cells, with numerous fine processes, which extend in all directions into the can- aliculi. In young developing bones the processes of adjacent cells anastomose. In adult bone the processes extend but a short allowing the outline of the distance into the canaliculi, and probably do lacuna to be seen. not anastomose. The basement substance or matrix has a fibrous structure, closely resembling that of fibrillar connective tissue, and it is in this fibrillar matrix that the lime salts are deposited. The fibrils are held together by cement substance into bundles. In most bone the bundles are fine and arranged in layers or lamellcc. Less commonly the fibre bundles are coarser and have an irregular arrangement. Fig. 50. — Bone Cell and Lacuna. (Joseph.) At a the cell body has shrunken. 102 THE TISSUES TECHNIC (i) For the study of the minute structure of bone a section of undecalcified or hard bone is required. Part of the shaft of one of the long bones is soaked for sev- eral days in water and all the soft parts are removed. It is then placed in equal parts alcohol and ether to remove all traces of fat and thoroughly dried (the handle of a tooth or nail brush frequently furnishes good material and is already dried). Thin longitudinal and transverse sections are now cut out with a bone saw. One surface is next ground smooth, first on a glass plate, using emerj^ and water, then on a hone. The specimen is now fastened polished side down on a block of wood or glass by means of sealing wax, and the other side polished smooth in the same manner as the first, the bone being ground as thin as possible. The sealing wax is removed by soaking in alcohol and the specimen looked at with the low power. If not thin enough, it is gently rubbed on a fine hone. It is then soaked in equal parts alcohol and ether, dried thoroughly, and mounted in hard balsam. This is accomplished by placing a small bit of hard balsam on a slide, melting, pushing a bit of the bone into the hot balsam, covering and cooling as quickly as possible. The object of the hard balsam and quick cooling is to prevent the balsam running into the lacunae and canaliculi and obscuring them by its transparency. The air imprisoned in the lacuna and canaliculi causes them to appear black when viewed by reflected light. (2) The structure of the bone cell is best studied in sections of decalcified bone which has first been carefully fixed. (See technic i, p. 196.) t * • • • CHAPTER IV THE BLOOD Blood is best considered as a tissue, the intercellular substance of which is fluid. This fluidity of the intercellular substance allows the formed elements or cells to move about freely, so that there is not the same definite and fixed relation between cells and intercellular substance as in other tissues. There are about 5 litres of blood in the adult body, blood thus constituting about ^- .^^^ one-thirteenth of the entire body weight. The fluid intercellular substance or plasma is slightly alkaline in reaction. It consists of serum albumen, globulin, fibrinogen and inorganic salts, chiefly the chlorid, carbonate, bicarbonate and phos- phate of soda. The reaction of blood is distinctly alkahne, due mainly to the phosphate of soda. Its specific gravity is about 1.030, while that of the whole blood is about 1.060. The bulk of the plasma is about equal to that of the red and the white cells. The formed elements of the blood are : (i) Red blood cells (red blood corpuscles, erythrocytes); (2) white blood cells (color- less corpuscles — leucocytes) ; (3) blood platelets (thrombocytes); (4) blood dust (ha^matokoniaj. I. Red blood cells (erythrocytes) (Fig. 51, i, 2, 3) are in man non-nucleated circular discs. ^ Their average diameter is about 7.5/<, their thickness 2// at the thin centre. A few red blood cells of a diameter of 8,« to8.5/t (macrocytes) and about the same number of red ceils only about one-half the usual diameter (microcytes) ' Some observers describe the red blood cell as bell- or cu|)-shiii)ed. (Lewis: Jour. Med. Research, N. S., vol. v, 1904; Radasch: Anal. Anz. xxviii, igc6. Weidcnreich: Krgcbn. d. Anat., 1903, 1904, 1909; Arch. f. mikr. Anal., Ixi, 1903, Ixix, igof).) 103 ' 8 Fig. 51. — Cells from Human Blood. X 600. (Technic 2, p. no.) I, Red blood cell seen on flat; 2,red blood cell seen on edge; 3, red blood cells forming rouleaux; 4, 4, small and large lymphocytes; 5, mononuclear leucocyte; 6, transitional leucocyte; 7, polymorphonuclear leucocyte, containing ncutrophile gran- ules; 8, polynuclear leucocyte, containing eosinophile gran- ules; 9, mononuclear leuco- cyte, containing basophile granules. 104 THE TISSUES are^usually present. They are biconcave, with rounded edges. Seen on the flat, the difference in thickness between centre and pe- riphery is evidenced by the difference in refraction (Fig. 51, i). Seen on edge, the shape resembles that of a dumb-bell (Fig. 51, 2). Singly or in small numbers, red blood cells have a pale straw color, due to the presence of haemoglobin. Redness is apparent only when the cells are seen in large numbers. If fresh blood be allowed to stand for a moment, the red cells are seen to adhere to one another by their flat surfaces, forming rows or rouleaux (Fig. 51, 3). Subjected to the usual technic, the red blood cell appears homo- geneous. By the use of special methods, this apparently homogene- ous substance can be separated into {a) a color-bearing proteid — hcemoglobin, and (b) a stroma, the latter representing the protoplasm of the cell. Peripherally the stroma probably forms an extremely delicate cell membrane, although the presence of any membrane whatever is denied by some. It is the haemoglobin which gives color to the corpuscles. Haemoglobin is a complex proteid, which can be resolved into a globulin and a pigment, hcematin. It is held in solu- tion 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 projec- tions 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, with a faint circular outline — the membrane of the cell. This separation of the haemoglobin from the corpuscle is also caused by freezing and thawing, by heat (6o°C.), by the addition of dilute acids, ether, or chloroform. Dilute alkaline solutions and bile first cause the red corpuscles to swell and^becom.e spherical, and then to dissolve. This is known as hcBfnolysis, and may also be effected by mixing the blood of one species with that of another. Dilute acetic acid causes swelling and fading of the red cells, with the form.ation of prism.atic crystals of hsemoglobin. THE BLOOD 105 The red blood cells number from 4,500,000 to 5,000,000 per cubic milKmeter of blood. 2. White blood cells (leucocytes) (Fig. 51, 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 five hundred red cells, or about 10,000 to the cubic milHmeter. 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. 51, 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 narrow zone around the nucleus. Lymphocytes constitute about 20-per-cent. of the white blood cells. (b) Mononuclear leucocytes (Fig. 51, 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. (c) Transitional leucocytes (Fig. 51, 6) 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. 51,7,8) 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 106 THE TISSUES down of the connecting cords, leaving several separate nuclei or nuclear segments. Granules in small numbers may be present in the protoplasm of any of the leucocytes, but the protoplasm of about 70 per cent, of all leucocytes is so distinctly and regularly granular that by some authors a prim.ary division into granular leucocytes and non-granular leuco- cytes is made. Under this classification, lymphocytes and some mononuclear leucocytes are placed in the non-granular group, while transitional leucocytes, polym.orphonuclear leucocytes and some mononuclear leucocytes, are placed in the granular group. Aniline dyes may be divided into acid, basic and neutral, according to whether the coloring matter is an acid, a base, or a combination of an acid and a base, and the granules of the granular leucocytes react in a definite manner to these dyes, thus allowing the following classification : f I neutrophile. Granular leucocytes] 2 acidophile (eosinophile). [ 3 basophile. As the neutrophile granules are fine and the eosinophile granules coarse, a classification of leucocytes into finely granular and coarsely granular is sometimes made. ' 1. Neutrophile Leucocytes. — These are the most numerous of all leucocytes, making up about 68 per cent. Their protoplasm is thickly studded with very fine granules which stain violet with a mixture'of eosin (acid) and toluidin blue (basic). Most neutrophiles are polymorphonuclear, a few are transitional. They have a wide distribution, being found not only in the blood itself, but in the spleen and lymph nodes and as wandering cells in various tissues and organs. 2. Acidophile Leucocytesl or, because the most common acid dye used is eosin, eosinophile. The granules in these cells are coarse and sharply defined. They stain strongly with acid dyes. Eosino- philes are mainly polymorphonuclear, m.ore rarely they are transi- tional. They make up from i per cent, to 4 per cent, of all leuco- cytes. In certain pathological conditions their number is greatly increased. 3. Basophile Leucocytes. — The granules in these cells are rather coarse and irregular in shape and are distributed unevenly through the cytoplasm. They stain strongly with basic dyes. They are THE BLOOD 107 present in small numbers (Ehrlich, 0.2 per cent, to 0.5 per cent.) in normal blood, or they may not be demonstrable. Ehrlich identifies them with the "mast cells" which are found in various tissues and organs, especially in areolar connective tissue, but this identity has been questioned. Upon the basis of the foregoing description the following classification of leucocytes on the basis of granulation may be made. 1^ Lymphocytes 22-25 per cent, f Xon-granular i I I Mononuclear leucocytes r-4 per cent. Leucocytes \ Neutrophile 65-72 per cent, (mainly polymorphonuclear, few transitional and mononuclear) Granular \ Acidophile 1-4 per cent, (mainly polymoiphonuclear, few transi- I tional and mononuclear) [ Basophile 0.2-o.s per cent, (mononuclear and transitional) The function of the red cells is primarily the carrying of oxygen from the lungs to the tissues and of carbonic acid from the tissues to the lungs. This oxygen-carrying ability is dependent upon the haemoglobin and is directly proportionate to the number of red cells and to the richness of the individual cells in haemoglobin. In the capillaries of the tissues and again in the capillaries of the lung the haemoglobin is undergoing constant change. The haemoglobin of arterial blood is known as oxy haemoglobin, of venous blood as re- duced haemoglobin. The difference is readily demonstrable by the spectroscope.^ That leucocytes possess in a marked degree the power of amoe- boid movement has been noted (p. 52). On account of this motility leucocytes are able (i) to leave the blood-vessels (diapedesis) and move about freely in the tissues (wandering cells), (2) to surround and take up substances from without (phagocytosis). Diapedesis. — This power is directly dependent upon motility and while possessed by all leucocytes is most markedly character- istic of the polymorphonuclear forms. (For details of amoeboid movement see p. 52.) Phagocytosis. — Phagocytic powers are not possessed equally In' all leucocytes, but are confined largely to the mononuclear and polymorphonuclear forms. Such cells can take up foreign sub- stances, bacteria, degeneration products, etc., carry them to other 'Of interest in this connection is the fact that in poisoninjf by illuminating j^as, a very definite and staljle combination of the carbon rlioxid with the hx-moglobin is ffirmcrl (( arboxyha.-moj^l(;binj. It is this substance which determines the darker red color ol the blood in this condition and is aijparenlly the cause of death. 108 THE TISSUES parts of the body or entirely outside the body (salivary corpuscles), or apparently absorb or digest them. (See also p. 50.)^ 3. Blood platelets (thrombocytes) are minute round or oval bodies from 2/^ to 4/^ in diameter. They are colorless and vary in shape. In fixed preparations they often appear stellate. Their num- ber has been variously estimated. The average is probably about 200,000 to 300,000 per cubic millimeter of blood. In the blood stream they are separate, but show a marked tendency to aggluti- nation directly the blood is drawn. Some comparatively recent observations tend toward considering the platelets as true cells. Thus they have been described as amoeboid, as having the same chemical composition as cells, and as containing either granules of chromatin or distinct nuclear structures. The so-called throm- bocytes of Ovipara are larger than those of man and are unques- tionably nucleated cells. Their appearance is, however, wholly un- like human thrombocytes and the identity of the two forms is doubtful. Various functions have been ascribed to the thrombocytes. It is fairly established that they have something to do with the for- mation of fibrin and the coagulation of blood. 4. The blood dust (haematokonia) occurs in the form of small refractive granules. In the blood of the lower mammals and in herbivorous animals small droplets of fat derived from the chyle are found. They are known as elementary granules and are not present in normal human blood. Development or the Blood. At an early stage of embryonic development certain mesodermic cells of the area vasculosa, which surrounds the embryo, become arranged in groups known as Uood islands. It is from these "islands" that both blood and blood-vessels develop. The peripheral cells arrange themselves as the primitive vessel walls, within which the central cells soon become free as the first Uood corpuscles. In this way vascular channels are formed, inside of which are developing blood cells. This division of the mesoblastic cells of the blood islands into (i) endothe- lial cells of the vessel walls and (2) progenitors of the blood cells or primitive blood ceUs, is quite generally accepted. From this point, however, opinions 1 It is to be noted that phagocytosis is not confined to leucocytes but has been ob- served in other cells, e.g., fixed connective-tissue cells including endothelium. It is also to be noted that phagocytic cells are not equally phagocytic to all substances. _ In other words phagocytes apparently have some powers of selection. Thus if two kinds of bacteria be presented to them, they may take up only one kind, or one kind much more readily than the other. THE BLOOD 109 diverge, the two main theories of blood-cell formation being known as the poly- phyletic and the monophyletic theories. According to the polyphyletic theory, after the original division of the meso- blast cells of the blood islands into vessel wall cells (endothelium) and blood cells (primary blood cells), the latter go on to the development of erythroblasts and these to the development of erythrocytes. Leucocytes develop later in con- nective tissue, in the liver, spleen and bone marrow, while lymphocytes have their origin in the germ centers of the lymphoid organs. There are thus three separate sources of blood cells: (i) Red blood cells, originally from erythroblasts of the blood islands; in adult life from the erythroblasts of the bone marrow and possibly of spleen; (2) leucocytes, first in connective tissue, liver, spleen and mar- row; in adult hfe in the marrow; (3) lymphocytes, in the lymphoid organs. Ac- cording to the polyphyletic theory these three types are genetically independent and remain so throughout life, the ceUs of each type undergoing mitotic division to produce new cells of the same type only. According to the monophyletic theory the blood-cell-forming elements of the blood islands fall into two groups as regards their future development: (i) cells which give rise to primary erythroblasts, these to secondary erythroblasts and these again to erythrocytes, thus completing the line of red blood cells, and (2) cells which remain undifferentiated, not only in the embryo but throughout Ufa, and retain the capabiUty of differentiating into erythroblasts or into white blood cells, either leucocytes or lymphocytes. According to Weidenreich^ and other supporters of this theory, red and white blood cells stand in very close genetic relation. He claims that an undifferentiated mother cell exists which is capable of differentiating either in the direction of the red cell or of the white cell; that this mother cell exists not only in the embryo but throughout life and is con- stantly giving birth to cells which are to develop into red blood cells or white blood cells to replace those being constantly used up and cast off. Morpho- logically the mother cell is apparently identical with the lymphocyte. In post- embryonic life, this mother cell is found in marrow in the germ center of lymphoid organs and possibly in connective tissue. On the other hand a once differentiated cell remains a differentiated cell, and while able to divide mitotically and thus give rise to cells of its own kind, it is never capable of producing cells of any other kind. Two views are held in regard to the manner in which the embryonic nucleated 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 and gradually secreted; 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 considering 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 proliferation. Before birth the splenic pulp and bone marrow become blood-forming organs. In the adult the bone marrow is prob- ably, under normal conditions, the main if not the sole seat of red-cell formation. ' Ant. Rec, vol. iv., Sept., 1910, 110 THE TISSUES During foetal 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 origin of the blood platelets is not known. They have been described as originating in the extruded nuclei of the red cells, as disintegrating leucocyte products, as red cells in process of development, as red cells in process of disin- tegration, as albuminoid precipitates, as a specific blood cell. According to Wright^, they are derived from the megakaryocytes of bone marrow and other blood-forming organs. TECHNIC (i) Fresh Blood. — Prick a finger with a sterile needle. Touch the drop of blood to the centre of the slide and cover quickly. For immediate examination of fresh blood no further preparation is necessary. Evaporation may be pre- vented 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. AUow the smear to become perfectly dry and stain by technic 1 1 , p. 3 1 . By this method the acidophile granules are stained red, basophile granules purple, and neutrophUe granules a reddish-violet. 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 solution of eosin, then in a rather weak aqueous solution of methylene blue. ^ Jour. Morph., xxi, 1910. CHAPTER V MUSCLE TISSUE While protoplasm in general possesses the property of contrac- tiKty, it is in muscle tissue that this property reaches its highest de- velopment. INIoreover, in muscle this contractiHty is along definite Fig. 52.^ — Isolated Smooth Muscle Cells from Human Small Intestine. X403. (Technic i, p. 124.) Rod-shaped nucleus surrounded by area of finely granular pro- toplasm; longitudinal 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. Involuntary Smooth Muscle. — This is the simplest form of muscle tissue and consists of long spindle-shaped cells (Fig. A B ¥\G. 53. — .Apparent Intercellular Bridges of Smooth Muscle. A, From longitudinal sectionof intestine of guinea-pig; B, from transverse section of intestine of rabbit. X420. a, Nerve cell; b, end of muscle cell. (Stohr.) 52) which are prismatic on transverse section (Fig. 53). The length of the smooth muscle cell varies usually from 30 to 200/<, its width from 3 to 8/'-,' except in the pregnant uterus where the cells fre- quently attain a much greater size. Less commonly the smooth ' In the walls of very small blood-vesselH smooth muscle cells 15 to 2o/( are found. (Apathy; . Ill 112 THE TISSUES 1\ m muscle cell is flattened, and, in tissues rich in elastic fibres, e.g., the media of some arteries, especially the aorta, is quite irregular in shape. The central thickest part of each cell contains an elliptical or long rod-shaped nucleus. The nucleus has a rather coarse intra- nuclear network and one or more nucleoh. In some cells a centrosome has been demonstrated. It lies out- side and usually just to one side of the nucleus. The irregular, wavy and twisted nuclei often seen are probably due to contractions of the cytoplasm. The nucleus is sur- rounded by an area of finely gran- ular cytoplasm, most abundant at the poles of the nucleus where it frequently forms a little pointed cap. The rest of the cytoplasm shows deHcate longitudinal striations, which probably represent a longitudinal arrangement of the spongioplasm. These fibrils are extremely fine, are frequently present in small numbers, are not arranged in bundles, are ap- parently homogeneous and anisotro- pic, and are often very difficult of demonstration. The fibrils lie in a less differentiated cytoplasm. The smooth muscle cell has no such dis- tinct envelop as the sarcolemma of striated muscle. The outer cyto- plasm is, however, modified to form a delicate cell membrane or at least a modified surface layer. The cells are united by a small amount of intercellular "cement" substance which reacts to silver nitrate. Intercellular "bridges" similar to those connecting epithelial cells have been described (Fig. 53) , but are regarded by many observers as artefacts. By others fine '\''| ,.'-,\ Fig. 54. — Preparation of Smooth Muscle Cells to show Fibrillar Struc- ture. From intestine of Triton. X2300. (Heidenhain.) MUSCLE TISSUE 113 intercellular fibrils are described as continuous from cell to cell, thus forming a syncytium. Between and surrounding the smooth muscle cells is a network of delicate connective- tissue fibrils (Fig. 55). ■Fig. 55. — ^Longitudinal Section of the Musculature of Cat's Intestine to show Intercellular Connective Tissue. (Bohemann.) These are not apparent in sections which have been subjected to the usual technic, but can be demonstrated either by digesting the smooth muscle cells by means of trypsin, thus bringing out the undigestible collagenous fibrils, or by special staining methods (p. 124). cf Fig. 56. — Elastic Fibres in the Smooth Muscle of Intestine of Cat. (Holmgren.) 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. 56). In such masses of smooth 8 114 THE TISSUES 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 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. - a " c Fig. 57. — Smooth Muscle from Longitudinal Section of Cat's Small Intestine, show- ing Portions of Inner Circular and Outer Longitudinal Muscle Coats with Intervening Connective Tissue. X350. (Technic 3, p. 124.) a, Transversely cut cells of inner circular layer; in comparatively few has the plane of section passed through the nucleus; b, longitudinally cut cells of outer longitudinal layer. In many of the cells the plane of section has not passed through the nucleus; c, intermuscular septum (connective tissue); d, smaU artery. Voluntary Striated Muscle. — This consists of cylindrical fibres from 50 to 130 mm. in length and from 10 to loops- in diameter. Generally speaking, the fibres of greater diameter are found in the larger animals. There is apparently no relation between the size of the muscle and either the length or diameter of its component fibres. There is also no relation of length to diameter, within the same muscle, fibres of many different sizes intermingling. Each muscle fibre consists of (a) a delicate sheath, the sarcolemma , enclosing (b) the muscle substance proper, in which lie (c) the muscle nuclei. The sarcolemma is a clear, apparently structureless, very elastic membrane, which adheres so closely to the underlying muscle sub- stance as to be indistinguishable in most preparations. In teased specimens it may frequently be seen at the torn ends of the fibres (Fig. 58). In staining reaction it differs from both white and elastic tissue and is apparently to be considered a distinct cell membrane. The muscle substance consists of ergasto plasm (fibrillce) and MUSCLE TISSUE 115 sarcoplasm (interjGibrillar substance) and shows two sets of striations (Fig. 59), longitudinal striations and cross striations. The longitudinal striations are due to parallel running ultimate fibrillae, which lie in and are more or less separated from one another by the sarcoplasm. Each fibrilla when examined, unstained, by re- flected light is seen to be composed of alter- nating Ught and dark segments. As like segments lie in the same transverse plane, the whole muscle fibre appears composed of alternate light and dark bands (Figs. 59 and 60), and this distinction is maintained even in stained specimens, as the light bands stain little if any, with most staining agents, while the dark bands react strongly to stains. The light band is composed of a singly re- fracting (isotropic) substance, the dark band of a doubly refracting (anisotropic) sub- stance. Through the middle of the light band runs a fine line (Krause's line), while an even finer line (Hensen^s line) can some- times be seen running through the middle lighter portion of the dark band. Both Krause's and Hensen's lines cross the inter- vening sarcoplasm as well as the fibrillae and extend to the sarcolemma, thus completely crossing the fibre. As all of these structures run through the entire thickness of the fibre, they in reality constitute discs of muscle substance (Fig. 60). 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 com- posed 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 uUimale fibrillce, transversely into muscle discs. Fig. 58. — Semidiagram- matic Drawing of Parts of two Muscle Fibres which have been broken, showing the relations between Mus- cle Substance Proper and Sarcolemma (Kanvier.) m, a, Retracted ends of muscle substance, between which is seen the sarcolem- ma with several adherent muscle nuclei; B, thin layer of muscle substance which has adhered to the sarco- lemma; 71, muscle nucleus; s, sarcolemma; p, space between sarcolemma and muscle sul)stance. 116 THE TISSUES What is known as the sarcous element of Bowman is that portion of a single fibrilla which is included in a single disc, i.e., between two adjacent lines of Krause (Fig. 60). The sarcoplasm is not evenly distributed throughout the fibre. On cross section irregular trabeculae of sarcoplasm are seen extend- ing in from the sarcolemma (Fig. 61). These separate the fibrillse into bundles, the muscle columns of Kolliker. A transverse section V»« "iS d ..... - -b d - Fig. 60. X350. (Technic 4, p. Fig. 59. Fig. 59. — Portion of Striated Voluntary Muscle Fibre. 125.) 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 indicates Hensen's line. The black line outlining the fibre represents the sarcolemma. a, Fibrillse; h, muscle nucleus; c, Krause's Hne; d, Hensen's line. Fig. 60. — Diagram of Structure of a Muscle Column of Kolliker. The appearance presented by the cross-cut muscle column = Cohnheim's field, a, Muscle fibrillae; h, sarcous element; c, Krause's line; d, Hensen's line; e, Cohnheim's field; /, muscle disc. of one of these columns presents the appearance of a network of sarcoplasm and of interfibrillar cement substance enclosing the fibrillse. This appearance is known as Cohnheim's field (Figs. 60 and 61). Many of the details of structure of striated muscle have been determined by studies upon the muscles of lower animals. These details are extremely com- plicated and the numerous terms used to designate the same structure very con- MUSCLE TISSUE 117 fusing. The following is the scheme of structure and the nomenclature accord- ing to Heidenhain (Fig. 62). The older terms muscle cell for the smooth muscle cell, and muscle fibre for the analogous mvdtinuclear element of striated muscle are retained, also Apathy's myofibril for the ultimate fibrilla, Kolliker's muscle column for the smallest bun- dles of fibrils, and sarcoplasm for interfibrillar substance. The muscle fibre is subdivided into a series of segments by transverse discs which completely cross the fibre, involving both fibrillse and sarcoplasm and are attached to the sarcolemma. This disc (ground membrane — membrane of Krause,) Heidenhain designates the telophragma (Z). That part of a fibre which lies between two telophragmata is an iiwkomma. The middle of each inokom- ma is crossed by a disc (Hensen's membrane) which also involves both fibrillae Fig. 61. — Semidiagrammatic Drawing of Transverse Section of a Voluntary Muscle Fibre, showing Sarcolemma; sarcoplasm separating fibrils into bundles, each bundle constituting a muscle column of KoUiker and the appearance of its cross-cut end being Cohnheim's field, a, Sarcoplasm; b, Cohnheim's fields; c, sarcolemma. and sarcoplasm and is attached to the sarcolemma. This membrane having apparently the same structure as the telophragma is designated the mesophragma (M). The mid-portion of the inokomma consists of anisotropic substance (Q), the ends of isotropic (J). That portion of Q which lies on either side of M is lighter than the rest of Q and is designated Qh (h = hell = clear) . That portion of J which lies close on either side of Z is darker than the rest of J and is designated Jd (d = dicht = thick). In the sarcoplasm between the fibrillae are'granules; those in J are arranged in a regular row and are known as J-granules, those in Q are more irregularly placed and arc known as Q-granules. In Fig. 62 arc also shown the " cross-fibrc-nets " which arc brought out by metallic impregnation and which possibly represent intracellular canals. Two varieties of striated voluntary muscle fibres are distinguished, while 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 arc less easily fatigued. In man white fibres are in the large majority, red fibres never occurring alone, but mingled with white fibres in some of the more active muscles, such as those of respiration 118 THE TISSUES 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 J^ yxs3K.-«ff^''' 6-^ — Telophragma J-granules Oh M — Qh J{ Jd Z / • M ## % # z - ^ 4j _^i pw ^'^'n M — - ■!""' K Q -granules Mesophragma Q-granules J-granules J-granules Cross-fibre nets Cross-fibre ' nets Fig. 62. — Scheme of Structure of Striated Voluntary Muscle Fibre with Nomenclature of Heidenhain. (Heidenhain.) in a rounded or blunt extremity, the sarcolemma being continuous with the tendon fibres (Figs. 63 and 64). Muscle fibres are usually unbranched. In some muscles — e.g., those of the tongue and of the eye — anastomosing branches occur. MUSCLE TISSUE 119 ^^. 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 large fibres containing hundreds or even thou- sands of nuclei.^ In the white fibres the nuclei are situated at the periphery just beneath the sarcolemma. In red fibres they are centrally placed. Involuntary Striated Muscle (Heart Mus- cle).— This, as its name implies, is a striated muscle not under control of the will. It occurs only in the cardiac musculature. Like voluntary muscle, heart muscle is striated both longitudinally and transversely. Like smooth muscle, on the other hand, it is com- posed of units which at least resemble cells and have long been called heart muscle cells. Heart muscle also resembles smooth muscle in that there is usually but one nucleus to a cell and this nucleus is centrally placed instead of lying at the periphery as in voluntary muscle. Also the sarcolemma if present at all is extremely delicate and is probably, as in smooth muscle, only a modification of the surface sarcoplasm. The amount of sarcoplasm throughout the cell is large. Around the nucleus is an area of sar- coplasm free from fibrillae. This area often ex- tends some distance toward the ends of the cell. Heart muscle fibres are not the parallel- running unbranched fibres of voluntary mus- cle, but, while having a generally parallel arrangement, give off side branches which anastomose, making it impossible to trace a single fibre for any great distance. These side branches have the same structure as the main fibre except that they are for the most part of smaller diameter and are nonnucleated. * Stiihr calls attention to the fact that such a structure may be considered either a multinuclear cell or a syncytium and that there is really no true distinction between the two. , c Fig. 63. — Semidia- grammatic Illustration of Endings of Muscle Fibres within a Muscle and in Tendon, ((iage.) 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 termi- nates above in pointed intramuscular ending, below in blunt ending connected with tendon. 120 THE TISSUES The striations of heart muscle are less distinct than are those of voluntary muscle, but apparently represent very similar structures. The longitudinal striations indicate fibrilla lying in the sarcoplasm. From the central mass of sarcoplasm which surrounds the nucleus, strands radiate toward the periphery. These strands, anastomosing, separate the fibrillse into columns, the muscle columns of KolUker. In cross section these present the appearance described under volun- tary muscle as Cohnheim' s fields . The disposition of the sarcoplasm, / Fig. 6 s. Fig. 64. — Two Muscle Fibres from Upper End of Human Sartorius, to show con- nection of muscle and tendon. X350. (Gage.) m, Muscle fibres; t, tendon fibres. Fig. 65. — Muscle Cells from the Human Heart (technic 6, p. 125), showing lateral branches and lines of union between cells. Xsoo. extending outward from the region of the nucleus like the spokes of a wheel, gives to the cross section a characteristic radiate appearance (Fig; 66), not seen in cross sections of voluntary muscle. The transverse markings represent, as in voluntary muscle, alternate light and dark discs. Through the middle of the light disc can be seen the membrane of Krause. Mc Galium describes Krause's membrane as crossing not only the fibrillas, but also the sarco- plasm, as in the voluntary muscle fibre. The sarcoplasm he describes as further subdivided by membranes, which are transversely con- tinuous with Krause's membranes, into minute discs. The centre MUSCLE TISSUE 121 of the cell around the nucleus is wholly composed of these little discs of sarcoplasm. Pecuhar to heart muscle are what appear in longitudinal sections to be dark lines which cross transversely both main fibres and side branches. These are known as intercallated discs and divide the muscle fibre into irregular, short, thick cylindrical segments. A disc may pass straight across a fibre, or it may cross it in a series of steps, or it may extend only part way across the fibre. It always touches at some point one of the ground membranes^ and sometimes Fig. 66. — Section of Heart Muscle. X350. (Technic 7, p. 125.) a, Cells cut longitudinally; b, cells cut transversely (only three nuclei have Ijeen included in the plane of section); c, ceUs cut obliquely; d, connective-tissue septum. entirely fills the space between two ground membranes which lie unusually close together. Special technic has demonstrated the fact that the fibrillae do not stop at the disc but are continuous through it, although they show in passage some modification of structure. The one question which has been most discussed in rcgcard to the structure of heart muscle and which remains still unanswered is whether heart muscle is cellular or a syncytium. The solution of the problem is of course dependent upon the determination of the significance of the intercallated discs — whether or not they represent true cell boundaries. The segments into which the heart muscle fibre is divided by the discs have long been described as cells. The fact that the nuclei are usually placed about midway between two discs; that the discs show the same staining reaction as intercellular substance when subjected to the action of silver nitrate, and the ease with which heart muscle may be ' Some authorities deny this. 122 THE TISSUES separated into "cells," especially in young animals and in lower vertebrates, by use of those chemicals usually used to break down intercellular cement; have all been used as arguments in favor of a cellular structure. On the other hand, the fact that it has not been proved that the segments bounded by the discs corre- spond to the original myoblasts; that in the later stages of development of heart muscle the type of nuclear division is usually amitotic, a type frequently unac- companied by division of the cytoplasm; that the fibrill« pass uninterrupted through the discs; that some discs only partially cross a fibre; that some segments contain more than one nucleus while others are non-nucleated; all favor a syncytial interpretation. How open the question still remains is shown by the fact that Heidenhain regards the "ceUs" as "growth segments"; that Marceau considers them non-contractUe "supports" for the fibriUae; that Jordan from his work on humming-birds concludes that they are a coarser development of the anisotropic bands; while Schaffer looks upon them as post-mortem contrac- tion artefacts. Development oe Muscle Tissue Smooth Muscle. — In the higher animals, muscle tissue with the exception of that connected with the sweat, lacrymal and mammary glands, which is of Fig. 67. — Myoblasts in DifEerent Stages of Development. (Godlewski.) The upper cell represents a myoblast with granular cytoplasm (from sheep embryo of 13 mm.) ; the middle, a myoblast with fibrils in process of formation (from guinea-pig embryo of 10 mm.); the lower, a myoblast with still further differentiated, segmented fibrils (from a rabbit embryo of 8.5 mm.). ectodermic origin, is derived wholly from mesoderm. The cells (myoblasts) which are to become smooth muscle cells develop in the general mesenchymal tissue among cells which are to become connective-tissue cells and with which they are at first apparently identical. In becoming a smooth muscle ceU the myoblast changes its shape, becoming greatly elongated, its nucleus at the same time becoming oval or rod shaped. Such cells anastomose freely. During these changes in shape, delicate longitudinal fibrils appear in the cytoplasm. Just how these fibrils originate is not known, but they probably represent a speciali- zation and rearrangement of the spongioplasm. Striated Vohmtary Muscle. — This develops from the mesoblastic somites. Each somite early divides into an outer part, sclerotome or cutis plate, and an inner part, the myotome or muscle plate. Cells of the myotome soon show changes MUSCLE TISSUE 123 which distinguish them as mj'oblasts or muscle-forming cells. These cells, which are at first spherical, become elongated and spindle shaped. The nucleus is at this stage centrally placed and the spongioplasm is in the form of a reticulum. '^■\ w . - Fig. 68. — From a Section of Developing Heart Muscle from a Rabbit Embryo of 9 mm. (Godlewski.) a, Cell body with granules arranged in series; b, cell body with centrosome and attrac- tion sphere; c, branching fibril; d, fibrils extending through several cells. Fibrillar arrangement of the spongioplasm first appears around the periphery where granules form in the cytoplasm and become arranged in rows lengthwise of the cell. The central portion of the cell is at this stage still occupied by retic- '"""^j^V'^j*^ -v^ """ xi:*'j-^-- ':::z^© ,.«a»*»**J****^-^V' ,, ^ Fig. 69. — From a Section oi I developing iicarL Aiuscle in a Kaijbit i'-mbryo of 10 mm. (Godlewski.) The fibrils are segmented, indicating the beginning of the cross striation charactis- tic of heart muscle. ular spongioplasm and the nucleus. These granules next unite to form deli- cate fibrils. New fibrils form both by longitudinal arrangement of more granules and by longitudinal splitting of fibrils already formed, until they finally fill 124 THE TISSUES the entire cell. After the union of the granules to form fibrils the latter are apparently homogeneous, but later differentiate into the alternate light and the dark substances which determine the cross striations characteristic of striated muscle. Just how this differentiation takes place is not known. During this process of fibrillation the nucleus has been undergoing mitotic division without corresponding division of the cytoplasm. In white fibres these nuclei migrate to the surface and come to lie just beneath the sarcolemma. The sarcoplasm in which the fibrils lie probably represents the remains of the undifferentiated pro- toplasm (hyaloplasm). Some authorities deny the origin of the musde fibre from a single cell, describ- ing it as derived from a number of myoblasts which unite to form a fibre. Heart muscle develops from mesenchyme. The myoblasts are at first small, spheroidal, and closely arranged. With the appearance of intercellular substance the cells become separated and irregular in shape, and anastomose to form a syn- cytium. A little later the cells become arranged in parallel columns and cross markings, the " intercaUated discs" appear, dividing the syncytium into the so-called "heart-muscle cells." Whether these coincide with the original myo- blasts is not known. As in voluntary muscle, fibrils first develop in the periphery of the cell apparently by union of granules which arrange themselves in lines lengthwise of the cell. The fibrils increase in number and invade the entire cell except the small area of undifferentiated protoplasm which remains around the nucleus. Later the fibrils show the alternate light and dark markings or cross striations. As with voluntary muscle the way in which these cross markings' develop is not known. Attention has already been called (p. 52) to the spongioplasm as the con- tractile element of protoplasm. It is to be noted that in the development of muscle no new element appears, the contractile fibrillce representing nothing more than a specialization of the already contractile spongioplasm. TECHNIC (i) Isolated Smooth Muscle Cells. — Place small pieces of the muscular coat of the intestine in o.i-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 (p. 19). This is poured off, the tissue again washed in water and preserved in eosin-glyc- erin,- 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 intes- tine 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 (p. 19). After twelve hours' staining, wash and transfer to eosin-glycerin. (3) Sections of Smooth Muscle. — Fix small pieces of intestine in formalin- MiiUer's (technic 5, p. 7) or in Zenker's fluid (technic 9, p. 8). Thin transverse MUSCLE TISSUE 125 or longitudinal sections are stained with haematoxylin-eosin (technic i, p. 20), 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 \'oluntary JMuscle 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 \'oluntary Muscle. — Fix a portion of a tongue in formalin- ]M tiller's fluid or in Zenker's fluid (p. 8). Thin sections are stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 21) and mounted in balsam. As the muscle fibres of the tongue run in aU directions, fibres cut transversely, longitudinally, and obliquely may be studied in the same section. The sarco- lemma, the pointed endings of the fibres, and the relation of the fibres to the con- nective tissue can also be seen. (6) Isolated heart-muscle cells may be obtained in the same manner as smooth muscle cells. (See technic i, p. 124.) (7) Sections of Heart Muscle. — These are prepared according to technic 3 (above). By including the heart wall and a papillary muscle in the same section, both longitudinally and transversely cut cells are secured. The stain may be either haematoxylin-eosin (technic i, p. 20), or haematoxylin-picro-acid-fuchsin (technic 3, p. 21). CHAPTER VI NERVE TISSUE The Neurone In most of the cells thus far described the protoplasm has been confined to the immediate vicinity of the nucleus. In the smooth muscle cell was seen an extension of protoplasm to a considerable distance from the nuclear region, while in the connective-tissue cells of the cornea the protoplasmic extensions took the form of distinct processes. Processes, often extending long distances from the cell body proper, constitute one of the most striking features of nerve- cell structure. Some of these processes are known as nerve fibres; and nerve tissue was long described as consisting of two elements, nerve cells and nerve fibres. With the establishment of the unity of the nerve cell and the nerve fibre, the nerve cell with its processes was recognized as the single structural unit of nerve tissue. This unit of structure is known as a neurone. The neurone may thus be defined as a nerve cell with all of its processes. In the embryo the neurone is developed from one of the ectoder- mic cells which ccnstitute the wall of the primitive neural canal. This embryonic nerve cell, or neuroblast, is entirely devoid of proc- esses. Soon, however, frcm 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, alf'o 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. 70) . An impoitant exception to this rule is presented by the cells of the cerebro- spinal ganglia (Fig. 292). Here the typical neurone has two processes, a periph- eral process, and a central process which enters the central nervous system. Both processes have a typical axis-cylinder structure. Some authorities have interpreted the peripheral process as a modified dendrite. I. The Cell Body. — Like most other cells, the nerve cell body consists of a mass of protoplasm surrounding a nucleus (Fig. 71), 126 NERVE TISSUE 127 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 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, many of the bodies of the cells w Ml Fig. 70. Fig. 71. Fig. 70. — Scheme of Peripheral JMotor Neurone. (Barker.) The cell body, protoplasmic processes, axone, collaterals, and terminal arborizations in muscle are all seen to be parts of a single cell and together constitute the neurone, c, Cytoplasm of cell body containing chromophilic bodies, neurofibrils, and perifibrillar substance; n, nucleus; «', nucleolus; d, dendrites; ah, axone hill free from chromophilic bodies; ax, axone; sf, branch (collateral;; m, medullary sheath; n R, node of Ranvier where branch is given ofT; si, neurilemma (probably not present in central nervous system); m', striated muscle fibre; lei, motor end plate. Fig. 7 r. — Large Motor Nerve Cell from Ventral Horn of Spinal Cord of Ox, showing Chromophilic Bodies. (From Barker, after von Lenhossek.) <7, Pigment; h, axone; c, axone hill; d, dendrites. 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, irregularly stellate. According to the number of processes given off, nerve cells are often referred to as unipolar, bipolar, or multipolar. The NUCLEUS of the nerve cell (Fig. 71) differs in no essential 128 THE TISSUES from the typical nuclear structure. It consists of (i) a nuclear mem- brane, (2) an intranuclear network of linin and chromatin, (3) an achromatic nucleoplasm, and (4) a nucleolus. The CYTOPLASM of the nerve cell consists of at least two distinct elements: (i) Neurofibrils, and (2) perifibrillar substance. In most nerve cells a third element is present, (3) chromophilic bodies. '^^v Fig. 72. — Ganglion Cells, Stained by Bathe's Method, showing Neurofibrils. A Anterior horn ceU (human); B, ceU from facial nucleus of rabbit; C, dendrite of human anterior horn cell showing arrangement of neurofibrils. (Bethe.) In B the chromophilic bodies are shown. In this picture the neurofibrils are shown as not anastomosing. (i) 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 (Figs. 72 and 73). (2) The perifibrillar substance (Fig. 72) is a fluid or semi-fluid substance which both in the cell body and in the processes surrounds NERVE TISSUE 129 and separates the neurofibrils. It is believed by some to be like the fibrils, continuous throughout the cell body and processes, by others to be interrupted at certain points in the axone (see p. 137). (3) The chromophilic bodies (Fig. 71) are granules or groups of granules which occur in the cytoplasm of all of the larger and of many of the smaller nerve cells. They are best demonstrated by means of a special technic known as the method of Nissl (page 38). \ Fig. 73. — Body of Large Pyramidal Cell from Cortex of Cat. Silver Method of Cajal. Shows nucleus pale and arrangement of neurofibrils within the cell; a, axone; b, main or apical dendrite. (Cajal). In this picture the neurofibrils are shown as anastomosing. When subjected to this technic, nerve cells present two very dif- ferent types of reaction. In certain small cells the amount of cytoplasm is extremely small and only the nuclei stain with the Nissl method. Such cells are found in the granule layers of the cerebellum, olfactory l(;be, and retina. They are known as caryo- chrome cells, and apparently consist wholly of neurofibrils and peri- fibrillar substance. Other cells react, both as to their nuclei and as to their cell bodies, to the Nissl stain. These cells are known as somalochrome cells. Taking as an example of this latter type of cell one of the motor cells of the ventral horn of the cord and subjecting 0 130 THE TISSUES it to the Nissl technic, we note that the cytoplasm is composed of two distinct elements: (a) a clear, unstained ground substance, and, scattered through this, (&) deep-blue-staining masses, the chromo- These bodies are granular in character and philic bodies (Fig. 71). Fig. 74. — Pyramidal Cell from Huro.an Cerebral Cortex. (Golgi bichlorid method. See 2, p. 36.) Golgi cell type I. a, Cell body; b, main or apical dendrite showing gemmules; c, lateral dendrites showing gemmules; d, axone with col- laterals. Only part of axone is included in drawing. differ in shape, size, and arrangement. They may be large or sm.all, regular or irregular in shape, may be arranged in rows or in an irregular manner, may be close together, almost filling the cell body, or quite separated from one another. Presenting these variations in different types of cells, the appearance of the chromophilic bodies in a particular type of cell remains constant, and has thus been used by Nissl as a basis of classification.^ It is important to note in studying the nerve cell by this method that somato- chrome cells of the same type frequently show marked variations in staining in- tensity. This appears to depend upon the size and closeness of arrangement of the chromophilic bodies, and this again seems dependent upon changes in the cytoplasm connected with functional ac- tivity. In cells stained by Nissl's method the cytoplasm between the chromophilic bodies remains unstained and apparently structureless, and it is this part of the cytoplasm that corresponds to the neuro- fibrils and a part at least of the perifibril- lar substance. The relation which the appearance of the Nissl-stained cell bears to the structure of the living protoplasm is still undetermined. According to some investigators the Nissl bodies exist as such in the living cell. Others believe that they are not present in the living cell, but represent precipitates due either to postmortem changes or to the action of fixatives. The significance of the Nissl picture from the standpoint of pathology lies in the fact that when subjected to a given technic, a particular type of nerve cell always pre- ■ ^ For this classification, the signiiicance of which is somewhat doubtful, the reader is referred to Barker, "The Nervous System and Its Constituent Neurones," p. 121. NERVE TISSUE • 131 sents the same appearance ("equivalent picture"), and that this appearance furnishes a norm for comparison with cells showing pathological changes, and which have been subjected to the same technic. Many nerve cells contain more or less brownish or yellowish pigment (Fig. 71). This pigment, which is a lipochrome, is not present in the cells of the ^new-born, but appears in increasing amounts with age. Its significance is not known. Another black pigment known as melanin is present in certain cells in the central nervous system (e.g., the substantia nigra). This pigment is said to increase in amount until adolescence. Fig. 75. — Golgi Cell Type II. from Cerebral Cortex of Cat. (Kolliker.) a;, Coarse protoplasmic processes with gemmules easily distinguishable from the more delicate, smoother axone, a. The latter is seen breaking up into a rich plexus of terminal fibres near its cell of origin, practically the entire neurone being included in the drawing. In addition to its characteristic structure, the nerve cell may contain many elements found in other cells (p. 43). Golgi, Holm- gren, Cajal, and others have also demonstrated a network of canals within the nerve cell similar to that found in other cells (p. 46, Fig. 4). n. The Protoplasmic Processes or Dendrites. — These have a structure similar to that (;f the cell body, consisting of neurofibrils, perifibrillar substance, and, in somatochrome cells, chromophiUc bodies (Figs. 71 and 72). Dendrites branch dichotomously, become 132 THE TISSUES rapidly smaller, and usually end at no great distance from the cell body (Figs. 74 and 75). III. The Axone.- — This differs from the cell body and dendrites in that it contains no chromophilic bodies (Fig. 71), consisting wholly of neurofibrils and perifibrillar substance. Not only is it entirely achromatic itself, but it always takes origin from an area of the cell body, the axone hill (Fig. 71), which is also free from chromophilic bodies. It is as a rule single, and while usually arising from the body of the cell may be given off from one of the larger protoplasmic trunks. Some few cells have more than one axone, and nerve cells without axones have been described. In Golgi preparations the axone is distinguished by its straighter course, more uniform diameter, and smoother outline (Fig. 74). It sends off few branches {collaterals), and these approximately at right angles. Both axone and collaterals usually end in terminal arborizations. In most cells the axone extends a long distance from the cell body. Such cells are known as Golgi cell type I or long axone neurones (Fig. 74). In others the axone branches and ends in the gray matter in the vicinity of its cell of origin — Golgi cell type II or short axone neurones (Fig. 75). As they leave the cell body the neurofibrils of the axone converge to a very narrow portion of the axone, where the perifibrillar sub- stance is much reduced in amount, or according to some, entirely interrupted. Beyond this the fibrils become more separated and the perifibrillar substance more abundant. Some axones pass from their cells of origin to their terminations as "naked" axones, i.e., uncovered by any sheath. Other axones are enclosed by a thin membrane, the neurilemma or sheath of Schwann. Still others are surrounded by a sheath of considerable thickness known as the medullary or myelin sheath. Depending upon the presence or absence of a medullary sheath, axones may thus be divided into two main groups — medullated axones and non-medullated axones. I. Non-medullated Axones (non-medullated nerve fibres) (Fig. 76). These are subdivided into non-medullated axones without a neurilemma and non-medullated axones with a neurilemma. (a) Non-medullated axones without a neurilemma are merely naked axones. Present in large numbers in the embryo, they are in the adult confined to the gray matter and to the beginnings and endings of sheathed axones, all of the latter being uncovered for a NERVE TISSUE 133 short distance after leaving the nerve cell body, and also just before reaching their terminations. {b) Non-meduUated axones with a neurilefnma — fibres of Remak (Fig. 76). In these the axone is surrounded by a delicate homo- geneous, nucleated sheath, the neurilemma or sheath of Schwann (see p. 135). These axones are described by some writers as having A I - "/III 11 Fig. 76. Fic. 7 7. Fig. 76. — Non-meduUated Ncr\-e Fibres with Neurilemma, only the nuclei of which can be seen. X300. Fig. 77. — A, Fresh nerve fibre from sciatic nerve of rabbit teased apart in normal salt solution, showing broad unshrunken axone and comparatively thin medullary sheath. B, Showing crenation of medullary sheath which occurs soon after placing fibres in salt solution. C, Same after fixation and staining with picro-acid-fuchsin, showing shrunken axone and broad medullary space. The latter usually contains irregular clumps of myelin, a, Node of Ranvier; b, incisures of Schmidt; c, nucleus of neurilemma. no true neurilemma, but merely a discontinuous covering of flat connective-tissue cells, which wrap around the axone and corre- spond to the end(meurium of the nerve trunk (see page 426). The majority of the axones of the cells of the sympathetic ganglia fall under this category. 2. Meuullated Axones (mcduUatcd or myelinated nerve fibres). — The.se, like the non-medullated, are subdivided according to the 134 THE TISSUES presence or absence of a neurilemma into meduUated axones with a neurilemma and meduUated axones without a neurilemma. (a) MeduUated axones with a neurilemma constitute the bulk of the fibres of the cerebro-spinal nerves. Each fibre consists of (i) an axone or axis-cylinder, (2) a medullary sheath, and (3) a neurilemma. (i) The axone is composed of neurofibrils continuous with those of the cell body, and like them lying in a perifibrillar substance or neuroplasm (Fig. 82). In the fresh condition the axone is broad, Fig. 78. — I, Fibres of Remak; 2, nerve fibre from, central nervous system (medul- lated but without sheath of Schwann) ; 3 and 4, nerve fibres from the sciatic of frog, showing nodes of Ranvier, and in 4 the incisures of Schmidt. and shows faint longitudinal striations corresponding to the neuro- fibrils, or appears homogeneous (Fig. 77, A). Fixatives usually cause the axone to shrink down to a thin axial thread, whence its older name of axis-cylinder (Fig. 77, C). A delicate membrane has been described by some as enveloping the axone. It is known as the axolemma or periaxial sheath (Fig. 80). (2) The medullary sheath (Figs. 77 and 82) is a thick sheath largely composed of a semi-fluid substance resembling fat and known as myelin. In the fresh state the myelin has a glistening homo- geneous appearance. It is not continuous, but is divided at intervals of from 80 to 6co/i by constrictions, the nodes or constrictions of Ranvier. That portion of a fibre included between two nodes is known as an internode (Fig. 80). The length of the internode is NERVE TISSUE 135 usually proportionate to the size of the jSibre, the smaller hbres having the shorter internodes. In fresh specimens the medullary sheath of an inter- node appears continuous (Fig. yj, A), but in fixed specimens it is broken up into irregular segments, Schmidt-La titer ma nn segments, by clefts which pass from neurilemma to the axolemma or axone, and are known as the clefts or incisures of Schmidt- Lantermann (Fig. 77, C). 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. 82, B, and 81) 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. Silver nitrate staining shows a black transverse line in the neurilemma at the node of Ranvier. This line represents the bound- ary between two neurilemma cells. Against the inner surface of the neurilemma, usually about midway between two nodes, is an oval-shaped nucleus, the nucleus of the neurilemma (Figs. 76, C, and 81). Each nucleus is surrounded by an area of granular protoplasm, and makes a little depression in the myelin and a sHght bulging of the neurilemma (Fig. 77, C). According to most observers no neurilemma is present in the central nervous system. An important exception is Cajal, who describes the medullated fibre of the central nervous system as having a neurilemma. In addition to the above-described sheaths, most medullated fibres of peripheral nerves have. Fi<;. 79. — Dia- grammatic Repre- scntalion of Portions of Two Medullated Nerve Fibres, as seen in Longitudinal Sec- tion, stained with OsmicAcid. (Length of intcrnode is pro- portionately short- _ encd.) /i,^, Nodes ~ of Ranvier, with axis cylinder passing through; n, neurilemma; c, nucleus surrounded by protojjlasm, lying at about the middle of the internode between the neurilemma and the mccluliary sheath. (From drawing by J. F. \eale in Quain's Anatomy.) 136 THE TISSUES k " h -.-. d \ f \ outside the neurilemma, a nucleated sheath of connective-tissue origin, known as the sheath of Henle (Fig. 8i). Two views as to the relation of the axolemma to the neurilemma are illus- trated in Fig. 80. According to one the neurilemma is continuous, merely dipping into the nodes of Ranvier, where it touches the axolemma or the axone. According to the second both neurilemma and axolem- ma are interrupted at the node, but unite with each other there to enclose completely the medullary substance of the internode. According to the views illustrated in Fig. 82, that part of the axone which lies between two nodes is enveloped by a cell, or by several cells forming a syncytium. The outer homogeneous membrane there pictured would thus be of the nature of a cell membrane or cuticle, and would cor- respond to the neurilemma. The trabeculse (protoplasmic strands and neurokeratin network, of which some of the larger strands would represent the incisures of Schmidt) would correspond to the spongio- plasm, and just along the outer side of the axone would constitute the axolemma. The myelin would thus be enclosed within the cylindrical neurilemma ceU which sur- rounds the axone. Recent experiments of Bethe and others tend to prove an interruption of the peri- fibrillar substance at the node of Ranvier. They consider the axone at the node as probably crossed by a sieve-like plate, through the holes of which the fibrils pass, but which completely interrupts the peri- fibrillar substance. The accuracy of these observations has been disputed. MeduUated nerve fibres vary greatly in size. The finer fibres have a diameter of from 2 to 4.^, those of medium size from 4 to lo/Jt, the largest from 10 to 20/^. They have few branches, and these are always given off at the nodes of Ranvier. (b) MeduUated axones without a neurilemma are the medullated Fig. Fig. 81. Fig. 80. — Diagram of Structure of a Medullated Nerve Fibre of a Peri- pheral Nerve showing two different views (one on each side) as to rela- tions of neurilemma and axolemma and their behavior at the nodes of Ranvier. (Szymonowicz.) a, Neuro- fibrils; b, cement substance; c, axone; d, incisure of Schmidt; e, nucleus of neurilemma; /, meduUary sheath; g, sheath of Schwann; h, axone; i, axo- lemma; y, sheath of Schwann; h, node of Ranvier. Fig. 81. — Piece of Medullated Nerve Fibre from Human Radial Nerve. X400. Osmic-acid fixation and stain. (Szymonowicz.) a, Me- dullary sheath; h, axone; c, sheath of Henle; d, nuclei of Henle's sheath; e, nucleus of neurilemma. XERVE TISSUE 137 nerve fibres of the central nervous system as described by most observers. Cajal, as already mentioned (p. 135), describes these fibres as having a neurilemma. Their structure is similar to the above-described structure of a medullated nerve fibre with a neuri- lemma, 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 centre of the neurone, 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 is probable that in most or perhaps all neurones the usual direction of conduction along the axone is cellulifugal, i.e., from cell body to terminal arborization. The dendrites and cell body would receive and probably integrate the various nerve impulses received by the neurone. In other words, the dendrites and cell body receive, the axone transmits and the axone branches and terminal arborizations distribute. This general direction of conduction indicates a certain polarity of the neurone. While the cerebro-spinal ganglion cell is obviously polar- FiG. 82. — Scheme of Structure of Medullated Per- ipheral Xerve Fibre of a Fish (XemilefF). A, Cross section; B, longitudinal section; on left, fibre is shown as stained intra vitam with methylene blue; on right, myelin is shown black as in osmic acid staining, with the incisures of Schmidt indicated; sz, cells of sheath of Schwann; n, their nuclei; ss, sheath of Schwann; sp, processes of the cells of sheath of Schwann or the myelin sheath network; Ic, larger trabeculx- of proto- plasmic framework of medullarj' sheath arranged obliquely to axis-cylinder and forming the so-called "funnels"; leo, clear streaks in fibres treated with osmic acid, corresponding to le, incisures of .Schmidt; mo, myelin blackened with osmic acid; ox, axis- cylinder; pa, periaxial s[)ace around axis-cylinder; gs, "coagulum sheath," granules [irobably representing coagulatcfl fluid in periaxial s[)a(e; />/, peri|)heral, non-fibrillar, part of axis-cylinder; /, neurofibrils of axis-cylinder; r, ring-like thickening of Schwann's sheath at node of Ranvier; o, cavity in r. 138 THE TISSUES ized, yet if its peripheral process be regarded as an axone, the latter would evidently constitute an exception to conduction along the axone being ceUu- lifugal. There are cases {e.g., unipolar cerebro-spinal ganglion cell) where the nervous impulse may apparently pass from one process to another without traversing the body of the cell. Regarding the chrom.ophilic substance, certain facts, such for example as the entire absence of chromophilic bodies in many nerve cells, which nevertheless undoubtedly functionate; the absence of these bodies in all axones; the diminu- tion 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 remains to be considered as possible factors in the transmission of the nervous impulse the neurofibrils and the perifibrillar substance. While a few investigators are inclined to magnify the importance of the latter, the ma- jority agree in considering the neurofibrils as the principal conducting 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, would, if true, be obviously in favor of this view. It is also obvious that if the neurofibrils are the sole conducting substance and if they do not anastomose within the body of the vertebrate neurone, we would be driven to conclude there must be an extracellular anastomosis. The proba- bility, however, is rather against both of these premises. The neurofibrils are probably a differentiation of the spongioplasm, while the perifibrillar substance and chromophilic bodies are specializations of the hyaloplasm. As to the manner in which neurones are connected, there are two main the- ories, 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 contiguity 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 Golgi, His, Forel, Cajal, and others, has been recently called in question by such prominent neurologists as Apathy, Bethe, Held, and Nissl, on the ground that in some cases the neurofibrils are continuous throughout a series of neurones. The point of contact between two neurones is called a synapsis (Fig. 83), and the conception that there is some kind of interruption or discontinuity in neural circuits involving more than one neurone has proved useful to physiologists. It affords an explanation of certain differences between conduction through a cir- cuit of two or more neurones and conduction through a nerve fibre alone. For example, an impulse takes longer to traverse the circuit than to traverse a nerve fibre of equal length. Also a stimulus may pass in either direction along a nerve fibre, but cannot be "reversed" along a circuit. Based upon the contact theory is the so-called "retraction hypothesis," which held that a neurone being asso- ciated with other neurones only by contact was able to retract its terminals, thus breaking the association and throwing itself, as it were, out of circuit. NERVE TISSUE 139 According to the continuity theory, while the perifibrillar substance is inter- rupted 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 netivork — 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 collaterals of other nerve cells. This connection is either direct, or, as some believe, through another general (diffuse) extracellular network. The neurofibrils are thus, accord- FiG. 83.— yl, B, C, Three cells of the Ventral Cochlear Nucleus of Rabbit, showing terminals of fibres of the cochlear nerve and their relations to the cell bodies (Cajal). a, a, a, Fibres of the cochlear nerve, which break up into terminal ari^orizations upon the cells; b, c, terminal rings. The i>oints of contact between the terminals of the axone of one neurone and the cell bodv and dendrites of the other neurone constitute a "synapsis." ing to this theory, continuous and form two or possibly three continuous net- works: (a) an intracellular network, (b) a pericellular network (Golgi), and (c) a more diffuse extracellular network, lying between the cells. The existence of (c) is extremely doubtful and it seems probable that the Golgi network is either non-nervous or an artefact. Thus the main point at issue is whether the neurofibrils, in such pericellular terminals as are illustrated in Fig. 83, are con- tinuous with the neurofibrils within the cell enveloped or are separate from the latter. The individuality of the ncun;ne and the interdef)cndence of its various parts arc strikingly shown by its behavior when injured. 'I'hat degenerative changes, which progress to complete flisappearance of the nerve structures, take place in 140 THE TISSUES the distal part of a nerve when that nerve is cut across has long been accepted as one of the fundamental laws of neuropathology (law of Wallerian degenera- tion). In terms of the neurone concept this would mean that an axone cut off from its cell of origin degenerates and disappears and this behavior of the axone would accord with the already stated fact that the cell body is the trophic center of the neurone. The degenerative changes in the axone are best shown in their earlier stages by an osmic acid (p. 7) or by a Marchi (p. 34) stain. Later, when the degenerated fibres have been largely replaced by connective tissue, the Weigert method is most satisfactory, especially in the central nervous system. In this case, of course, the degeneration is indicated by an absence of stain, while in the Marchi method a positive picture of the degenerating myelin sheaths is B C Fig. 84. — A, Normal Nerve Fibres from Sciatic Nerve of Rabbit, osmic acid fixa- tion and stain; each fibre shows node of Ranvier. B, Two fibres from distal part of rabbit's sciatic five days after cutting the nerve; shows segmentation of myelin; C, three fibres from distal part of rabbit's sciatic three weeks after cutting nerve; most of the myelin has been absorbed and only traces of the axones remain. seen. When the nerve is cut the first changes affect the cut ends and seem to be of a traumatic character such as formation of bulbous enlargements of the ends of the axones, increase and separation of neurofibrils and formation of sprouts. This phase is soon passed over in the distal stump, while in the central it may pass over into a beginning regeneration of the nerve fibres. In the distal portion a degeneration of the nerve fibres throughout their length and including their termi- nal arborization now takes place. All parts of the nerve fibre are affected. In the medullary sheath the changes consist in segmentation of the sheath, break- ing up of the segments into granules and finally complete absorption (Fig. 84). While undergoing these physical changes chemical changes are also taking place in the myelin which result in its breaking down into simpler fatty substances which give the fat reaction to the Marchi stain. At the same time the neuro- fibrils become irregular and granular and the axis-cylinders finally disappear. XERVE TISSUE 141 The neurilemma cells of the peripheral nerve fibres are peculiar in that they do not degenerate; instead of this their protoplasm increases, and their nuclei pro- liferate. These cells are apparently concerned in the disintegration and absorp-. tion of the myelin. They also form protoplasmic bands which play an important part in the regeneration of the nerve (see below). The same rule holds good for dendrites as for axones as far as it has been possible to determine, namely, that cut oflf from their cells of origin they undergo complete degeneration. At the time the law of Wallerian degeneration was established it was believed that the central portion of the nerve and the cell bodies remained intact after division of the nerve. More recently the method of Marchi (for nerve fibres) and Fig. 85. — Two ]M(jtor Cells from Ventral Horn of Dorsal Cord of Rabbit; fifteen days after cutting major sacro-sciatic nerve. A, Cell in which the chromophilic bodies appear disintegrate.! and nucleus eccentric; B, cell showing more advanced chromatoly- sis, the chromophilic substance being present only in the dendrites and around the nucleus in the form of a homogeneous mass; nucleus causes bulging of surface of cell. the method of Nissl (for neurone bodies) have shown marked degenerative changes in the parts proximal to the lesion. The extent and rapidity of these changes are dependent mainly upon three factors (i) the type of neurone — some neurones being apparently more resistant than others to injury; (2) the character of the injury — e.g., tearing the nerve causing the greatest reaction; cutting the nerve, a reaction of less intensity; pinching the nerve, the least degree of reaction; and (3) the location of the injury, an injury near the cell body caus- ing more effect centrally than one at a distance, in other words, the effect de- I)ends upon the fjerccntage of the neurone cut ofT. If the injury is very near the cell body the latter may ultimately disappear, the proximal portion of the nerve fibre which remains attached to it undergoing a final degeneration similar to that of the distal severed portion. In the cell body there is an initial turgescence^ 142 THE TISSUES followed by disintegration and disappearance of the chromophilic bodies begin- ning near the centre of the cell, and a displacement of the nucleus toward the periphery. This reaction on the part of the cell body to injury to its axone — "central chromatolysis " and "nuclear eccentricity" — is sufi&ciently characteristic to have led to the designation "axonal degeneration" (Fig. 85). If the neurone body survive the injury regeneration takes place. This con- sists in a reappearance of the chromophilic substance, beginning near the nucleus, a return of the nucleus to its normal position, and a slow subsidence of the tur- gescence. While this is going on the axones of the central stump grow across the scar to the protoplasmic bands beyond formed by the neurilemma cells of the peripheral stump, and find their way along or in them to their former termina- tions. According to some authorities the neurilemma cells can do more than this and form, in young animals at least, new nerve fibres which, however, do not persist unless they form connections with the central stump. The outgrowth (centrogenetic) theory seems more probable than the latter (autogenetic) theory. A B Fig. 86. — A, Neuroglia Cell — Spider Type — Human Cerebrum. B, Neuroglia CeU — Mossy Type — Human Cerebrum. The rapidity of the above degenerative and regenerative changes varies according to the metabolic activity of the animal, i.e., they are much more rapid in warm-blooded than in cold-blooded animals, and in summer than during hibernation. They are also influenced by the age of the animal. The importance of these degenerations from the standpoint of anatomy lies in the fact that, by using such methods as Nissl, Weigert, and Marchi, one is enabled to trace the connections between cell bodies and nerve fibres throughout the nervous system. Neuroglia This is a peculiar form of connective tissue found only in the nervous system. Unlike the other connective tissues, neurogha is of ectodermic origin, being developed from the ectodermic cells which line the embryonic neural canal. These cells, at first mor- phologically identical, soon differentiate into neuroblasts or future XERVE TISSUE 143 neurones, and spongioblasts or future neuroglia cells, the latter most probably being in the form of a syncytium. Later this syncytium differentiates fibres, the neuroglia fibres, which, according to Weigert and others, may be entirely separate from the cells (Fig. 87), but more probably lie wholly or at least partly within them. In the syncytium there are also to be distinguished an endoplasm of granular protoplasm and a clear ectoplasm which may perhaps be regarded as a matrix. (See also Fig. 313.) The structure of neuroglia would thus be analogous to that of fibrous connective tissue, i.e., composed Fig. 87. — Neuroglia Cells and Fibres from the White Matter of the Human Cere- bellum stained by Weigert's neuroglia stain. A, Neuroglia cell; B, blood-vessel cut longitudinally, and C, blood-vessel cut transversely, showing enveloping neuroglia fibres; a, neuroglia fibres; b, cytoplasm of neuroglia cell. (Cajal.) of cells, the neuroglia cells, a fibrillar substance, the neuroglia fibres, and a ground substance. The Golgi method apparently reveals a great variety of neurogha cells which may be divided into cells with straight radiating unbranched processes, spider cells, and rough thick branching cells, mossy cells. It seems probable that in the former both cells and fibres are stained while in the latter only por- tions of the cells or syncytium, this method not differentiating between cells and fibres. An increased activity of the neuroglia, due it may be to some pathological cause, is indicated by increase in the endoplasm, by proliferation of the nuclei, and by some cells detaching themselves 144 THE TISSUES from the syncytium and becoming amoeboid. Such amoeboid cells may convey various products of degeneration to the vicinity of the perivascular lymph spaces. There may also result a temporary in- crease in the ghal fibres (ghosis). According to good authorities a continuous glial membrane forms the outer boundary of the ectodermal elements of the central nervous system, i.e., the nerve tissue and glia. This glial membrane is of course in apposition with the mesodermic connective tissue which forms the pia or inner covering of the brain and cord and also with the connective tissue of the adventitia of the blood-vessels which penetrate the central nervous system. The ghal membrane is formed by superficial flattened cells and processes of other glial cells which come into contact with the connective tissues. The lining of the cavity of the neural tube is formed by the ependymal cells which are a form of neuroglia cells. These cells are columnar epi- thelial cells each with a process penetrating a variable distance into the wall of the neural tube and joining the general neurogha syncy- tium. Their nuclei lie near the central cavity of the tube and they contain glia fibres. The inner lining of the chorioid plexuses of the brain is a single layer of cuboidal epithelial cells which have prob- ably a secretory function and contribute to the production of the cerebro-spinal fluid. It is probable that the ependyma cells also have a secretory activity. It is probable that the neurilemma cells also originate from the neural tube. If this is the case it is evident that they may be regarded as a special form of neuroglia cells. It has already been seen that in pathological changes they behave similarly to the neuroglia cells in the central nervous system. TECHNIC (i) Pieces of the cerebral cortex are stained by one of the Golgi methods. If the rapid or mixed silver method is used, sections must be mounted in hard bal- sam without a cover; if the slow silver or the bichlorid method is used, the sec- tions may be covered. Sections are cut from 75 to loo/^ in thickness, cleared in carbol-xylol or oil of origanum and mounted in balsam. This section shows only the external morphology of the neurone. It is also to be used for studying the different varieties of neuroglia cells as demonstrated by Golgi's method (see page 143). (2) Thin transverse slices from one of the enlargements of the spinal cord are fixed in absolute alcohol. Thin sections (5 to lo/*) are stained by Nissl's method (page 38)<-and mounted in balsam. This section is for the purpose of NERVE TISSUE 145 studying the internal structure of the nerve cell and processes as demonstrated by the method of Xissl. (3) MeduUated Xerve 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 lon- gitudinally. 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 Ran- vier. 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. 7), and hardened in alcohol. Small strands are stained twenty minutes in strong picro-acid-fuchsin solution (technic 2, p. 20), washed thoroughly in strong alcohol, cleared in oil of origanum, thor- oughly teased longitudinally and mounted in balsam. General References for Further Study of Tissues Barker: The Nervous System. Bethe: Allgemeine .^natomie und Physiologic des Nervensystem. Cabot: A Guide to the Clinical Examination of the Blood for Diagnostic Purposes. Ewing: Clinical Pathology of the Blood. Hertwig: Die Zelle und die Gewebe. Kolliker: Handbuch der Gewebelehre. Prenant, Bouin et Maillard: Traite d'Histologie. Ranvier: Traite Technique d'Histologie. Van Gehuchten: Le Systeme nerveux de I'homme. Wood: Laboratory Guide to Clinical Pathology. 10 PART IV THE ORGANS A tissue such as any one of those described in the precechng chap- ters scarcely exists in the pure state, that is alone by itself, in the adult body. Two or more tissues are always associated and such an associa- tion of two or more tissues for the purpose of performing a definite function constitutes an organ . In practically all cases one of the tissues is connective tissue, the main function of which is to form a supportive framework for the more active specific tissue of the organ. In many cases this connec- tive tissue forms a definite covering or capsule. From the capsule strands of connective tissue frequently extend down into the organ which branch and form its connective tissue framework. Sometimes the subdivision of the organ by connective tissue is quite regular macroscopic subdivisions being marked off by coarse connective tissue septa, and these again subdivided by liner septa. In such case the former are known as lobes, the smaller as lobules. But while an organ has been defined as consisting of two or more tis- sues and while in general one of these tissues is connective tissue frame- work and the other the specific functioning tissue of the organ, such a simple combination of two tissues does not actually exist, for all organs are suppHed with blood and lymph which are distributed to them and through them b>^ the blood and lymph vessels. There are thus car- ried into the organs not only the blood and lymph themselves, but also the tissues which compose the walls of the blood and lymph vessels. Also every organ has its nerve supply; thus nerve tissue is distributed through all organs. Many organs are hollow tubes and the study is the study of the structure of the walls of the tube, such, e.g., are the stomach and intestines, the heart and the blood-vessels, the trachea and bronchi. Other organs not so apparently tubular still show their tubular struc- ture on closer analysis, such, e.g., are the lungs and the duct glands. As an organ has a specific function and as this function is performed mainly by the si)ecinc cells of the organ, these cells show variations in structure dependent ujK)n whether the organ is at vvork or at rest. Many adult organs are so com[)lex that it is i)ossible to understand them only by reference to their development from more simple structures. 1 1<) 150 THE ORGANS From the foregoing it follows that in studying an organ there are to be considered primarily: (i) The specific tissue of the organ. (2) The connective tissue framework. (3) The blood supply. (4) The nerve supply. (5) Function. (6) Development. CHAPTER I THE CIRCULATORY SYSTEM The circulatory apparatus consists of two systems of tubular structures, the blood-vessel system and the lymph-vessel system, which serve, respectively, for the transmission of blood and lymph. THE BLOOD-VESSEL SYSTEM This consists of (a) a central propelling organ, the heart; (b) a series of efferent tubules — the arteries — which by branching constantly increase in number and decrease in calibre, and which serve to carry the blood from the heart to the tissues; (c) minute anastomosing 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; (^) 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 Hning, which consists of a single layer of endothe- lial cells. Of the capillaries this single layer of cells forms the entire wall. In the heart, arteries, and veins, the endothelium serves simply as the Hning for walls of muscle and connective tissue. Capillaries It is convenient to describe these first on account of their simplicity of structure. A capillary is a small vessel from 4.5 to i6,« in diameter. Its wall consists of a single layer of endothelial cells. The cells are somewhat elongated in the long axis of the vessel. The smaller the caUbre of the capillary the more elongated are the cells. Two endo- thelial cells suffice to complete the circumference of the smaller capillaries, while the larger requires three or four. The protoplasm of the cells is clear or finely granular. The nuclei are oval, with their long axes in the long axis of the vessel. In fixed material the 1.51 152 THE ORGANS nuclei bulge into the lumen. In the living condition the lining of the capillary is probably nearly smooth. According to some investiga- tors a deUcate cuticle limits the cytoplasm on the side toward the lumen. The edges of the cells are serrated and are united by a small amount of intercellular substance (p. 77), which can be demonstrated by the silver nitrate stain. ^ In certain capillaries — those of the early embryo, of the kidney glomeruli, of the chorioid coat of the eye, of the liver — no cell boundaries can be made out. In these capillaries the endothelium appears to be of the nature of a syncytium (p. 68). Capillaries branch without diminution in calibre, and these branches Fig. 88. — ^Large and Small Capillaries. Silver-nitrate and hematoxylin stain (technic 7, p. 79), to show outlines of endothelial cells and their nuclei. anastomose to form capillary networks, the meshes of which differ in size and shape in different tissues and organs (Figs. 88, 89, 90). The largest meshed capillary networks are found in the serous mem- branes and in the muscles, while the smallest are found in the glands, as, e.g., the liver. As to calibre, the largest are found in the liver, the smallest in muscles. In such thin membranous parts as the web of the frog's foot, or the wall of the frog's bladder, the blood may be observed as it flows through the arteries,^ capillaries, and veins. The current is seen to be fastest in the arteries, and faster in the centre of the vessel than at its periphery. It is slower in the veins and slowest in the capillaries. In the case of the frog's bladder, the mere expo- sure to the air acts as a sufficient irritant to cause slight inflammatory changes, and the leucocytes may be seen adhering to the walls of the capillaries and passing through them into the tissues. The capillary, both from the thinness of its wall and from the slowness with which the blood passes through it, is peculiarly adapted for the interchange of material between the blood and the tissues, and it is probable that it is in the capillary that all such interchange takes place. ^ Some authors describe delicate protoplasmic anastomoses between the cells and describe an irregular precipitation of the silver nitrate corresponding to the spaces be- tween the anastomosing threads- These spaces are interpreted as intercellular channels through which the leucocytes and plasma can pass. Intracellular spaces reacting to silver nitrate and supposed to have a similar function have also been described. THE CIRCLTLATORY SYSTEM 153 Arteries The wall of an artery consists of three coats: (i) An inner coat, the intima. (2) The middle coat, the media. (3) An outer coat, the advent ilia. The intima consists of a single layer of endothelial cells, continu- ous with and similar to that forming the walls of the capillaries, or, fed Fig. 89. — Diagram of Capillaries and Small Artery showing their structure and relations, a. Capillaries; /;, nuclei of ca[)illary endothelium; c, precapillary arteries; d, arteriole; e, large capillary;/, small artery. in arteries of considerable size, of this layer plus more or less Qonnec- tive tissue. The middle coat consists mainly of smooth muscle, the outer of connective tissue. The structure of these three coats varies according to the size of the artery, and while the transition between them is never abruj)t, it is convenient, for purposes of description, to distinguish (a) small arteries, (b) medium sized arteries, and (c) large arteries. Small Arteries.- — Pas.sing from a cajnllary to an artery, the first change is the addition of a thin sheath of connective tissue, the fibres 154 THE ORGANS of which are disposed longitudinally, around the outside of the endo- thelial tube. A little farther back isolated smooth muscle cells, circularly arranged, begin to appear between the endothelium and the connective tissue. Such an artery is known as a precapillary artery. The next transition is the completion of the muscular coat, the muscle cells now forming a continuous layer. Such an artery, consisting of three distinct coats, the middle coat composed of a single continuous layer of smooth muscle cells, is known as an arteriole (Fig. 89, d; Fig. 90, h). Medium-sized Arteries. — This group comprises all the named arteries of the body with the exception of the aorta and the pulmo- FiG. 90. — Capillary Network from Human Pia Mater, showing also an arteriole in " optical section" and a small vein. X3S0. (Technic i, p. 160.) a, Vein; b, arteriole; c, large capillary; d, small capillaries. nary. Their walls are formed of the same three coats found in the arteriole, but the structure of these coats is more elaborate. I. The INTIMA consists of three layers (Fig. 91). {a) An inner endothelial layer already described. (h) A middle layer, the intermediary layer of the intima. This is composed of delicate white and elastic fibrils which run longi- tudinally, and connective-tissue cells. (c) An outer layer, the elastic layer of the intima, or membrana elastica interna — a thin fenestrated membrane of elastic tissue. This membrane is intimately connected with the media and marks the boundary between the latter and the intima. In the smallest of THE CIRCLTLATORY SYSTEM 155 the medium-sized arteries the intermediary layer is often wanting, the endothelial cells resting directly upon the elastic membrane. Owing to the extensive amount of elastic tissue in their walls, there is a postmortem contraction of arteries which results in the intima being thrown up into folds. For this reason the elastic membrane presents, in transverse sections of an artery, the appearance of a wa\y band (Fig. 91). Fig. 91. — From Cross-section through Walls of Medium-sized Artery and its .'\ccompanying Vein. Xys. (Technics, p. 160.) yl, Intima of artery; a, its endothe- lial layer; b, its intermediary layer; c, its elastic layer; B, media of artery; C, adv-entitia, the upper part belonging to the artery, the lower to the vein; within the adventitia are seen the vasa vasorum; D, media of vein; E, intima of vein; i, its intermediary layer; j, its endothelial layer. 2. The MEDIA is a thick coat of circularly disposed smooth muscle cells (Fig. gi,B). Its thickness depends largely upon the size of the vessel, though varying somewhat for different vessels of the same size. A small amount of fibrillar connective tissue supports the muscle cells. Elastic tissue is present in the media, the amount being usually pro- portionate to the size of the vessel. In the smaller of the medium- sized arteries, the elastic tissue is disposed as delicate fibrils among the muscle cells. In lafger arteries many coarse fibres are intermin- gled with the fine fibrils. When much elastic tissue is present the muscle cells are separated into more or less well-defined groups. In such large arteries as the subclavian and the carotid, elastic tissue 156 THE ORGANS occurs not only as fibrils but also as circularly disposed plates or fenestrated mem.br anes. 3. The ADVENTiTiA (Fig. 91, C) is composed of loose fibrous connective tissje with some elastic fibres. Occasionally there are scattered smooth muscle cells. Both smooth muscle cells and elastic fibres are arranged longitudinally. The adventitia does not form a definitely outlined coat like the media or intima, but blends ^^ ■4 ~^&>^^^--^'&- /f --------- Fig. 92. — From Transverse Section of Dog's Aorta. X60. (Technic 4, p. 160.) a, Intima; h, media; c, adventitia; d, vasa vasorum; e, elastic tissue;/, endothelium. externally with the tissues surrounding the artery and serves to attach the artery to these tissues. In some of the larger arteries the elastic tissue of the adventitia forms an especially well-defined layer at the outer margin of the media. This is known as the membrana elastica externa. In general, it may be said that the thickness of the adventitia and the amount of elastic tissue present are directly proportionate to the size of the artery. Large arteries like the aorta (Fig. 92) have the same three coats as small and medium-sized arteries. The layers are not, however, so distinct. This is due mainly to the excessive amount of elastic THE CIRCULATORY SYSTEM 157 tissue in the media (Fig. 93), which makes indistinct the boundaries between intima and media, and between media and adventitia. The walls of the aorta are thin in proportion to the size of the vessel, in- creased strength being obtained by the decided increase in the amount of elastic tissue. Of the intima, the endothelial cells are short and polygonal; the intermediary layer similar to that of a medium-sized artery; the elastic layer less distinct and often broken up into several Fig. 93.— From Transverse Section of Dog's Aorla, lo show Elastic Tissue X60. rrechnic 7, p. 160.) Elastic tissue stained black, a, Intima; b, media; c, adventitia. thin layers. The media consists mainly of elastic tissue arranged in circular plates or fenestrated membranes. Between the elastic- tissue plates are groups of smooth muscle cells and some fibrillated connective tissue. The adventitia resembles that of the medium-sized artery. There is no external elastic membrane. Certain arlerics have structural peculiarities. The arteries of the brain and cord are thin-walled in proportion to their calibre, the inner elastic membrane is especially well defined, and there are few elastic fibres in the media. In the renal, crjeliac, mesenteric, and external iliac arteries there is little or no connective 158 TH-E ORGANS tissue separating the endothelium from the media. In the subclavian, the media contains longitudinal muscle cells. Longitudinally running muscle cells occur in the adventitia of the umbilical arteries, in the iliac, splenic, renal, superior mesenteric and dorsahs penis. The radial, femoral and coeliac arteries have comparatively little elastic tissue, while in the common iliac, carotid, and axillary the elastic tissue is in excess of the muscular. Veins The walls of veins resemble those of arteries. There are the same three coats, intima, media, and adventitia, and the same elements enter into the structure of each coat (Fig. 91). Venous walls are not, however, so thick as those of arteries of the same calibre, and the coats are not so distinctly differentiated from one another. The transition from capillary through the precapillary vein to the small vein is similar to that described under arteries (page 154). Unlike the artery, the thickness of the wall of a vein and its structure are not directly proportionate to the size of the vessel, but depend also upon other factors such as the position of the vein and the support given to its walls by surrounding structures. Of the INTIMA the endothelial layer and the intermediary layer are similar to those of the artery. The elastic layer is not always present, is never so distinct, and is not wavy as in the artery (Fig. 91). The result is a lack of demarcation between intima and media, the connective tissue of the intermediary layer of the intima merging with the mixed muscle and connective tissue of the media. Project- ing at intervals from the inner surface of the wall of some veins are valves. These are derived entirely from intima and consist of loose fibrous and elastic tissue covered by a single layer of endothelium. Valves are especially large and strong in the larger veins of the lower limbs. They are absent in the veins of the brain and cord and their membranes, in the veins of bones, in the umbilical vein, and in most of the visceral veins with the exception of some branches of the portal. The MEDIA of veins is thin as compared with that of arteries of the same size. It consists of fibrous and elastic tissue and smooth muscle cells. The amount of muscle is comparatively small and the cells are arranged in groups through the connective tissue. The ADVENTITIA is well developed in proportion to the media. It consists of mixed fibrous and elastic tissue and usually contains along its inner margin small bundles of longitudinally disposed smooth muscle cells. THE CIRCULATORY SYSTEM 159 The media is thickest in the veins of the lower extremities, especially the popliteal, and in the veins of the skin. In the veins of the head and abdomen the media is very thin, while in the sub- clavian and superior vena cava and in the veins of bones, of the pia mater, dura mater, and retina, there is an almost entire absence of media. Arteries are as a rule empty after death, while veins contain blood. The absence of much elastic tissue in the walls of the veins prevents any such extensive post-mortem contraction as occurs in the arteries. Veins tend to collapse after death, but are usually prevented from doing so by the presence of blood in them. In the iliac and femoral veins, longitudinally disposed muscle occurs in the inner part of the media. The umbUical vein, like the corresponding artery, has three distinct muscular coats. Longitudinal muscle fibres are present in the adventitia of the superior vena cava (hepatic and abdominal portion) and of the portal and hepatic veins. In the upper portion of the inferior vena cava, in the superior vena cava, the jugular, innominate, and subclavian, there is little muscle tissue in any of the coats, while in the veins of the brain and its mem- branes, the retina, the placenta and the bones, no muscle is present. Vasa Vasorum. — Medium and large arteries and veins are sup- plied mth small nutrient vessels — vasa vasorum. These vessels run in the adventitia, small branches penetrating the media (Figs. 91 and 92). Lymph channels are found on the outer surface of many blood- vessels. Some of the smaller vessels are surrounded by spaces Hned by endothehum — perivascular lymph spaces. These communicate with the general lymphatic system. Nerves.— The walls of the blood-vessels are supplied with both medullated and non-mcdullated fibres. The latter are axones of sympathetic neurones. As these nerves control the calibre of the vessels they are known as vasomotor nerves. They form plexuses in the adventitia, from which are given off branches which pene- trate the media and terminate on the muscle cells. The medul- lated fibres are the peripheral arms of spinal or cranial ganglion cells. The larger fibres run in the connective tissue outside the adventitia. From these are given off branches which enter the media, divide re- peatedly, lose their medullary sheaths, and terminate mainly in the media, although some fibres have been tracerl to their terminations in the intima. 160 THE ORGANS TECHNIC (i) Capillaries, Arterioles, Small Arteries, and Veins.— Fix an entire brain, or slices about an inch thick from its surface, in formalin-Miiller's fluid for twenty- four hours (technic 5, p. 7). Remove the pia mater, especially the thinner parts which He in the sulci between the convolutions, and harden in graded alcohols. Select a thin piece, stain with haematoxylin (lightly) and eosin (strongly) (technic I, p. 20), and mount in balsam or in eosin-glycerin. The veins, having thin walls and being usually well filled with blood, appear distinct and red from the eosin-stained red cells. The arteries, having thicker walls, in which are many hemoglobin-stained nuclei, have a rather purple color. Between the larger vessels can be seen a network of anastomosing capillaries with their thin walls and bulging nuclei. Some are filled with blood ceUs; others are empty with their collapsed walls in apposition. Note the appearance of an arteriole, first focusing on its upper surface, then focusing down through the vessel. In this way what is known as an "optical section" is obtained, the artery appearing as if cut longitudinally. Trace the transition from arteriole to precapillary artery and the breaking up of the latter into the capillary network. Similarly follow the convergence of capillaries to form a small vein. (2) Instructive pictures of the relations of arteries, capillaries, and veins in living tissues may be obtained by curarizing a frog, distending the bladder with normal saline introduced through a small catheter or cannula, opening the abdo- men and drawing out the bladder, which can then be arranged upon the stage of the microscope. The passage of the blood from the arteries through the capillary network and into the veins is beautifully demonstrated. (3) For studying the structure of the walls of a medium-sized artery and vein remove a portion of the radial artery, or other artery of similar size, and its accom- panying vein, together with some of the surrounding tissues. Suspend the ves- sels, with a small weight attached, in formalin-Miiller's fluid (technic 5, p. 7). Sections should be cut transversely, stained with haematoxylin-eosin (technic I, p. 20), or with haematoxylin-picro-acid fuchsin (technic 3, p. 21), and mounted in balsam. The vessels of the adventitia — vasa vasorum — are con- venient for studying the structure of arterioles and small veins. (4) Fix a piece of aorta in formalin-Miiller's fluid, care being taken not to touch the delicate endothelial lining. Stain transverse sections with haema- toxylin-eosin or with haematoxylin-picro-acid fuchsin and mount in balsam. (5) The outlines of the lining endothelial cells may be demonstrated as fol- lows: Kill a small animal, cut the aorta, insert a glass cannula and, under low pressure, thoroughly wash out the entire vascular system with distilled water. Follow the water by a one-per-cent. aqueous solution of silver nitrate. Remove some of the smaller vessels, split longitudinally, mount in glycerin, and expose to the direct sunlight. After the specimen has turned brown examine with the low power. The outlines of the cells should appear brown or black. (6) The endothelium of the smaller vessels and capillaries may also be demon- strated in the specimen described under technic 8, p. 79. (7) The elastic tissue of the blood-vessels is best demonstrated by means of Weigert's elastic-tissue stain. Prepare sections of medium-sized vessels and of the aorta, as above described (3), and stain as in technic 3, p. 28. THE CIRCULATORY SYSTEM 161 The Heart The heart is a part of the blood-vessel system especially differ- entiated for the purpose of propelHng the blood through the vessels. The main mass of the heart wall consists of a special form of muscle tissue already described as heart muscle (page 119). This constitutes the myocardium. On its inner and outer sides the myo- cardium is covered by connective-tissue membranes Uned, respect- ively, with endotheUum and mesotheHum and known as the endo- cardium and epicardium. The MYOCARDIUM varies in thickness in different parts of the heart, being thickest in the left ventricle, thinnest in the auricles. A ring of dense connective tissue, the auriculo-ventricular ring, com- pletely separates the muscle of the auricles from that of the ventri- cles, except in the median septum where the auricular and ventricu- lar muscle is continuous. The auricular muscle consists of an outer coat common to both auricles, the fibres of which have a transverse or somewhat obHque direction, and of an inner coat, independent for each auricle, the fibres of which are longitudinally disposed. Bun- dles of these fibres stand out as ridges along the inside of the auricles. They are more strongly developed in the right than in the left auricle. Between the two coats bundles of muscle fibres are frequently found which run in various directions. The disposition of the muscle tissue of the ventricles is much more compHcated. It is usually described as composed of several layers, the fibres of which run in different directions. The meaning of these fibre layers becomes apparent when we study the arrangement of the fibres in embryonic hearts in which the connective tissue has been broken down by maceration. Thus dissected, the muscle of the ventricles is seen to consist mainly of two sets of fibres, a super- ficial set and a deep set. These run at approximately right angles to each other. Both sets of fibres begin at the auriculo-ventricular rings. The superficial fibres wind around both ventricles in a spiral manner, becoming constantly deeper, to terminate in the papillary muscles of the opposite ventricle. The deeper fibres pass from the auriculo-ventricular ring around the ventricle of the same side, through the interventricular septum and terminate in the papillary muscles of the opposite ventricle. The ENDOCARDIUM covcrs the inner surface of the myocardium and forms the serous lining of all the chambers of the heart. M. 11 162 THE ORGANS the arterial and venous orifices it is seen to be continuous with and similar in structure to the intima of the vessels. It consists of two layers: (a) an inner composed of a single layer of endotheHal cells, corresponding to the endothelial lining of the blood-vessels; and (b) 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 and elastic tissue, surround the openings between auricles and ventricles. Similar but more delicate rings encircle the openings from the heart into the blood-vessels. The heart valves are attached at their bases to the annuli fibrosi. They are folds of the endocardium, and like the latter consist of fibrous and elastic tissue continuous with that of the rings and covered 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 structure. It consists of a layer of mixed fibrous and elastic tissue covered over by a single layer of mesothelial cells. Beneath the epicardium 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 endocardium 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 (vagus) and sympathetic systems (cervical gangha) and consist of both medullated and non-meduUated fibres. Sympathetic gangHon cells are distributed in groups throughout the myocardium, the largest, lying in the epicardium near the base of the heart, being known as the cardiac ganglion or ganglion of Wrisberg. Among these cells the nerve fibres form plexuses from which both motor and sensory THE CIRCULATORY SYSTEM 163 terminals are given off to the muscle. (For nerve endings in heart muscle see page 440.) TECHNIC (i) The Heart. — Cut pieces through the entire thickness of the wall of one of the ventricles, care being taken not to touch either the serous surface or the lining endothelium. Fix in formalin-Miiller's fluid (technic 5, p. 7). Cut trans- verse and longitudinal sections; stain with haematoxylin-eosin (technic i, p. 20) 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 condi- tion 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. Developmext of the Circulatory System The blood-vessels and the heart begin their development separately and afterward become united. Both are derived from mesoderm, but while the heart develops within the embryo, the earliest blood-vessels and blood are developed from extra-embryonic mesoderm. 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, appearing in the chick by the end of the first day of incubation. The superficial cells of these islands become flattened to form the endothelium, which is at this stage apparently a syncytium through which the nuclei are scattered and showing no evidences of cell boundaries. This endotheUal syncytium surrounds the remaining more central cells, from which blood cells are developed. These represent the earliest blood-vessels. The channels, which are at first unconnected, anastomose and give rise to a network of channels which are the earliest capillaries. These develop rapidly in the area vasculosa and some of them increase in size to become arteries and veins, the smooth muscle and connective tissue of their walls being differentiated from the surrounding mesoderm. These grow toward, and finally into, the embryo where they unite with the heart. In granulation tissue, and in new growths in general, both normal and pathological, new blood-vessels apparently develop as off-shoots from other vessels. These are at first solid extensions of endothelium which become hollowed out. In regard to the origin of the later vessels in the extraembryonic area, and those within the embryo, there are two views: (i) that they represent outgrowths from the original capillaries; (2) that they arise in silii in the same manner as the earliest capillaries and unite secondarily to form networks. The weight of evidence at present favors the latter view. The entire vascular system is at first represented by a network 164 THE ORGANS of capillaries. As development proceeds some of the channels enlarge to form the arteries and veins. The heart first appears as an endotheUal tube, the primitive endocardium, at a very early age of embryonic life, in the chick embryo during the first day of incubation. It apparently begins its rhythmic contraction before any contractile fibrils can be distinguished in its walls and before any connection with blood-vessels has been estabhshed. The origin of the cardiac endothelium is not definitely known. It is believed by some to be of entodermic origin, by others of m.esodermic, by still others to be partly derived from each of these layers. Around this endothelial tube, but separated from it by a space, there develops from mesoderm an entirely distinct muscular tube, the primitiv Coelom Parietal mesoderm Ectoderm i'*^^*. ^ Visceral mesoderm Blood islands Fig. 94. — Section of Blastoderm of Chick of 42 Hours' Incubation. Photograph- The cells of the blood islands are difierentiated into nucleated red blood ceUs (erythro. blasts) and the endothelium of the vessels. myocardium. These two tubes are at first united only in places by bands of connective tissue. Later they unite so that the inner tube, the endocardiume becomes a lining for the outer tube, the myocardium. The union of the two heart tubes occurs very early. The foregoing description applies to the chick and to those mammals thus far studied. In the earliest human embryos (2 to 3 mm.) the heart is already a single, slightly coiled tube connected at its cephalic end with the ventral aorta and caudally with the omphalo-mesenteric veins. The epicardium, as the visceral layer of the pericardium, has a sepa- rate origin, being constricted off from that portion of the mesoderm which hnes 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. These are folds of the intima, and THE CIRCULATORY SYSTEM 165 always occur in pairs. They are arranged with considerable regu- larity and in small vessels where the intima forms the entire wall there is a distinct bulging just above the valve which gives the vessel a quite characteristic appearance. They are capable of great distention, and when empty collapse so that their thin walls are in apposition. The largest of the lymph vessels, the thoracic duct, has three well- defined ccats: 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 which occur in groups separated by connective tissue containing a few elastic fibres and an advent it ia of longitudinally running connective tissue contain- ing elastic fibres and 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 canaliculi of the cornea and of bone (pages 83 and loi). 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 ccelom, body cavity, or pleuro peritoneal cleft. This cleft is lined with mesothelium beneath which the stroma is formed. These mem- branes 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 have been described as communicating with the free surfaces by means of openings — slomata. Recent observations, however, would seem to indicate that these stomata are artefacts. 166 THE ORGANS That the lymph-vessels form a definite and closed system of channels and are not in direct communication with the lymph spaces has been clearly demonstrated. The origin of the lymphatic vessels is not clear. According to some investi- gators they originate as evaginations from the vascular system, starting as four buds, two from the veins of the neck, and two from the veins in the inguinal region. The two anterior buds appear first (pig embryo of 14.5 mm.) and are in communication with the anterior cardinal vein. A little later the posterior buds connected with the posterior cardinals appear. From these buds the entire lymphatic plexus develops by a process of evagination. According to other investigators, the lymphatic vessels first appear as min- ute spaces in the mesenchyme of the axilla and groin (during second month in human embryo). These spaces enlarge and by sending off branches give rise to the network of lymphatic vessels. According to this veiw, the connection between lymphatic vessels and veins is secondary. TECHNIC (i) 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. 79. 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. (2) The Thoracic Duct. — Remove a portion of the thoracic duct, fix in forma- lin-Muller's fluid (technic 5, p. 7), and stain sections with haematoxylin-eosin (technic i, p. 20). General References for Further Study of the Circulatory System KoUiker: Handbuch der Gewebelehre des Menschen, vol. iii. Stohr: Text-book of Histology. Schafer: Histology and Micros,copic Anatomy, in Quain's Elements of Anat- omy, tenth edition. CHAPTER II LYMPHATIC ORGANS Lymphatic Tissue^ So-called lymphatic tissue consists of reticular connective tissue and a special type of cells, lymphoid cells, which fill in the meshes of the reticulum. Lymphoid cells are small spheroidal cells, each hav- ing a single nucleus which almost completely fills the cell. Lym- phatic tissue may be diffuse or circumscribed. In diffuse lymphatic tissue the cells are not closely packed and there is no distinct de- marcation between the lymphatic and the surrounding 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 the cells are very closely packed, often completely obscuring the reticulum. There is also a quite distinct demarca- tion between the lymphatic and the surrounding tissues. Such a circumscribed mass of lymphatic tissue is known as a lymph nodule. 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 trabeculcB or septa into the organ. The capsule and septa constitute the connective-tissue framework of the node, and serve as a support for the lymphatic tissue (Fig. 95). The capsule is composed of fibrous connective tissue. Toward the surface of the capsule the fibres are loosely arranged and there are comparatively few elastic fibres. This outer layer of the capsule 'In the preceding editions " lymphatic tissue" was placed according to name rather than structure among the tissues. As it consists of a connective tissue framework suj)- porting a special type of cell which has a specific function, it is more properly classi- fied as an organ. On account of the peculiar structure and wide dislril)ulion of the "tissue" and on account of convenience and long usage, the term " lym[)hatic tissue" is still retained. 1G7 168 THE ORGANS blends with the surrounding tissues and serves, Hke the fibres of the arterial adventitia, to attach the organ to them. The inner layer of the capsule consists of a more dense connective tissue, is richer in elastic fibres, and contains some smooth muscle cells. At one point, known as the hilum (Fig. 95), there is a depression where the con- nective tissue of the capsule extends deep into the substance 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. h g f Fig. 95. — Section through Entire Human Lymph Node, including Hilum. XiS. (Technic i, p. 172.) Dark zone, cortex; light central area, medulla, a, Lymph nodule of cortex; b, germinal centres; c, trabeculse containing blood-vessels; d, capsule; e, hilum; /, lymph sinus of medulla; g, lymph cords of medulla; h, lymph sinuses of medulla and cortex. The connective-tissue septa, which extend from the capsule into the interior of the node incompletely divide it into irregular inter- communicating compartments. In the peripheral portion of the node these compartments 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. Within the compartments formed by the capsule and the septa is the lymphatic tissue. Near the capsule where the compartments are large and spheroidal or pear-shaped, the lymphatic tissue is arranged in masses which correspond in shape to the compartments. LYMPHATIC ORGANS 169 These are known as lymph nodules (Fig. 95). In the centre of each nodule is usually an area in which the cells are larger, are not so closely packed, and show marked mitosis. As it is here that active proliferation of lymphoid cells takes place, this area is known as the germinal centre (Figs. 95 and 96). Immediately surrounding the germinal centre is a zone in which the lymphoid cells are more closely packed than elsewhere in the nodule (Fig. 96). 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 surrounding this light area is the darkest Fig. 96. — Section through Cortex and Portion of Medulla of Human Lymph Node (Technic 2, p. 172.) a, Capsule; h, lymph sinus; c, trabecula; d, closely packed cells at outer border of lymph nodule; e, germinal centre; /, lymph cords in medulla. portion of the nodule (Fig. 96). From the inner sides of the nodules strands of lymphoid tissue extend into the center of the node. These are known as lymph cords, and anastomose freely. The regular arrangement of the lymph nodules and trabecular in the peripheral portion of the node contrasts strongly with their irregu- lar arrangement in the centre. This determines a division of the nodule into two zones, an outer peripheral zone or cortex and a cen- tral zone or medulla. In both cortex and medulla the lymphoid tissue is always separated from the cai)sule or from the septa by a distinct space — the lymph sinus — which is bridged over by reticular tissue containing comparatively few lymphoid cells (Fig. 96). These sinuses form a continuous system of anastomosing channels throughout the node. 170 THE ORGANS The regular arrangement of trabeculse, and lymph nodules with sinuses between, which is characteristic of the cortex, makes this part of the organ easily understood. To appreciate the structure of the medulla it must be borne in mind that all of these cortical struc- tures extend down into the medulla, the trabeculas as anastomosing networks of connective tissue, the lymph nodules as cord-Hke struc- tures which divide and anastomose, the sinuses as more or less clear channels which always separate the connective tissue from the lymph cords. These parts — -trabecula, sinus, lymph cord — all anas- tomosing freely and most irregularly in the medulla, always main- tain the same relation to one another as in the cortex, namely, that sinus is always interposed between lymph tissue and trabecula. The reticular connective tissue (page 94), which forms a part of the lymphatic tissue proper, is continuous with the fibrous connec- tive-tissue framework of the organ in such a manner that it is im- possible to determine any demarcation between the two tissues. In the lymph nodules, and wherever the lymphoid cells are densely packed, the underlying reticular network is almost completely ob- scured. Crossing the sinuses, especially 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 trabec- ulse. 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- 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 lymphatic tissue, where they break up into dense capillary net- LYMPHATIC ORGAN 171 works. Some of the arteries, instead of passing directly to the lym- phatic tissue, follow the septa, supplying these and the capsule, and also sending 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. L5rmphatics. — The afferent lymph vessels enter the node on its convex surface opposite the hilum, penetrate 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 Efferent lymph, ves. '•' . -t -"--,^'''^>i ' ""iViN Bloodvessel ■ Hilus -Marginal sinus )^'^^®S;#SW ••'^-Capsule Anereni, ^.i.^VV If'' c ('.tfti-jfe lymph, ves. \<7% m,' r, .'. WW*^ Afferent Fig. 97. — From a Section through the Axilla of a Human Embryo of 125 mm. (4-5 months), showing an Earl}' Stage of a Lymph Gland. (Kling.) 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. Both medullated and non-medullated fibres occur. Their exact modes of termination are not known. Development. — The first indications of lymph node formation are found in the axilla and groin (pig embryo of about 30 mm.; human toward end of third month) in the connective tissue in which the lymphatic vessels arc best developed (p. 166). Here groups of more closely packed cells appear. As the tissue is richly vascular it seems impossible to determine whether the closely packed cells originate within the blood-vessels or develop from fixed connective tissue cells. Each group of cells is the anlage of a lymph node (Fig. 97). The immediately surrounding connective ti.ssuc forms the capsule, while the lymph channels just 172 THE ORGANS beneath form the subcapsular or marginal sinus (Fig. 98). The point of main connection with outside blood-vessels becomes the hilum. As the lymph node grows outward, parts of the capsule remain within to form trabeculas while the lymph channels within the nodule apparently develop as ingrowths from the marginal sinus (Figs. 97 and 98). TECHNIC (i) Remove several lymph nodes from one of the lower animals (ox, cat, dog, rabbit), fix in formalin-Miiller's fluid (technic 5, p. 7), and harden in alcohol. Cut thin sections through the hilum, stain with hasmatoxyUn-eosin (technic i, p. 20), or with hsematoxylin-picro-acid-fuchsin (technic 3, p. 21), and mount in balsam. Afferent lymphatic vessels Marginal sinus Capsule Dense lymph, tissue — Marginal sinus (plexus) Capsule Trabecula •Reticular tissue Intermediary plexus Efferent lymph, vessel Blood vessels Fig. -Diagram Illustrating a Stage (Later Than Fig. 97) in the Development of a Lymph Gland. (Stohr.) (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-Miiller's fluid until the node becomes tense. By now slightly increasing the pressure the fluid may be made to pass into the second node, and so through the entire chain. The nodes are then carefully dissected out and placed for twenty-four hours in formalin-Miiller's fluid, then hardened in alcohol. Sections are cut through the hilum, stained with haematoxylin-eosin or with hsematoxylin-picro-acid-fuchsin and mounted in balsam. Near the centre of the chain are usually found nodes in which the lymph sinuses are properly dis- tended. 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 ceUs have been washed out, especially in the medulla. (3) Human lymph nodes may be treated by either of the above methods. LYMPHATIC ORGANS 173 Owing to the coalescence of their cortical nodules their structure is not so easily- demonstrated as that of the lymph nodes of lower animals. Haemolymph Nodes These are lymphoid structures which closely resemble ordinary lymph nodes, but with the essential difference that their sinuses are hlood 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 cap- Fig. 99. — Section through Human Haemolymph Node, including Hilum, showing cap- sule, trabeculse, sinuses filled with blood, and lymph nodules. (Warthin.) sule Iraheciiloe of the same structure pass down into the node, forming its framework (Fig. 99). Beneath the capsule is a blood sinus, which may be broad or narrow, and usually completely surrounds the node. Less commonly the sinus is interrupted by lymphoid tissue extending out to the capsule. From the peripheral sinus branches extend into the interior of the node, separating the lymphoid tissue into cords or islands. The relative proportion of sinuses and lymphoid tissue varies greatly, some nodes being composed almost wholly of sinuses, while in others the lymphoid tissue predominates. There is usually a fairly distinct hilum. In many glands no differentiation into cortex and medulla can be made. Where there arc a distinct medulla 174 THE ORGANS and cortex the peripheral lymphoid tissue is arranged in nodules as in the ordinary lymph node. Reticular connective tissue crosses the sinuses and supports the cells of the lymph nodules and cords (Fig. loo). The cellular character of the lymphoid tissue has led to the sub- division of haemolymph nodes into splenolymph nodes and marrow- lymph nodes. In the splenolymph node the lymphoid tissue resembles that of the ordinary lymph node of the spleen. In the marrow- lymph node, which is the much less common form, the lymphoid Vt^-^XSL Fig. ioo. — Section through Superficial Portion of Human Haemolymph Node (Marrowlymph Node). (Warthin.) Capsule, trabeculse, 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. tissue resembles red marrow. There are no distinct nodules, and there is a quite characteristic distribution of small groups of fat cells. The most numerous cells are eosinophiles and mast cells (see page 1 06). 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 haemolymph 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. LYMPHATIC ORGANS 175 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 structure betwen the haemo- lymph node and the ordinary lymph node, between the splenolymph node and the marrowlymph node, and between the splenolymph node and the spleen. Under normal conditions the h^molymph 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 lymphatic system is not known, and like ignorance exists as to its innervation. The development of the haemolymph nodes is probably much the same as that of the lymph nodes, except for the relation of the latter to the lymphatic vessels, the sinuses of the haemolymph nodes being developed from blood-vessels. TECHNIC Same as for lymph nodes (technic i, p. 172). 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. 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 fully developed thymus presents the following structure. The entire gland is surrounded by a rather delicate and loose connective-tissue capsule which attaches it to the surrounding tissues. From the capsule septa extend down into the 176 THE ORGANS organ. These branch and subdivide the gland into lobes, and these into larger and smaller lobules. From the perilobular connective tissue, septa extend into the lobule, incompletely separating it into a number of chambers. Each lobule .consists of a cortical portion and a medullary portion. The cortex consists of nodules of compact lymphatic tissue, composed of reticular tissue and lymphoid cells, similar to those found in the lymph node. These occupy the chambers formed by the connective-tissue septa. The medulla consists of the same elements only more loosely arranged, .-_ h •*^^ft^> c Fig. ioi. — From Section of Human Thymus, showing parts of five lobules and interlobular septa. X20. (Technic, page 178.) a, Cortex; 6, medulla; c, interlobular septum. the cells being much less closely packed, thus forming a more diffuse lymphatic tissue. There are also in the medulla no connective-tissue septa. In some lobules the more dense cortical substance completely encloses the medulla. It is common, however, for the medullary tissue to extend to the surface of a lobule at one or more points and to be there continuous with the medullary substance of an adjacent lobule. These interlobular connecting strands of medullary substance are known as medullary cords. In the medulla are found a number of spherical or oval bodies composed of concentrically arranged epi- thehal cells. These are known as HassalVs corpuscles (Fig. 102), and represent the only remains of the original glandular epithehum. LYMPATHIC ORGANS 177 They are characteristic of the thymus. The central cells of the cor- puscles are usually spherical and contain nuclei, while the peripheral cells are flat and non-nucleated. As the entire corpuscle takes a bright red stain with eosin-haematoxylin, the corpuscles stand out sharply from the surrounding bluish or pinkish lymphatic tissue. With low magnifications they are apt to be mistaken for blood- vessels. '^''" . ^.I,.^ Unlike the other lymphatic %^ organs, the lymph nodules of the thymus contain no germinal centres. Mitosis can, however, ;-.- % ■' usually be seen in the lymphoid ; cells. No definite lymph sinuses #''^ have been demonstrated. Nu- ......,,,.. v i^' cleated red blood cells occur in ^--m^-^- Fig. I02. — Hassall's Corpuscle and Small the thymus. The thymus must Portion of Surrounding Tissue. X6oo. 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. The thymus originates in the entoderm in the region of the third bran- chial groove, first as two hollow evaginations of the endothelium of the pharyngeal cavity, which later become solid cords, and then separate entirely from the pharynx. The thymus thus begins its fcetal existence as a typical epithelial gland. Into this epithelial structure mesodermic cells grow and difTerentiate into lymphatic tissue. This almost completely replaces the epi- thelial tissue, only rudiments of which remain as Hassall's corpuscles. Stcihr denies the mesodermic invasion of the thymus and consequently the lymphatic character of the gland. He considers the specific cells of the thymus as modified epithelial cells which have become "deceptively like lymphoid cells." This explanation has not, however, been generally accepted. 12 178 THE ORGANS TECHNIC Fix the thymus of a new-born infant in formalin-Miiller's fluid (technic 5, p. 7), and harden in alcohol. Stain sections with h£ematoxylin-eosin (technic i, p. 20), or with hsematoxylin-picro-acid-fuchsin (technic 3, p. 21), and mount in balsam. The Tonsils The Palatine Tonsils or True Tonsils. — These are lymphatic organs, essentially similar in structure to those already described. / d ^<^ Fig. 103. — Vertical Section of Dog's Tonsil through Crypt. X15. (Szymonowicz.) a, Lymph nodule; b, epithelium of crypt; c, blood-vessel; d, crypt; e, connective-tissue capsule;/, mucous glands; g, epithelium of pharynx. The usual fibrous capsule is present only over the attached sur- face, where it is firmly adherent on the one side to the tonsillar tissue and on the other to the surrounding structures from which it separates the tonsils. 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 LYMPHATIC ORGANS 179 stratified squamous epithelmm of the pharynx (Fig. 103) . This epithe- lium presents the same structure as elsewhere in the pharynx, flat surface cells, beneath which are irregular cells, while the deepest cells are more or less distinctly columnar. The latter rest upon apapillated stroma from which they are separated by ihebasement membrane. At several places on the surface of the tonsil deep indentations or pockets occur. These are from ten to twenty in number, are known as the crypts of the tonsil (Fig. 103), and are lined throughout by a continua- tion of the surface epithelium which becomes thinner as the deeper part of the crypt is reached. 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. ^'''¥o'-0*, fj v.. (^"a Fig. 104. — Vertical Section through Wall of Crypt in Dog's Tonsil, showing lymphoid infiltration of epithelium. Xiso. (Bohm and von Davidoff.) o, Leucocytes in epithe- lium; 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. The stroma consists of diffuse lymphatic tissue in which are found 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 nod- ules may have a fairly definite arrangement, forming a single layer beneath the epithelium of the crypts or may be arranged quite irregu- larly, several nodules uniting to form masses of dense lymphatic tissue. At various points on the surface of the tonsil, and especially in the crypts, occurs what is known as lymphoid infiltration of the epithelium (Tig. 104). This consists in an invasion of the epithelium by the underlying lymphoid cells. It varies from the presence of only a few lymphoid cells scattered among the epithelium, to an almost 180 THE ORGANS complete replacement of epithelial by lymphoid tissue. In this way the latter reaches the surface 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. In the connective tissue adjacent to the tonsil are numerous mucous glands, the ducts of which empty into the tonsillar crypts. The Lingual Tonsils — ^Folliculi Linguales. — These are small lymphatic organs situated on the dorsum and sides of the back part of the tongue between the circumvallate papillae and the epiglottis. They are similar in structure to the true tonsils. Each Ungual tonsil has usually one rather wide-mouthed deep crypt {Xh^ foramen cacum lingucE) which may be branched and which is lined with a continuation of the surface stratified squamous epithelium. Into these crypts frequently open the ducts of some of the mucous glands of the tongue. The Pharyngeal Tonsils.^ — These are lymphatic structures which he in the naso-pharynx. They resemble the lingual tonsils, except that they are, as a rule, not so sharply circumscribed. Hypertrophy of the pharyngeal tonsils, with consequent obstruction of the nasal openings, is common especially in children, constituting what are known as adenoids. The blood-vessels have a distribution similar to those of the lymph nodes, but enter the organ along its entire attached side and not at a definite hilum. Of the lymphatics of the tonsil Httle is known. The nerves which are branches of the glosso-pharyngeal and of the spheno-palatine ganghon also enter the organ along its attached side. The palatine tonsils make their first appearance during the third month of intra-uterine life as hollow evaginations of entoderm which grow down into the underlying mesenchyme, in the region of the second inner branchial groove. The earliest of the tonsillar lymphoid cells are apparently white blood cells which have migrated from the vessels of the stroma of the mucosa and have infiltrated the surrounding connective tissue. Further development of the tonsil is by proliferation of these cells. The crypts are at first solid ingrowths of surface epithelium. These later become hollowed out. The lingual and pharyngeal tonsils begin their development during the later months of intra-uterine life. In the pharyngeal tonsils, definite nodules appear about the time of birth or during the first or second year. In the lingual tonsils the nodules are not fully formed until the fifth or sixth year. LYMPHATIC ORGANS 181 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. 172, care being taken that sections pass longitudinally through one of the crypts. The Spleen The spleen is a lymphatic organ and might be quite properly classed as a hasmolymph node. Its peculiar structure appears to depend largely upon the arrangement of its blood-vessels. Except where attached, the spleen is covered by a serous mem- brane, the peritoneum (page 267). Beneath this is a capsule of fibrous tissue containing numerous elastic fibres and some smooth muscle Fig. 105. — Section through Portion of Cat's Spleen.^to show general topography. XiS- (Technic i, p. 187.) a, Capsule; b, septa containing blood-vessels; c, germinal centres; d, septa; e, lymph nodules. cells. From the capsule strong connective-tissue septa, similar to the capsule in structure, extend into the interior of the organ. These branch and unite with one another to form very incomplete anastomosing chambers. The capsule and septa form, as in the .lymph node, the connective-tissue framework of the organ (Fig. 105). The chambers incompletely bounded by the connective-tissue septa are filled in with tissue resembling lymphatic tissue, composed of reticular connective tissue, lymphoid cells, and other varieties of cells described on p. 184. This tissue constitutes the substantia propria of the organ and is everywhere traversed by thin-walled vascular channels, the tissue and vascular channels together con- 182 THE ORGANS stituting the splenic pulp (Fig. io6). Compact lymphatic tissue occurs in the spleen as spherical, oval, or cyHndrical aggregations of closely packed lymphoid cells. These are known as Malpighian bodies or splenic corpuscles (Figs. 105 and 106) and are distributed throughout the splenic pulp. Each splenic corpuscle contains one or morejsmall arteries. These usually run near the periphery of the corpuscle; more rarely they lie at the centre. Except for its relation to the blood-vessels, the splenic corpuscle is quite similar in structure to a lymph nodule. It consists of lymphoid cells so closely packed as completely to obscure the underlying reticulum. In a S5 Fig. 106. — Section of Human Spleen, including portion of Malpighian^body with its artery and adjacent splenic pulp. X300. (Technic 2, p. 187.) a, Malpighian body; b, pulp cords, c, cavernous veins; b and c together constituting the splenic pulp. child's spleen the centre of each corpuscle shows a distinct germinal centre (see page 169). In the adult human spleen germ centres are rarely seen. The blood-vessels of the spleen have a very character- istic arrangement, which must be described before considering further the minute structure of the organ. The arteries enter the spleen at the hilum and divide, the branches following the connective-tissue septa. The arteries are at first ac- companied by branches of the splenic veins. Soon, however, the arteries leave the veins and the septa, and pursue an entirely separate course through the splenic pulp. Here the adventitia of the smaller arteries assumes the character of reticular tissue and becomes in- filtrated with lymphoid cells. In certain animals, as e.g., the guinea- LYMPHATIC ORGANS 183 pig, this infiltration is continuous, forming long cord-like masses of compact lymphoid tissue. In man, the adventitia is infiltrated only at points along the course of an artery. This may take the form of elongated collections of lymphoid cells — the so-called spindles — or of distinct lymph nodules, the already mentioned splenic corpuscles. Although usually eccentrically situated with reference to the nodules, these arteries are known as central arteries. They give rise to a few capillaries in the spindles, to a larger number in the nodules. Be- Spleen sinus Sheath artery Pulp vein Beginning of in- _ erlobular vein Capillary net- ^ A work of nodule "■ ~ Pulp artery Trabecula Penicillus Central artery Interlobular vein > Lobule Hiius ■ Reticulum Spleen nodule Capsule Fig. 107. — Scheme of Human Spleen, x, Opening of arterial capillaries into' spleen sinus; xx, interruption of closed blood course at ends of arterial capillaries, at margin of nodule, xxx. For sake of clearness, sinus is placed too far from margin of nodule. (Stohr.) yond the latter the arteries divide into thick sheathed terminal arteries — pulp arteries — which do not anastomose, but lie close to- gether like the bristles of a brush or penicillus. The pulp arteries break up into unusually thick walled arterial capillaries wliich still retain an adventitia, the so-called sheathed arteries. These are of remarkably uniform diameter — 6-8 cc — and empty into broader spaces from 10 to 40/' in diameter — the spleen, sinuses or ampullcB — which in turn empty into the cavernous veins of the si)lenic pulp (Fig. 106). 184 THE ORGANS A £ J) II The Splenic Pulp.— The anastomosing cavernous veins break up the diffuse lymphatic tissue of the spleen into a series of anasto- mosing cords similar to those found in the medulla of the lymph node. These are known as pulp cords (Fig. io6), and with the caver- nous veins constitute, as already mentioned, the splenic pulp. The pulp cords consist of a dehcate framework of reticular connective tissue, in the meshes of which are found, in addition to lymphoid cells, the following (Fig. io8) : (i) Red blood cells, including nucleated red blood cells and fragments of red cells in process of disintegration. (2) White blood cells. (3) Mononuclear cells, the so-called spleen cells. These are rather large, granular, spher- ical, or irregular cells. From the fact that blood pigment and red blood cells in various stages of disintegration are found in their cytoplasm, these cells are beheved to be concerned in the G J Fig. 108. — Isolated Spleen Cells. X 700. (Kolliker.) A, Cell containing red blood cells; &, blood cell; k, nucleus; B, leucocyte with polymorphous nucleus; C, "spleen" cell with pigment granules; D, lymphocyte; £, large cell with lobulated nucleus (megalo- cyte); F, nucleated red blood cells; G, red blood cell; U, multinuclear leucocyte; /, cell containing eosinophile granules. destruction of red blood cells. (4) Multinuclear cells. These are most common in young ani- mals. Each cell contains a single large lobulated nucleus, or more frequently several nuclei. These cells resemble the osteoclasts of developing bone and the multinuclear cells of bone-marrow. In macerated splenic tissue or in smears from the spleen, there are found, in addition to the above varieties of cells, long spindle- shaped cells with bulging nuclei. These come from the walls of the cavernous veins. Two views have been held regarding the vascular channels of the pulp. According to one, these channels have complete waUs, the blood-vessel system of the splenic pulp being a closed system as in other organs. According to the other, the cavernous veins or spleen sinuses into which the arterial capillaries, open, have fenestrated walls — open system. These fenestra are of sufficient size to allow fluid and formed elements of the blood to pass out freely into the pulp cords and elements of the pulp cords to pass freely into the vessels. From these open- walled sinuses the veins proper take origin. The smaller veins, even those within the trabeculae, have only endothelial walls. These uniting, form veins. LYISIPHATIC ORGANS 185 which enter the septa and ultimately converge to form the splenic veins which leave the organ at the hilum. MoUier^ describes the walls of the spleen pulp sinuses as having the following structure, (i) A protoplasmic endothelial syncytium, with scattered oval nuclei (their long diameter lying in the long diameter of the vessel) and no evidence of cell boundaries. This syncytium is arranged as a network, the meshes of which are sometimes quite irregular, at other times quite regular and Fig. 109. — Section of Monkey's Spleen. Two pulp sinuses are shown and between them some of the reticular tissue of the pulp. The sinus in the upper right corner is cut transversely so that the longitudinal fibres of its wall are cut across and appear as a row of dots. The prominent dark nuclei belong to the endothelium. The other sinus is cut obliquely and a branch from it is seen partly in longitudinal section. It shows well the disposition of both longitudinal and circular fibres. (Alollicr.) rectangular, with the long diameter of the mesh running in the long diameter of the vessel. This protoplasmic syncytium lines the sinus. (2) Outside the endothelial syncytium are clo.sely-placed longitudinal fibres which lie upon the lengthwise-running strands of the protoplasmic reticulum (Fig. 109). (3) Out- side the lengthwise-running strands transverse fibres or ring fibres which at rather longer intervals encircle the tube (Fig. no). Both longitudinal and ring fibres arc connective tissue (reticular), and the reticulum which they form is 'Arch. f. mikr. Anat., Hd. Ixxvi, 1910-1 1. 186 THE ORGANS everywhere continuous with the reticulum of the pulp cords and apparently- identical with it, except that to form walls, the reticulum is flattened out and usually has a more regular arrangement. The strands of the fibre reticulum lie upon the strands of the protoplasmic reticulum in such a way that the meshes correspond. A thin structureless fenestrated membrane lying just outside the endothehal syncytium has been described. The sinus walls probably possess a certain elasticity or contractility and undoubtedly change their diameter to accord with varying functional conditions. It follows that the meshes of the reticulum may be at one time widely open (con- gested spleen), at another partly open, at still I another entirely closed. 0 The main functions of the adult spleen in , ^ health appear to be the production of leuco- I II cytes and the destruction of red blood cells. It is possible that to a limited degree the spleen produces red blood cells. During foetal life the spleen is actively engaged in the development of both red and white cells. Also in adult life a severe secondary anaemia or a pernicious anaemia may stimulate the spleen to resume production of red and white cells. The spleen can be re- moved without apparently seriously interfering with the body functions. Enlargement of lymph nodes and increased blood-forming activity of bone marrow as a result, have been described. The ultimate origin of the spleen in man has not been definitely determined. The anlage of the spleen can be seen in embryos of five weeks as a thickening of the mesenchyme in the left dorsal mesogastrium and it was believed that the organ developed wholly from this mesen- chyme. The mesothelium covering the thick- ened mesenchyme also shows thickening, and demarcation between the two layers becomes almost entirely lost. Recent investigations have made it seem probable that the spleen develops in part at least from these mesothelial cells which grow down into the mesenchyme. In this case it is probable that mesen- chyme gives rise to capsule, trabeculae, and reticular connective tissue — the connective tissue framework of the organ — while mesothelium is responsible for some at least of the various cellular elements of the splenic pulp. Hi Fig. iio.^ — Diagram of a Sinus of the Splenic Pulp, showing Re- lations of Longitudinal Fibres, Circular Fibres and Endothelial Nuclei. (Mollier.) Lymphatics are not numerous. In certain of the lower animals large lymph vessels occur in the capsule and septa. These are not well developed in man. Lymph vessels are present in the connective tissue of the hilum. They probably do not occur in the splenic pulp or in the splenic corpuscles. LYMPHATIC ORGANS 187 Nerves.- — These are mainly non-meduUated, although a few meduUated fibres are present. Among the latter are dendrites of sensory neurones whose cell bodies are situated in the spinal gan- glia. They supply the connective tissue of the capsule, septa, and blood-vessels. The non-medullated fibres — axones of sympathetic neurones — accompany the arteries, around which they form plexuses. From these plexuses terminals pass to the muscle cells of the arteries, to the septa, to the capsule, and possibly also to the splenic pulp. The exact manner in which both medullated and non-medullated fibres terminate is as yet undetermined. TECHNIC (i) The spleen of a cat is more satisfactory for topograpliy tfian the human spleen, as it is smaller, contains more connective tissue and its Malpighian bodies are more evenly distributed and more circumscribed. Fix in formalin-Muller's fluid (technic 5, p. 7), and harden in alcohol. Cut sections through the entire spleen. Stain with haematoxylin-eosin (technic i, p. 20), or with haematoxylin- picro-acid-fuchsin (technic 3, p. 21). (2) Human Spleen. — Small pieces are treated as in technic (i). (3) Human Spleen (Congested). — Congested human spleens are usually easy to obtain from autopsies. Treat as in technic (i). The cavernous veins being distended with blood, the relations of the veins to the pulp cords are more easily seen than in the uncongested spleen. The contrasts are especially sharp in sections stained with haematoxylin-picro-acid-fuchsin. (4) The cells of the spleen may be studied along the torn edges or in the thin- ner parts of any of the spleen sections. Or a smear may be made in a man- ner similar to that described in technic (page no), by drawing the end of a slide across a freshly cut spleen surface and then smearing the tissue thus ob- tained across the surface of a second slide. Dry, fix in equal parts alcohol and ether (one-half hour), stain with ha^matoxylin-eosin and mount in balsam. Or the cut surface of the spleen may be scraped with a knife, the scrapings trans- ferred to Zenker's fluid, hardened in alcohol, stained with alum-carmine (pages 19 and 63) and mounted in eosin-glycerin. General References for Further Study KoUiker: Handbuch der Gewebelehre des Menschen, vol. iii. Szymonowicz and MacCallum: Histology and Microscopic Anatomy. Warthin: Haimolymph Glands (with bibliography). Reference Handbook of the Medical Sciences, vol. iv. Mall: Lobule of the Spleen. Bui. Johns Hopkins Hospital, vol. ix.— Archi- tecture and Blood-vessels of the Dog's Spleen. Zeit. f. Morph. u. Anth., Bd. ii. Oppel: Ueber Gitterfasern der menschlichen Leber und Milz. Anat. Anz., 6 Jahrg., S. 165. CHAPTER III THE SKELETAL SYSTEM The skeletal system consists of a series of bones and cartilages which are united by special structures to form the supporting frame- work of the body. Under this head are considered: (i) bones, (2) marrow, (3) cartilages, (4) articulations. The Bones A bone considered as an organ consists of bone tissue laid down in a definite and regular manner. If a longitudinal section be made through the head and shaft of a long bone, the head of the bone and also part of the shaft are seen to be composed of anastomosing bony Fig. III. — Section of Spongy Bone. X7S- (Technic 3, p. 197.) a, Marrow space; h, group of fat cells; c, blood-vessel; d, trabeculae of bone. trabeculae enclosing cavities. This is known as cancellous or spongy hone. The shaft of the bone consists of a large central cavity sur- rounded by spongy bone, which, however, passes over on its outer side into a layer of bone of great density and known as hard or compact hone. Spongy bone forms the ends and lines the marrow cavities of the long bones, and occurs also in the interior of short bones and flat 188 THE SKELET.\L SYSTEM 189 bones. Compact bone forms the bulk of the shafts of the long bones and the outer layers of the fiat and short bones. In compact hone the layers or lamellae of bone tissue have a defi- nite arrangement into systems, the disposition of which is largely dependent upon the shape of the bone and upon the distribution of its blood-vessels. In spongy hone (Fig. 1 1 1) there is no arrangement of the bone tissue into systems. The trabecular consist wholly of bony tissue laid down b^r^A>\* Fig. 112. — Longitudinal Section of Hard (Undccalcitied) Bone: Shaft of Human Ulna. X90. (Szymonowicz.) Haversian canals, lacuna;, and canaliculi in black. in lamella;. These trabeculae anastomose and enclose spaces which contain marrow and which serve for the passage of blood-vessels, lymphatics, and nerves. On examining a longitudinal section of compact bone (Fig. 112) 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- plete system of intercommunicating tubes. They are known as Haversian canals, contain marrow elements, and serve for the trans- mission of blood-vessels, lymi)hatics, and nerves. They anastomose not only with one another, but are in communication with the sur- 190 THE ORGANS face of the bone and with the central marrow cavity. Between the Haversian canals most of the lamellas run parallel to the canals. In a cross section through the shaft of a long bone (Fig. 113), three distinct systems of lamellae are seen. These are known as Haversian lamellcB, interstitial lamellce, and circumferential lamellce. ':v«r;/ / / * • ». '^ If. •^; «..■ .w.'V^' ^.-^..^'!%.^;..;.-^- »- ' •■'■\'' ': "4 > ^ '1 1,'- ^^^ /■ . ' - — " . -fc-i .'^•:v.'^-.-.v V Fig. 115. — Section of Red Bone-marrow from Rabbit's Femur. X700. (Technic 4, p. 197.) a, Red blood cells; &, myeloplax; c, fat space; d, nucleated red blood cells; e, myelocytes; /, reticular connective tissue; g, leucocytes. tion 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. (See also p. 109.) At the same time it is quite probable that it functionates as a place where blood cells are destroyed. It is also active in the development of bone. THE SKELETAL SYSTEM 195 Yellow marrow (Fig. ii6) consists almost wholly of fat cells, which have gradually replaced the other marrow elements. Under certain conditions the yellow marrow of the bones of the old or greatly emaciated undergoes changes due for the most part to the absorption of its fat. Such marrow becomes reddish and assumes a somewhat gelatinous appearance. It is known as '^ gelatinous '^•^W. ' % '^^\^ ^ ^.&^ s.. ^ ^ ^~ ,d f e Fig. ii6. — Yellow Marrow from Rabbit's Femur. Xs6o. (Technic 4, p. 197.) a, nucleated red blood cells; b, myeloplax; c, fat cells; d, myelocytes; e, reticular connec- tive tissue; /, leucocytes. marrow." Under certain conditions, e.g., fracture of shaft of long bone, yellow marrow may assume the character of red marrow and take an active part in the process of repair. It also serves as a storage place for fat. The large marrow cavities, such as those of the shafts of the long bones, are lined by a layer of fibrous connective tissue, the endosteum. Blood-vessels. — 7'hc blood-vessels of bone pass into it from the periosteum. Xear the centre of the shaft of a long bone a canal passes obliquely through the compact bone. This is known as the 196 THE ORGANS nutrient canal and its external opening as the nutrient foramen. This canal serves for the passage of the nutrient vessels — usually one artery and two veins — to and from the medullary cavity. In its passage through the compact bone the nutrient artery gives off branches to, and the veins receive branches from, the vessels of the Haversian canals. Each of the flat and of the short bones has one or more nutrient canals for the transmission of the nutrient vessels. ' In addition to the nutrient canals the surface of the bone is every- where pierced by the already mentioned (page 191) Volkmann's canals, which serve for the transmission, of the smaller vessels. In compact bone these vessels give rise to a network of branches which run in the Haversian canals. In spongy bone the network lies in the marrow spaces. Branches from these vessels pass to the marrow cavity, and there break up into a capillary network, which anasto- moses freely with the capillaries of the branches of the nutrient artery. The capillaries of marrow empty into wide veins without valves, the walls of which consist of a single layer of endothelium. So thin are these walls that the veins of marrow were long described as pass- ing over into open or incompletely walled spaces in which the blood came into direct contact with the marrow elements. These veins empty into larger veins, which are also valveless. Some of these con- verge to form the vein or veins which accompany the nutrient artery; others communicate with the veins of the Haversian canals. Lymphatics with distinct walls are present in the outer layer of the periosteum. Cleft-like lymph capillaries lined with endothe- lium accompany the blood-vessels in Volkmann's and in the Haver- sian canals. The lacunce and canaliculi constitute a complete sys- tem of lymph channels which communicate with the lymphatics of the periosteum, of Volkmann's and the Haversian canals, and of the bone-marrow. Nerves, — Both medullated and non-medullated nerves accom- pany the vessels from the periosteum through Volkmann's canals, into the Haversian canals and marrow cavities. Pacinian bodies (page 433) occur in the periosteum. Of nerve endings in osseous tissue and in marrow Httle definite is known. TECHNIC (i) Decalcified Bone.— Fix a small piece of the shaft of one of the long bones —human or animal— in formalin-Mxiller's fluid (technic 5, p. 7) and decalcify THE SKELET.\L SYSTEM 197 in hydrochloric or nitric acid solution (page lo). After decalcifying, wash until all traces of acid are removed, in normal saline solution to which a little ammonia has been added. Dehydrate, and embed in celloidin. Transverse and longi- tudinal sections are made through the shaft, including periosteum and edge of marrow cavity. Stain with hsematoxylin-eosin (technic i, p. 20) and mount in eosin-gylcerin. (2) Hard Bone. — Transverse and longitudinal sections of undccalcified bone may be prepared as in technic i, p. 102. (3) Spongy Bone. — This may be studied in the sections of decalcified bone, technic (i), where it is found near the marrow cavity. Or spongy bone from the head of one of the long bones or from the centre of a short bone may be prepared as in technic (2). (4) Red Marrow.— Split longitudinally the femur of a child or young ani- mal, and carefully remove the cylinder of marrow. Fix in formalin-Miiller's fluid and harden in graded alcohols. Cut sections as thin as possible, stain with haematoxylin-eosin, and mount in balsam. (5) ]\Iarrow: fresh specimen. — By means of forceps or a vice, squeeze out a drop of marrow from a young bone, place on the centre of a mounting slide, cover and examine it immediately. (6) Place a similar drop of marrow on a cover-glass and cover with a second cover-glass. Press the covers gently together, slide apart and fix the specimen by immersion for five minutes in saturated aqueous solution of mercuric chlorid. Wash thoroughly, stain with haematoxylin-eosin, and mount in balsam. Development of Bone The forms of bones are first laid down either in cartilage or in embryonic connective tissue. The bones of the trunk, extremities, and parts of the bones of the base of the skull develop in a matrix of cartilage. This is known as in- tracarlilagi>ious or endochondral ossification. The flat bones, those of the vault of the cranium and most of the bones of the face, are developed in a matrix of fibrillar connective tissue — intraniemhranous ossification. A form of bone development, similar in character to intramembranous, occurs in connection with both intramembranous ossification and intracartilaginous ossification. This consists in the formation of bone just beneath the perichondrium— 5«&- perichondrial ossification— ot, as with the development of bone perichondrium becomes periosteum — subperiosteal ossification. There are thus three forms of bone development to be considered: (i) Intramembranous, (2) intracartilaginous, and (3) subperiosteal. I. Intramembranous Development (Fig. 117).— In intramembranous ossifi- cation the matrix in whith the bone is developed is connective tissue. The process of bone formation begins at one or more points in this matrix. These are known as ossilication centres. Here some of the bundles of white fibres be- come calcified, i.e., become impregnated with lime salts. There is thus first established a centre or centres of calcification. Between the bundles of calcified fibres the connective tissue is rich in cells and vascular, and from its future r61e in bone formation is known as osleogenclic tissue (Fig. 117). .Mong the surfaces 198 THE ORGANS of the calcified fibres certain of the osteogenetic cells arrange themselves in a single layer (Figs. 117 and 118). These are now known as osteoblasts or "hone formers." Under the influence of these osteoblasts a thin plate of bone is formed between themselves and the calcified fibres. This plate of bone at first contains no cells, but as the lameUa of bone grows in thickness, the layer of osteoblasts becomes completely enclosed by bone. The osteoblasts are thus transformed into hone cells (Fig. 118), the spaces in which they lie becoming honelacunoe. The bone cell is thus seen to be derived from the embryonic connective-tissue cell, the osteoblast being an intermediate stage in its development. In this way ~:^h Fig. 1 1 7. — Intramembranous Bone Development. Vertical section through parietal bone of human foetus. X160. (Technic i, p. 203.) a, Osteoblasts; b, bone trabeculse; c, osteoclasts lying in Howship's lacunae; d, internal periosteum; e, bone cells;/, calcified fibres; g, osteogenetic tissue; h, external periosteum (pericranium.) irregular anastomosing trabeculae of bone are formed enclosing spaces (Fig. 117). The bony trabeculas at first contain remains of calcified connective- tissue fibres, while the spaces, which are known as primary marrow spaces, con- tain blood-vessels, osteogenetic tissue, and developing marrow. The osteo- blasts ultimately disappear and the spaces are then occupied by blood-vessels and marrow. The connective-tissue membrane has now been transformed into cancellous or spongy bone (Fig. iii). The bone thus formed is covered on its outer surface by a layer of con- nective tissue, a part of the membrane in which the bone was formed, but which from its position is now known as the periosteum, or, in the case of the cranial bones, as the peri- or epicranium (Fig. 117). THE SKELETAL SYSTEM 199 In this form of bone development, occurring as it does in the bones of the skull, provision must be made for increase in the size of the cranial cavity to ac- commodate the growing brain. This is accomplished in the following manner: Along the surface of the bone, directed toward the brain, large multinuclear cells — osteoclasts or "bone breakers^' — -make their appearance (Fig. ii8). The origin of these cells is not clear. Similar cells are conspicuous elements of adult, marrow. They have been variously described as derived from leucocytes from osteoblasts, or directly from the connective-tissue cells. A recent theory holds that they are derived by a process of budding from the endothelial ceUs, which form the walls of the capillaries. These osteoclasts apparently possess the power of breaking down bone. They are found mainly along its inner sur- face, and can be seen lying in little depressions — ■Howship's lacunce (Fig. ii8) — 1 e d c Fig. ii8. — Intramembranous Bone Development. Vertical section through parietal bone of human foetus. X350. (Technic i, p. 203.) a, Osteoblasts; b, calcified fibres; c, osteogenetic tissue; d, osteoclast lying in Howship's lacuna; c, bone lacunae; /, bone. which they have hollowed out in the bone. Between the outer surface of the bone and the pericranium is a layer of osteogenetic tissue, the innermost cells of which are arranged as osteoblasts along the outermost osseous lamellae. Here they are constantly adding new bone beneath the pericranium. This new bone is laid down, not in flat, evenly disposed layers, but in the form of anastomos- ing trabecular enclosing marrow spaces. It is thus seen that subperiosteal bone, like intramembranous, is at first of the spongy variety, and that with the development of the cranium the original intramembranous bone is entirely absorbed, together with much of the sub- periosteal. 2. Intracartilaginous Development. — In this form of ossification an embry- onal type of hyaUne cartilage precedes the formation of bone, the cartilage corre- sponding more or less closely in shape to the future bone (Fig. 119). Covering the surface of the cartilage is a membrane of fibrillar connective tissue, the peri- chondriiwt or primary periosteum. In most of the long bones the earliest changes take place within the cartilage at about the centre of the shaft (Fig. 119). Here the cartilage cells increase in 200 THE ORGANS / size and in number in such a way that several enlarged cartilage cells come to lie in a single enlarged cell space, and the cartilage assumes the character of hyaline cartilage. The cell groups next arrange themselves in rows or columns, which at first extend outward in a radial manner from a common centre, but later lie in the long axis of the bone. During these changes in the cells there is _ -- an increase in the intercellular matrix and a de- posit there of calcium salts. In this way the car- tilage becomes calcified, the area involved being known as the calcification centre. Further growth of cartilage at the calcification centre now ceases and, as growth of cartilage at the ends of the bone continues, the central portion of the shaft appears constricted. The changes up to this point seem to be preparatory to actual bone formation. Ossification proper begins by blood-vessels from the periosteum^ pushing their way into the calci- fied cartilage at the calcification centre, carrying with them some of the osteogenetic tissue from beneath the periosteum. These blood-vessels with their accompanying osteogenetic tissue are known as periosteal buds (Fig. 120). Osteoblasts now develop from the osteogenetic tissue and appear to dissolve the calcified cartilage from in front of the advancing vessels. In this way the septa between the cartilage cell spaces are broken down, the cartilage cells disappear, and a central cavity is formed — the primary marrow cavity. From tjie region of the primary marrow cavity blood-vessels and osteogenetic tissue push in both directions toward the ends of the cartilage which is to be replaced by bone. These break down the trans- verse septa between the cell spaces, whUe many of the longitudinal septa at first remain to form the walls of long anastomosing channels, the primary marrow spaces (Fig. 121). As in intramembranous bone, these contain blood-vessels, embryonal mar- row, and osteoblasts, all of which are derived from the osteogenetic tissue brought in from the periosteum by the periosteal buds. The osteo- blasts next arrange themselves in a single layer along the remains of the calcified cartilage, where they proceed to deposit a thin layer of bone between themselves and the cartilage (Fig. 122). As this increases in thickness some of the osteoblasts are enclosed within the newly formed bone to become hone cells, while the remains of the cartilage diminishes in amount and finally disappears. The calcification centre ^ The term "periosteum" is admissible from the fact that the first bone actually formed is beneath the perichondrium, which thus becomes converted into periosteum Fig. 119. — Intracartilagin- ous Bone Development. Longitudinal section of one of the bones of embryo sheep's foot, showing ossification cen- tre. X20. (Technic 2, p. 203.) a, Periosteum; h, blood- vessels; c, subperiosteal bone; d, intracartilaginous bone; e, osteogenetic tissue; /, carti- lage; g, ossification centre; h, calcification zone. THE SKELET.\L SYSTE:\I 201 has now become the ossification centre, and its anastomosing osseous trabecule, with their enclosed spaces containing osteogenetic tissue and marrow, constitute primary spongy hone. At either end of the ossification centre the cartilage presents a special struc- ture. Nearest the centre the cell spaces are enlarged, flattened, arranged in rows and contain shrunken cells. Some of the walls break down and irregular spaces are formed. The ground substance is calcified. Passing away from the ossification centre,, the cell spaces become less flattened, still arranged in rows, V k ■V-:^ m d-. "-"M^^^^ Fig. 1 20. Fig. 121. Fig. 120. — Intracartilaginous Bone Development. X3S0. Showing osteogenetic tissue pushing its way into the cartilage (periosteal bud) at the ossification centre. a, Periosteum; /;, cartilage cell spaces; c, periosteal bud; d, blood-vessel; e, cartilage cells;/, cartilage matrix. Fig. 121. — Intracartilaginous Bone Development. Same specimen asFig. iJ9(X3So), showing osteogenetic tissue pushing its way into the cartilage and breaking it uj) into trabeculae; also formation of primary marrow si)aces and disintegration of cartilage cells, a, Disintegrating cartilage cells; 6, cartilage trabccula; c, osteogenetic tissue in primary marrow space; d, blood-vessels; e, cell spaces;/, cartilage cells. the contained cells larger, and there is a lesser degree of calcification. This area passes over into an area of hyaline cartilage which blends without distinct demarcation with the ordinary embryonal cartilage of the rest of the shaft. The area of calcified cartilage at cither end of the os.sification centre is known as the calcification zone and everywhere precedes the formation of true bone (Fig. no). 3. Subperiosteal or subperichondrial development (Fig. 119) has already been largely described in connection with intramembranous ossification, and 202 THE ORGANS differs in no important respect from the latter. It always accompanies one of the other forms of ossification. Bone appears beneath the perichondrium some- what earlier than within the underlying cartilage. Beneath the perichondrium is a layer of richly cellular osteogenetic tissue. The cells of this tissue 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 carti- lage in the same manner as in intramembranous ossification. Intracartilaginous and subperiosteal bone can be easily differentiated 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 Haversian spaces. i h Fig. 122. — Intracartilaginous Bone Development. Same specimen as Fig. 119. (Xssc), showing bone being deposited around one of the trabeculse of cartilage, a, Blood-vessel; h, bone; c, cartilage remains; d, bone cell; e, cartilage cell space;/, osteo- blasts; g, osteogenetic tissue; h, lamella of bone; i, connective-tissue cells; j, cartilage cell. 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. In this way are formed the Haversian canals and the Haversian systems of lamellcB. Some of the interstitial lamellae are the remains of the spongy bone which was not quite removed in the enlargement of the primary marrow spaces to form the Haversian spaces; other interstitial 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 ossification are more apparent than real. In intracartilagi- nous ossification the bone is developed in cartilage but not from cartilage. As in intramembranous and in subperiosteal ossification, intracartilaginous bone is developed /roTO osteogenetic tissue. This osteogenetic tissue is a differentiation of embryonal connective tissue, in this case carried into the cartilage from the THE SKELET.\L SYSTEM 203 periosteum in the periosteal buds. In intramembranous ossification the bone is developed untJiin and directly from the embryonal connective tissue of which the membrane is composed. In intracartilaginous ossification there is the same embryonal connective-tissue membrane, but within this membrane 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 perichon- drium. It is from tissue which grows into the cartilage from this membrane — ■ embryonal connective tissue — that the bone, although developed in cartilage, is formed. Marrow develops from the mesenchymal tissue which enters the cartilage anlage in the periosteal buds, at the beginning of ossification. Growth of Bone The growth of intramembranous bone by the formation of successive layers beneath the periosteum has been already described (page 198). Intracartilaginous bones grow both in diameter and in length. Growth in diameter is accomplished by the constant deposition of new layers of bone beneath the periosteum. During this process, absorption of bone from \A'ithin 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 intra- cartilaginous bone being completely absorbed. Growth in length takes place in the following manner: Some time after the beginning of ossification in the shaft or diaphysis, independent ossification centres appear in the ends of the bone (epiphyses). So long as bone is growing, the epiphyses and diaphysis remain distinct. Between them lies a zone of grow- ing 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. TECHNIC (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. ig6, except that the sections which arc cut per- pendicular to the surface of the bone should be stained with ha^matoxylin-picro- acid-fuchsin (technic 3, p. 21) 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. (Fcetal pigs from five to six inches long are very satisfactory.) Treat as in technic (i). 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 ossification centres, stain with hiematoxylin-picro-arid-fuchsin, and mount in balsam. Cut away the ends of one or two of the embedded bones, leaving only the ossification centres. Block so as to cut transverse sections through the ossification centre. Stain and mount as the preceding. 204 THE ORGANS 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 there 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. Articulations Joints are immovable (synarthrosis) or movable (diarthrosis) . In synarthrosis union may be 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 Hgaments. 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 artic- ular cartilages, (b) the glenoid ligaments and interarticular cartilages, (c) the joint capsule. (a) Articular cartilages cover the ends of the bones. They are of the hyahne variety,^ being the remains of the original cartilag- inous 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 1 In the acromio-clavicular, sterno-clavicular, costo- vertebral, and maxillary articu- lations the cartilage is of the fibrous form. The same is true of the cartilage 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. THE SKELET.\L SYSTEM 205 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 hyahne cartilage (page 98) ; 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 fihrosum, and an inner 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 abun- dant. The cells are scattered among the fibres and are stellate branch- ing cells like those usually found in fibrous connective tissue. On the free surface, however, the cells He close together, in places forming a single surface layer, in other places being disposed in three or four lay- ers. Formerly described as endothelium, they are now generally considered connective- tissue cells or "mesenchymal epithelium." From the free surfaces of synovial membranes, processes {syno- vial villi — Haversian frifiges) Y^ioitct 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. 196, making longitudinal sections through the entire 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 206 THE ORGANS of the ligament in formalin7Muller's fluid (technic 5, p. 7), make sections per- pendicular to the surface, stain with haematoxylin-eosin (technic i, p. 20), and mount in balsam. General References for Further Study Braunca: Precis d'Histologie. KoUiker: 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^ The voluntary muscular system consists of a number of organs — the muscles — and of certain accessory structures — the tendons, tendon sheaths, and hursa. 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 epimysium (Fig. 123). This sends trabeculae Fig. 123. —From a Transverse Section of a Small Human Muscle, showing relations of muscle fibres to connective tissue, a, Epimysium; b, perimysium; c, muscle fibres; d, arteries; e, endomysium. of more loosely arranged connective tissue into the substance of the muscle. These divide the muscle fibres into bundles or fascicles. Around each fascicle the connective tissue forms a more or less definite * Definite arrangements of smooth muscle, such as arc found in the stomach and intestines, also the muscle of the heart, are properly a part of the muscular system. They are, however, best considered under tissues and in connection with the organs in which they occur. 207 208 THE ORGANS ' ( /, ( 1 1 1 1 i I \i envelope, the perifascicular sheath or perimysium. From the latter delicate strands of connective tissue pass into the fascicles between the individual muscle fibres. This constitutes the intrafascicular 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 endo-mysium all being connective tissue grad- ing 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 sur- rounded by a connective- tissue capsule, wliich sends septa into the organ, dividing it into a number of compart- ments 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 91). Tendon sheaths and burses are similar in structure, con- sisting of mixed white and elastic fibres. Their free surfaces are usually lined by flattened connective tissue-cells. At the junction of muscle and tendon, the muscle fibre with its sarcolemma ends in a rounded or blunt extremity (Fig. 63, p. 119). 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 peri-mysium. This has been for a long time the accepted idea of the transition from muscle to tendon, and is perhaps more nearly correct where the insertion is oblique. It has been more recently proved, however, that for some muscles at least, and especially where muscle and tendon join end to end a much more intimate union of elements occurs. This is shown in Figs. 125 and 126 from Stohr. lUj I Fig. 124. — From a Longitudinal Section through Junction of Muscle and Tendon. XiSo. (Bohm and Davidoff.) a, Tendon; b, line of union showing increase in number of muscle nuclei; c, muscle. THE MUSCULAR SYSTEM 209 In Fig. 125 the individual muscle fibrillae are seen passing over into tendon fibrillae with no line of demarcation. In Fig. 126 a similar continuity is shown except that groups of muscle fibrillse are seen to be continuous with groups of tendon fibrillae. Both sarcoplasm, which appears somewhat augmented at this point, and sarcolemmae, extend between the tendon bundles beyond the line of cross muscle striations. Along the fine of union of muscle and tendon the muscle nuclei are more numerous than elsewhere (Fig. 124, b), and it has been suggested that there is here a zone of indifferent or formative Jjj ■;:,,-, mV' ill ta teiMi Sa ,- Krause line , Dark band Transition zone .v- Tendon fibrils Nucleus Fig. 125. — Longitudinal Section through Muscle- tendon Junction: Human Intercostal. X750. Only part of a fibre is shown, without sarcolemma. (Stohr.) tissue which is capable of developing on the one hand into muscle, on the other into the connective tissue of tendon. 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 muscle, 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 (sarcostylcs), some of which contain nuclei while others are non-nucleated. The sarcostylcs next divide into smaller fragments, and finally completely disintegrate. This is followed by a process of absorption and com- plete disappearance of the fibre. From the free sarcoplasm new 14 210 THE ORGANS 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 (p. 122). 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 'i '^ Sarcoplasm , Tendon fibre bundles Transition zone y Sarcolemma ' v^ -* . Tendon nucleus Fig. 126. — Junction of Muscle and Tendon showing Continuity of Fibrils and_Ex- tension of Sarcolemma beyond the Limits of Cross Striations. Rectus abdominis of frog. X7SO- (Stohr.) 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 supphed with valves. In tendons blood-vessels are few. They run mainly in the con- nective tissue which surrounds the fibre bundles. Tendon sheaths and bursse, 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. THE MUSCULAR SYSTEM 211 Nerves. — The terminations of nerves in muscle and tendon are described under nerve endings (page 434). TECHNIC (i) A Muscle. — Select a small muscle, human or animal, and, attaching a weight to the lower end to keep it stretched, fix in formahn-Miiller's fluid (tech- iiic 5, p. 7), and harden in alcohol. Stain transverse sections with haematoxyUn- picro-acid-fuchsin (technic 3, p. 21) and mount in balsam. (2) Junction of Muscle and Tendon. — Any muscle-tendon junction may be selected. Fix in formalin-Miiller'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 bal- sam. 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 prepa- ration in the fixative removing the muscle-tendon from the bone after fixation. CHAPTER V. GLANDS General Structure and Classification Attention was called in describing the functional activities of cells (page 51) to the fact that certain cells possess the power of not only carrying on the nutritive functions necessary to maintain their own existence, but also of elaborating certain products either neces- sary for the general body functions (secretions) or for the body to eliminate as waste (excretions) . Such cells are known as gland cells or glandular epithelium, and an aggregation of these cells to form a definite structure for the purpose of carrying on secretion or excretion is known as a gland. A gland may consist of a single cell, as, e.g., the mucous or goblet cell on the free surface of a mucous membrane or the unicellular glands of invertebrates. Such a cell undergoes certain changes which result in the production within itself of a substance which is to be used outside the cell. The appearance which this cell presents depends upon the stage of secretion. It is thus possible to differen- tiate between a " resting' ' and an " active " cell or between an "empty " and a loaded" cell. The mucous secreting cell of the intestine is one of the simple columnar cells which constitute the epithelium of the mucous membrane. It is distinguishable as a mucous or goblet cell only after secretion begins. The resting cell is granular and takes a rather dark cytoplasmic stain. As the cell becomes active, part of the cytoplasm is transformed into, or is replaced by, a clear substance which does not stain like cytoplasm, but reacts to hsematoxyhn. The mucus collects first in the free end of the cell, and gradually increases in amount until the entire cell is filled, with the exception of a small area at the base, where a little unchanged protoplasm sur- rounds a flattened nucleus. The cell at this stage is much larger than in the resting state, and finally ruptures on the free surface and pours out its secretion. OpiDions differ as to the further behavior of this cell. According to some, its life history is now ended, and its place is taken by other cells which pass through the same process. Others 212 GLANDS 213 believe that in most cases the cell is reconstructed from the nucleus and unchanged cytoplasm, and again passes through the process of secretion. In stratified epithelium secretion may begin while the cell is still deeply situated, but is completed only as the cell reaches the surface, where its mucus is to be discharged. In describing the vital properties of cells (p. 50) attention was called to the fact that all cells take up from the surrounding blood and lymph substances required for their own nutrition, and give off waste products (excretions). In such sense all cells secrete and excrete. What distinguishes the gland cell is that in addition to carrying on its own metabolism (p. 51) it manufactures a specific substance not for its own use but to be extruded from the cell, and used elsewhere in the body (secretion, e.g., gastric juice), or discarded (excre- tion, e.g., urine). Such a cell takes up from blood or lymph the substances required, more or less completely as- similates them, and finally transforms them into its specific secretion. Certain changes, other than those described as seen in the mucous secret- ing cell, may occur in the protoplasm of actively secreting gland cells. In many cells there appears at the onset of secretion a modification of the cytoplasm which has been designated ergastoplasm (Fig. 127). In some cells the ergasto- plasm takes the form of slender threads near the base of ih.t ctW, basal filaments, in others of minute rods. Other form shave been described as mitochondria, cytosomes, pseudochromosomes, etc. Many gland cells have intracellular secretory canals (see p. 46). In some gland cells a body known as the paranucleus appears at the beginning of secretion. It is a rather large, usually irregular mass, differing somewhat in staining qualities from both nucleus and ergastoplasm. Its relation to either of these structures, to the cytoplasm, or to the secretion is not known. The function of the nucleus in secretion is apparently of great importance. Non-nucleated portions of cells are probably not able to elaborate any true secretion. When (see below) the entire cell enters the secretion as in the mammary gland, the entire nucleus of course becomes a part of the secretion. In any event the onset of secretion is apt to be evidenced in the nucleus by en- largement and irregularity, in some cases by the giving off of nuclear material to the cytoplasm, in others by amitotic division. According to some observers both ergastoplasm and paranucleus arc of nuclear origin. In connection with the mucous cell Cp. 212) it was noted that according to some authorities the cell dies in secreting, while others believe that the cell Fig. 127. — Gland Cell from Pancreas of Salamander; n, Nucleus; cs, caryosomes; pi, plasmosomes; cp, paranucleus; erg, ergastoplasm filaments; g, secretory gran- ules. Xsoo. (Prenant.) 214 THE ORGANS reconstructs itself and can again secrete. In certain glands, e.g., the mammary and sebaceous, the cast off cells themselves form the secretion. More commonly the cell merely gives off its secretion, the remainder of the cell recovering and again goin'g through the same process. Most glands are composed of more than one cell, -usually of a large number of cells, and these cells, instead of lying directly upon the sur- face, Hne more or less extensive invaginations into which they pour their secretions. In the simplest form of glandular invagination all 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 secreting portion of a gland the epithe- lium usually rests upon a more or less definite basement membrane or membrana propria (page 70). Beneath the basement membrane, separating and supporting the glandular 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 tubular 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. Intermediate forms have been described as tubulo-alveolar glands. 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 sys- tems with their terminal tubules or saccules — compound gland. A few glands, e.g., the thyreoid and thymus, have no ducts, and are known as ductless glands. All compound glands are surrounded by connective tissue which forms a more or less definite capsule. From the capsule connective- tissue septa or trabeculce 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 interlobar 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 GLANDS 215 one or more terminal ducts. The glandular (epithelial) tissue is known as the parenchyma of the gland, in contradistinction to the connective or interstitial tissue. The relations of the glandular tissue proper to the connective tissue are best understood by reference to development. All glands, simple and compound, originate as simple evaginations from a surface lined with epithehum. The epithelial evagination grows down into the underlying connective tissue. In a compound gland this invagination tubule becomes the main excretoiy duct. As the tubule grows, it divides and subdivides to form the larger and smaller ducts and finally the secreting tubules or alveoli. During the development of the gland tubules, the connective tissue is also developing, but is being largely re- placed by the more rapidly growing tubules. The gland tubules do not develop irregularly, but in definite groups, each group being dependent upon the tubule (duct) from which it originates. Thus the invagination tubule (main excretory duct) gives rise to a few large branches (lobar ducts), each one of which gives oflf the subdivisions which constitute a lobe. From each lobar duct there arise within the lobe a large number of smaller branches (lobular ducts) each one of which gives rise to the subdivisions included in a lobule. As the lobe groups and lobule groups of tubules develop, the largest strands of connective tissue are left between adjacent lobes (interlobar connective tissue), smaller strands between lobules (interlobular connective tissue), and the finest connective tissue between the tubules or alveoli within the lobule (intralobular connective tissue). Glands may thus be classified as follows: A. Duct glands or glands of external secretion. I. Tubular glands. r straight. (a) Simple tubular \ coiled. branched. {h) Compound tubular. 2. Alveolar or saccular glands. {a) Simple alveolar. {h) Compound alveolar, saccular or racemose. B. Ductless glands or glands of internal secretion. Duct Glands 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 the cells may be secreting 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 highly developed of the simple tubular glands we distinguish a mouth, open- 216 THE ORGANS ing upon the surface, a neck, usually somewhat constricted, and a 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. (i) A straight tubular gland is one in which the entire tubule runs a straight unbranched course, e.g., the glands of the large intestine (Fig. 128, i). (2) A coiled tubular gland is one in which the deeper portion of the tubule is coiled or convoluted, e.g., the sudoriferous glands of the skin (Fig. 128, 2). 6 Fig. 128. — Diagram Illustrating Different Forms of Glands. Upper row, tubidar glands; i, 2, and 3, simple tubular glands; 4, compound tubular gland. Lower row, alveolar glands; la, 2a, and 3a, simple alveolar glands; 40, compound alveolar gland. For description of la, 2a, and 3a, see simple alveolar glands in text. (3) A forked or branched tubular gland is a simple tubular gland in which the deeper portion of the tubule branches, the several branches being lined with secreting cells and opening into a superficial 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 giving rise to a number of secondary ducts, from which are given off the terminal tubules. The mucous glands of the mouth, oesophagus, trachea, and bronchi are examples of these more elaborate simple tubular glands (Fig. 128, 3). GLANDS 217 (b) 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 sali- vary glands, Uver, kidney, and testis (Fig. 128, 4). In certain compound tubular glands, as, e.g., the liver, extensive anastomoses of the terminal tubules occur. These are sometimes called reticular glands. 2. Alveolar Glands. — (a) Simple Alveolar Glands. — The sim- plest 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. 128, i a). 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. 128, 2 a), are represented by the smaller sebaceous glands. Simple branched alveolar glands, in which a common duct gives rise to a number of saccules (Fig. 128, 3 a), are seen in the larger sebaceous glands, and in the Meibomian glands. (b) Compound Alveolar Glands.- — These resemble the com- pound tubular glands in general structure, consisting of a large number 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 tubules of uniform lumen, as in a tubular gland, end in sac-hke dilatations, the alveoli or acini (Fig. 128, 4 a). The best example of a compound alveolar gland is the mammary gland, although the lung is constructed on the principle of a compound alveolar gland. Ductless Glands Certain structures remain to be considered which are properly classified as glands, but in which during development the excretory duct has disappeared. Such glands are known as ductless glands. The ovary is a ductless gland, the specific secretion of which, the ovum, is under normal conditions taken up by the oviduct and carried to the uterus. This is known as a dehiscent gland. Other ductless glands, such as the thyreoid, hypophysis and adrenal, are known as glands of internal secretion, their specific secretions passing directly into the blood or lymph systems. A few glands, e.g., the liver and pancreas, have both an internal secretion, and an external secretion. 218 THE ORGANS General Structtire of Mucous Membranes The alimentary tract, the respiratory tubules, parts of the genito- urinary system, and some of the organs of special sense are lined by mucous membranes. While differing as to details in different organs, the general structure of all mucous membranes is similar. The essential parts are (i) surface epitheUum, (2) basement membrane, and (3) stroma or tunica propria. The epithelium may be simple columnar, as in the gastro-intestinal canal; ciHated, as in the bronchi; stratified squamous, as in the oesophagus, etc. The epithelium rests upon a basement membrane or membrana propria which, Hke the same membrane in glands, is described by some as a product of the epithe- Uum, by others as a modification of the underlying connective tissue. Beneath the basement membrane is a connective- tissue stroma, or tunica propria. This usually consists of loosely arranged fibrous tissue with some elastic fibres. It may contain smooth muscle cells and lymphoid tissue. In addition to the three layers above described there is frequently a fourth layer between the stroma and the underlying connective tissue. This consists of one or more layers of smooth muscle, and is known as the muscularis mucosce. A mucous membrane usually rests upon a layer of connective tissue rich in blood-vessels, lymphatics, and nerves — the submucosa. CHAPTER VI THE DIGESTIVE SYSTEM The digestive system consists of the alimentary tract and certain associated structures such as glands, teeth, etc. The alimentary tract'is a tube extending from lips to anus. Dif- ferent parts of the tube present modifications both as to caHbre and as to structure of wall. The embryological subdivision of the canal into headgut, foregut, midgut, and endgut admits of further subdivision upon an anatomical basis as follows: I. Headgut: (a) Mouth, including the tongue and teeth. (b) Pharynx. II. Foregut: (a) (Esophagus. (b) Stomach. III. Midgut: Small intestine. IV. Endgut: (a) Large intestine. (b) Rectum. The entire canal is lined by mucous membrane, the modifications of which constitute the most essential difference in structure of its several subdivisions. Beneath the mucosa is usually more or less connective tissue, which in a large portion of the canal forms a definite submucosa. Muscular tissue is present beneath the submucosa throughout the greater part of the canal. In most regions it forms a definite, con- tinuous, muscular tunic. The upper and lower ends of the tube — mouth, pharynx, oesoph- agus, and rectum — are quite firmly attached by fibrous tisue to the surrounding structures. The remainder of the tube is less firmly attached, lying coiled in the abdominal cavity, its surface covered, except along its attached border, by a serous membrane, the visceral peritoneum. 219 220 THE ORGANS I. THE HEADGUT The Mouth The Mucous Membrane of the Mouth. — This consists of stratified squamous epithelium lying upon a connective- tissue stroma or tunica propria. The latter is thrown up into papillcs, which do not, however, appear upon the free surface of the epithehum. The submucosa is a firm connective- tissue layer with few elastic fibres. The thickness of the epithehum, the character of the strom.a, and the height of the papillae vary in different parts of the mouth. There is no muscularis mucosae. At the junction of the skin and m.ucous membrane (red margin of the Hps) the epitheHal layer is much thickened, the stroma is thinned, and the papillae are very high. At this point the stratum corneum of the skin passes over into the softer nucleated epithelium of the mouth, while the stratum lucidum and stratum granulosum of the skin terminate (see skin, page 383). The mucous m.embrane of the gums has prominent, long, slender papillae, the sum.mits of which are covered by a very thin layer of epithehum. This nearness of the vascular stroma to the surface accounts for the ease with which the gums bleed. That portion of the gums which extends over the teeth is devoid of papil- lae. The submucosa of the gums is firmly attached to the underlying periosteum. The mucous membrane fining the cheeks has low, small papillae, and the submucosa is closely adherent to the muscular fibres of the buccinator. Covering the hard palate, the mucous membrane is thin and the short papillse are obHquely placed, their apices being directed ante- riorly. The submucosa is firmly attached to the periosteum. Over the soft palate the papillae of the mucous membrane are low or even absent. They are somewhat higher on the uvula, the pos- terior surface of which shows a transitional condition of its epithe- hum, areas of stratified squamous alternating with areas of stratified columnar cihated epithehum. Throughout the mucous membrane of the soft palate, uvula, and fauces, the stroma and subm.ucosa contain diffuse lymphatic tissue. In some places the lymphoid cells are so closely placed as to form distinct nodules. Glands of the Oral Mucosa.^^ — Distributed throughout the ^ For description of the larger salivary glands see page 276. THE DIGESTIVE SYSTEM 221 oral mucosa are small branched tubular glands. Only in those parts of the mucous membrane which are closely attached to underlying bone, as on the gums and hard palate, are mucous glands few or entirely absent. While the deeper portions of the glands are in the submucosa, some of the tubules usually he in the stroma of the mucous membrane. The ducts open upon the surface and are hned with a continuation of the surface stratified squamous epithehum as far as the first bifur- cation. Here the epithelium becomes stratified columnar, and this, as the smaller branches are approached, passes over into the simple columnar type. Not infrequently ducts of small secondary glands empty into the main duct during its passage through the mucosa. According to the character of their secretions, the oral glands are divided into : (a) Mucous glands, which secrete a mucin-containing fluid (mucus) ; {h) Serous glands, which secrete a serous (albuminous) fluid; (c) Mixed glands, the secretion of which is partly mucous and partly serous. Morphologically, also, a similar distinction can be made in regard to the glandular epithehum which lines the terminal tubules, the tubules of mucous glands being hned with "mucous" cells, those of serous glands with "serous cells," while of the mixed glands the cells of some tubules are mucous, of others serous. In certain tubules both mucous and serous cehs occur. Th% appearance which these cells present depends largely upon their secretory condition at the time of death. Serous cells when resting have a shghtly granular protoplasm, which in the fresh condition is highly refractive, giving the cells a transparent appearance. With the beginning of secretion the gran- ules increase in number and the cehs become darker. Stained with hacmatoxyUn-eosin, serous tubules have a purphsh color. The nuclei are spherical or oval, and are situated between the centre and base of the cell (Fig. i8i, p. 279). Mucous cells are in the quiescent state rather small cuboidal or pyramidal colls, with cloudy cytoplasm and nuclei situated at the base of the cell. When active the mucous cells are much larger, with clear cytoplasm and with nuclei flattened against the basement membrane. The protoj)lasm of the fresh unstained mucous cell is less highly refractive than that of the serous cell. It consequently 222 THE ORGANS appears darker and less transparent. Mucous tubules are larger and more irregular in shape than serous tubules, and when stained with haematoxylin-eosin either remain almost wholly unstained or take a pale blue hsematoxylin stain (Fig. i8i, p. 279). Many mucous tubules have in addition to the mucous cells a peculiar, often crescentic-shaped group of cells on one side of the tubule, between the mucous cells and the basement membrane. These cells are granular and stain very much Uke serous cells with haematoxyhn- eosin, thus resembling the latter in appearance. On account of the shape of the groups, they are known as the crescents of Gianuzzi or demilunes of Heidenhain (Fig. 181, p. 279). The cells of the crescents are connected with the lumen by means of secretory canals, which pass between the mucous cells and end in branches within the proto- plasm of the crescent cells. It is quite possible that some of the crescents are not serous cells but mucous cells in the non-active condition which have been pushed away from the lumen by the more active cells. Such cell groups are not connected with the lumen of the gland by intercellular secretory canals. Peculiar irregular branching cells have been described, extending from the basement membrane in between the mucous cells. They are known as "basket" cells and are supposed to be supportive in character. The cells of both mucous and serous tubules rest upon a membrana propria, outside of which, separating the tubules, is a cellular connect- ive-tissue stroma. Of the small glands of the mouth, a group near the root of the tongue are of the mucous variety, some "lingual" glands in the region of the circumvallate papillae are serous, while the remainder are of the mixed type. Blood-vessels. — The larger vessels run mainly in the submucosa. The arteries of the submucosa give off one group of branches to the tunica propria, where they break up into a dense subepithelial capil- lary network, sending capillary loops into the papillae. A second group of arterial branches pass to the submucosa, where they give rise to capillary networks among the tubules of the mucous glands. From the capillaries veins arise which accompany the arteries. Lymphatics. — The larger lymph vessels he in the submucosa. These send smaller branches into the tunica propria, where they open into small lymph capillaries and spaces. Nerves. — MeduUated nerve fibres form plexuses in the submucosa THE DIGESTIVE SYSTEM 223 and deeper parts of the mucosa. From these plexuses, branches are given off which lose their medullary sheaths and form a second plexus of non-medullated fibres just beneath the epithelium. From this subepithelial plexus, branches pass in between the epithehal cells to terminate in end brushes or in tactile corpuscles. The nerves belong to the cerebro-spinal system, and are dendrites of sensory ganglion cells. Axones of sympathetic neurones are also present in the oral mucosa, destined mainly for the muscle-tissue of the blood-vessels. TECHNIC (i) The superficial cells of the oral mucous membrane may be prepared for examination as in technic i, page 63. (2) For the study of the mucous membrane of different parts of the mouth, fix smaU pieces in formalin-Miiller's fluid (technic 5, p. 7), cut sections perpen- dicular to the surface, stain with haematoxylin-eosin (technic i, p. 20), and mount in balsam. (3) Small mucous and serous glands of the mouth may be studied in the preceding sections. The Tongue The tongue is composed mainly of striated muscle fibres, sup- ported by connective tissue and covered by a mucous membrane. Fungiform papillae Fig. 129. — Surface View of Tongue showing filiform |),ii)ill;e and k 5 Fig. 137. — From Longitudinal Section of Crown of Human Premolar (X200) (Sobotta), showing junction of enamel and dentine. S, Enamel; D, dentine; Sp, enamel prisms; Dk, dental canals; Jg, interglobular spaces. A few dentinal fibres are seen passing beyond the limits of the dentine into the enamel. The obHque dark bands in the enamel are the lines of Retzius. tissue of the gum, to the alveolar wall or to the cementum of an adjacent tooth, and short fibres which fill in the interstices between the long fibres. Many of the long fibres are continued as calcified fibres into the cementum on the one hand and into the bone of the alveolus on the other. They are analogous to Sharpey's fibres of bone (p. 192). As they enter the cementum or bone, the fibres are grouped in bundles but in the central portion of the membrane these bundles break up and their fibres interlace in all directions. The long fibres differ in direction in different parts of the membrane. Those which spring from the cementum near its junction with enamel, pass out at right angles and then turn sharply toward the free margin of the gum which they support. Passing toward the apex, the next fibres instead of bending toward the free margin of the gum pass out at right angles and blend with the gum connective tissue. All fibres THE DIGESTIVE SYSTE:M 235 to the gums are more developed on the lingual than on the labial side. Between the teeth, fibres of this level pass from the cementum of one tooth to that of the next adjacent, here also supporting the free margin of the gum. Still further apically the fibres are grouped in coarser bundles to form the so-called dental ligament, and pass over or into the edge of the alveolus; between the teeth into the edge of the septum oi" into the cementum of the adjacent tooth. Frcm ihe dental ligament for about one-third the depth of the root, rather coarse bundles of fibres pass from cementum almost straight to alveolar wall. For most of the remaining two-thirds, th^ direction of the fibres is away from the apex, the bundles tending to break up and to radiate to a more extended inser- tion into the alveolar bone. The apical fibres also radiate to their insertion into bone. Among the connective-tissue fibres are found fixed connective-tissue cells, groups or cords of cells which are apparently epithelial, and, during development and sometimes sparingly later, osteoblasts, osteoclasts, and cementoblasts. The fixed connective-tissue cells are described on p. 83. The epithelioid cells are mostly arranged in cords which anastomose to form a network. The cells themselves are granular and have oval or round nuclei rich in chromatin. In some places their arrangement resembles tubules, but whether they have a glandular character is not known. Osteoblasts and osteoclasts are the same as in developing bone (p. 198). The latter are present wherever absorption is taking place. They are especially numerous during the absorption of the roots of the milk teeth to make way for the permanent set. Cementoblasts are cells which lie against the cementum and are analogous to the osteoblasts, from which however they differ morphologically. They are flattened and their protoplasm fills in the spaces between the ends of the fibres so that the cell bodies show the indentations of the fibres lying against them. From the cementoblasts processes extend into the cementum in much the same manner as processes of bone cells extend into their canaliculi. Blood-vessels. — The arteries which supply the tooth and peri- dental membrane, enter the apical portion of the latter from the ad- jacent bone of the alveolus (Fig. 134). On entering the membrane the vessels divide into two main sets one of whichpasses through _tl}£- -- foramina of the apex to supply the dental pulp, while the other passes along the outside of the tooth in the peridental membrane which it supplies (Fig. 134). The vessels in the membrane anastomose with vessels from the bone of the alveolar wall and at the margin of the alveolar cavity with vessels from the gums. From the capillary net- work veins follow the arteries back to the apical portion of the mem- brane and into the bone at the bottom of the alveolus. The arteries to the I)ulp run mainh' through its centre giving off brunches which iorm a capilhiry network whi( h is especially rich at the periphery of the pulp (Fig. 138). I-rom these capillaries arise veins which j)ass 236 THE ORGANS back through the apical foramina into the peridental membrane where they unite with veins from the membrane and pass into the bone of the alveolus. Enamel, cementum, and dentine are non-vascular. A marked feature of the pulp vessels is the thinness of their walls, ves- sels of much greater calibre than are usually classed as capillaries having walls of capillary thinness. Lymph Vessels. — Most author- ities have denied the existence of definite lymph channels in the pulp. Schweitzer on the other hand describes an arborization of small lymph vessels in the pulp of the crown, converging to a few larger lymph vessels in the root pulp and accompanying the blood- vessels through the foramina of the apex. Nerves. — The distribution of nerves to the tooth follows quite closely that of the blood-vessels. Bundles of medullated fibres from the bone at the bottom of the alveolus enter the peridental mem- brane just beneath the apex where they divide into two main sets, one of which follows the vessels of the membrane while the other passes with the vessels through the apical foramina to the dental pulp. The membrane surrounding the tooth also receives nerves from the bone of the side wall of the alveolus. The branches to the pulp pass up through its center, giving off branches which are mostly non-medullated and which radiate toward the periphery where they form a plexus in the layer of Weil just 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.^ ^Noyes calls attention to the extreme sensitiveness of dentine in spite of the fact that no nerve fibres have been demonstrated in its canals, and believes that sensation is carried to the nerve fibres through the processes (dentinal fibres) and cell bodies of the odontoblasts. Fig. 138.- -Diagram of Blood-vessels of Pulp. (Stowell). THE DIGESTIVE SYSTEM 237 /c■r^t•■^>°A■:V^•."■''' \i>jv" "^ ■■^V;'.'' Fig. 139. Fig. 140. V'"'.!'.- ' , , . , . .'■. . / f-^^- Fig. 141. Fig. 142. Figs. 139, 140, 141, 142.— Four Stages in the Development of a Tooth (from lower jaw of sheep embryo;. (Bohm-DavidolT.) Fig. 139, Beginning of enamel organ show- ing connection with epithelium of mouth; Fig. 140, Later stage showing same with first trace of papilla; Fig. 141, Later stage showing papilla well formed, the dilTercnlialion of the enamel i>ulp and of the inner and outer enamel cells can l)e seen; odontoblast appearing along periphery of the pai)illa; Fig. 142, shows also beginning enamel organ of permanent tooth; Figs. 139, 140, Mi.Xiio, Fig. 142, X40- o. Ei)ithclium of mouth; h, its basal layer; c, suijcrficial cells of enamel organ; d, enamel pulp; />, dental papilla; 5, enamel cells; 0, odontoblasts; S, enamel organ of permanent tooth just beginning to diflerentiate; v, remains of enamel ledge of milk tooth; «, surrounding connective tissue. 238 THE ORGANS Development. — The enamel of the teeth is of ectodermic origin, the re- mainder of mesodermic. The earliest indication of tooth formation occurs about the seventh week of intra-uterine life (embryos 12 to 15 mm.). It consists in a dipping down of the epithelium covering the edge of the jaw into the underlying connective tissue (mesoderm) where it forms the dental shelf, or common dental germ. Soon after the formation of the dental shelf, a groove appears along the margin of the jaw where the ingrowth of epithelium occurred. This is known as the dental groove. The epithehum of the dental shelf is at first of uniform thickness. Soon, however, at intervals along the outer side of the dental shelf, the cells of the shelf undergo proliferation and form thickenings, ten in h .--- ('i e -- — :%—T- Fig. 143. — Developing Tooth from Three-and-one-half-months' Human Embryo. X65. (Szymonowicz.) a, Epithelium of gums; b, neck of enamel organ; c, dental germ of permanent tooth; d, bone of lower jaw; e, dental papilla; /, inner enamel cells; g, enamel pulp; h, outer enam.el cells. the upper and ten in the lower jaw, 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 side of each special dental germ there occurs about the end of the third month (embryos of about 40 mm.) an invagination of the underlying con- nective tissue. In the upper jaw the invagination takes place on the upper and inner side, in the lower jaw on the lower and inner side, of each dental germ. Each invagination forms a dental papilla (Fig. 141), over which the tissue of the THE DIGESTIVE SYSTEM 239 special dental germ forms a sort of cap, the latter being known from its sub- sequent function as the enamel organ, the papilla itself giving rise to the pulp and dentine. The dental germs are at this stage connected with each other by remaining portions of the dental shelf, and with the surface epithelium by remains of the original invagination. The next step is the almost complete separation of the special dental germs and ridge from the surface epithelium (Fig. 142), and the formation around each special dental germ of a vacsular membrane, the dental sac. The attenuated strand of epithelial cells, which Epithelium of mouth Enamel cells Dental sac Bone of jaw Blood-vessel Papilla Fig. 144. — Longitudinal Section of a Developing Tooth of a New-born Puppy. (Bonnett.) Late Stage. still maintains a connection between the dental germs and the epithelium of the gums, is known as the neck of the emanel 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. 143, c). Into the latter, connective-tissue papilla; extend as in the case of the milk teeth. There are thus present as early as the fifth month of foetal existence the germs of all milk and of some permanent teeth. The ENAMEL is formed by the enamel organ. At the stage represented in Fig. 145, it consists of three layers: (i) The outer enamel cells, somewhat fiat- 240 THE ORGANS tened; (2) the inner enamel cells, high columnar epithehum; (3) a layer of enamel pulp, situated between the other layers, and consisting of stellate anastomosing cells with considerable intercellular substance (Figs. 145 and 146). A mem- brane, the cuticular membrane, is first laid down between the inner enamel cells and the dentine. Each of the inner enamel cells now sends out a process, Tomes^ process, from its inner end. The processes are separated by a considerable amount of cement, and are the beginnings of the enamel prisons. Calcification now takes place both in the prisms and in the cement substance, beginning in the ends nearest Enamel Dentine | Enamel prisms Papilla ItoTT*' Cuticle Basal memb. of enam.el cells Fig. 145. — Section through Border of a Developing Tooth of a New-born Puppy. (Bonnett.) the papilla. As this proceeds outward, the prisms become much elongated and the cement substance reduced in amount. Further growth in thickness of enamel occurs by lengthening of the enamel prisms. During the formation of the enamel, the enamel pulp and the external enamel cells disappear. The formation of enamel in the milk teeth begins about the end of the fourth month and continues until the eruption of the teeth. The extent of the enamel organ is considerably greater than that of the enamel, the former covering the entire tooth, both root and crown, with the exception of the base of the papilla where the latter is connected with the underlying connective tissue. As the tooth develops,, the enamel organ disappears over the root, remaining to form the enamel only over that part of the tooth which is to be subsequently exposed. The function of the enamel pulp is not known. It disappears as the tooth grows. It has been suggested that it may furnish nutrition or serve as an avenue through which nutrition reaches the non-vascular enamel organ. It may serve as an area of least resistance through which the tooth grows. THE DIGESTIVE SYSTEM 241 a^l-a d® @ 0 ■f^'0®®#'s iiSStfai^itfiVa!f.JSliii>S&Bat&:fcs*^"^^^ The DENTINE is the first of the dental tissues to become hard. Both dentine and pulp develop, as noted on p. 238, from the mesoderm of the dental papilla. When the latter is first formed it is of the same structure as the surrounding mesoderm with which it is continuous, except that it is somewhat more dense; later it assumes more the character of embryonal connective tissue, blood-vessels and nerves growing into it from the underlying connective tissue. The most peripheral cells of the pulp, those lying nearest the enamel organ, differentiate from the rest of the pulp to form a single layer of columnar or pyramidal cells — the odontoblasts. The outer end of each cell is broad, while the inner end which contains the nucleus narrows to a point from which a dehcate process extends into the pulp and probably anastomoses with other cell processes. These cells are analogous to the osteoblasts of de- veloping bone and like them appear to determine the deposition of lime salts. The lime salts are first laid down in a membrane-like structure — the mem- bratia preformativa — which the odonto- blasts apparently form between them- selves and the enamel. From this membrane the transformation of the connective tissue into dentine progresses inward toward the pulp, additional dentine continuing to be laid down in layers, each new layer internal to the preceding. In this way the dental papilla is reduced in size to form the pulp cavity. In the outer part of the dentine spaces remain in which no lime salts are deposited. These are the in- terglobular spaces. As calcification pro- ceeds the odontoblasts do not become enclosed within the dentine as do the osteoblasts within bone. They leave merely long slender processes, the dental fibres, lying in minute channels through the dentine, dental canals, while the bodies of the cells form a single layer along the inner margin of the dentine. There are thus no lacunae and no cells within the dentine. This relation of odontoblasts to dentine, and probably the original odontoblasts, persist, not only throughout embryonic but through adult life. While the tooth lies within the gum, the somewhat condensed connective tissue which surrounds it constitutes the dental sac. As the germs of the milk teeth develop, the dental shelf broadens by extend- ing inward toward the tongue. Ahmg this inner margin appear the germs of the permanent teeth, the development of the various teeth structures from the germs being identical with the process described in the case of the milk teeth, 16 Fig. 146. — From Cross-section through a Developing Tooth. X 720. (Bohm and von Davidoff.) Note close relationship between odontoblasts and tissue of dental pulp, a, Dental pulp; b, odontoblasts; c, dentine; d, inner enamel cells; e, enamel pulp. 242 THE ORGANS Twenty of the permanent teeth correspond in position to the twenty milk teeth, whUe twelve new molar germs, six in the upper and six in the lower jaw, represent the true molars of the adult. The first permanent tooth germ to appear is that of the first molar, about the beginning of the fifth month (embryos of about i8o mm.). It lies just behind the second molar of the milk dentition. The germs of the incisors and canines appear about the end of the sixth month, those of the premolars, which replace the milk molars, in the beginning of the eighth month. The germs of the second and third (wisdom teeth) molars do not appear until after birth, those- of the former at about six months, of the latter during the fifth year. The CEMENTUM is developed by ossification of that part of the dental sac which covers the root, its development being similar to subperiosteal bone formation (p. 201), without the formation of Haversian systems. TECHNIC (i) Teeth are extremely difficult organs from which to obtain satisfactory material for study. Sections of hard (undecalcified) and of decalcified teeth may be prepared in the same manner as sections of bone— technics i, p. 196; 2, p. 197. The decalcified tooth should include if possible the alveolar margin of the jaw, so that in longitudinal sections the mode of implantation and the relation of the tooth to the surrounding structures can be seen. (2) For the study of developing teeth, embryo pigs, sheep, cats, dogs, etc., are suitable. For the early stages foetal pigs should be five to six inches long; for the intermediate, ten to twelve inches. The later stages are best obtained from a small new-born animal, e.g., kitten or small pup. The jaw — preferably the lower — or pieces of the jaw are fixed in formalin-Miiller's fluid (technic 5, p. 7), hardened in alcohol, and decalcified (page 10). Subsequent treatment is the same as for developing bone (technic i, p. 203). The Pharynx The wall of the pharynx consists of three coats — mucous, muscu- lar, and fibrous. I. The mucous membrane has a surface epithelium and an un- derlying stroma. The EPITHELIUM is stratified squamous except in the region of the posterior nares, where it is stratified columnar ciKated, continuous with the similar epithelium of the nasal mucosa. The STROMA, or tunica propria, consists of mixed fibrous and elastic tissue infiltrated with lymphoid cells. In certain regions these cells form distinct lymph nodules (see pharyngeal tonsils, page 180). Beneath the stratified squamous epithelium the stroma is thrown up into numerous low papillae. These are absent in regions THE DIGESTIVE SYSTE:M 243 covered by ciliated cells. Bounding the stroma externally is a strongly developed layer of longitudinal elastic fibres, the elastic limiting layer, which separates the stroma from the muscular coat and sends stout bands in between the muscle bundles of the latter. 2. The muscular coat hes beneath the elastic layer and is formed of very irregularly arranged muscle fibres belonging to the constrictor muscles of the pharynx. 3. The fibrous coat consists of a dense network of mixed fibrous and elastic tissue. It has no distinct external Hmit, and binds the pharynx to the surrounding structures. The distribution of blood-vessels, lymphatics, and nerves is similar to that in the oral mucosa. Small, branched, tubular, mucous glands are present in the stroma, and extend down into the intermuscular connective tissue. They are most numerous near the opening of the Eustachian tube. TECHNIC For the study of the structure of the walls of the pharynx, material should be prepared as in technic 2, p. 223. n. THE FOREGUT The CEsophagus The walls of the oesophagus are continuous with those of the phar- ynx and closely resemble the latter in structure. They consist of four layers, which from within outward are mucous, submucous, muscular, and fibrous (Fig. 147J. 1 . The mucous membrane resembles that of the pharynx except that beneath the stroma is a well-developed nmscularis mucosce com- posed of smooth muscle cells arranged longitudinally. The muscu- laris mucosae forms a complete coat only in the lower part of the cesophagus. The epithelium is stratified squamous and rests upon a papillatcd stroma. 2. The submucosa is composed of loosely arranged fibrous and elastic tissue. It contains mucous glands, the larger blood-vessels, lymj>hati(s, aiul ncr\'cs. 3. The Muscular Coat. In the uj>j)cr \H>rl.'um of the (X'soj)hagus this coat is composed of striated muscle fibres; in the middle j)orti()n, of mixed striated and smooth muscle. In the lower j)ortion there are 244 THE ORGANS two distinct layers of smooth muscle, an inner circular and an outer longitudinal. The latter is not continuous. 4. The fibrous coat consists of bundles of white fibrous tissue with many elastic fibres. It serves to connect the oesophagus with the surrounding structures. Two kinds of glands occur in the oesophagus. Fig. 147. — Transverse Section through Wall of Dog's (Esophagus. X18. (Bohra and von Davidoff.) a, EpitheHum; b, stroma; c, muscularis mucosse; d, submucosa; e, circular muscle layer; /, longitudinal muscle layer; g, fibrous layer. (i) Mucous Glands. — These are of the same structure as those of the tongue, but much smaller. They he in the submucosa and are distributed throughout the entire oesophagus, though most numerous in its upper third. The ducts pass obUquely downward on their way to the surface. Just before entering the muscularis mucosse the duct widens out to form a sort of ampulla. Beyond this it again becomes narrow and enters the epithelium in the depression between two adjacent papillee. A small lymph nodule is usually attached to the duct as it passes through the tunica propria. (2) Simple Branched Tubular Glands. — These resemble the glands of the cardiac end of the stomach, but branch much more THE DIGESTIVE SYSTE:M 245 profusely. Some contain both chief and acid cells, others only chief cells (see stomach, page 249) . They He in the tunica propria, and are for the most part confined to a narrow zone at the lower end of the oesophagus and to the level of the fifth tracheal ring. Scattered groups also occur in other regions. The distribution of blood-vessels, lymphatics, and nerves in the oesophagus is similar to their distribution in the mouth (p. 222). Be- tween the two muscular coats the nerve fibres form plexuses in which are many sympathetic nerve cells. These plexuses are analogous to the plexuses of ^Sleissner in the stomach and intestine. TECHNIC Remove a portion of the wall of the oesophagus, wash carefully in normal salt solution, and pin out, mucous-membrane side up, on a piece of cork. Fix in formalin-Miiller's fluid and harden in alcohol (technic 5, p. 7). Transverse or longitudinal sections should be cut through the entire thickness of the wall. If the details of the muscular coat are to be studied, sections from at least three different levels should be taken: one near the upper end, one at about the middle, and the other in the lower third. Stain with haematoxylin-eosin or hjematoxyhn-picro-acid-fuchsin (technic i, p. 20; 3, p. 21) and mount in balsam. General Structure of the Walls of the Gastro-intestinal Canal The walls of the stomach and intestines are made up of four coats (Fig. 148) . These from the lumen outward are mucous, submucous, muscular, and serous. I. The mucous membrane (Fig. 148) consists of surface epithe- lium, glands, stroma, and muscularis mucosae. The surface epithe- lium is simple columnar and rests upon a distinct basement membrane. The arrangement of the glands and the nature of the gland cells differ in different parts of the tract. The stroma is a loosely arranged, richly cellular connective tissue, which in many places is so infiltrated with lymphoid cells as to constitute diffuse lymphatic tissue. In other places it contains circumscribed masses of lymphatic tissue, lymph nodules. The amount of stroma depends upon the closeness with which the glands are packed. The muscularis mucosa consists of smooth muscle cells, which have a generally longitudinal arrange- ment. Where, however, the muscularis mucosae is thick there are usually two distinct layers — an inner circular and an outer longitudi- 246 THE ORGANS nal. Folds of considerable extent occur in the mucous membrane. Those of the stomach are known as rugm, and are not constant, depending upon the degree of distention of the organ. Those of the small intestine are much more definite, and are known as valvules conniventes. Fig. 148, — Diagram of Structure of Wall of Gastro-intestinal Canal. A, Mucous membrane; a, glands; b, epithelium; c, goblet cells; d, stroma; e, inner circular;/, outer longitudinal layers of g, muscularis mucosae. B, Submucosa. C, Muscular coat; h, its inner circular layer; j, its outer longitudinal layer; i, intermuscular connective- tissue septum. D, serous coat; k, its connective-tissue layer; /, its endothelial layer. 2. The submucosa (Fig. 148) is a loose connective-tissue stucture. It contains the larger blood-vessels, lymphatics, and nerves. 3. The muscular coat (Fig. 148) consists of two layers of smooth muscle, which in the intestine are sharply differentiated into an inner circular and an outer longitudinal. In the stomach the direction of the layers of the muscular coat is less definite. A narrow layer of THE DIGESTIVE SYSTEM 247 connective tissue separates the two layers of muscle. From this, septa extend into the muscle tissue, separating it into bundles. 4. The serous coat (Fig. 148) is the visceral layer of the perito- neum. It consists of a thin layer of connective tissue covered by a single layer of mesothelium. Along the attachment of the mesentery the serous coat is wanting. \, J) cd Fig. 149. — Section through Junction of Oisophagus and Stomach of Man. X121. (Schafer.) Oe, (Esophagus; M, stomach; cd, cardiac glands; wd, dilated ducts of cardiac glands; S, stroma; E, stratified squamous epithelium of oesophagus; mm, muscularis mucosae; cd, irregularly cut tubules of cardiac glands; dd, cardiac glands in lower end of the oesophagus; ii, limit of stratified oesophageal epithelium. The subdivisions of the gastro-intestinal canal dilTcr from one another mainly in regard to the structure of their mucous membranes, and especially in regard to the structure of the glands of the niucous membrane and submucosa. The Stomach At the junction of oesophagus and stomach there is an abrupt transition from the stratified squamous epil helium of the former with 248 THE ORGANS its smooth surface to the simple columnar epithelium of the latter with its elevations and depressions. In the deeper structures the line of demarcation is not so abrupt, the muscularis mucosae of the oesoph- agus being continuous with that of the stomach, and glands of the stomach type extending up under the stratified epitheUum' of the oesophagus (Fig. 149). I . The mucous membrane of the stomach is folded into ridges or rugcs, the height and number of which depend, as already noted, upon Fig. 1 50. — Outline Diagram of Stomach to showLocation of Different Kinds of Glands- c, Cardia; p, pylorus; vvvvvv, cardiac glands; , fundus glands; + + + ++, pyloric glands; 000000, intestinal type glands (of Lieberkuhn). (Jouvenal.) the degree of distention of the organ. The rugae are most prominent in the collapsed organ, almost absent when the organ is fully dis- tended. In addition to the rugae the entire mucous membrane is studded with minute depressions barely visible to the naked eye, the so-called gastric pits (Fig. 152, Mg). These mark the openings of the gastric glands. In the fundus they are comparatively shallow, ex- tending through about one-fifth the thickness of the mucosa; in the pylorus the pits are much deeper, extending through half or more of the thickness of the mucous membrane (compare Figs. 152 and 156). The Epithelium. — This is of the simple columnar type, covers the entire surface of the gastric mucosa and extends down into the pits (Fig. 152). The cells are of the high, clear, mucous type (Fig. 153, M and M'). The end of the cell toward the lumen is clear, usually THE DIGESTIVE SYSTE.M 249 consists mostly of mucus, and consequently stains lightly. There is no such distinct cuticle as in the intestine. The basal end of the cell contains the spheroidal, oval, or sometimes flattened nucleus, is granular, and takes a darker stain. The amount of mucus in the cell depends upon its functional condition. The cells rest upon a distinct basement membrane. The Gastric Glands.— Extending from the bottoms of the pits, their epithelium continuous with that of the pits themselves, are the gastric glands. These are of two main kinds, fundus or peptic glands, Gastric ^^^B::^lBHBhiB^ jS^HHESH^^^^^^B Gastric pits Fig. 151. — Surface View of Mucous Membrane of Stomach showing gastric pits (Spalteholz). distributed through the greater part of the gastric mucosa, and py- loric glands, confined to the immediate region of the pylorus. The fundus glands (Fig. 152) are simple, sometimes branched, tubular glands, of which from three to seven open into each gastric pit. They extend through the entire thickness of the stroma, to the muscularis mucosae. Each gland consists of (i) a mouth opening into the pit; (2) a constricted portion, the neck; (3) the body or main portion of the tubule; and (4) a slightly dilated and bent blind extremity, the fundus (Fig. 152). The mouth marks the transition from the higher epithelium of the pit to the low cuboidal of the neck (Fig. 153, h). In the body and fundus of the gland two types of cells are found: (a) chief cells (central, peptic, or adelomorphous), and {b) parietal cells facid, oxyntic, or delomorphous). The chief cells (Figs. 153, 154) are the more numerous. They are of the low columnar type, often pyramidal with apices directed toward the lumen. Their bases rest either on the basement membrane or 250 THE ORGANS against the parietal cells. The appearance which these cells present depends upon their functional condition (p. 272). They usually appear clear and granular and take a light stain. The parietal cells (Fig. 153,) are oval or polygonal in shape, and . lie here and there against the base- ment membrane. The nucleus is spherical, somewhat larger than that of the chief cell, and is usually situ- ated at the center of the cell. There may be two nuclei. The protoplasm is finely granular and in the fresh unstained condition appears clearer than that of the chief cells. The parietal cells stain intensely with the aniline dyes with the result that in Fig. 152. Fig. 153. Fjg 152.— Vertical Section through the Mucous Membrane of the Fundus of the Stomach. X8s. (KoUiker.) Mg, Gastric pits; h, neck; k, body; g, fundus of peptic glands; h, chief cells; h, parietal cells; w, muscularis mucosa. Fig. 153.— Cross-sections at Various Levels of Peptic Glands of Stomach. X400. (KoUiker.) M, Section through gastric pit near surface; M , section through gastric pit near bottom; h, mouth of gland; k, neck; g, body near fundus; a, chief ceUs; b, parietal cells, j stained "specimens the two kinds of "cells are in marked contrast, the parietal cells being much darker than the chief cells. Although lying against the basement membrane and frequently pushing it out so as THE DIGESTIVE SYSTEM 251 to form little protuberances beyond the even line of the gland tubule, the parietal cells always maintain a connection with the lumen. This is accomplished by means of little clefts between the chief cells, intercellular secretory tubules, which extend down to the parietal cells. By means of the method of Golgi may be demonstrated not only the intercellular secretory tubules, but also the fact that upon reaching the cells these are continuous with a network of minute spaces within the cell — the intracellular secretory tubules (Figs. 154 and 155). Parietal Fig. 154. — Sections through Different Parts of a Fundus Gland of Human Stomach. A, mouth of gland; cells resemble surface cells except that they are shorter. B, neck of gland; cp, principal cells; cpm, mucous cells; ch, parietal cells, one contain- ing two nuclei. C, middle portion of gland; cp, chief cells; ch, parietal cells. Z?, deeper part of gland; cp, chief cells containing secretory granules, and at their bases ergastoplasm filaments itrg): at c are shown intracellular secretory canals i)cne- trating base of cell; ch, parietal cells, one of which shows canaliculus leading to lumen of gland (\), the latter being cut twice in section owing to its irregular course. /s, blind end of gland; cp, chief cells with secretion granules and ergastoplasm fila- ments (erg); ch, parietal cells, one containing three nuclei; cpm, mucous cells; /, lumen of gland. X2S0. (Prenant.) cells are not distributed uniformly throughout the gland, but are most numerous in the body, where they frequently almost obscure the chief cells. In the fundus of the gland parietal cells are less nu- merous. For this reason and because of the wider lumen of the fundus, transverse and longitudinal sections of this part of the tubule are most satisfactory for the study of the relations of the two kinds of cells (Figs, 152 and 153). Mitosis is most active at the junction of 252 THE ORGANS the neck and body of the tubule which has consequently been desig- nated the "growing point" of the tubule. Lying near the basement membrane among the bases of the colum- nar epithelial cells are small spherical or irregular cells with dark nuclei. These are young epithelial cells which from their function are known as "replacing cells" (see page 73). The PYLORIC GLANDS (Figs. 156 and 157) are simple branched tubular glands, several of which open into each of the deep pyloric pits. The glands, though short, are quite tortuous, so that in sections the tubules are seen cut mainly transversely or obliquely. In most of the pyloric glands but one type of cell is found. These resemble the chief cells of the fundus, but present a more uniform appear- ance, probably due to the absence of parietal cells. As in the fundus, "re- placing cells" Ue between the bases of the columnar epitheUal cells. Parietal cells are not always entirely absent, but occur here and there in the pyloric tubules, especially near the fundus. The transition from fundus to pylorus is not abrupt, but is marked by a "transitional border zone," in which fundus and pyloric glands are inter- mingled, and in which are also found single glands which resemble in structure both cardiac and pyloric. In the transition zone between oesophagus and stomach are found glands which resemble those found in the lower end of the oesophagus (p. 244). Their cells are clear resembling those of the pyloric glands. Some of the tubules have parietal cells. They have been designated cardiac glands and pass over by gradual transition into the fundus glands. There are also found in the stomach glands which are appar- ently identical in structure with those of the intestine (p. 260) . Most of these resemble the glands of Lieberkiihn and are distributed in small groups mainly in the pylorus and lesser curvature. Glands resembling Brunner's glands have also been described. The STROMA (Figs. 152 and 156) -or tunica propria, in which the glands are embedded, consists of mixed fibrillar and reticular connective Fig. 155. — ^Longitudinal Section of Fundus of Gland from Pyloric End of Dog's Stomach. (Golgi method. See 5, p. 29.) a, Lumen of gland; h, intracellular canals in parietal cells; c, cut-off portion of parietal cell; d, chief cells; e, inter- cellular canals leading from lumen of_^gland to canals in parietal cells. THE DIGESTIVE SYSTE:\I 253 tissue infiltrated with lymphoid cells. In the fundus of the stomach the glands are so closely packed that the stroma is reduced to thin strands, which pass up between the glands and also separate them from the muscularis mucosae. In the pylorus the glands are more widely separated and the stroma is correspondingly greater in amount. In both fundus and pylorus thicker strands of stroma surround a num- My mm iySr^c^A^^-^^*^ mm mj^^ W^ f Fig. 156. Fig. 157. Fig. 1 56.— Vertical Section through Mucous Membrane of Pyloric End of Stomach. X85. (Kolliker.) Mg, Gastric pit; b, blood-vessel in stroma; d, longitudinal section of body of gland; m, muscularis mucosae. Fig. 157. — Pyloric Gland from Vertical Section through Wall of Dog's Stomach. (Ebstein.j m, Gastric pit in which are seen some transversely cut cells; n, neck of gland; /, fundus cut transversely. her of ghmd tubules, thus separating them into more or less well- defined groups. In addition to the diffuse lymphatic tissue of the stroma, closely packed aggregations of lymphoid cells are found in the shaj)e of distinct nodules, known as "solitary follicles." These occur throughout the entire gastric mucosa, but are most numerous in the pylorus. The nodules are usually egg-shaped, their apices 254 THE ORGANS lying just beneath the epithelium, their bases resting upon the muscu- laris mucosae. Less commonly they lie partly in the submucosa. Over the nodules the epithelium is more or less infiltrated with migra- tory leucocytes. Most of the nodules contain germinal centres around which the lymphoid cells are more closely packed than else- where (see page 169). The MUSCULARis MUCOSA (Figs. 152 and 156, m) may consist of a single layer of smooth muscle with cells arranged longitudinally or obliquely, or there may be two distinct layers, an inner circular and an outer longitudinal. From the muscularis mucosae single cells and groups of cells extend into the stroma between the gland tubules. 2. The submucosa consists of connective tissue, loosely arranged, many elastic fibres, and some fat. It contains the larger blood- vessels and nerves, including the plexus of Meissner (p. 271). 3. The muscular coat is usually described as consisting of three layers, an inner oblique, a middle circular, and an outer longitudinal. In the fundus the muscle bundles run in various directions, so that the division of the muscular coat into layers having definite directions .can be made out only in the pylorus. Here the inner and middle layers are thickened to form the sphincter pylori. In the connective tissue which separates the groups of muscle cells are collections of sympathetic nerve cells and fibres, which while much less distinct, are analogous to Auerbach's plexus of the intestine. 4. The serous coat consists of a layer of loosely arranged connec- tive tissue covered by a single layer of mesothelium. TECHNIC (i) Remove a human stomach (not more than two or three hours after death) or that of a recently killed dog. Open along the lesser curvature, and carefully remove the excess of mucus by washing with normal saline. Cut pieces through the entire thickness of the wall, one from the fundus and one from the pylorus; pin out, mucous membrane side up, on pieces of cork, fix in formalin-Miiller's fluid (technic 5, p. 7) or in Zenker's fluid (technic 9, p. 8), and harden in alcohol. Sections are cut as thin as possible, care being taken that the plane is such that the glands are cut longitudinally, stained with hsematoxylin-eosin (technic i, p. 20), and mounted in balsam. (2) Instead of removing pieces of stomach and pinning them out on cork, as suggested in the preceding technic, the entire stomach may be filled with the fixative, the ends being tied, and then placed in a large quantity of the fixing fluid. After fixation, pieces are removed and hardened in graded alcohols. If this method is used, great care must be taken not to overdistend the organ, only THE DIGESTIVE SYSTEM 255 very moderate distention being desirable. Further treatment is the same as in the preceding technic (i). (3) For comparison of resting with active gastric cells, preparations should be made from the stomach of an animal that has been for from twenty-four to forty- eight hours without food, and from a stomach during active digestion. Fix in Zenker's fluid as in technic (i), above. Examine unstained sections and sections stained with ha?matoxylin-eosin. (4) Sections through the junction of oesophagus and stomach and through the junction of stomach and duodenum furnish instructive pictures. They should be prepared as in technic (i). (5) For the study of the distribution of the blood-vessels sections of an injected stomach should be made. This is best accomplished by selecting a small animal, such as a rat or guinea-pig, and injecting in toto through the as- cending aorta, or by injecting only the hind part of the animal through the ab- dominal aorta. Technic, p. 25. III. THE MIDGUT The Small Intestine On passing from stomach to small intestine the rugae of the former disappear, but are replaced by much more definite foldings of the mucosa, the valvulcB conniventes (Fig. 159). These folds involve the entire thickness of the mucous membrane and part of the submucosa. They are in general parallel to one another, and pass in a circular or obUque manner, partly around the lumen of the gut. The entire surface of the intestine, including the valvula), is studded with minute projections just visible to the naked eye, and known as villi (Figs. 160 and 161). These involve only the epithelium and stroma, although they also contain some muscular elements derived from the muscularis mucosae. The viUi differ in shape in the different parts of the small intestine, being leaf-shaped in the duodenum, rounded in the jejunum, club-shaped in the ileum. The valvulai conniventes and the viUi are characteristic of the small intestine. It is important to note that while the pits of the stomach are depressions in the inucous membrane, the intestinal vilh* are definite projections above its general surface (Fig. 158). The wall of the intestine consists of the same four coats described as constituting the wall of the stomach, mucosa, submucosa, muscu- laris, anrl serosa. 1. The mucosa, a.-, in the stomach, is com])oseJ^» 0 ^-^ ■ ■» ft Fig. 181.— Section of Human Sublingual Gland. X2S2. (Stohr.) a, Excretory duct; h, lumina of serous and mucous tubules; c, mucous tubule; d, demilune; e, serous tubule; /, cross section mucous tubule; g, interstitial connective tissue. 280 THE ORGANS The l3miphatics begin as minute capillaries in the connective tissue 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, and consist of both medullated and non-medullated fibres. The medullated fibres are afferent, probably the dendrites of cells located in the geniculate ganglion. Small bundles of these fibres accompany the ducts. Single fibres leave the Fig. 182. — Section of Human Submaxillary Gland. X252. (Stohr.) a, Mucous tubule; b, serous tubule; c, intermediate tubule; d, "secretory" tubule; e, demilune;/, lumen; g, interstitial connective tissue. bundles, lose their medullary sheaths, and form a non-medullated subepithehal plexus, from which delicate fibrils pass to end freely among the epithelial cells. Efferent impulses reach the gland through the sympathetic. The fibres are axones of cells situated in small peripheral ganglia; the cells sending axones to the submaxillary lying upon the main excretory duct and some of its larger branches; those sending axones to the sublingual being situated in a small gang- lion— the sublingual — lying in the triangular area bounded by the chorda tympani, the lingual nerve, and Wharton's duct; those sup- plying the parotid probably being in the otic ganglion. Axones from these cells enter the glands with the excretory duct and follow its branchings to the terminal tubules, where they form plexuses THE DIGESTIVE SYSTEM 281 beneath the epitheHum. From these, terminals pass to the secreting cells. It is probable that the saHvary glands also receive sympathetic fibres from cells of the superior cervical ganglia. TECHNIC (i) The salivar)^ glands should be fixed in Flemming's fluid (technic 7, p. 7), or in formalin- JNIiiller's fluid (technic 5, p. 7). Sections are cut as thin as possi- ble, stained with haematoxylin-eosin (technic i, p. 20), 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. 8). Examine some sections unstained and mounted in glycerin, others stained with hasma- toxylin-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 solution, 4 volumes; i-per-cent. osmic acid, i volume), and then transferred with- out washing to a o.7S-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 j)ancreas 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 liuman liver. Rarely the human pancreas is distinctly l()])ulated. The gland has a main excretory duct, the pancreatic duct or dud of Wirsung. In many cases there is also a secondary excretory dud, 282 THE ORGANS 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 ofif 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. 183). The excretory ducts are lined with a simple high columnar epitheHum which rests upon a basement membrane. Outside of this is a connective-tissue coat, the thickness of which is directly proportionate to the size of the duct. In the pancreatic duct goblet cells are present, and the accompanying connective tissue of the main duct and of its larger branches contains small mucous glands. As the ducts decrease in size, the epitheHum be- comes lower until the intermediate tubule is reached where it becomes fiat. The terminal tubules themselves are most of them very short, frequently almost spher- ical. This and the fact that several terminal tubules are given off from the end of each in- termediate 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. 184 and 185). The appearance of these cells depends upon their functional condition. Each cell consists of a central zone bordering the lumen, which contains numerous granules known as zymogen granules, and of a peripheral zone next to the basement membrane, which is homogeneous and contains the nucleus (Fig. 185). The zymogen granules are quite large granules and as they are highly refractive stand out distinctly even in the fresh, unstained condition and under low magnification. The relative size of these zones depends upon whether the cell is in the active or resting state (compare Fig. 186, A and B). During rest (fasting) the two zones are of about equal size. During the early c Fig. 183. — Diagram to illustrate Structure of Pan- creas. (Stohr.) a, Excretory duct; b, intermediate tubule; c, c, terminal tubules. THE DIGESTIVE SYSTEM 283 stages of activity (intestinal digestion) the granules largely disappear and the clear zone occupies almost the entire cell. During the height of ^digestion the granules are increased in number to such an extent ^^f^^^^ ^^SA Fig. 184. — Section of Human Pancreas. X112. (Kolliker.) af, Alveoli; a, inter- lobular duct surrounded by interlobular connective tissue; L, islands of Langerhans; -a, small vein. that they almost fill the cell, while after prolonged secretion they are again almost absent. The cell now returns to the resting state in which the two zones are about equal. The increase and disappear- ance 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 zymogen granules are the intracellular representatives of the secretion of the gland. In sections of the gland there are seen within the lumina of many of the secret- ing tubules one or more small cells of which little but the nucleus can usually be made out. These cells lie in contact with the secreting cells, and resemble the flat cells which line the intermediate tubule. They arc known as the centro-acinar icenlro-luhular) cells of Langerhans (Fig. 185, c). Fig. 185. — From Section of Human Pancreas. X700. (KoUiicer.) a, Gland cell; 6, basement membrane; s, intermediate tubule; c, centro- acinar cells; sk, intracellular secretory tubule. 284 THE ORGANS Their significance is not definitely known. Langerhans believed that they were derived from the intermediate tubule, the epithe- lium of which, instead of directly joining that of the terminal Fig. i86. — 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 (b) being much larger; outlines of cells and of lumen much more distinct. tubule as in the submaxillary gland, was continued over into the lumen of the terminal tubule (Fig. 185). This interpretation has been quite generally accepted. Cells which differ from the secreting cells are frequently found Fig. 187. — Sections through AlveoU of Human Pancreas — Golgi Method — (Dogiel), to show intracellular secretory tubules, a, Intermediate tubule giving off several terminal tubules, from which pass off minute intracellular secretory tubules; b, gland cells lining terminal tubules. 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 secreting THE DIGESTIVE SYSTEjM 285 cells are minute intracellular secretory tubules. These are demonstra- ble only by special methods (Golgi) (Fig. 187). The pancreas also contains pecuHar groups of cells, the cell-islands of Langerhans, having a diameter from 200 to 300/^ (Figs. 184, 188, and 189). The "island" cells differ quite markedly both in arrange- ment and structure from those which line the terminal tubules (Fig. 188), They contain no zymogen granules. They are arranged in anastomosing cords or strands which are separated from one another by capillaries. There are no ducts and the method of Golgi shows no inter- or intra-cellular secretory tubules. Their protoplasm is un- © ,^- FiG. 188. — Island of Langerhans and few surrounding Pancreatic Tubules. (Bohm and von DavidofT.) a, Capillary; b, tubule. stained by basic dyes, but stains homogeneously with acid dyes. Their nuclei vary greatly in 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 connec- tive tissue containing a few elastic fibres (Fig. 188). Others blend with the surrounding tis.sucs. The origin, structure, and function of these islands have been subjects of much controversy. For some time they were considered of lymjjhoid origin. They are now believed to be epithelial tells having a develo[)mental history similar to the cells lining the secreting lubules. Each cell-island consists of, 286 THE ORGANS in addition to the cells, a tuft or glomerulus of broad tortuous anastomosing capillaries, which arise from the network of capillaries surrounding the secret- ing 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 histology of the pancreas and adds the results of his own careful researches. He concludes that the cell-islands of Langerhans are definite structures "formed in embryological life," that "they possess an anatomical identity as definite as the glomeruli of the kidney or the Malpighian body of the spleen, and that they subserve some Fig. 189. — From Section of Pancreas, the blood-vessels of which had been injected (Ktihne and Lea), showing island of Langerhans with injected blood-vessels, surrounded by sections of tubules. Zymogen granules are distinct in inner ends of cells. 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 parathyreoid bodies. From his study of the pancreas in diabetes, Opie concludes that the islands of Langerhans are concerned in carbohydrate metabolism. 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 axones of cells in sympathetic gangHa, outside the pancreas; others, of cells situated in small ganglia within the substance of the gland. They pass to plexuses among the secreting tubules, to which and to the walls of the vessels they send dehcate terminal fibrils. THE DIGESTIVE SYSTE:M 287 TECHNIC (i) The general technic for the pancreas is the same as for the salivary- glands (page 281). (2) Zymogen granules may be demonstrated by fixation in formalin-Miil- ler's fluid (technic 5, p. 7), and staining with picro-acid-fuchsin (technic 2, p. 20), or with Heidenhain's iron haematoxylin (technic 3, p. 18). (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 25) . The Liver The Hver 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. Fic. 190. — Section of Lobule of Pig's Liver X60 (technic i, p. 295), showing lohulc completely surrounded by connective tissue, a, Portal vein; b, bile duct; c, hepatic artery; d, [wirtal canal; e, capillaries;/, central vein; /j, cords of liver cells; //, hepatic vein. The liver is surrounded by a connective-tissue capsule, the capsule oJGlisson. At the liilurn tliis capsule extends deep into the substance of the hver, giving off broad connective-tissue sepia, which divide the organ into lobes. I*>om the capsule and from these interlobar septa, 288 THE ORGANS trabeculae pass into the lobes, subdividing them into lobules. In some animals, as for example the pig, each lobule is completely invested by connective tissue (Fig. 190). In man, only islands of connective tissue are found, usually at points where three or more lobules meet (Fig. 191). The lobules are cylindrical or irregularly polyhedral in shape, about i mm. in breadth and 2 mm. in length. Excepting just^beneath the capsule, where they are frequently arranged with B P H I, ' -'. ' - '-, '* ** ^^\ •"> « '\V /l ^/- °° ^'.''/''°°/<' ' « i' •" \f'"--^ , i* .«? Fig. 191. — Section of Human Liver. X80. (Hendrickson.) P, Portal vein; R, hepatic artery; B, bile duct. P, H, B constitute the portal canal and lie in the connec- tive tissue between the lobules. their apices toward the surface, the liver lobules have an irregular arrangement. The lobule (Fig. 190) 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-vessels of the liver must therefore be first considered. The BLOOD SUPPLY of the liver is peculiar in that in addition to the ordinary arterial supply and venous return, which all organs possess, THE DIGESTIVE SYSTEM 289 the liver receives venous blood in large quantities through the portal vein. There are thus hco 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 network. These intralobular capillaries all con- verge toward the center of the lobule, where they empty into the central vein (Fig. 190). The central veins are the smallest radicles of the hepatic veins, which are the efferent vessels of the hver. Each central vein begins 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 constantly 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 (Fig. 198). The hepatic artery accompanies the portal vein, following the branchings of the latter through the interlobar and interlobular con- nective tissue, where its finer twigs break up into capillary networks. Some of these capillaries empty into the smaller branches of the portal vein; others enter the lobules and anastomose with the intralobular portal capillaries. 'J'he M/\iN p:xcretory Duci'—hepatic duct — leaves the liver at the hilum near the entrance of the portal vein and hepatic artery. Within the liver the duct divides and subdivides, giving off interlobar, and these in turn interlobular branches. These ramify in the connective tissue, where they always accompany the branches of the jjortal vein Fig. 192. — Portal Canal. X315. (Klein and Smith.) a, Hepatic artery; F, portal vein; 6, bile duct. 290 THE ORGANS and hepatic artery. These three structures — the hepatic artery, the portal vein, and the bile duct, which always occur together in the con- nective tissue which marks the point of separation of three or more lobules — together constitute the portal canal (Fig. 192). From the interlobular ducts short branches pass to the surfaces of the lobules. From these are given off extremely narrow tubules, which enter the lobule as intralobular secreting tubules. The walls of the ducts consist of a single layer of epithelial cells resting upon a basement membrane and surrounded by connective Fig. 193. — Part of Lobule of Human Liver, showing capillaries and anastomosing cords of liver cells. X350. a, Liver cells; b, capillaries. tissue (Fig. 192). The height of the epithelium and the amount of connective tissue are directly proportionate to the size of the duct. In the largest ducts there are usually a few scattered smooth muscle cells. The walls of the secreting tubules are formed by the liver cells. The LIVER CELLS (Fig. 193) are irregularly polyhedral in shape. They have a granular protoplasm which frequently contains gly- cogen, pigment granules, and droplets of fat and bile. Each cell contains one or more spherical nuclei. Like other gland cells, the granularity of the protoplasm depends upon its functional condition. Within the cells are minute irregular canals, some of which can be THE DIGESTIVE SYSTE.M 291 injected through the blood-vessels, while others are apparently continuous with the secreting tubules (Fig. 195, A and B). The capillaries of the portal vein, as they anastomose and con- verge from the periphery to the centre of the lobule, form long-meshed Fig. 194. — Part of Lobule of Human Liver, Golgi Method (technic 3, p. 295), to show relations of bile duct to intralobular secretory tubules and of the latter to the liver cells, a, Bile duct; b, cords of liver cells; c, blood capillaries; d, central vein; e, secretory- tubules. capillary networks. In the meshes of this network lie the anasto- mosing secreting tubules. On account of the shape of the capillary network, the liver cells, which form the walls of these tubules, are Fig. 195. — A, Cell from human liver showing intracellular canals (Browicz); c,, intracellular canal; n, nucleus. Ji, From section of rabbit's liver injected 'jirough portal" vein, showing intracellular canals (continuous with inten cllular blood ca|)illaries). (Schafer.) arranged in anastomosing rows or ( ords, known as hcpalic cords or cords of liver cells (Fig. 193). The secreting tubules (Fig. 194) are extremely minute channels, the walls of which are the liver cells. A secretory lubulc always runs, 292 THE ORGANS between two contiguous liver cells, in each of which a groove is formed. The Uood capillaries, on the other hand, are found at the corners where three or more Uver 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. 195, A). At the surface of the lobula.there is a modification of some of Fig. 196. — Liver Lobule, to show Connective-tissue Framework. (Mall.) the liver cells to a low cuboidal type, and these become continuous with the lining cells of the smallest bile ducts, the secretory tubule being continuous with the duct lumen. 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. 196). Special technical methods also show the presence of stellate cells — cells of Kupfer — within the lobule. These are interpreted THE DIGESTIVE SYSTE^NI 293 by Kupflfer as belonging to the endothelium of the intralobular capillaries. Comparing the Uver ^Nath other compound tubular glands, it is seen to present certain marked peculiarities which distinguish it and which make its structure as a compound tubular gland difficult to understand. The most important of these are the following: (Figs. 197 and 198). The extremely small amount of connective tissue; in the human liver not enough interlobular connective tissue to outhne the lobules, _ e Fig. 197. Fig. iq8. Fig. 197. — Scheme of an Ordinary Compound Tubular Gland. In lobule 3 only the ramifications of the excretory duct, without endpieces, arc shown. (Stohr.) a, Branches of excretory duct; b, artery; c, vein; d, terminal tubules; e, capillaries. Fig. 198. — Scheme of Liver. In lobule r, only the direction of the endpieces is shown; in lobule 2 only their branching; in 3 only the excretory ducts. (Stohr.) a, Branches of excretory duct; b, i)ortal vein; c, terminal tubules (hepatic cords); d, capillaries; e, vein (central and sublobular). while intralobular connective tissue demon.strable by ordinary stain- ing methods is wholly absent. There is thus no connective tissue seen separating the cells of one tul)ulc from those of another as, for example, in such a gland as the submaxillary. The result is that cells of neighboring tubules lie sicle by side, and back to back as it were, with no intervening connective tissue. The fact that unlike the tubules of other glands, the liver tubule con- sists of only two rows of cells, between which lies the lumen. The latter is thus never in touch with more than two cells. 294 THE ORGANS The end-to-end anastomosis of the secreting tubules, there being jio true terminal tubules; anastomosis of neighboring tubules by tneans of side branches ; the arrangement of the bile capillaries in such a manner that a single liver cell abuts upon more than one capillary. The more intimate relation of the liver cell to the blood capillaries . Thus most gland cells have one side on the lumen, one side only in contact with a capillary blood-vessel, the remaining sides being in contact with other cells of the same tubule. A hver cell, on the other hand, may and usually does come in contact with several blood capillaries. The arrangement of both blood-vessels and tubules within the lobule. In the submaxillary, for example, the terminal tubules are con- voluted and run in all directions. In the liver the terminal tubules are straight and run in a definite direction from the periphery of the lobule toward the centre. Again, while in other glands both intra- lobular arteries and ducts are distributed outward from the centre of the lobule, and the blood is returned through veins which pass to the periphery of the lobule, in the liver the interlobular ducts pass to the periphery of the lobule and give off secreting tubules which pass in toward the centre of the lobule. The afferent vessels also (portal veins) take the blood to the periphery of the lobule and distribute it to a capillary network which converges to an efferent vessel (hepatic vein) at the centre of the lobule. The veins are also pecu- liar in that they do not follow the arteries in leaving the liver but pursue an entirely independent course. Blood-vessels. — These have been already described. Lymph vessels form a network in the hver 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 hepatic, 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 epithehum resting upon a basement membrane THE DIGESTIVE SYSTEM 295 and a stroma which contains smooth muscle cells and small mucous glands. The suhmucosa is a thin layer of connective tissue. Hen- drickson describes the muscular coat as consisting 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 common duct, there are thickenings of the circular fibres to form sphincters. In the cystic duct occur folds of the mucosa — the Heisterian valve — into which the muscularis extends. The Gall-Bladder The wall of the gall-bladder consists of three coats — mucous, muscular, and serous. The mucous membrane is thrown up into small folds or rugcs, which anastomose and give the mucous surface a reticular appearance. The epithehum 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 pig's liver in formalin-Muller's fluid (technic 5, p. 7). Cut sections near and parallel to the surface. Stain with ha^m- aloxylin-picro-acid-fuchsin (technic 3, p. 21) and mount in balsam. In the pig's liver the lobules arc completely outlined by connective tissue and the yellow picric-acid-stained lobules are in sharp contrast with the red fuchsin-staincd connective tissue. (2) For the study of the human liver treat small pieces of perfectly fresh tissue in the same manner as the preceding, but slain with hiumatoxylin-eosin (technic i, p. 20). (3) The secretory tubules and smaller bile ducts may be demonstrated by technic, 5. p. 2q. .\ light eosin stain brings oul 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 convciiicnl to 296 THE ORGANS inject from the heart directly through the right auricle into the vena cava. Sections should be rather thick and may be stained with eosin, or even lightly with h^matoxyhn-eosin (technic i, p. 20), and mounted in balsam. (5) For demonstrating the intralobular connective tissue, Oppel recommends fixing fresh tissue in alcohol, placing for twenty-four hours in a 0.5-per-cent. aqueous solution of yellow chromate of potassium, washing in very dilute silver nitrate solution (a few drops of 0.75-per-cent. solution to 50 c.c. of water) and then transferring to 0.75-per-cent. silver nitrate solution, where it remains for twenty-four hours. Embed quickly in celloidin. The best tissue is usually found near the surfaces of the blocks. A similar result is obtained by fixing fresh^ tissue in o . 5-per-cent. chromic-acid solution for three days, then trans- ferring to o . 5-per-cent. silver nitrate solution for two days. V Development or the Digestive System In the development of the digestive system all the layers of the blastoderm are involved. Mesoderm and entoderm are, however, the layers most con- cerned, as the ectoderm is used only in the formation of the oral and anal orifices. The primitive alimentary canal is formed by two folds which grow out from the ventral surface of the embryo and unite to form a canal, in a manner that is quite similar to the formation of the neural canal. In this way the primitive gut is lined with cells which previously formed the ventral surface of the embryo, i.e., entoderm. A portion of the mesoderm accompanies the entoderm in the formation of the folds. This is known as the visceral layer of the mesoderm. The primary gut is thus a closed sac or tube. It is connected with the umbilical vesicle, but has no connection with the exterior. These connections are formed later by oral and anal invaginations of ectoderm which extend inward and open up into the ends of the hitherto imperforate gut. The ends of the ahmentary tract, including the oral cavity and all of the glands and other structures connected with it, are of ectodermic origin. The epithelial lining of the gut and the parenchyma of all glands connected with it are derived from entoderm. The muscle, the connective tissue, and the mesothelium of the serosa are developed from mesoderm. The mesodermic elements show little variation throughout the gut, the peculiarities of the several anatomical divisions of the latter being dependent mainly on special differentiation of the entoderm (epithelium). Beneath the entodermic cells is a narrow layer of loosely arranged tissue which later separates into stroma, muscularis mucosae, and submucosa. Outside of this a broader mesodermic band of firmer structure represents the future muscularis. The stomach first appears as a spindle-shaped dilatation about the end of the first month. Its entodermic cells, which had consisted of a single layer, increase in number and arrange themselves in short cylindrical groups. These are the first traces of tubular glands. They increase in length and extend downward into the mesodermic tissue. For a time the cells lining the peptic glands are all apparently alike, but at about the fourth month the differentia- tion into chief cells and parietal cells takes place. In the intestines a proliferation of the epithelium and of the underlying THE DIGESTIVE SYSTEM 297 stroma results in the formation of villi. These appear about the tenth week, in both small and large intestines. In the former they increase in size, while in the latter they atrophy and ultimately disappear. The simple tubular glands of the intestines develop in a manner similar to those of the stomach. The mesothelium of the serosa is derived from the mesodermic cells of the primitive body cavity. The development of the larger glands, connected with the digestive tract, takes place in a manner similar to the formation of the simple tubular glands. All originate in extensions downward of entodermic cords into the underlying mesodermic tissue. From the lower ends of these cords, branches extend in all directions to form the complex systems of tubules found in the compound glands. The salivary glands being developed from the oral cavity, originate in similar invaginations of ectodermic tissue. The pancreas originates as three separate evaginations from the entoderm of the future duodenum. Of these, one atrophies, while the other two unite to develop the adult pancreas. The two evaginations account for the two pancreatic ducts. The evaginations take place into the underlying mesenchyme, which develops the connective-tissue framework of the organ. The growths are at first solid cords of cells which later become hollowed out to form tubules and differentiate chief and centro-acinar cells, the former early showing zymogen granules. Increase in size of the gland is accomplished by continuous budding and extension of tubules. In some of the tubules the epithelial cells become darker and arranged as a wall around a mass of developing blood cells. These structures are known as primary islands. Neither their function nor subsequent history is known. Later in development some of the gland tubules lose their lumina, the zymogen granules disappear from their cells and the whole becomes transformed into a secondary island or island of Langerhans of the adult pancreas. According to Lageusse a transformation of islands into tubules also occurs, and this process of transformation of tubules into islands and islands into tubules persists throughout life. Claude on the other hand derives both primitive islands and islands of Langerhans from the mesenchyme. Recent observations tend to support the former of these views. The liver originates as a downgrowth of the entoderm of the ventral wall of the future duodenum into the mesoderm of the transverse septum. At its cephalic end the downgrowth is solid and gives rise to the liver; at the caudal end it is hollow and gives rise to the gall-bladder. As the evagination increases in size it becomes almost completely separated from the intestine, the slender connection which does remain becoming the ductus cholcdochus. Between the latter and the liver anlage a slender connection also remains, the cystic duct. The mesenchyme surrounding these structures develops their connective-tissue framework. As the liver outgrowths develop they come into relation with the omphalomesenteric veins in such a manner that the vessels are broken up into an anastomosing network of smaller vessels, while the liver cells dcvelof) into anastomosing hepatic cylinders. These resemble the tubules of other glands in that the walls are formed by many cells. The manner in which these are trans- formed into the slender hepatic cords of the adult liver is not known. The 298 THE ORGANS change begins to take place about the time of birth in man. The breaking up of the larger vessels into smaller by ingrowth of the cords of liver cells gives rise to the so-called sinusoidal circulation. General References for Further Study Oppel: Lehrbuch der vergleichenden mikroskopischen Anatomie. KoUiker: Handbuch der Gewebelehre des Menschen. Opie: The Pancreas. Stohr: Salivary Glands, in Text-book of Histology. CHAPTER VII THE RESPIRATORY SYSTEM The respiratory apparatus consists of a system of passages — nares, larynx, trachea, and bronchi, which serve for the transmission of air to and from the essential organ of respiration, the lungs. The Nares The nares. or nasal passages, are divided into vestibular, respira- tory, and olfactory regions, the differentiation depending mainly upon the structure of their mucous membranes. The VESTIBULAR REGION marks the transition between skin and mucous membrane (page 220). Its epithehum is of the stratified squamous variety and rests upon a basement membrane, which is thrown into folds by papillae of the underlying stroma. The latter is richly cellular, and contains sebaceous glands (page 391) and the fol- licles of the nasal hairs. The RESPIRATORY REGION is much larger than both the vestibular and olfactory regions. Its epithelium is of the stratified columnar variety. The cells of the surface layer are ciliated and are inter- spersed with goblet cells. The stroma is distinguished by its thick- ness (3 to 5 mm. over the inferior turbinates) and by the presence of networks of such large veins that the tissue closely resembles erectile tissue. It contains considerable diffuse lymphoid tissue and here and there small lymph nodules. In the stroma are small simple tubular glands lined with both serous and mucous cells. There is no sub- mucosa, the stroma being connected directly with the periosteum and perichondrium of the nasal bones and cartilages. The mucous membrane of the accessory nasal sinuses is similar in structure to that of the respiratory region of the nares, Init is Ihimur and contains fewer glands. The (OLFACTORY REGION Can be distinguished with the naked eye by its brownish-yellow color, in contrast with the reddi.sh tint of the surrounding respiratory mucosa. The ej)itli("liiini is of the stratilK-d 2H\) 300 THE ORGANS columnar type, and is considerably thicker than that of the respira- tory region. The surface cells are of two kinds: (i) sus tentacular cells, and (2) olfactory cells. (i) The sustentacular cells are the more numerous. Each cell consists of three parts: {a) A superficial portion, which is broad and cylindrical, and contains pigment, and granules arranged in longi- tudinal rows. The cells have well-marked, striated, thickened free borders, which unite to form the so-called membrana limitans olfactoria. (b) A middle portion which contains an oval nucleus. As the nuclei of these cells all he in the same plane, they form a distinct narrow band, which is known as the zone of oval nuclei, (c) A thin filament- ous 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 apparently anastomoses with processes of other cells to form a sort of proto- plasmic reticulum. (2) The olfactory cells Ue 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 sev- eral minute hair-Hke 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 426). 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 elastic fibres, and connective-tissue cells. Embedded in the stroma are large numbers of simple branched tubular glands, the glands of Bow- man. Each tubule consists of a duct, a body, and a fundus. The secreting cells are large and irregular and contain a yellowish pigment, 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 fre- quently extend beyond the limits of the olfactory region. THE RESPIRATORY SYSTEM 301 The Larynx The larynx consists essentially of a group of cartilages united by strong fibrous bands and Hned by mucous membrane. The epithelium covering the true vocal cords, the laryngeal sur- face of the epiglottis, and the anterior surface of the arytenoid carti- lages is of the stratified squamous variety with underlying papillas. With these exceptions the mucous membrane of the larynx is lined \\'ith stratified columnar ciHated epitheHum similar to that of the re- spiratory portion of the nares. Numerous goblet cells are usually present, and the epithehum rests upon a broad basement membrane. On the posterior surface of the epiglottis many taste buds (see Fig. 297 and page 587) are embedded in the epithehum. The stroma is especially rich in elastic fibres. The true vocal cords consist almost wholly of longitudinal elastic fibres covered by stratified squamous epithehum. Lymphoid cells are present in vary- ing numbers. In some places they are so numerous that the tissue assumes the character of diffuse lymphoid tissue. Distinct nodules sometimes occur. Owing to the absence of a muscularis mucosae the stroma passes over with no distinct fine of demarcation into the submucosa. This is a more loosely arranged, less cellular connective tissue, and con- tains simple tubular glands Hned 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 perichondrium. Of the cartilages of the larynx, the epiglottis, the middle part of the thyreoid, 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 thyreoid and the cri- coid cartilages are hyaline. After the twentieth year, more or less ossification is usually found in the cricoid and thyreoid cartilages. The Trachea The walls of the trachea consist of three layers — mucosa, submu- cosa, anrl fil^rosa (Fig. 199). The mucosa is continuous with that of the larynx, which it closely resembles in structure. It consists of a stratified columnar ciliated 302 THE ORGANS 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 '■U'.iii^iiuUMaUiiJ±JtV,,,':-.\-.::'.-.i ii^g~ '^'.^/••^'t; - h --d Fig. 199. — From Longitudinal Section of Human Trachea. X40. (Technic 3, p 304.) a, Epithelium; b, stroma; c, cartilage; d, fibrous coat; e, serous tubules;/, mucous tubules; g, glands in submucosa; h, ducts. 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 frequently penetrate the muscle and extend into the fibrosa. The fibrosa is composed of coarse, rather loosely woven connect- ive-tissue fibres embedded in which are the tracheal cartilages. These are incomplete rings of hyahne cartilage shaped Hke the letter C (Fig. 200). They are from sixteen to twenty in number and encircle about four-fifths of the tube, being open posteriorly. The openings THE RESPIRATORY SYSTEM 303 between the ends of the cartilaginous rings are bridged over by a thickened continuation of the fibrous coat, strengthened by a layer of smooth muscle (Fig. 200, m). The bundles of muscle cells run mainly in a transverse direction, and extend across the intervals between adjacent rings as well as between their open ends. There ^■ '^ YiG. 200. — Transverse Section of Human Trachea through One of the Cartilage Rings. X8. (KoUiker.) E, EpithcHum of {s) mucous membrane; dr, glancls; us, gland duct; ad, adenoid tissue; A', cartilage; m, smooth muscle cut longitudinally, extending across between ends of cartilage ring. are frequently a few bundles of obliquely disposed cells and, most external, some few that run longitudinally. Outside the fibrous coat proper is a looser, more irregular con- nective tissue, which serves to attach the trachea to the surrounding structures. Blood-vessels, lymphatics, and nerves have a similar distribution in larynx and trachea. The larger vessels pass directly to the sub- muc(jsa. From these, smaller branches pass to the; different coats, where they break up into capillary networks. 304 THE ORGANS Lymphatics form plexuses in the submucosa and mucosa, the most superficial lying just beneath the subepithelial capillary plexus. The nerves of the larynx and trachea are derived from both cere- bro-spinal and sympathetic systems. The cerebro-spinal nerves are afferent, the dendrites of spinal ganghon cells. They form a sub- epithehal plexus from which are given off fibrils which pass into the epithelium and terminate freely among the epithelial cells. Other afferent fibres of cerebro-spinal nerves pass to the muscular coat of the trachea. Sympathetic nerve fibres form plexuses which are inter- spersed with minute groups of ganglion cells. Axones from these ganghon cells have been traced to the smooth muscle cells of the trachea. Sympathetic axones also pass to the glands of the trachea and larynx. On the under surface of the epiglottis small taste buds are found. TECHNIC (i) For the study of the details of structure of the walls of the nares and larynx, fix small pieces of perfectly fresh material from different regions in formalin-Miiller's fluid (technic 5, p. 7), harden in alcohol, stain sections with haematoxylin-eosin (technic i, p. 20), and mount in balsam. (2) The general relations of the parts can be studied by removing the larynx, upper part of the trachea, and corresponding portion of the oesophagus of an animal or of a new-born child, fixing and hardening as above, and cutting longi- tudinal sections through the entire specimen. (3) Trachea. — Remove a portion of the trachea and treat as in technic (i). Both longitudinal and transverse sections should be made; the longitudinal in- cluding at least two of the cartilaginous rings; the transverse being through one of the rings. The Bronchi The primary bronchi and their largest branches have essentially the same structure as the trachea except that the cartilaginous rings are not as complete. Bronchi branch at acute angles and also give off small side branches. As they decrease in caHbre, the following changes take place in their walls (Figs. 201 to 204). (i) The epitheUum gradually becomes thinner. In a bronchus of medium size (Fig. 201) it has become reduced to three layers of cells, which Kolhker describes as an outer "basal" layer, a middle "replacing" layer, and a surface layer of ciliated and goblet cells. In the smaller bronchi (Figs. 203, 204) the epithelium is reduced to a THE RESPIRATORY SYSTEM 305 single layer of ciliated cells. These are at first high, but become gradually lower as the bronchi become smaller, until in the terminal branches the epithelium is simple cuboidal and non-cihated. Among the cihated cells are varying numbers of mucous or goblet cells. (2) The stroma decreases in thickness as the bronchi become smaller. It consists of loosely arranged white and elastic fibres. This layer with the epithehum is folded longitudinally (Fig. 204). There is considerable diffuse lymphatic tissue, and in some places small nodules occur, over which there may be lymphoid infiltration Fig. 201. — Transverse Section through two Medium-size Bronchi of the Human Lung. X 15. (Technic 2, p. 317) In the fibrous coat are seen the bronchial arteries and veins, a, Epithelium; b, stroma; c, muscularis mucosae; d, lung tissue; e, fibrous coat; /, plates of cartilage. of the epithelium (see Tonsil, page 179). Near the root of the lung many small lymph nodules are found, which show different degrees of pigmentation. (3) With decrease in thickness of the epithelium and of the stroma, the thickness of the mucosa is maintained by the appearance of a layer of smooth muscle. In the larger bronchi this is a contin- uous layer of circularly disposed smooth muscle, and lies just external to the stroma, forming a muscularis mucosa? (Fig. 202) . It reaches its greatest thickness relative to the size of the bronchus in the bronchi of medium size. As the bronchi become smaller it becomqs thinner, then discontinuous, and in the smallest bronchi consists of only a few scattered muscle cells. These continue into the walls of the alveolar ducts, but are absent beyond this j)oint. (4) The submucosa decreases in thickness with decrease in the 306 THE ORGANS calibre of the bronchi. It consists of loosely arranged connective tissue. Mixed glands (Fig. 202) are present until a diam.eter of about . I mm. is reached, when they disappear. They lie in the submucosa and frequently extend through between the cartilage plates into the fibrous coat. The ducts pass through the muscular coat and open into pit-Hke depressions lined with a continuation of the surface ciliated epithelium. (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 (Figs. 201 and 202). They Muscle Epithelium Stroma coat Alveol Nerve Blood-vessel Fat Cartilage ^ \ \ >/-A\ f Excretory duct Fig. 202. — Cross Section of Human Bronchus (of a child) of 2 mm. diameter. X30. (Stohr.) are frequently fibrocartilage rather than hyaline. These plates de- crease in size and number, and are absent after a diameter of i mm. is reached. Cartilage and mucous glands thus disappear at about the same time, although it is common for glands to extend over into smaller bronchi than do the cartilage plates. The bronchi down to a diameter of from 1.5 to i mm. are inter- lobular, and belong to the duct system down to a diameter of about THE RESPIRATORY SYSTEM 307 0.5 mm. From the small interlobular bronclii are given off the ter- minal bronchi. These are respiratory in character and are described with the lungs. Fig. 203. — Transverse Section of Small Bronchus from Human Lun^. XiiS- (Tech- nic 2, p. 317.) a, Stroma; b, epithelium; c, muscularis mucosa;; d, fibrous coat. :s ^ Fig. 204. — Transverse Section throuj^h small lirom hus of Human i>unK. (Sobotta.) Simple columnar ciliated eijithelium ; no cartilage; no Khm''^; mucosa folded longitudi- nally; elastic tissue stained with W'eigert's elastic tissue stain. In studying the bronchi it is convenient to arbitrarily divide ihcm into large, medium-sized, and small bronchi. Lar^e bronchi have essentially the same structure as llie trachea except for somewhat thinner walls. Medium-sized bronchi (P"ig. 201) have an ejiiilH lium aixjut three layers deep, disconnected plates of cartilage, a continuous iayir of sinootli iiuiscle disposed circularly as a muscularis mucosx- and tubular glands. 308 THE ORGANS Small bronchi have a single layer of ciliated epithelium, a thinner muscular coat, no glands, and no cartilage. (Figs. 203, 204.) The Lungs The lung is built upon the plan of a compound alveolar gland, the trachea and bronchial ramifications corresponding to duct systems, 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 Fig. 205. — From Lung of an Ape. The bronchi and their dependent ducts and alveoli have been fiUed with quicksilver. X15. (KoUiker, after Schulze.) Z>, Terminal bronchus; a, alveolar duct; i, alveoli. wall as the parietal pleura. It consists of fibrillar connective tissue containing fine elastic fibres which are more numerous in the visceral than in the parietal layer. From the capsule broad connective- tissue septa pass into the organ, dividing it into lobes. From the capsule and interlobar septa are given off smaller septa, which sub- divide the lobes into lobules. The human pulmonary lobule (Figs. 206 and 208) is the anatomic unit of lung structure. Each lobule is complete in itself, having its own bronchial system, its own vascular system, and being more or less completely separated from its neighbors by connective tissue. In the young and in some lower animals the lobule is quite plainly outlined by connective tissue, but in the human adult the amount of connective tissue is extremely small, and the lobules, especially the more central, difficult of definition. The lobules are best observed at the surface of the lung where their bases which lie against the pleura can be seen with the naked eye. This is especially true in the aged THE RESPIRATORY SYSTEM 309 where carbon deposits in the interlobular connective tissue assist in outhning the lobules. Thebase of the peripheral lobule is four- toeight- sided and from i to 1.5 cm. in diameter. It is pyramidal in shape, narrowing to an apex about i to 1.5 cm. from the base. In the inte- rior of the lung the lobules are not pyramidal but irregularly poly- hedral, the apex being, however, distinguishable by the entrance of the lobular bronchus. The apex of each lobule is the point of entrance of the lobular bronchus (branch of inter- or sublobular) (Figs. 205, and 208, hi) which is about 0.5 mm. in diameter, and of the lobular branch of the Bronchial artery Pumonary vein Pulmonary artery "Terminal bronchus T. ■ Alveolar passage Pleural capillaries Fig. 206. — Scheme of a Pulmonary Lobule and its Blood Sui)ply. (Stohr.) The two main branches of the pulmonary vein are seen lying in the interlobular connective tissue. pulmonary artery. Accompanied by the artery (Fig. 206) the bron- chus passes through the central axis of the lobule, giving off collateral branches (Fig. 205, a, b), to about the middle of the lobule, where it divides into two branches (Fig. 206). These branches and also the collaterals branch dichotgrnously giving rise to from 50 to 100 ter- minal or alveolar bronchi ("terminal", as being the last subdivision of the bronchial tree which preserves its identity as a bronchus; "alveolar", as it gives off especially toward its distal end, some alveoli). From each terminal bronchus open from three to six narrow 310 THE ORGANS passages^alveolar passages or alveolar ducts — from which are given off the alveoli — air vesicles or air cells. The somewhat dilated distal end of the alveolar passage is sometimes designated the alveolar sac or infundibulum. Laguesse divides the lobule schematically into three parts a proximal part containing the intralobular bronchus and collaterals, a middle part in which the main division of the Alveolar sacs Blood-vessel Respiratory bronchus Alveoli Small bronchus Fig. 207. — ^Section of Cat's Lung (Szymonowicz), surface lobule; respiratory bronchus opening into alveolar duct from which are given off two alveolar sacs. lobular bronchus occurs, and a third part containing the alveolar bronchi and alveoH. The terminal bronchus. The proximal portion of the terminal bronchus is lined by a simple columnar ciliated epithelium, resting upon a basement membrane. Beneath this is a richly elastic stroma containing bundles of circularly disposed smooth muscle cells. The epitheUum becomes gradually lower and non-ciliated, and near THE RESPIRATORY SYSTEM 311 the distal end of the terminal bronchus there appear small groups or islands of flat, non-nucleated epithelial cells — respiratory epithelium f^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 terminal u § -^6. A r "^Ji-c' ./•'^ I -4< r -"X^r?" Fig. 208. — Section of Lung of Rat to show Arrangement of Bronchial Ramifications and of Alveoli within a Single Lobule. W, Lobular or sublobular bronchus; 6i/, intralobular bronchus; ba, terminal bronchus; v, dilatation sometimes called vestibule; ca, alveolar canal; i, portion sometimes called infundibulum; d, alveoli, some of which are so cut as to show their openings into the infundibulum and alveolar canals, etc., while others api)ear closed. X60. (Prenant.; bronchus, consisting of delicate libro-clastic tissue with scattered smooth muscle cells. The basement membrane is extremely thin. The alveolus. The epithelium of the alveolus consists of two kinds of cells, respiratory cells and so-called "fcBlal" cells (see Develop- ment, page 3 1 5 j. The respiratory cells (Fig. 209) are some of ihcni large, Hat, non- nucleated j)lates, while others are much smaller, non-nucleated ele- 312 THE ORGANS 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. 78). The "fcetar' cells are granular, nucleated cells which are scattered among the respiratory cells. Their position appears to be less super- ficial than that of the respiratory cells, the foetal cells lying in the meshes of the capillary network, the respiratory cells covering the capillaries. In the embryonic lung and in the lungs of a still-born Fig. 209, — ^From Section of Cat's Lung Stained with Silver Nitrate. (Klein.) (Technic i, p. 78.) Small bronchus surrounded by alveoli, in which are seen both flat cells (respiratory epithelium) and cuboidal cells (foetal cells). child the air passages and alveoli contain only this type of cells, the small flat plates apparently resulting from a flattening out of the cuboidal cells due to pressure from inspiration, and the large flat plates to union of a number of small plates. Delicate elastic fibrils support the respiratory and foetal cells. Around the opening of the alveolus the elastic fibres are more numerous, forming a more or less definite ring. The disposition of elastic tissue in the wall of the alveoli is undoubtedly of importance in determining the contraction and expansion of the alveoli under varying conditions of pressure. It has been estimated that on forced inspiration an alveolus can expand to three times its resting capacity. Each alveolus communicates not only with its alveolar passage, or alveolar bronchus, by means of a THE RESPIRATORY SYSTEM 313 broad opening, but alveoli are connected with one another by minute openings in their walls. The interalvcolar connective tissue, while extremely small in amount, serves to separate the alveoli from one another. Somewhat thicker connective tissue separates the alveoli of one alveolar passage from those of another. Still stronger connective-tissue bands as already noted separate adjacent lobules. Fig. 2IC. — Section Through Three Alveoli of Human Lung._ X235. Weigert's elastic-tissue stain (technic 3, p. 28) 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, alveolus in which either the bottom or top is included in section. Blood-vessels. — Two systems of vessels distribute blood to the lungs. One, the bronchial system, carries blood for the nutrition of the lung tissue. The other, the much larger pulmonary system, carries blood for the respiratory function (Fig. 206). The bronchial artery and the pulmonary artery enter the lung at its hilum. Within the lung the vessels branch, following the branchings of the bronchi, which they accompany (Fig. 206). 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. 201). The pulmonary lobule forms a distinct "blood-vascular unit." A branch of the pulmonary artery enters the apex of each lobule close 314 THE ORGANS to the lobular bronchus, and almost immediately breaks up into branches, one of which passes to each alveolar passage (Fig. 206). 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-meshed, and invests the alveoli on all sides (Fig. 211). Similar networks invest the walls of the respiratory bronchi, the alveolar ducts, and their j. alveoli. All of these capillary networks freely anastomose. --^ Fig. 211. — Parts of Four Alveoli from Section of Injected Human Lung. X200. (Technic 5, p. 317.) a, Wall of alveolus seen on flat; c, same, but only small part of alveolar wall in plane of section; b, alveoli in which plane of section includes only side walls; alveolar wall seen on edge. There are thus interposed between the blood in the capillaries and the air in the alveoli only three extremely thin layers: (i) The thin endothelium of the capillary wall; (2) the single layer of flat respiratory epithelial plates; and (3) the delicate basement membrane upon which the respiratory epithelium rests together with an extremely small amount of fibrous and elastic tissues (see diagram, Fig. 212.) The veins begin as small radicles, one from the base of each alve- olus (Fig. 206). These empty into small veins at the periphery of the lobule. These veins at first run in the interlobular connective tissue THE RESPIRATORY SYSTEM 315 away from the artery and bronchus. Later they empty into the large pulmonary 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 divisions, 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. Air y'- a Blood ""•- C Fig. 212. — Diagram of Tissues Interposed Between Blood and Air in Alveolus, a, Respirator}- epithelium; b, fibro-elastic tissue; c, endothelium of capillary. As h does not form a continuous membrane, the capillary wall is in many places in direct apposi- tion with the respiratory epithelium, so that only two layers, a and b, are interposed between blood and air. 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 pul- monary 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-meduUated 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 or the Respir.'Vtory System The epithelium of the respiratory system develops from entoderm, the con- nective-tissue elements from mesoderm. The first differentiation of respiratory system appears as a dipping down of the entoderm of the floor of the primitive pharynx (some investigators describe two original evaginations, one for each lung). The tubule thus formed divides into a larger and longer right branch, which subdivides into three branches corresponding to the three lobes of the future right lung, (Fig. 213, b, b, b) and a smaller and shorter left branch, which subdivides into two branches corresponding to the two lobes of the future left 316 THE ORGANS lung. By repeated subdivisions of these tubules the entire bronchial system is formed. Up to this point (about six months in human foetus) the development is that of a compound alveolar gland, (Fig. 213). The last to develop are the respiratory divisions of the bronchi with their alveolar passages and alveoli. The appearance of the alveoli is wholly characteristic of lung (Fig. 213, vpd.) The epithelium of the alveoli is at first entirely of the foetal-cell type, the large flat respiratory plates appearing only late in foetal life. Just how and when _ IiG. 213.— Scheme of Development of Lung (Right), h, b, b, thethree primary bron- chial buds; b', b', b', collateral branches and secondary buds, terminating in vpp, the primary vesicles; vpd, pulmonary vesicles proper or alveoli; ca, alveolar canals. The broken line limits the stage of the three primary lung buds; between this line and the Hne only the three primary buds, their collaterals and secondary buds; between the hne and hne , the stage of dichotomous division and of termination in primary vesicles. Up to this point the development is that of a com- pound alveolar gland. From this hne to the surface represents the final period of development, which is peculiar to the lung and results in the formation of the pulmonary alveoli. (Prenant.j the flattening of the epithelial cells takes place is not definitely known. The accepted theory has been that the cells become flattened rather suddenly at birth as a result of the first inspiration. Some authors describe a gradual thinning of the ceUs from the sixth foetal month on. Bikfalir describes a gradual thinning which is completed rather rapidly on inspiration. The foetal and respiratory cells of the adult lung have the same embryonic origin. During the early stages of lung development the mesodermic tissue predominates, but with the rapid growth of the tubules the proportion of the two changes until in the adult lung the mesodermic tissue becomes restricted to the inconspicuous pulmonary framework and the blood-vessels. THE RESPIRATORY SYSTEM 317 TECHNIC (i) The technic for the largest bronchi is the same as for the trachea (technic 3, p. 304). 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-Miiller's fluid (technic 5, p. 7), 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 haema- toxylin-eosin (technic i, p. 20), 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 pulmonary lobules is best seen in sections near and horizontal to the sur- face. Sections perpendicular to and including the surface show the pulmonary pleura. (3) Respiratory Epithelium (technic i, p. 78). (4) Elastic Tissue of the Lung (technic 3, p. 28). (5) Blood-vessels. — For the study of the blood-vessels, especially of the cap- illary networks of the alveoli, sections of injected lung should be made. A fresh lung is injected (page 25) 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. 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 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 foetus septa extend from the cap- sule 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 lohe (Fig. 214). In the adult human kidney the division into lobes is not complete, the peripheral parts of the differ- ent lobes blending. Rarely the foetal division into lobes persists « , ,, ^.32^.,™^^ in adult life, such a kidney being V^i ' ?3. known as a "lobulated kidney." On the mesially directed side of the kidney is a depression known as the hilum (Fig. 214). This serves as the point of entrance 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. 214 and 215). 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 tubules are convoluted, while in the medulla they run in parallel radial hues 318 Fig. 214. — Longitudinal Section Through Kidney of Guinea-pig, includ- ing hilum and beginning of ureter. XS- (Technic i, p. 331.) a, Pelvis; h, papilla; c, wall of pelvis; d, ureter; e, ducts of Bellini; /, cortical pyramids; g, medullary rays; h, cortex; i, medulla; j, renal cor- puscles. THE URINARY SYSTEM 319 alternating with straight blood-vessels. The medullary portion of the kidney projects into the pelvis, or upper expanded beginning of the ureter (Figs. 214 and 215) in the form of papillcB. The number Fig. 215. Fig. 216. Fig. 215. — Longiludinal Section of Kidney Through Hilum. a, Cortical pyramid; b, medullary ray; c, medulla; d, cortex; e, renal calyx;/, hilum; g, ureter; //, renal artery; i, obliquely cut tubules of medulla; j and k, renal arches; I, column of Bertini; m, connective tissue and fat surrounding renal vessels; 71, medulla cut obliquely; 0, papilla; p, medullary pyramid. CMerkel-Henle.) Fig. 216. — 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, Glomer- ulus; he, Bowman's capsule; N, neck; I'C, proximal convoluted tubule; S, spiral tubule; D, descending arm of Henle's loop; L, Henle's loop; A, ascending arm of Hcnle's loop; I ,DC, distal convoluted tubule; AC, arched tubule; SC, straight collecting tubule; ED, duct of BeUini; A, arcuate artery, and V , arcuate vein, giving off interlobular vessels to cortex and vasa recta to medulla; a, afferent vessel of glomerulus; e, elTercnt vessel of glomerulus; c, capillary network in cortical labyrinth; .v, stellate veins; vr, vasa recta and caijillary network of medulla. fPearsol.) of j)apilla; varies from ten to fifteen, corresponding to the number of lobes in the fcxtal kidney. The pyramidal segment of medulla, the apex of which is a papilla — in other words, the medullary 320 THE ORGANS 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 renis. Radiating lines — medullary rays or pyramids of Ferrein — - extend outward from the base of each Malpighian pyramid into the cortex (Fig. 215). As the rays extend outward 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 number of long tortuous tubules, the uriniferous tubules. Each URINIFEROUS TUBULE begins in an expansion known as Bowman's capsule (Figs. 216, BC, and 217, 3, 4, 5). This encloses a f Fig. 217. — Diagrams Illustrating Successive Stages in Development of the Renal Corpuscle, i and 2, Approach of blood-vessel and blind end of tubule; 3, invagination of tubule by blood-vessels; 4 and 5, later stages, showing development of glomerulus and of the two-layered capsule of the renal corpuscle, the outer layer being the capsule of Bowman continuous with the epithelium of the first convoluted tubule. tuft of blood capillaries, the glomerulus. Bowman's capsule and the glomerulus together constitute the Malpighian body or renal corpuscle (Fig. 218). As it leaves the Malpighian body the uriniferous tubule becomes constricted to form the neck (Figs. 216, N, 217, and 218, b). It next broadens out into a greatly convoluted portion, the first con- voluted tubule (Fig. 216, PC, and Fig. 219). The Malpighian body, the neck, and the first convoluted tubule are situated in the cortical pyramid (Fig. 216). The tubule next takes a quite straight course downward into the medulla — descending arm of Henle's loop (Fig. 216, D) — turns sharply upon itself — Henle's loop (Fig. 216, L) — and passes again toward the surface — ascending arm of Henle's loop (Fig. 216, A ) — through the medulla and medullary ray. Leaving the medullary ray, it enters the same cortical pyramid from which it took origin to become the second convoluted tubule (Fig. 216, DC). This tubule is in THE URINARY SYSTEM 321 close proximity to the Malpighian body from which it started, lying, however, on the side of the afferent and efferent blood-vessels, i.e., on the side opposite its point of origin. The second convoluted tubule 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 col- lecting tubule receives other arched tubules. As it descends it be- comes broader, enters the papilla, where it is known as the duct of Bellini (ED), and opens on the surface of the papilla into the kid- ney pelvis. About twenty ducts of Bellini open upon the surface of each papilla, their openings being known as the foramina papillaria. / Fig. 2i8. — Malpighian Body from Human Kidney. X280. (Technic 2, p. 331.) a, Bowman's capsvde; i, neck; c, first convoluted tubule; d, afferent and efferent vessels. Each tubule consists of a delicate homogeneous membrana propria upon which rests a single layer of epithelial cells. The shape and structure of the epithelium differ in different portions of the tubule. I. The Malpighian body is spheroidal, and has a diameter of from 120 to 200//. The structure of the Malpighian body can be best understood by reference to its development (Fig. 217). During the development of the uriniferous tubules and of the blood-vessels of the 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. 217). The result is that the end of the vessel 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, dipping down into it and separating the 21 322 THE ORGANS groups of capillaries (see p. 326). This layer by modification of the original epithelium of the tubule is finally composed of a single layer of flat epithelial cells with projecting nuclei. The outer layer of the capsule lies against the delicate connective tissue which surrounds the Malpighian body. This layer consists of a similar though slightly 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. 218), the epithelium of Bowman's capsule being directly continuous with that of the neck of the tubule. B A Fig. 219. — Proximal Convoluted Tubules of Human Kidney. X350. (Technic 2, P- 33I-) ^, Cross-section; B, oblique section. • 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 fiat 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 (Fig. 218). 3. The first convoluted tubule (Fig. 219) measures from 40 to 70/* in diameter. It is lined by irregularly cuboidal or pyramidal epithe- lium, with very indistinct demarcation between the cells. The cyto- plasm 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. 220, i), 10 to 15/^ in diameter. It is lined by a simple fiat epithelium. The THE URINARY SYSTEM 323 part of the cell which contains the nucleus bulges into the lumen, and as the nuclei of opposite sides of the tubule usually alternate, the lumen is apt to present a wavy appearance in longitudinal sections. 5. Henle's Loop. — The epithelium here changes from the flat of the descending arm to the cuboidal of the ascending arm. The exact point w^here the transition occurs varies. It may take place during the turn of the loop, or in either the ascending or descending arm. M #i M. s^ Fig. 220. — Tubules of Human Kidney. Xs6o. From longitudinal section. (Technic 2, p. 331.) i, 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. 6. The ascending arm of Henle's loop (Fig. 220, 2) is broader than the descending, measuring from 20 to 30/( in diameter. Its epithelium is cuboidal with granular striated protoplasm. The cells thus resemble those of the convoluted tubule, but are smaller, more regular, and less granular. 7. The second 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. 8. The arched tubule has a somewhat narrower lumen (about 25/O than the seccmd convoluted. It is lined with a low cuboidal epithe- lium with only slightly granular cytoplasm. 324 THE ORGANS 9. The straight or collecting tubule (Fig. 2 20, 3) has at its commence- ment at the apex of a medullary ray a diameter of from 40 to 50/t. As it descends it receives other arched tubules, and increases in diameter until in the ducts of Bellini (Fig. 220, 4) of the papilla it has a diameter of from 200 to 300/^ and a widely open lumen. The epithelium is at first low and gradually increases in height. In the X ■■:^ Fig. 2 2i.^Cross Section Through Cortex of Human Kidney. X60. (Technic 2, P- 33I-) o> Convoluted tubules of cortical pyramid; b, interlobular artery; c, medullary rays; d, Malpighian bodies. ducts of Bellini it is of the high columnar type. The cytoplasm of these cells contains comparatively few granules, thus appearing transparent in contrast with the granular cytoplasm of the ascending arms of Henle's loops and of the convoluted tubules. Location in kidney Cortical labyrinth . . . . < Portion of tubule Epithelium Bowman's capsule. . Flat with bulging nuclei. Neck Cuboidal, granular. First convoluted Pyramidal, granular; large cells with granules in rows, giving striated appearance; striated free border; indis- tinct cell outline. Second convoluted. . Similar to preceding, but cells not so distinctly striated and more regular in shape. THE URINARY SYSTEM 325 Location in kidnev Medullary ray in cor- tex Portion of tubule Epithelium Arched (passing from Rather clear cuboidal cells, labyrinth to ray) Part of ascending Cuboidal, granular, regular, arm of Henle's loop Collecting tubule. . . Cuboidal or columnar, clear; varying in height with diameter of tubule. Descending arm, Clear flat cells with bulging Henle's loop nuclei. Henle's loop Usually like descending, rarely like ascending arm. *! Part of ascending Cuboidal, granular. I arm Henle's loop Collecting tubule. . . Cuboidal or columnar, clear; I varying in height with di- [ ameter of tubule. Papilla Ducts of Bellini Clear, cuboidal or columnar cells according to diameter of tubule. Medulla. The epithelium of the uriniferous tubule rests upon an apparently structureless basement membrane. Rlihle describes the basement membrane as consisting of delicate longitudinal and circular connec- tive-tissue fibrils. He regards the fibrils as merely a more regular arrangement of the interstitial connective tissue. According to Riihle the epithelium simply rests upon the basement membrane, being in no way connected with it. In the cortex the tubules are closely packed and the amount of interstitial connective tissue is extremely small. In the medulla the connective tissue is more abundant. (Jf the function of the different parts of the uriniferous tubule our knowledge is extremely limited. The water of the urine is secreted in the Malpighian body, some specific action of the cells covering the glomerulus, allowing the water, normally free from albumen, to pass from the capillaries into the lumen of the tubule. The urinary solids are secreted mainly or wholly by the cells of the convoluted tubule and of the ascending arm of Henle's loop. Blood-vessels (diagram, Fig. 223).— The blood supply to the kidney is rich and the blood-vessels come into intimate relations with the tubules. The renal artery enters the kidney at the hilum, and 326 THE ORGANS immediately splits up into a number of branches — the interlobar arteries (Fig. 223, g). These give off twigs to the calyces and to the capsule, 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 cortex and medulla, form a series of arches, the arteries arciformes or arcuate arteries (Fig. 223, d). From the arcuate arteries two sets of vessels arise, one supplying the cortex, the other the me- dulla (Figs. 216 and 223). d < b c Fig. 222 — Cross Section through Medulla of Human Kidney. X465. (Technic 2, p. 331.) a, Capillaries; b, collecting tubule; c, ascending arms of Henle's loops; d, de- scending arms of Henle's loops. The arteries to the cortex spring from the outer (Fig. 223, h) sides of the arterial arches, and as interlobular arteries pursue a quite straight course through the cortical pyramids toward the surface, about mid- way between adjacent medullary rays. From each interlobular artery are given off numerous short lateral branches, each one 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 THE URINARY SYSTEM 327 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 appearance. From the smaller glomerular capillaries the blood passes into some- Fir 22^ — Diacram to IllusLralc (left) the Course of the Uriniferous Tubule; (right) g, interlobar artery; h, medulla; /, medullary ray;;, cortex. what larger capillaries, which unite to form the efferent vessel of the gh)merulus. As afferent and efferent vessels He side by side, the glomerulus has the appearance of being suspended from this pomt (Figs 2 1 6 2 1 8) . rite entire vascular system of the glomerulus is arterial. 328 THE ORGANS 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 (Fig. 223, c). These accompany the interlobular arteries to the boundary between cortex and medulla, where they enter the arcuate veins, which accompany the arcuate arteries (Fig. 223, d). The main arteries to the medulla arise from the inner concave sides of the arterial arches. They pass straight down among the tubules of the medulla and are known as arterice rectce. Branching, they give rise to a long-meshed capillary network which surrounds the tubules. This capillary network is also supplied by (i) efferent vessels from the more deeply situated glomeruli (false arteriae rectae) and (2) by medullary branches from the interlobular arteries. The veins of the medulla arise from the capillary network and follow the arteries back to the junction of medulla and cortex, where they empty into the arcuate veins (Fig. 223, d). 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 w^hich accompanies the arteries to the parenchyma of the organ. 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. THE URIXARY SYSTEM 329 Plexuses of fine non-medulla ted 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 oxcretory duct of the kidney. Their walls consist 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 distinctly 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 tissue frequently occurs in the stroma, especially of the pelvis. Occasionally 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 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 330 THE ORGANS enter the mucosa where they lose their medullary sheaths. Terminals 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 compara- tively smooth, according to the degree of distention of the organ. The epithelium is of the same general type — transitional epithelium Fig. 224. — Vertical Section through Wall of moderately distended Human Bladder. X60. (Technic 5, p. 331.) a, Epithelium, b, stroma, of mucous membrane; c, sub- mucosa; d, inner muscle layer; e, middle muscle layer;/, outer muscle layer. (see page 74) — 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 cuboidal 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 THE URIXARY SYSTEM 331 become flatter and the entire epithelium thinner (Fig. 224). In the distended organ there is still further flattening of the superficial cells and thinning of the entire epithelium. The stroma consists of line loosely arranged connective tissue containing many lymphoid cells and sometimes small lymph nodules. It merges without distinct demarcation into the less cellular siihmucosa (Fig. 224, 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. 224, d, e,f). The fibrous layer is similar to that of the ureter, and attaches the organ to the surrounding structures. The blood- and l5rmph -vessels have a distribution similar to those of the ureter. Nerves. — 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 plexuses in the fibrous coat, where they are interspersed with numerous 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 376. TECHNIC (i) Fix the simple kidney of a rabbit or guinea-pig in formalin- Aliiller's fluid (technic 5, p. 7). Make sections through the entire organ including the papilla and pelvis, stain with haematoxylin-eosin (technic i, p. 20), 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- Mullcr's fluid or in Zenker's fluid. Thin sections should be made, some cutting the tubules longitudinally, others transversely, stained with ha;matoxylin-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 26). (4) Ureter. — Cut transversely into short segments, fix in formalin-Muller's fluid (technic 5, p. 7), and stain transverse sections with ha;matoxylin-cosin (technic i, p. 20), or with hacmatoxylin-picro-acid-fuchsin (technic .3, p. 21). Mount in balsam. (5) Bladder (technic i, p. 254, or technic 2, [>. 254). By the latter method any desired degree of distention may Ijc obtained. 332 THE ORGANS General References for Further Study Kolliker: Handbuch der Gewebelehre, vol. iii. Gegenbauer: Lehrbuch der Anatomic des Menschen, vol. ii. Henle: Handbuch der Anatomic dcs 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, 1899. 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 alhuginea (Fig. 225, 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 parietal layer lines the inner surface of the scrotum. Posteriorly the serous sac is wanting, the testis lying behind and outside of the tunica vaginalis. As the latter is derived from the peritoneum, being brought down with and invagi- nated 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 vasculosa. Posteriorly the tunica albu- ginea is greatly thickened to form the corpus Highmori, or mediastinum testis, from which strong connective-tissue septa radiate (Figs. 225, m and 226, b). Fig. 225. — Diagram illustrating the Course and Relations of the Semi- niferous Tubules and their Excretory Ducts. (Piersol.) a, Tunica albu- ginea; b, connective-tissue septum between lobules; w, mediastinum; t, convoluted portion of seminiferous These septa pass completely through tubule; s, straight tulmlc; r, rate fViA nr, paradidymis. way the mterior of the testis is subdi- vided into a number of pyramidal chambers or lobules, with bases directed toward the periphery and apices at the mediastinum (Figs. 225 and 226). Behind the testis and outside of its tunica albuginea is an elongated body — the epididymis (Figs. 225, c and 226, c), consisting of convoluted 333 334 THE ORGANS >. ■'■I^^i^^*^ tubules continuous with those of the mediastinum. The epididymis is divided into three parts: an expanded upper extremity, the head or globus major (Figs. 225 and 226, c) ; a middle piece, the body (Fig. 226, d) ; and a shghtly expanded lower extremity, the tail or globus minor. From the last named passes off the main excretory duct of the testis, the vas deferens (Fig. 225, vd). All of the tubules of the epididymis are continuous on the one e a i - .„ hand with the tubules of the testicle, and on the other with the vas deferens. They thus constitute a portion of the complex sys- tem of excretory ducts of the testicle. The seminiferous tubule may be divided with reference to structure and location into three parts, (i) A much convo- luted part, the convoluted tubule, which begins at the base and occupies the greater portion of a lobule of the testis (Fig. 229, a). As they approach the apex of a lobule several of these convoluted tubules unite to form (2) the straight tubule (Fig. 225, s, 229). 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. 229, c). I. 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 150 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. 229, a). The wall of the convoluted tubule (Fig. 227) consists of three .-— ^ Fig. 226. — Longitudinal Section through Human Testis and Epididymis. X2. (Bohm and von Davidoff.) The Hght strands are connective- tissue septa, a, Tunica albuginea; h, mediastinum and rete testis; c, head of epididymis; d, body of epididymis; e, lobule; s, straight tubules; t, vas epididymis. THE REPRODUCTIVE SYSTEM 335 layers: (a) An outer layer composed of several rows of flattened con- nective-tissue cells which closely invest the tubule; (b) a thin base- ment membrane; and (c) a lining epithelium. The epithelium con- sists of two kinds of cells, the so-called supporting or sustentacular cells and the glandular cells proper, the spermatogenic cells. The sustentacular cells, or columns of Sertoli, are irregular, high, epitheHal structures, whose bases rest upon the basement membrane, sf SC ijs sf M Fig. 227. — Cross Section of Convoluted Portion of Human Seminiferous ".Tubule. X480. (Kollikcr.) M, Basement membrane; i, its inner homogeneous layer; fs, its outer fibrous layer; s, nucleus of Sertoli cell; sp, spermatogone; sc, spermatocyte; sc' , spermatocyte showing mitosis; ,s/, nearly mature spermatozoon; sf , spermatozoon free in lumen of tubule; d, degenerating nucleus in lumen;/, fat droplets stained byi^osmic acid. and which extend through or nearly through the entire epithelium (Fig. 228, s). Their sides show marked irregularities and depressions, due to the pressure of surrounding spermatogenic cells. Their nuclei are clear, being poor in chromatin and their protoplasm contains browni.sh fat droplets. The cells of Sertoli have long been considered as sustentacular in character. It has recently been suggested that these cells are derived from the spermatogenic cells, but that, in- stead of developing into spermatozoa, they undergo retrograde changes. 336 THE ORGANS their protoplasm mingling with- the intercellular substance, their nuclei becoming lost and the cells finally disappearing. According to this theory the tuft-like arrangement of the spermatozoa about the ends of the Sertoli cells is due to pressure by surrounding spermato- genic cells (Figs. 228, A and 230, /). h s b h f ^ ! L^ ^/ / r * ^ St sc -- St. — — m /'yd 'W0^^^ ■ i '' " ^P Fig. 228. — Parts of Transverse Section of three Seminiferous Tubules from Testis of White Mouse. X600. (Szymonowicz.) 5, SertoU cell with nucleus; i-^, spermatogone, resting state; sp', spermatogone in mitosis; sc, spermatocyte; st, spermatid; sf, spermatid developing into spermatozoon; h, head of spermatozoon; /, tails of developing sper- matozoa; b, blood-vessel; c, interstitial cell; m, basal membrane;/, fat droplets. 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 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 : (i) Spermatogones (Figs. 227 and 228, sp). — These are small cu- boidal cells which he against the basement membrane. Their nuclei are spherical and rich in chromatin. By mitotic division of the spermat- ogones are formed the cells of the second layer, the spermatocytes. THE REPRODUCTIVE SYSTEM 337 (2) Spermatocytes (Figs. 227 and 228, so). — These are larger spher- ical cells with abundant cytoplasm and large vesicular nuclei showing various stages of mitosis. They form from two to four layers to the inner side of the spermatogones, and are sometimes differentiated into spermat- ocytes of the first order and spermato- cytes of the second order. By mitotic division of the innermost spermatocytes ^-^ - - are formed the spermatids. (3) The spermatids (Figs. 227 and 228, >o st) are small round cells which line the Q 'D llG. 230. Fig. 229.— Passage of Convoluted Part of Seminiferous Tubules into Straight Tu- bules and of these into the Rete Testis (Milhalicowicz.) a, Convoluted part of tubule; b, fibrous stroma continued from the mediastinum testis; c, rete testis. Fig. 230. — Spermatoblast with some Adjacent Sperm Cells, from Testis of Sparrow. (From Kcilliker, after Etzold.) M, Basement membrane; s, nucleus of SertoU cell; sp, spermatogones; ic, spermatocyte; s^i and 5/2, spermatids lying along the surface of the Sertoli cell, s' and sh; at 5/3 are seen the nearly mature spermatozoa; /, tuft-like arrangement of bodies of spermatids around free end of Sertoli cell, with two mature spermatozoa. lumen of the seminiferous tubule. They are the direct progenitors of the spermatozoa. (For details of spermatogenesis see page 344.) 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. 227, sf and 230). There are also found in the lumen many small 22 338 THE ORGANS cells with dark nuclei. These are spermatids which have become free and which degenerate without forming spermatozoa. Separating and supporting the convoluted tubules is a small amount of interstitial connective tissue in which are the blood-vessels and nerves. Among the usual connective-tissue elements are found c Fig. 231. — From Section through Human Mediastinum and Rete Testis. X96. (Kolliker.) A, Artery; V, vein; L, lymph space; C, canals of rete testis; s, cords of tis- sue projecting into the lumina of the tubviles and so cut transversely or obliquely; Sk, section of convoluted portion of seminiferous tubule. groups of rather large spherical cells with large nuclei — interstitial cells. They are believed to represent remains of the Wolffian body (Fig. 228, c). 2. The Straight Tubule .---With the termination of the con- voluted portion, the spermatogenic tissue of the gland ends, the remainder of the tubule constituting a complex system of excretory ducts. The straight tubule is much narrower than the convoluted, THE REPRODUCTIVE SYSTEM 339 having a diameter of from 20 to z|o/<. It is lined by a single layer of cuboidal cells resting upon a thin basement membrane. At the apex of the lobule the straight tu- bules become continuous with ^■ the tubules of the rete testis. ^^ ' 3. The Tubules of the Rete Testis. — These are ir- regular canals which vary greatly in shape and size. They are lined with a single layer of low cuboidal or fiat epithehal cells (Fig. 231, 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 w'holly 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 Fig. 232. — Part of a Cross Section through a Vas Efferens of the Human P^pididymis. X140. (KoUiker.) F, High columnar cifiated epithelium; d, lower non-ciliated epithelium, presenting appearance of a gland; d' , the same cut obliquely. m ' ■^.i'^^% /'/,■' -'-^^ c ■'';",,',"- ■■ m Fig. 233. — From Cross Section through Head of E|)ididymis. y.Z'^. (KoUiker.) 6, Interstitial connective tissue; c sections through tubules of epididymis, showing two- layered columnar epithelium; g, blood-vessel. 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 eferenlia, or efferent ducts of the 340 THE ORGANS testis (Fig. 225, e). Eacli vas effexens piusnes a tortuous course, is separated from its fellows by connective tissue, and forms one of the lobules of the bead of tbe epidid3Tnis. The epithelium of the ^^asa efferentia consists of two kinds of cells, high columnar cihated cells (Fig. 232, F), and, interspersed among these, low cuboidal non- ciliated cells (Fig. 232, d). Occasionall}' some of the high ceOs are free from ciha and some of the cuboidal cells ma3" bear dlia. The cuboidal cells he in groups between groups of the higher cells, often giAing the appearance of ciypt- I ■ . ''}\ \ h like depressions. Th^e liave *'''^*=-~----===^_^________^^ ...:>--' — '' been referred to as intraepithelial ■' :=:,,;:^ glands. The}' do not, howeveTj I ■ ' . , ;! ' K3 invaginate the underlying tissues. The epithehum rests upon a base- ment membrane, beneath which are several layers of circularly disposed smooth musde ceis. The vasa efferentia converge to "" form the i^as epididymis (Fig. 233). Waiwl- ¥4^e''of"Epid!SS". "^xt. ^ere the epitheli™ is "of the (Xolliker.) (Fig. 233 more highly magni- stratified "^^uriety, there being two fied.) &, Connective-tissue and smooth , r n tt'Tl. musde cells; r, basal layer of epithehal O^ three rowS Ol celis. Ihe SW~ ^eUs; /, high columaiar epithelial cells; p, f^^g ^g^g ^^^ narrow, high, and pigment granules m columnar cells; r. _ _ . . cuticula; /?, ciha. cihated, and their nuclei are placed at different levels (Fig. 234). The ciha are long and each ceU has onl}' 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 approached the muscular coat becomes thickened, and is sometimes strengthened by the addition of scattered bundles of longitudinal^ disposed cells. The Vas Deferens. — The walls of the vas deferens consist of four coats — mucosa, sub mucosa, muscularis, and fibrosa (Fig. 235), The mucosa is folded longitudinall}', and is composed of a stroma and a lining epithehum. The epithehum 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 cihated varies greatly. In some cases the entire vas is cihated, in others only the upper portion, in still others no ciha are present bej^ond the epi- did3'mis. The epithehum rests upon a basement membrane beneath THE REPRODUCTIVE SYSTEM 341 which is a libro-elastic cellular stroma. The stroma merges without distinct demarcation into the more vascular subniucosa. The ninscularis consists of two strongly developed layers of smooth muscle, an inner circular and an outer longitudinal (Fig. 235), 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, and situated between the inner circular laver and the submucosa. ■•-,w:-' - - C Fig. 235. — Cross Section of Human Vas Deferens. X37. (Szymonowicz.) a, Epithelium; b, stroma; c, submucosa; d, inner circular muscle layer; e, outer longitudinal muscle layer; /, fibrous layer; g, blood-vessels. The fibrosa consists of fibrous tissue containing many elastic fibres. Near its termination the vas dilates to form the ampulla, the walls of which present essentially the same structure as those of the vas. The lining epithelium is, however, frequently markedly pig- mented and the mucosa ccjntains branched tubular glands. The Seminal Vesicles and Ejaculatory Ducts.^ — The seminal 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 simple 342 THE ORGANS columnar epithelium. Beneath the epithelium is a thin stroma, out- side 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 ejaciilatory ducts are lined with a single layer of columnar cells. The muscularis is the same as in the ampulla except that the inner circular layer is thinner. In the prostatic portion of the duct the muscularis is indistinct, merging with the muscle tissue of the gland. The ducts empty either directly into the urethra or into the urethra through the vesicula prostatica. Rudimentary Structtires Connected with the Development of the Genital System. — Connected with the testicle and its ducts are remains of certain fcEtal structures. These are: (i) The paradidymis, or organ of Gir aides, situated between the vessels of the spermatic cord near the testis. It consists of several blind tubules lined with simple columnar ciliated epithelium. (2) The ductus aberraus Halleri, found in the epididymis. It is lined with simple columnar ciliated epithelium and opens into the vas epididymis. In- stead of a single ductus aberrans, several ducts may be present. (3) The appendix testis (stalked hydatid or hydatid of Morgagni), in the upper part of the globus major. It consists of a vascular connective tissue surrounding a cavity lined with simple columnar ciliated epithelium. (4) The appendix epididymidis, a vascular structure, not always present, lying near the appendix testis. It resembles the latter in structure. The paradidymis and ductus aberrans Halleri probably represent remains of the embryonal mesonephros. The appendix testis and the appendix epididy- midis are believed by some to be derived from the primitive kidney, by others from the embryonal duct of jNIiiller. Blood-vessels. — Branches of the spermatic artery ramify in the mediastinum and in the tunica vasculosa. These send branches into the septa of the testicle, which give rise to a capillary network among the convoluted tubules. From the capillaries arise veins which accompany the arteries. L5miph 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-meduUated 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 mth the epithelium has not been determined. In the epididymis THE REPRODUCTIVE SYSTEM 343 Head Anterior end knob Posterior end knob are found small sympathetic ganglia. The walls of the vasa effer entia, vas epididymis, and vas deferens contain plexuses of non-medul- lated nerve fibres, which give off terminals to the smooth muscle cells and to the mucosa. The Spermatozoa. — The spermatozoa are the specific secre- tion of the testicle. Human spermatozoa are long, slender flagellate bodies, from 50 to 70/f in length, and are suspended in the semen, which is a secretion of the accessory sexual glands. The general shape is that of a tadpole; and by means of an undulatory motion of the tail, the spermato- zoon is capable of swimming about freely in a suitable medium. It has been estimated that the human spermatozoa average about sixty thousand per cubic millimetre of semen. The human spermatozoon con- sists of (i) a head, (2) a middle piece or body, and (3) a tail or flagellum (Fig. 236). 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 mainly of chromatin de- rived from the nucleus of the parent cell. Enveloping the nu- clear material of the head is a thin layer or delicate membrane of cyto- plasm, the galea capitis. The front of the head ends in a sharp edge, the apical body or acrosome. In some lower forms the acrosome is much more highly developed than in man and extends forward as a pointed or barbed spear, the perjoralorium. The acrosome is differentiated from the nuclear Main segment of tail Fig. 236. — Diagram of a Human Sper- matozoon. (Meves, Bonnet.) 344 THE ORGANS portion of the head by taking an acid dye, the chromatin, of course, taking a basic stain. The body is cyHndrical, about the same length as the head, and consists of a iibrillated central core, the axial thread, surrounded by a protoplasmic capsule. A short clear portion, the neck, unites the head and body. Just behind the head the axial thread presents a bulbous thickening, the anterior end knob, which fits into a depression in the head. At the junction of neck and body are one or several posterior end knobs to which the axial thread is attached. The latter leaves the body through a perforated ring, the end ring or end disc. Delicate fibrils — spiral fibres — run spirally around the body portion of the axial thread. The tail consists of a main segment, from 40 to 6o,« in length, and a terminal segment having a length of from 5 to io;<. The main seg- ment has a central iibrillated axial thread which is continuous with the axial thread of the body. This is enclosed in a thin cytoplasmic membrane or capsule continuous with the capsule of the body. This membrane, inconspicuous and apparently structureless in man, is remarkably developed in some lower forms, e.g., the membrana undulata of birds. The terminal segment consists of the axial thread alone. The motility of 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 usually be differentiated, except by the use of special methods and very high-power objectives. Development of the Spermatozoa. — As already noted in describing the testicle, the spermatozoa are developed from the epithelial ceUs of the semi- niferous tubules. The most peripheral of the tubule cells, the spermatogones (Fig. 227, sp and Fig. 228, sp) are small round cells with nuclei rich in chromatin. By mitosis the spermatogone gives rise to two daughter cells, one of which remains at the periphery as a spermatogone, while the other takes up a more central position as a spermatocyte (Fig. 228, sc and Fig. 230, 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. 228, st and Fig. 230, st). These are small spherical cells, which line the lumen of the tubule and are the direct progenitors of thu spermatozoa. In the transformation of spermatocyte into spermatid an extremely important change takes place in the nucleus. This consists in a reduction of its chromosomes to one-half the number specific for the species (page 58) . The transformation of the spermatid into the spermatozoon differs somewhat in different animals and the details of the processmust be regarded as not yet definitely determined. The THE REPRODUCTIVE SYSTEM 345 nucleus of the spermatid first becomes oval in shape, and its chromosomes be- come 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 themselves 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. 230). The body or middle piece of the spermatozoon is described by most investigators as derived from the centrosome, while the tail is a derivative of the cytoplasm. Head Anterior end knob Posterior end knob Head Anterior end knob Posterior end knob End ring Nucleus Cytoplasm .:— Proximal centrosome 7 Distal centrosone Fig. 237. — Transformation of a Spermatid into a Siiermatozoon (human). Schematic (Meves, Bonnet.) The details of the transformation of the spermatid into the spermatozoon are illustrated in Fig. 237. The centrosome either divides completely, forming two centrosomes, or incompletely, forming a dumb-bell-shaped body. The nuclear material becomes very compact ancl passes to one end of the cell, forming the bulk of the head. Both centrosomes help to form the body. They first become disc-shaped. 'J'he one lying nearer the centre of the cell becomes attached to the posterior end of the head as the anterior end-knob, the other takes a position a 346 THE ORGANS little more posteriorly as the posterior end-knob and from it grows out the axial filament. Some of this centrosome passes to the posterior limits of the body and there becomes the end ring. As the two parts of this centrosome separate the delicate cytoplasm between them forms the spiral fibres. During these changes the axial filament has been growing and projects beyond the limits of the cell. Most of the cytoplasm of the spermatid is not used in the formation of the spermatozoon, but is cast off and degenerates. A small amount is used for the sheath of the body, and for the galea capitis. The sheath of the main part of the filament appears to develop from the filament itself. The significance of the different parts of the spermatozoon has been brought out in describing its development. From this it is seen that the spermatozoon, like the mature ovum, is a true sexual element with one-half the somatic number of chromosomes. The head and body, containing the chromatin and the centro- somes, are the parts of the spermatozoon essential to fertilization. The acro- some is an accessory which in some forms at least aids the spermatozoon in attaching itself to and in entering the ovum. The tail is an accessory structure which provides motion, enabhng the spermatozoon to move about freely in the semen and in the fluids of the female generative tract. Considering their minuteness, their speed is considerable, having been estimated at from 1.5 to 3.5 mm. per minute; enough to allow them to ascend through uterus and oviduct against the adverse action of the cilia. When in a favorable environment, such as the fluids of the female generative tract, the spermatozoon is capable of living for some time after leaving the testicle. Living spermatozoa have been found in the uterus or tubes three and a half weeks after coitus. 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 formahn-M tiller's fluid (technic 5, p. 7). Antero-posterior longitudinal sections through the entire organ and in- cluding the epididymis should be stained with hsematoxylin-picro-acid-fuchsin (technic 3, p. 21) or with haematoxylin-eosin (technic i, p. 20) and mounted in balsam. (2) The testis of a young adult is removed as soon after death as possible, is cut into thin transverse shces, which include the epididymis, and is fixed in forma- lin-Miiller's or in Zenker's fluid (technic 9, p. 8). 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 haematoxyhn-eosin (technic i, p. 20) and mount in balsam. (3) For the study of spermatogenesis fix a mouse's testis in chrome-acetic- osmic mixture (technic 7, p. 7). Harden in alcohol and mount thin unstained sections in balsam or in glycerin. (4) Spermatozoa. — -Human spermatozoa may be examined fresh in warm normal sahne solution or fixed in saturated aqueous solution of picric acid and THE REPRODUCTIVE SYSTE:\I 347 mounted in glycerin. ^lammalian 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- "---W ■* , /■ -•'■::^''-^ ' "'^ -V 'V 't:\ '^""'^'^^'% h -■■-"' " \ * . FiG. 238. — Section of Human Prostate. X150. (Technici,p. 349.) a, Epithelium of lubule; h, interstitial connective tissue; c, corpora amylacea. 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 trabeculce of the same structure as the capsule pass into the gland. The amount of connective tissue is large. It is less in the prostate of the young than of the old. The hypertrophied prostate of age is due mainly to an increase in the connective-tissue elements. The tubules have wide lumina and are lined with simple cuboidal epithelium of the serous type, resting upon a delicate basement mem- 348 THE ORGANS brane (Fig. 238). Less commonly the epithelium is pseudo-strati- fied. The ducts are lined with simple columnar epithelium until near their terminations where they are hned with transitional epithe- lium similar to that lining the urethra. Peculiar concentrically laminated bodies, crescentic corpuscles, or corpora amylacea, are fre- quently present in the terminal tubules (Fig. 238, 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 {utriculus prostaticus — uterus masculinus). It represents the remains of a foetal structure, the Mullerian duct 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 secre- tion is serous. The blood-vessels of the prostate ramify in the capsule and tra- beculse. 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. The lymphatics begin as blind clefts in the trabeculae and fol- low 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 medullated fibres ending within capsular structures of fiat 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 large fibre. Cowper's Glands The bulbo-urethral glands, or glands of Cowper, are small, com- pound tubular glands. Both tubules and ducts are irregular in diam- eter so that some of them have more the character of alveoli than of tubules. They are lined with mucous cells. The smaller ducts are lined with simple cuboidal epithelium. They unite to form two main THE REPRODUCTIVE SYSTEM 349 excretory ducts which open into the urethra and are lined with stratified columnar epithelium consisting of two or three layers of cells. In the main duct, as well as in its branches, smooth muscle occurs. TECHNIC (i) Fix small pieces of the prostate of a young man in formalin-Miiller's fluid (technic 5, p. 7). Stain sections with haematoxylin-eosin (technic i, p. 20) 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 (i). 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. 239). All three are enclosed in a common connective-tissue capsule which , is loosely attached to the over- lying skin. In addition each corpus has its own special cap- sule or tunica albuginea, about a millimetre in thickness, and composed of dense connective tissue containing many elastic fibres. The corpus spongiosum and corpora cavernosa have essen- tially the same structure, being composed of so-called erectile tissue (Fig. 240). This consists of thick trabeculae of inter- mingled 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 condition of the organ these sinuses are empty and their sides are in apposition. In erection these sinuses become filled with venous Ijlood. I-'iG. 239. — Tran.-.\ i-r.-.c Section through Human Penis. a, Skin; b, subcutaneous tissue; c, fibrous tunic; d, dorsal vein; e, corpora cavernosa; /, corpus spongiosum; g, urethra. 350 THE ORGANS 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 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. The blood may therefore pass either through the usual course— arte- ries, capillaries, veins — or, under certain conditions, may jpass through the cavernous sinuses. This determines the flaccid or the erect con- dition of the organ. The veins arise partly from the capillaries and ^'' ■-■■.' .':■.•■';'.'■■■' ,■ Z-:^"- . \ \ --^ > h Fig. 240. — Erectile Tissue of Corpus Spongiosum of Human Penis. X60. a, Trabeculse of connective tissue and smooth muscle; b, cavernous sinuses; c, groups of leucocytes in sinus. partly from the cavernous sinuses. They pass through the tunica albuginea and empty into the dorsal vein of the penis (Fig. 239). In the corpus spongiosum there is probably no direct opening of arteries into sinuses. Both trabeculae and sinuses are also smaller. Of the lymphatics of the penis little definite is known. The nerve endings, according to Dogiel, consist of: (a) free sensory endings, (b) deeply situated genital corpuscles, (c) Pacinian corpus- cles and Krause's end-bulbs in the more superficial connective tissue, and (d) Meissner's corpuscles in the papillae;. (For details see pages 432 and 433.) The glans penis consists of erectile tissue similar in structure to THE REPRODUCTIVE SYSTEM 351 that of the corpus cavernosum, except that the venous spaces are smaller and more regular. The mucous membrane is very closely attached to the fibrous sheath of the underlying erectile tissue. A few small sebaceous glands, unconnected with hairs — the glands of Tyson — are found in the mucous membrane of the base of the glans penis. The prepuce is a fold of skin which overlies the glans penis. Its inner surface is lined with mucous membrane. Fig. 241. Fig. 242. Fig. 241. — From Transverse Section of Urethra and Corpus Spongiosum, including Mucous Membrane and part of Submucosa. X15. The dark spots represent the cavernous veins. Fig. 242. — Vertical Section through Portion of Wall of Human Male Urethra. X350. A, Mucous membrane; B, submucosa; a, epithelium; b, stroma; c, cavernous veins; d, connective tissue of submucosa. The Urethral The MALE URETHRA Is divided into three parts — prostatic, mem- branous, and penile. The wall of the urethra consists of three coats — mucous, submuccjus, and muscular. The structure of the wall differs in the different parts of the urethra. 'J"he mucous membrane (Fig. 242) consists of epithelium and stroma. The epithelium of the prostatic part is stratified squamous * The female urethra, while not so distinctly divisible into sections, presents essen- tially the same structure as the male urethra. The epithelium begins at the bladder as stratified squamous of the transitional ty|)e, changes to a two-layered stratified or pscudostratified, and finally jiasses over into stratified stjuamous near the uretiiral opening. Glands of Eittr6 are jirescnt, but are fewer than in the male. 352 THE ORGANS (transitional), resembling that of the bladder. In the membranous part it is stratified columnar or pseudostratified. In the penile portion it is pseudostratified up to the fossa navicularis, where it changes to stratified squamous. The epithelium rests upon a base- ment membrane, beneath which is a thin stroma rich in elastic fibres and having 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 character of erectile tissue (Fig. 242). The muscular coat is thickest in the prostatic and membranous portions. 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 disposed smooth muscle cells are found in the outer part of the submucosa 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. TECHNIC (i) 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 formalin-Mxiller's fluid (technic 5, p. 7), cut rather thick sections across the entire penis, stain with haematoxylin-picro-acid-fuchsin (technic 3, p. 21) or with haematoxyhn-eosin (technic i, p. 20) 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, leaving only the corpus spongiosum and contained urethra, and treat as above. Sections 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. n. 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 THE REPRODUCTIVE SYSTEM 353 directly 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 hilum, to 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 Fig. 243.- — Ovary opened by Longitudinal Incision. Ovum has Escaped ihrough Tear in Surface. Cavity of follicle filled with blood clot (corpus haemorrhagicum) and irregular projections compo.sed of lutein cells. (KoUmann's .\tlas.) stroma. This consists of fibrous connective tissue rich in elastic fibres and containing many smooth muscle cells. In the deeper cen- tral portion of the organ, stroma alone is found. Here it contains many large blood-vessels, and constitutes the medulla or zona vas- culosa of the ovary (Fig. 244, 2). From the medulla the stroma radi- ates toward the surface of the ovary and becomes interspersed with glandular elements forming the ovarian cortex (Fig. 244, 3, 3'). At the surface of the ovary, just beneath the peritoneum, the stroma forms a rather dense layer (;f fibrous tissue, the tunica albuginea. At the margin of the peritoneal surface of the ovary the connective tissue of the peritoneum becomes continuous with the stroma of the ovary, while the flat mesothelium of the general peritoneum is replaced by a single layer of cuboidal cells, which covers the surface of the 354 THE ORGANS ovary and is known from its function as the germinal epithelium (Fig. 245, he). The parenchyma or secreting portion of the ovary consists of peculiar glandular elements, the Graafian follicles. The structure of the Graafian follicle can be best appreciated by studying its development. The follicles originate from the ger- minal epithelium during foetal life. At this time the germinal epithe- lium is proliferating, and certain of its cells differentiate into larger Fig. 244. — Semidiagrammatic Drawing of Part of Cortex and Medulla of Cat's Ovary. (From Schron, in Quain's" Anatomy.") i, 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 cor- tex; 5, small (primitive) Graafian follicles near surface; 6, same deeper in cortex; 7, later stage of Graafian folHcle, beginning of cavity; 8 and 8', still later stages in development of follicle; 9, mature follicle; a, stratum granulosum; h, germ hill; c, ovum; d, nucleus (germinal vesicle); e, nucleolus (germinal spot). spherical cells — primitive ova (Fig. 245, op). The primitive ova pass down into the stroma accompanied by a considerable number of the undifferentiated cells of the germinal epithelium. A cord-like mass of cells is thus formed, extending from the surface into the stroma. This is known a.5 Pfliiger's egg cord (Fig. 245). Each cord usually contains several ova. In some cases the differentiation 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 (Fig. 245). During this process, proliferation of the epithelial cells of the cords and nests has been going on, and each ovum surrounded. THE REPRODUCTIVE SYSTEM 355 by a layer of epithelial cells becomes separated from its neighbors (Fig. 245, fp). This central o\aim surrounded by a single layer of epithehal cells (follicular cells) is the primitive Graafian follicle (Fig. 245,//?, Fig. 246, and Fig. 247, a). Rarely a follicle may contain more than one o\-um, of which, however, only one goes on to maturity, the others degenerating. The follicle increases in size, mainly on account of proHferation of the follicular cells, which soon form several layers instead of a single layer, but also partly on account of growth of Fig. 245. — From Transverse Section of Ovarjr of Xevv-burn Child. X280 (Sobotta). Shows primitive ova in Kerminal e])ithelium; Pfliiger's egg cords and nests of cells; c, capillaries; he, germinal epithelium; sir, stroma;//), primitive follicles; op, primitive ova. the ovum itself (Fig. 247). 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. 247, e and Y\g. 244, 7). 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 gramdosiim. With increase in the liquor folliculi the ovum becomes still further i)ressed to one side of the follicle, where, surrounded by an accumulation of foliic ulur ( ells, it forms a distinct projection into the cavity (Fig. 249, and I''ig. 244, 8 and 9). This 356 THE ORGANS is known as the germ hill (discus proligerus — cumulus ovigerus) . The cells of the germ hill nearest the ovum become columnar and arranged in a regular single layer around the ovum — the corona radiata (Fig. 250). The ovarian stroma immediately surrounding the Graafian follicle becomes somewhat modified to form a sheath for the follicle — Fig. 246. — Vertical Section through Cortex of Ovary of Young Girl. X 190. (Bohm and von Davidoif.) a, Germinal epithelium; b, tunica albuginea; c, follicular epithe- lium; d, ovum; e, primitive Graafian follicles in ovarian cortex; /, granular layer of large Graafian follicle. the theca folliculi (Fig. 248). This consists of two layers, an outer more dense fibrous layer, the tunica fibrosa, and an inner more cellular and vascular, the tunica vasculosa. Between the theca folHculi 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 THE REPRODUCTIVE SYSTEiNI 357 is a spherical cell, having a diameter of from 40 to yo/f 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. 247). It is surrounded by a double-contoured nuclear membrane, and contains a distinct chromatic network and nucleolus or gcnninal 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 numer- ous in the ovarv of the infant and child (Fig. 246). farqf^ Fig. 247. — From -Section through Cortex of .\pe's Ovary. X150. (Szymonowicz.) a, Primitive follicle; b, ovum, with nucleus and nucleolus; c, zona pellucida; d, follicular epithelium; c, follicular cavity;/, ovarian stroma; g, blood-vessel in stroma. 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. 'Ihese contain processes of the cells of the corona radiata. A narrow cleft, the perivitelline space, has been described as separating the ovum from the zona pellucida. During the growth of the ovum its cyto- plasm becomes coarsely granular from the development of yolk or deutoplasm granules (Fig. 250). Immediately surrounding the nucleus, and just beneath the zona i)elluci^^'/ .'j^ granulosum ■«=>>■ . v^. Tunica albugmea \'< ^^ ^^ ^ Germ hill -p^eca folhculi with ovum (vascular layer) Theca foUicuIi (fibrous layer) . It J Fig. 251. — Iruin icLliun uf Human Ovar\-, showing mature Graafian follicle ready- to rupture. (Kollmann's Atlas.) Point of rupture Lutein cells Corpus haemorrhagicum Blood vessel of theca •Cavity of follicle ■Theca folliculi Ovarian stroma Stratum granulosum Fig. 252. — From Section of Human Ovary, showing early stage in formation of Cor[)us I^uteum. (Kollmann's .\tlas.) 362 THE ORGANS tilized. 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 advanced by Born, that the corpus luteum is a gland having an inter- Point of rupture Connective tissue Remnant of corpus hasmorrhagicum Blood vessel of theca Lutein cells Connective tissue from theca Theca f ollicuH Blood vessels of theca Fig. 253. — From Section of Human Ovary, showing later stage of Corpus Luteum than Fig. 252. (Kollmann's Atlas.) nal secretion, which appears to have some influence upon the attach- ment of the fecundated ovum to the uterus and upon its nutrition during the first few weeks of its development. According to Fraenkel the 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. THE REPRODUCTIVE SYSTEM 363 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, to- gether with their follicles, retrogressive changes known as atresia of the foUicle. The nucleus of the ovum, as well as the nuclei of the folHcular cells, passes through a series of chromatolytic changes, or in some cases apparently simply atrophies. The cell bodies un- dergo fatty or albuminous degeneration and the cells become 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 oft" branches which pass to the cortex and end in a capillary network in the tunica albuginea. In the outer layer of the theca folHculi 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 folHcuh. 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 ffjctal 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 it opens into the vagina. Both paroophoron and c])ooj)horon are 364 THE ORGANS 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: (i) The isthmus, beginning at the uterus and extending about one-third the Fig. 254. — Cross Section of Oviduct near Uterine End. a, Mucous membrane; b, circular muscle coat; c, longitudinal muscle coat; d, connective tissue of serous coat. (Orthmann.) length of 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: (i) Mucous, (2) muscular, and (3) serous (Figs. 254 and 255). 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. 255). The epithelium lining the tube is of the simple columnar ciliated type, and completely covers the foldings of the mucous mem- THE REPRODUCTIVE SYSTEM 365 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 longi- tudinal layer. The latter is a comparatively thin layer in the isth- mus, consists of discontinuous groups of muscle cells in the ampulla, and in the fimbriated extremity is frequently absent. 'V- '- ^r!;\-'''n?^ ' -I* % Fig. 255. — Cross Section of Oviduct near Fimbriated Extremity, showing complicated foldings of mucous membrane. (Orthmann.) 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 (i) Child's Ovary.— Remove the ovary of a new-born child, being careful not to touch the surface epithcHum, fix in Zenker's fluid (technic 9, p. 8), and harden in alcohol. Cut sections of the entire organ through the hilum. Stain with ha;niatoxylin-eosin (technic i, p. 20) and mount in balsam. (2) For the purpose of studying the Graafian follicle in the different stages of 366 THE ORGANS its development remove an ovary from an adult cat or dog and treat as above. Technic (i). These sections also, as a rule, are satisfactory for the study of the corpus luteum. (3) The adult human ovary is little used for histological purposes on account of the few foUicles 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 possi- ble a normal human ovary should be obtained from a young subject for purposes of comparison with the above. Technic (i). (4) For studying the egg cords of Pfiiiger and their relation to the germ epithelium, ovaries of the human foetus, and of very young cats, dogs, and rabbits are satis- factory. Technic (i). (5) Sections of the frmbriated end of the oviduct are usually found in the sections of ovary. For the study of other parts of the tube, cut out thin pieces from different regions, fix in formalin-MuUer's fluid, stain transverse sec- tions with h^ematoxylin-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 muscularis consists of bundles of smooth muscle cells separated 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. Fig. 256. — Muscle cells from (a) non- pregnant uterus; b, pregnant uterus; drawn to same scale. (Sellheim.) THE REPRODUCTIVE SYSTEM 367 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 6o,«. During preg- nancy the muscular tissue of the uterus is greatly increased. This is due 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 6oo/<. (Fig. 256.) The mucous membrane. As the mucosa presents marked variation in structure, dependent upon the functional condition of the organ, it is necessary to describe: 1. The mucosa of the resting uterus. 2. The mucosa of the men- struating uterus. 3. The mucosa of the prci^ nant uterus. I. The Mucosa of the Res i iNG Uterus This is from i to 2 mm. thick and consists of a stroma, glands. and a lining epithelium (Fig. 257). The stroma resembles embryonal connective tissu( consisting of fine fibrils and long, irregular branching cells which form a sort of network, the me.shes of which are filled in with lymphoid cells and leuco- cytes. The epithehum is of the simple high columnar ciliated variety, the ciliary motion being toward the cervix. A basement membrane separates the epithelium from the underlying stroma. The glands are simple forked tubules lined by a single layer of columnar ciliated cells resting upon a basement membrane and continuous with the surface cells. The glands extend completely through the stroma. Near the surface they run a comparatively 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 mu- FiG. 257. — From Uterus of Young Woman. (Bohm and vonDavidoff; preparation by Dr. J. Amann.) X34. a, Mucous membrane; b, surface epithelium; c, gland; e, muscle. 368 THE ORGANS cous membrane is thicker and presents numerous folds — the pliccB palmatce. The epithehum is higher than in the body of the organ. In addition to glands like those found in the body of the uterus, the cervical mucosa contains peculiar short, sac-like invaginations, lined with a continuation of the surface epithelium, which secrete a glairy J- mucus. Closure of the mouths ,,-• " -..^ of some of these sacs frequently -''..--■,-:'- ^^''^0^ occurs, leading to the formation jk of retention cysts, the so-called , 'm ovula Nabothi. At about the ^ - -.|l^ — - - - _- Vr"' " ' junction of middle and lower h |v - - ' '. i-^ '^ thirds of the cervical canal a II, \ / ' change takes place in the epi- ^ '"""X , , , -- ^ thelium. Here the simple ..-"^|; -if columnar ciliated epithehum / ' ..^.:^-.-,..-:^r.^, '"" h of the upper part of the cervix Fig. 258.-From Section of Dog's Cervix, gradually passes over into a X4- (Technic 2, p. 379.) a, Cervical canal; stratified Squamous epithe- h, mucosa; c, folds of mucosa (plicae pal- mata;); d, muscle layers of cervix; e, epithe- hum. Near the external OS Hum of vagina and vaginal surface of cervix; papiH^ appear, the vaginal /, vagmal epithelium; g, vaginal mucosa; h, ^ ^ ^^ ' _ ° ^ submucosa and muscularis of vagina; i, blood- surface of the cervix being covered with a stratified squamous epithelium with underlying papillae similar to and con- tinuous 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 may be 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 hypersemia determining a swelling and growth of the entire mucosa. THE REPRODUCTIVE SYSTEM 369 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 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 de- cidua menstrualis (Fig. 259). {h) 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 rup- ture of the vessel walls, partly by diapedesis. The hemorrhage 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 destruction during menstruation, some claiming that the entire epithelium is de- stroyed with each menstrual period, others that the epithelium re- mains almost intact. Complete destruction of the e])ithelium is hardly compatible with the restoration of the ejnthelium which always follows menstruation. While there is undoubtedly destruction of most or all of the surface epithelium and of the glands to some con- siderable dei)th, the deeper portions of the glands always remain to take |)art in the succeeding regenerative phenomena. {c) Tin-; Stac;!-: of Rki'Akaiion. — After from three to live days the blecfjing from the uterine mucosa ceases and the return to the resting condition begins. This is marked by disaj)pearan(e ot the Fig. 259. — Section Membrane of Virgin Uterus during First Day of Menstruation. X30. (Sciiaper.) <;, Surface epithelium; b, disintegrating surface; r, pit-like depression in mucous membrane; d, excretory duct; e, blood- vessels; g, gland tubule; //, dilated gland tulnile; m, muscularis. 370 THE ORGANS congestion, by decrease in thickness of the mucosa and in the size of the glands, and by restoration of the surface epithelium. 3. The Mucosa of the Pregnant Uterus. This is known as the decidua graviditatis, and presents changes in structure somewhat similar to those which occur during menstrua- tion, but more extensive. It is divided into three parts: (a) The decidua serotina or decidua basalis — that part of the mucosa to which the ovum is attached. (b) The decidua refiexa or decidua capsularis — that part of the mucosa which surrounds the ovum. (c) The decidua vera — which consist of all the remaining mucosa. The development of the decidua vera resembles the changes which take place in the mucosa during menstruation. There is the same thickening of the mucosa, and this thickening is due to the same factor, i.e., distention of the blood-vessels and proliferation of the tissue elements. These changes are, however, much more extensive than during menstruation. The superficial part of the stroma between the mouths of the glands becomes quite dense and firm, forming the compact layer. The deeper part of the stroma contains numerous cavities, which are the lumina of the now widely distended and tortuous glands. This is known as the spongy layer. Within the stroma, especially of the compact layer, develop the so-called decidual cells. These are peculiar typical cells derived from connective tissue. They are of large size (30 to 100/^), vary greatly in shape, and in the later months of pregnancy have a rather charac- teristic brown color, which they impart to the superficial layers of the decidua vera. They are mostly mononuclear, although poly- nuclear forms occur. During the latter half of pregnancy there is a gradual thinning of the decidua vera, due apparently to pressure. The necks of the glands in the compact layer disappear, and the gland lumina in the spongy layer are changed into elongated spaces, which lie parallel to the muscular layer. The decidua refiexa and decidua serotina have at first the same structure as the decidua vera. The decidua refiexa undergoes hyaline degeneration during the early part of pregnancy, and by the end of gestation has either completely disappeared (Minot) or has fused with the decidua vera (Leopold). THE REPRODUCTIVE SYSTEM 371 The decidua serotina undergoes changes connected with the development of the placenta. The Placenta^ The placenta consists of two parts, one of which is of maternal origin- — placenta uterina — the other of foetal origin — the placenta fostalis. As it is in the placenta that the interchange takes place between the maternal and the foetal blood, the relations between the maternal and foetal parts of the placenta are extremely intimate. This relation consists essentially in the growing out from the foetal placenta of tinger-like projections — villi — which penetrate the mater- nal placenta, the latter being especially modified for their reception. The Placenta Fcetalis. — This is a differentiated portion of the chorion. On the surface directed toward the fcetus the chorion after the third month is covered by a delicate foetal membrane, the placen- tal portion of the amnion. This consists of a surface epithelium, resting upon a layer of embryonal connective tissue which attaches it to the chorion. The chorion consists of (a) a compact layer — the membrana cJiorii — composed at first of embryonal, later of fibrous, connective tissue, and containing the main branches of the umbilical vessels and {b) an inner villous layer, which gives rise to finger-like projections which extend down from the foetal into the maternal placenta and serve to connect the two. The chorionic villi first appear as short projections composed entirely of epithelium. Each of these primary villi branches dichoto- mously, giving rise to a number 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 maternal placenta pass down among the villi and separate them into groups or cotyledons. The main or primary villi run a quite straight course from the chorion into the maternal placental tissue, apparently serving to secure firm union between the two. They are thus known as roots of attachment or fastening villi (Fig. 260) . The secondary villi are given off laterally from the primary villi, end freely in the spaces between the latter intervillous si)accs (Fig. 260) — and are known -d?, free, terminal or floating villi (Fig. 260). The chorionic villus thus ccmsists of a central core of connective ' I-'or many facts as to ihc structure of the placenta, the writer is indebted lo the excellent chapter f)n the subject aflrlcd \)y I'rof. Alfred Schai)er to the liftli edition of Stohr's "Textbook of Mistoloj^y." 372 THE ORGANS THE REPRODUCTIVE SYSTEM 373 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 outHned cells and an outer layer of fused cell bodies-a syncytium (Fig. 262, A, a)-containing small scattered nuclei. The villi of the later months of pregnancy have no defimte epithelial covering, but are surrounded by a dehcate homogeneous membrane, probably the remains of the syncytium. At various points on the surface of the villus are groups of nuclei. These stam "Giant" cell Syncytium Trophorierm mass Stroma of villus mass vulus ->^ Fig. 261.— Section of Chorion of Human Embryo of one monlh (9 mm.). (Grosser.) intensely, are surrounded by a homogeneous protoplasm, and form knob-Uke projections above the general surface of the villus. They are known as cell patches, or more properly as nuclear groups (Fig. 262 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 resemble more closely in structure the younger villus, being frequently covered by a nucleated syncytium. Portions of the syncytium, especially of older villi, sometimes bec(;me changed into a peculiar hyaline sub- stance containing numerous channels. This is known as canalized 374: THE ORGANS d—f-. fibrin, and may form dense layers upon the surface of the chorion. (Fig. 261.) The Placenta Uteeina. — 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 foetal pla- centa, as described above, separat- ing 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 suhchorionic pla- cental 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 between the maternal and the foetal circulations, the arrangement of the placental blood-vessels is of espe- cial 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. 260) their walls lose their muscular and connective- tissue ele- ments and become reduced to epithehal 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 vilH. 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 foetal blood. Inter- change of materials must therefore always take place through the capillary walls and through the walls of the chorionic vilH. (Fig. 260.) Fig. 262. — Cross Sections of Human Chorionic Villi at End of Pregnancy. X250. (Schaper.) A, Small villus; B, larger villus, a, Protoplasmic coat (syncytium); h, epithelial nucleus; c, nuclear groups; d, small artery; e, small vein; /, capillaries. THE REPRODUCTIVE SYSTEM 375 Blood-vessels. — The arteries enter the uterus from the broad ligament and pass to the stratum vasculare of the muscularis, where they undergo extensive ramification. From the arteries of the stra- tum vasculare branches pass to the mucosa and give rise to capillary networks, which surround the glands and are especially dense just beneath 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 vas- culare. Thence the veins accompany the arteries, leaving the uterus through the broad Ugament. 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 longitudinal 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 papillatcd connective-tissue stroma of mixed fibrous and elastic tissue. The stroma usually con- tains diffuse lymphoid tissue and more rarely solitary nodules. Cover- ing the stroma is a stratified squamous epithelium, the surface cells of which are extremely thin. The surface of the mucosa is not smooth, 376 THE ORGANS but is folded transversely, forming the so-called rugcB. Most au- thorities 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 sub- mucosa, muscularis, and stroma. The vascular networks have a general direction parallel to the surface. The capillaries empty into veins which form a plexus of broad venous channels in the muscularis. The L5mipliatics. — 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 — glandulce vestih- ulares minores — especially numerous around the clitoris and opening of the urethra. Larger mucous glands — glandulce vestihulares ma- jores, 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 epithelium with underlying papillae, and is richly supplied with nerves having highly specialized terminations. Development of the Urinary and Reproductive Systems The development of the genito -urinary system is complicated by the appear- ance, and disappearance for the most part, of two sets of urinary organs, and the final formation of the permanent set. The three sets, in the order of their appearance, are the pronephroi, mesonephroi, and metanephroi. The first two sets, which are present only in the embryo in the higher animals, are the representatives of organs that function in the adult in the lower vertebrates. They are also intimately concerned in the development of the efferent duct system of the male reproductive organs in higher animals. The metanephroi,. generally known as the kidneys, are the functional urinary organs in the majority of reptiles and in all birds and mammals. The pronephroi are represented in the human embryo of 3 to 5 mm. by one or two small, condensed masses of mesoderm just lateral to the primitive segments THE REPRODUCTIVE SYSTEM 377 on each side in the cervical region. These masses, which are probably derived from the mesothelial lining of the body cavity, may or may not become hollow, but do not connect with the pronephric duct, and soon disappear. The pro- nephric duct appears about the same time as a derivative of the mesoderm just lateral to the primitive segments. It extends from the cervical region to the caudal region of the embryo where it bends mesially and opens into the gut. These ducts persist and become the ducts of the mesonephroi. The mesonephroi begin to develop almost as soon as the pronephroi and just caudal to them. Condensations appear in the mesodermal tissue lateral to the primitive segments and become more or less tortuous. Lamina appear in these condensations, thus forming tubules which then connect at one end with the pronephric duct (now the mesonephric or Wolffian duct). The tubules develop progressively from before backward and finally form a series extending from the cervical region to the pelvic region of the embryo. At the distal end of each tubule a glomerulus, containing branches from the aorta, develops. The tubules increase in length and number and come to form a pair of large struc- tures which project into the dorsal part of the body cavity. These are often spoken of as the Wolffian bodies. They reach the height of their develop- ment during the fifth or sixth week. During the period of their existence in the embryo in higher animals, the mesonephroi functionate as urinary organs, not only through the agency of the glomeruli but also by means of the epithelium of the tubules themselves, among which numerous branches of the posterior cardinal veins ramify. From the sixth week on, and coincident with the development of the meta- nephroi or kidneys, the mesonephroi atrophy, leaving finally only certain parts which differ in the two sexes. In the male some of the tubules in the cephahc portion persist as the vasa efferentia, while a few in the caudal portion remain as the paradidymis and vasa aberrantia; the duct persists as the vas epididy- midis, vas deferens, and ejaculatory duct. In the female the mesonephric tu- bules disappear for the most part, only a few remaining to form the epoopho- ron and paroophoron, while the duct persists in part as Gartner's canal. Each kidney begins in embryos of 5 to 6 mm. as a hollow bud on the dorsal side of each mesonephric duct near its opening into the gut. This bud grows dorsally, and then turns cranially in the mesoderm between the vertebral column and the mesonephros. The proximal portion remains more slender as the ureter, while the distal end becomes dilated to form the primitive renal pelvis. From this dilated end a number of secondary evaginations grow out to form the papillary ducts and straight collecting tubules. The mesodermal tissue surrounding the outgrowths from the primitive renal pelvis becomes conden.scd in places and gives ri.se to the convoluted portions of the uriniferous tubules and to Ilenle's loops. A glomerulus develops in connection with the distal end of each convoluted tubule. The portion of each tubule derived from the mesodermal tissue unites secondarily at the arched (junctional) tubule with the portion derived from the renal pelvis. Thus the kidney tubules arc derived in j)art directly from undifferentiated me.sotlerm (convoluted tubules and Ilenle's loop.s) and in part from an outgrowth from tlie mesonephric duct (straight collecting tubules and papillary ducts). 378 THE ORGANS During foetal life the kidneys are distinctly lobulated, but after birth the surface becomes quite smooth. The genital gland on each side appears on the mesial surface of the meso- nephros as a thickening of a narrow band of the mesothelial lining of the body cavity. The cells in the band become differentiated into two kinds — small cuboidal cells which stain rather intensely, and larger spherical cells with clearer cytoplasm and vesicular nuclei. The latter are the sex cells which are destined to give rise to the ova in the female or the spermatozoa in the male. The whole thickened band is known as the germinal epithelium. The cells of the germinal epithelium increase in number by mitosis and soon become differentiated into two layers — a superficial layer which retains its epithelial character and contains the sex cells, and a deeper layer which is des- tined to give rise in part to the stroma of the genital gland. The elevation caused by the increased number of cells projects into the body cavity as the genital ridge. From the superficial layer containing the sex cells a number of plugs or columns of cells grow down into the deeper layer carrying some of the sex cells with them. Thus far (up to about the fifth week) the changes are common to both sexes, and only later does the sex differentiation occur. After about the fifth week certain changes occur in the genital ridge which differ accordingly as the ridge is to become an ovary or a testicle. In the case of the ovary a layer of loose connective tissue grows in between the surface epithelium and the cell columns or plugs mentioned above. The cell columns are thus pushed farther from the surface and constitute the medullary cords. These ultimately disappear. The surface epithelium again sends plugs of cells (Pfliiger's egg cords) down into the underlying tissue. These cords are made up for the most part of epithelial cells which give rise to the follicular cells, but contain also a considerable number of sex cells (primitive ova). The egg cords then become broken up into smaller masses each of which contains a single primitive ovum (rarely more) and constitutes a primitive Graafian follicle. The sex cell grows in size and becomes the primary oocyte (and finally the mature ovum), while the epithelial cells around it give rise to the stratum granulosum and germ hill of the mature Graafian follicle. During these processes the stroma also increases in amount, while the original germinal epithelium becomes reduced to a single layer of cuboidal cells. The formation of egg cords is usually com- pleted before birth. (See also p. 354.) In the case of the testicle a layer of dense connective tissue, the tunica albuginea, develops between the germinal epithelium and the sex cords. The epithelium becomes reduced to a single layer of flat cells. The sex cords which first grew into the underlying tissue and which contain the sex cells, are destined to give rise to the convoluted seminiferous tubules. This phase of development differs from that in the ovary inasmuch as in the latter case the first formed sex cords (medullary cords) disappear, Pfliiger's egg cords being formed later and having no homologue in the testicle. The sex cords of the testicle become more and more convoluted and the sex cells (spermatogonia) proliferate rapidly. Beginning after birth and continuing up to the time of puberty, lumina appear in the sex cords and they thus give rise to the convoluted seminiferous tubules. The supporting cells (of Sertoli) are probably derived from the undifferentiated epithelial cells of the sex cords. (See also p. 335.) THE REPRODUCTIVE SYSTEM 379 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. 8) or in formalin- ]MiiIler's fluid (technic 5, p. 7). For topography these shces are cut in half through the middle of the uterine cavity and sections made through the entire half organ. These are stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 21) and mounted in balsam. For details of the mucous membrane, cut away most of the muscle from around the half sHce, being careful not to touch the mucous surface; make thin sections, stain with haematoxjdin-eosin (technic i, p. 20), 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 (i). (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-Miiller's fluid. The blocks of tissue should be so arranged that sections include the utero-placental junc- tion. They may be stained with haematoxylin-eosin or with h«matoxylin-picro- acid-fuchsin (see above). (5) Treat pieces of the human vagina according to technic i, p. 254. General References for Further Study Ballowitz: Weitere Beobachtungen liber den feineren Bau der Saugethier- Spermatozoen. Zeit. f. wiss. Zool., Bd., Hi, 189 1. Hertwig: Lehrbuch der Entwickelungsgeschichte des ^Menschen und der Wirbelthiere, Jena, 1896. Kolliker: Handbuch der Gewebelehre des Menschen. Nagel: Das menschhche Ei. Arch. mik. Anat., Bd. xxxi, 1888. Ruckert: Zur Eireifung der Copepoden. Anat. Hefte, I. Abth., Bd. iv, 1894. Schaper: Chapter on the Placenta in Stohr's Text-book of Histology, 5th ed. Sobotta: Ueber die Bildung des Corpus luteum bei der Maus. Arch. f. mik. Anat., Bd. xlvii, 1896. — Ueber die Bildung des Corpus luteum beim Kaninchen. Anat. Hefte, I. Abth., Bd. viii, 1897. CHAPTER X THE SKIN AND ITS APPENDAGES The Skin The skin or cutis consists of two parts: (i) The derma, corium, or true skin, and covering this, (2) the epidermis or cuticle. The derma is a connective-tissue derivative of the mesoderm, the epider- mis an epithelial derivative of the ectoderm. •""•;\ .-f'!'^?'^. - — d ws^.^ ^a- ff CI-^Xj- '--" /^,lier' 0 Fig. 263. — -Vertical Section of Thin Skin, Human. X60. (Technic 2, p. 385. )'a, Epidermis; h, pars papillaris of derma; c, papillas; d, pars reticularis of derma; e, duct of sweat gland; /, sweat gland; g, subcutaneous fat. The Derma. — This is divided into two layers which blend without distinct demarcation. The deeper is known as the pars reticularis, the more superficial as the pars papillaris (Fig. 263), The -pars reticularis is made up of rather coarse, loosely arranged 380 THE SKIN AND ITS APPENDAGES 381 white and elastic fibres with connective-tissue cells in varying num- bers. The fibres run for the most part parallel to the surface of the skin. The pars papillaris is similar in structure to the preceding, but both white and elastic fibres are finer and more closely arranged. 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 A\ Fig. 264.— Thick Vertical Seclion through Skin of Finger Tip. (Merkel-Henle.") A, Epidermis; B, derma; C; subcutis. a, Stratum corneum; b, duct of sweat gland; c, stratum lucidum; d, stratum germinativum; e, papilla of derma; /, derma; g, blood- vessel; h, sweat gland; /, fat lobule; p, sweat pore. 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 termmiiiions— -nerve papilla (Fig. 265). 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 382 THE ORGANS 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 panniculus adiposus. The Epideemis. — This is composed of stratified squamous epithelium. In the comparatively thin skin of the general body sur- face the epidermis is divided into two sublayers: (i) One lying just above the papillary layer of the derma, and known as the stratum af^^SZ J>^ 1 ?^'^-^.r^Jt^%^t ^>. ? -S'. ««, fe^ ^If ^l Fig. 265. — From Vertical Section through Skin of Human Finger Tip. X200. (Schafer.) a, Stratum corneum; b, stratum lucidum; c, stratum granulosum; d, stratum germinativum. To the left a vascular papilla; to the right a nerve papillalconta.mmg tactile corpuscle. germinativum (stratum mucosum — stratum Malpighii) ; (2) the other constituting the superficial layer of the skin — the horny layer or stratum corneum. In the thick skin of the palms and soles two ad- ditional layers are developed; (3) the stratum granulosum; and (4) the stratum lucidum (Fig. 264). (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 THE SKIN AND ITS APPENDAGES 383 from the derma. The membrane and cells follow the elevations and depressions caused by the papilla. 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. 266, c). The cells of the stratum germinativum are usually in a state of active mitosis. (2) The stratum grannlosum is well developed only where the skin is thick. It consists of from one to three layers of flattened polygonal cells. The protoplasm of these cells contains deeply staining granules — keratohya- line granules — which probably repre- sent a stage in the formation of the horny substance — keratin — of the corneum cells. The nuclei of these cells always show degenerative changes, and there is reason for be- lieving that this karyolysis is closely associated with the formation of the keratohyaline granules (Fig. 266, b). (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 transparency of the cells is due to the presence of a substance known as elcidin, and derived from the keratohyaline granules of the stratum granulosum (Fig. 266, a). (4) The stratum corneum varies greatly in thickness, reaching its greatest development in the skin of the i)ahns and soles. The cells **^^ ' ^ @ i r^^^ Vu,. 266. — F'rom Vertical Section through Thick Skin. (Merkcl- Henle.) _— - g n ^ A & \\C!!);|£^ -^yn \£j v^ c Fig. 279. — A , Mammary gland cells — secreting stage; B, same — excreting stage; the secretion having separated from the cells; C, resting stage; w = nucleus. ^ = ergasto- plasm filaments. (Simon.) Lymphatics. — ^Lymph capillaries form networks among the alveoli and terminal ducts. The lymph capillaries empty into larger lymph- atics 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 THE SKIX AXD ITS APPENDAGES 401 gland, the larger trunks follomng the interlobar and interlobular connective-tissue septa. The nerve terminals break up into plexuses which surround the alvech just outside their basement membranes. From these plexuses dehcate 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 lacta- tion the alveoU 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 connective tissue ultimately almost wholly replacing the glandular elements. TECHNIC (i) Fix thin sUces of an inactive mammary gland in formalin-Muller's fluid (technic 5, p. 7). Stain sections with haimatoxylin-eosin (technic i, p. 20), and mount in balsam. (2) Prepare sections of an active mammary gland, as in preceding technic (i). (3) Fix very thin small pieces of an active gland in one per cent, aqueous solution 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 Fiirther Study Kolliker: Handbuch der Gewebelehre des Menschen. McMurrick: Development of the Human body. Ranvier: Traite Technique d'Histologie. Schafer: Essentials of Histology. Spalteholz: Die Vertheilung der Blutgefasse in der Haut. Arch. Anat. u. Phys., Anat. Abth., 1893. CHAPTER XI THE THYREOID AND PARATHYREOID, THE PITUITARY BODY, THE PARAGANGLIA AND THE ADRENAL. The Thyreoid The thyreoid (Fig. 280) is a ductless structure built upon the general principle 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 :'?' w Fig. 280. — Section of Human Thyreoid. Most of the alveoli contain colloid. lobules. From the perilobular connective tissue finer strands extend into the lobules, separating the alveoli. The latter are spherical, oval, or irregular in shape. They vary greatly in diameter (40 to 120/^) 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 402 THE THYREOID AXD PARATHYREOID 403 or double layer of cuboidal epithelial cells. Two types of cells are recognized. One of these is actively secreting colloid and is known as a secreting or colloid cell. The other contains no colloid and is known as a resting or reserve cell. It is probable that their names indicate the relation of these cells to each other and that the reserve cell ultimately becomes a colloid-secreting cell. The colloid cell appears in some cases simply to pour out its colloid secretion into the lumen, after which it may assume the character of a resting cell; in other cases the cell appears to be completely trans- formed into colloid, its place being taken by proliferation of the rest- ing cells. In certain alveoli which are much distended with colloid the lining epithelium is flattened. That the thyreoid exerts a decided influence upon general body metabolism is shown by the symptoms resuhing from congenital absence of the gland (con- genital myxcedema or cretinism) and by the effects of complete removal, the latter giving rise to a train of symptoms known as the "cachexia strumipriva." The blood supply of the thyreoid 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 alveoH. Terminals to the secreting cells end in club-like dilatations against the bases or between the epithelial cells. A few afferent meduUated fibres are found in the plexuses surrounding the blood-vessels. Development. — The thyreoid originates as a diverticulum from the ento- derm of the primitive pharynx. It first appears in embryos of 3 to 5 mm. and grows ventrally into the mesoderm of the ventral wall of the neck. Here it forms a mass which lies transversely across the neck. It is composed of solid cords of cells which become hollow to form the alveoli of the gland. At first the gland is connected with the surface by the thyreo-glossal duct. This either disappears entirely or is represented in the adult by such rudimentary structures as the so-called prehyoid, suprahyoid, and accessory thyreoid glands. The gland at first con.sists 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 occurs with the formation of colloid. 404 THE ORGANS The Parathyreoids These are small ductless glands which usually lie upon the posterior surface of the lateral lobes of the thyreoid. There are commonly two pairs, a superior and an inferior, on each side. The number is, however, subject to variation. Each gland is from 6 to 8 mm. long, about half that in breadth, and 2 mm. thick. Small groups of cells having the structure of the parathyreoids have been found below the thyreoid and within the thyreoid and thymus. t>^. ^' * *' " • '-^'Vti— ^.-^- > > jiffere/itnerinhera/ neu- ^ Fig. 288. — Diagram illustrating an arc transversing only the segmental, and an arc transversing the suprasegmental part of the nervous system. All reactions performed by the nervous system must ultimately take effect upon some part of the body or effector and are usually ini- tiated by changes in some receptor. Such a circuit from receptor to effector may be termed a neural arc (Fig. 288), and will involve affer- ent peripheral neurones, efferent peripheral neurones and usually intermediate neurones. The complexity of such arcs depends largely upon the number and character of intermediate neurones intercalated in the arc between the afferent and efferent peripheral neurones. Such arcs may obviously traverse centrally only the cord or segmental brain or may also traverse one or more of the suprasegmental parts of the nervous system. Intermediate neurones which link together different parts of the same segment may be termed intrasegmcntal neurones. Intermediate neurones which link together different segments of the cord and segmental brain may be termed interseg- mental neurones. Other intermediate neurones form conduction paths to and from suprasegmental ])arts (ajfcrciit and rjfrrnil supra- 422 THE ORGANS segmental neurones) and still others are suprasegmental associative neurones, confined to suprasegmental structures (Fig. 288). 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, a part of the latter being often referred to as a separate membrane, the arachnoid (Fig, 289). The dura mater is the outer of the two membranes and consists of dense fibrous tissue. The cerebral dura serves both as an investing 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 {h) an outer layer, which forms the periosteum and is similar in structure to the inner layer, but much richer in blood-vessels 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 separate peri- osteum. 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 spinal dura is said to contain lymphatics which open on both of its surfaces. Beneath the spinal dura, between it and the arachnoid, is the subdural space, a narrow cleft containing a fluid probably of the nature of lymph. It is stated that this space communicates by lymph clefts with the lymph spaces in the sheaths of nerves and also communicates with the deep lymph- atic vessels and glands of the neck and groin. It has no direct com- munication with the subarachnoid space (see below). 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 lamellas 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 space beneath the arachnoid is the subarachnoid space and contains cerebrospinal fluid. There is direct communication between these cavities and the subarachnoid space in the roof of the fourth THE NERVOUS SYSTEM 423 ventricle. The subarachnoid space communicates with the lymphatic spaces within the nerve sheaths. It is also stated that it communi- cates with the lymph spaces contained within the vascular connective tissue (media and adventitia) of the blood-vessels which penetrate the central nervous system. Whether there is normally a peri- vascular lymph space between adventitia and glial membrane is doubtful and the existence has also been denied, by good authorities, of lymph spaces within the glia, the glial membrane forming in normal conditions a continuous structure. The inner himclhe of the i)ia are more loosely arranged, are more cellular and more vascular. Espe- 424 - THE ORGANS daily conspicuous are large, irregular cells with delicate bodies and large distinct nuclei. 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 longitudinal fissure. They are composed of fibrous tissue, and fre- quently 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 membranes attached, in formalin-Miiller's fluid (technic 5, p. 7) and stain sec- tions with haematoxylin-eosin (technic i, p. 20). THE PERIPHERAL NERVES The peripheral nerves are divided into spinal nerves and cranial nerves, the former being connected with the cord, the latter with the brain. Each spinal nerve consists of two parts — a motor or efferent 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. 451, and Figs. 308 and 318) and axones of sympathetic ganglion cells. The former leave the cord as separate bundles, which join to form the motor or efferent root. The afferent fibres are peripheral processes of cell bodies situated in the spinal ganglia (p. 429 and Figs. 308, 292). These leave the ganglion and join with the fibres of the motor root to form the mixed spinal nerve (Fig. 308, /) . The connection of the ganglion with the cord is by means of the central processes of the spinal ganglion cells, which enter the cord as the posterior root. Among the afferent fibres of the posterior root are also found, in some animals, a few efferent fibres (Fig. 308, c), processes of cells in the cord. Some fibres from the spinal ganglion and from the efferent roots form the white ramus THE NERVOUS SYSTEM 425 communicans to the sympathetic ganglia. Fibres from the sympa- thetic ganglia form the gray ramus communicans to the mixed spinal nerve (Figs. 289 and 302). For further details regarding cranial nerves see pp. 474, 475. The peripheral nerve consists of nerve fibres supported by con- nective tissue (Fig. 290). Enclosing the entire nerve is a sheath of 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 tissue forms a fairly distinct sheath, the ^e/- Fig. 290. — Erom Transverse Section of Human Nerve Trunk. (Osmic acid fi.xa- tion.) f Quain.) cp. Nerve sheath or epineurium surrounding the entire nerve and con- taining blood-vessels (v) and small groups of fat cells (/); pri\ perifascicular sheath or perineurium surrounding each bundle or fascicle of nerve fibres; oid, interior of fascicle showing supporting connective tissue, the endoneurium. perifascicular sheath or perineurium, which is covered with a layer of flat cells. From the perineurium, delicate strands of connective tissue pass into the fascicle, separating the individual nerve fibres. This con- stitutes the inlrafascicular connective tissue or endoneurium. In the connective-tissue layers of the perineurium are blood-vessels, and lymph spaces lined with endothelium, which communicate with lymph channels within the fascicle. When nerves branch, the con- nective tissue sheaths foHow the branchings. When the nerve becomes reduced to a singh- fibre, th(; jx-rineural and endoneural 426 THE ORGANS connective tissue still remaining constitutes the sheath of Henle. The space between the neurilemma and sheath of Henle has been described as a lymph space communicating with the rest of the lymphatic system of the nerve. On emerging from the central nervous system, the root fibres receive an investment of connec- tive tissue (endoneurium and perineurium) as they pass through the pia mater. This is reinforced by additional connective tissue (epineurium,) as the nerve passes through the dura mater. For description of medullated and non-medullated nerve fibres see Chap. VI. 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- Miiller's fluid (technic 5, p. 7). Stain transverse sections in haematoxylin-picro-acid-fuchsin (technic 3, p. 21), or haematoxylin-eosin (technic i, p. 20), and mount in balsam. Pieces of nerve should also be fixed in osmic acid imbedded, cut, and mounted without further staining. Pieces of fresh nerve may be tested and examined without staining. THE AFFERENT PERIPHERAL NEURONES These comprise, as already stated, the bodies and processes of the cerebro-spinal ganglion cells or cerebro-spinal ganglionic neurones, probably some of the sympathetic ganglion cells, certain cells of the retina and certain cells of the olfactory mucous membrane. The two last named are described in connection with the organs of special sense. The Cerebro-spinal Ganglia A cerebro-spinal ganglion contains the bodies of the cerebro-spinal ganglionic neurones whose processes form the afferent root of the cerebro-spinal nerve. It lies in the dorsal root, a dorsal root being that part of the nerve which lies between the exit of the dorsal root fibres from the cord and their junction with the ventral root. Each ganglion is surrounded by a connective-tissue capsule which is continuous with the perineurium and epineurium of the peripheral nerves. (Fig. 289.) From this capsule connective-tissue trabeculae extend into the ganglion, forming a connective-tissue framework. Within the ganglion the nerve cells are separated into irregular groups by strands of connective tissue and by bundles of nerve fibres. Each ganglion cell contains a centrally located nucleus and a dis- THE NERVOUS SYSTEM 427 tinct nucleolus, and is surrounded by a capsule of flat, concentrically- arranged cells which are probably derived from the neural plate (p, 418) and are often called am pJdcytes or satellite cells (Fig. 293). Stained by Xissl's method the cytoplasm is seen to contain rather small, finely granular chromophilic bodies, which show a tendency to concentric arrangement around the nucleus. Pigmentation is common, the granules usually forming a group in the vicinity of the point of origin of the main process of the cell. The majority of the Fig. 291. — Longitudinal Section through a Spinal Ganglion. X20. (Stohr.) a, Ventral nerve root; b, dorsal nerve root; c, mixed spinal nerve; d, groups of ganglion cells; e, nerve fibres;/, perineurium; g, fat; h, blood-vessel. cells of the spinal ganglia have one principal process which, at some distance from the cell body, divides into a peripheral branch which becomes an afferent fibre of the peripheral nerve and a central branch which enters the cord as a dorsal root fibre (p. 420). These cells are usually called unipolar cells. The principal process usually becomes medullated soon after emerging from the cell cap- sule (Fig. 292). Among these cells a number of forms have been distinguished by Dogiel: (a) Cells with only a principal process. This process may pass almost directly from the capsule, but often takes several turns around the cell body, forming a "glomerulus," before emerging from the capsule (Fig. 292, i and Fig. 293, .1). This form is usually represented as the typical cerebro-spinal ganglion cell, but constitutes only a minority of these cells, (b) The principal i)rocess gives off collaterals. These lie within the capsule or are given off outside the capsule and 428 THE ORGANS terminate in other parts of the gangUon and its covering, either in terminal arborizations or in terminal enlargements or bulbs. The collaterals may branch. Some of the terminations may be around the capsules of other cells. There may be one or several collaterals, sometimes a number of very short intracapsular collaterals. This last variety of cell may have more than one main process. (Fig. 292, 2 and 3, Fig. 293, B). (c) Cells with split processes. Here the main process divides into a numter of fibres which reunite; this may be repeated. The splitting may be intracapsular or extracapsular. A variation of this is where there are two to six processes from the cell which form a complicated intra- capsular network, finally uniting to form the single main process (Fig. 292, 4 and 5 and Fig. 293, C and Z)). (d) Cells with a number of dendrite-like processes Fig. 292. — Various Types of Cells and Nerve Terminations found in a Spinal Ganglion. Schematized from Dogiel. g.r., gray ramus communicans; sy.c, sympa- thetic cell; wj'., white ramus communicans. a. Spinal ganglion; b, dorsal or afferent root; c, ventral or efferent root; d, sympathetic ganglion; g, spinal nerve. For further explanation see text (pp. 427-429). . which divide, forming an intracapsular network which finally fuses into the main process (Fig. 292, 6). (e) Cells whose principal process divides as usual, but the peripheral branch terminates by arborizations or bulbs in the ganglion and in its covering, or in the neighborhood of the dorsal root (Fig. 292, 7). (f) Bipolar cells (Fig. 292, 8). (g) Multipolar cells with a number of intracapsular dendrites and a main process (Fig. 292, 10; Fig. 293, E andi^). (h) Cells with a principal process which probably enters, the dorsal root and a number of proc- esses which may be dendrite-hke in character, but also become meduUated in places, and which branch and terminate in arborizations or bulbs in various parts of the ganglion. These latter apparently collectively represent the per- ipheral process which here ends in the ganglion (Fig. 292, 9) The various endings in the ganglion of collaterals and other processes of ganglion cells often have capsules and resemble the terminations in receptors in THE NERVOUS SYSTEM 429 other parts of the body (corpuscles of Pacini, end bulbs, etc.) and, together with their envelopes, may represent the receptors of the ganglion itself and of the connected nerves. Sj-mpathetic fibres enter the ganglion and form a plexus within it from which fibres pass and terminate within the capsules of the various ganglion cells. P P D E F Fig. 293. — Cerebro-spinal Ganglion Cells and ihcir Capsules. (Cajai.) A (adult man), Unipolar cell with single process forming a glomerulus; B (man), cell with short process ending in intracapsular bulb and main process giving oil intracapsular collateral; C Cdog), "fenestrated" cell with .several jjroces.-^es uniting to form main process; D (ass), more comiilicated form of the same; li (man), cell with short bulbous dendrites; /'' (man), cell with bulbous dendrites and envelo])efl with jjcrii ellular arborizations {p.a.) of fdires (a.f.) terminating around cell; c, collateral; d, di-ndrite; p, i)rincipal process; s.p., short process. (Cajal's silver slain.) The periphp:ral processes of the cerebro-spinal ganglion CELLS are the ajjereni fibres of the cerebro-spinal nerves (p. 424). The modes (;f termination of these i)eripherai processes in receptors 430 THE ORGANS (p. 419) are extremely varied and complicated. These peripheral terminations are always free, in the sense that, while possibly some- times penetrating cells, they probably never become directly con- tinuous 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, the axis-cylinder splits up into minute fibrils (terminal ar- borization) which pass in between the cells and terminate there, often in Httle knob-like swellings (Fig. 294). Such terminal Fig. 294. — Free Endings of Afferent Nerve Fibres in Epithelium of Rabbit's Bladder. (Retzius.) 0, Surfa.ce epithelium of bladder; b g, subepithelial connective tissue; n, nerve fibre entering epithelium where it breaks up into numerous terminals among the epithe- lial cells. fibrils do not penetrate among the squamous cells. Similar "dif- fuse^' endings are found in serous membranes and in simple epithelia, also in connective tissue. In the case of glandular epithelia such endings may, in part at least, be terminations of efferent (secretory) fibres from sympathetic ganglion cells (p. 439). Diffuse endings have also been described on endothelial surfaces such as endocardium and in smooth muscle. The latter are to be distin- guished from the regular motor endings described below. An impor- tant form of diffuse ending is that found encircling and ending in the outer root sheaths of hair follicles (Fig. 295). Nerve endings are abundant in the pulp of teeth. There has been some dispute THE NERVOUS SYSTEM 431 as to whether they penetrate the dentine. In addition to such com- paratively 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. Fig. 295. — Nerves and Nerve Endings in ihe Skin and Hair Follicles. (After G. Retzius.) As, Outer root sheath; c, most sui)crficial ncrvc-fibre plexus in the cutis: dr, sebaceous glands; //, the hair itself; hsl, stratum corncum; is, inner root sheath of hair; n, cutaneous nerve; rm, stratum gcrminativum IMal])ighii. 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 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 432 THE ORGANS receive the nerve terminations. These are known as compound tac- tile cells, the axis cylinder ending in a flat tactile disc or discs between the cells. Of the tactile corpuscles (Fig. 296) those of Meissner, which occur Fig. 296. Fig. 297. Fig. 296. — Tactile Corpuscles of Meissner, tactile cell and free nerve ending. (Mer- kel-Henle.) a, Corpuscle proper, outside of which is seen in the connective-tissue cap- sule, h, fibre ending on tactile cell; c, fibre ending freely among epithelial cells. Fig. 297. — Taste Bud from Circumvallate Papilla of Tongue. (Merkel-Henle.) a, Taste pore; h, nerve fibres entering taste bud and ending upon neuro-epithelial cells. On either side fibres ending freely among epithelial cells. in the skin of the fingers and toes, are the best examples. These corpuscles lie in the papilte of the derma. They are oval bodies, surrounded by a connective-tissue capsule and composed of flattened cells. From one to four medullated nerve fibres are distributed to Fig. 2c I. — -End Bulb from Conjunctiva. (Dogiel.) o, Medullated nerve fibre, axone of which passes over into dense end skein. each corpuscle. As a fibre approaches a corpuscle, its connective- tissue sheath becomes continuous with the fibrous capsule, the medullary sheath disappears, and the fibrillae of the terminal abori- zation pass in a spiral manner in and out among the epithelial cells. THE NERVOUS SYSTEM 433 These terminal fibrils usually end in a flattened expansion consisting of neurofibrils and perifibrillar substance. Tactile corpuscles are also found on the volar surface of the forearm, eyelids, lips and tip of the tongue. 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 usually branched axis cyHnder, surrounded by a mass of finely granular, nucleated protoplasm — the inner bulb— the whole enclosed in a capsule of flattened connective-tissue cells continuous with the sheath of Henle. Other end bulbs may be compound. End bulbs are found also in the mucous membrane of the tongue, epiglottis, nasal cavities, lower end of rectum, peritoneum, serous mem- branes, tendons, ligaments, connective tissue of nerve trunks, synovial mem- branes of certain joints and external geni- tals, especially the glans penis and clitoris.^ The Pacinian bodies (Fig. 299) are laminated, elliptical structures which differ „ t^ • • t. j , I'lG- 299. — racinian Body from the more simple end bulbs already from Mesentery of Cat. (Ran- j •! J • 1 • .1 i 11 vier.) c, Lamina of capsule; described, mamly m the greater develop- j, epithelioid cells lying between ment of the perineural capsule. The cap- laminie of capsule; n, nerve fibre, consistmg of axis cylinder SUle IS formed by a large number of con- surrounded by Henle's sheath, entering Pacinian body; /, perineural sheath; m, mner bulb; n, terminal fibre which lireaks uj) at a into an irregular bulbous terminal arborization. As in the simpler end centric lamellae, each lamella consisting of connective-tissue fibres lined by a single layer of flat connective-tissue cells. The lamcllse are separated from one another by a clear fluid or semifluid substance, bulbs, there is a cylindrical mass of protoplasm within the cap- sule known as the inner bulb. Extending lengthwise through the centre of the inner bulb and often ending in a knob-like ex- tremity is the axis cylinder. Arteries in the vicinity of the cor- puscle break up into capillary networks surrounding the corpuscle. ' This description of the distril>ution of the various rc'rc,i)tors is principally taken from the excellent account in Sihafer's "Text-Mook of Mic roscojjic /Nnalomy." An admirable and still more tlelailcd account is to be found in " 'J'lie Nervous System," by Barker. 434 THE ORGANS From this network capillaries enter the corpuscle, usually near the nerve fibre, and penetrate among the lamellae, even reaching the distal end of the corpuscle. They do not enter the inner bulb. The Pacinian bodies are found in the subcutaneous fat, especially of the hand and foot, in the parietal peritoneum, mesentery, penis, clitoris, urethra, nipple, mammary gland, in the vicinity of tendons, ligaments and joints. In voluntary muscle afferent nerves terminate in Pacinian cor- puscles, in end bulbs, and in complicated end organs called muscle spindles, or neuromuscular bundles. The muscle spindle (Fig. 300) is an elongated cyhndrical structure within which are muscle fibres, connective tissue, blood-vessels, and medullated nerves. The whole is enclosed in a connective-tissue sheath which is pierced at various points by nerve fibres. A single spindle contains several muscle fibres and nerve fibres. The muscle fibres differ from ordi-* nary fibres, being fine and containing more nuclei and sarcoplasm in the middle. The muscle spindles are more numerous in the limb muscles than in those of the trunk, and in the distal than the proximal part of the limb. There are few in eye muscles and in some muscles they have not been detected. According to Ruffini, there are three modes of ultimate terminations of the nerve fibres within the spindles: one in which the end fibrils form a series of rings which encircle the individual muscle fibre, termed annular termina- tions; a second in which the nerve fibrils wrap around the muscle fibres in a spiral manner — spiral terminations ; a third in which the terminations take the form of dehcate expansions on the muscle fibre — arborescent terminations . At the junction of muscle and tendon are found the elaborate afferent terminal structures, known as the muscle-tendon organs of Golgi (Fig. 301). This is a spindle-shaped body composed of several tendon bundles. Into this there enter one or several nerve fibres which break up into complicated terminal arborizations upon the tendon bundles. Other forms of nerve endings enclosed within connective- tissue sheaths are corpuscles of Ruffini found in the subcutaneous tissue of the finger and corpuscles of Golgi-Mazzoni found on the surfaces of tendons and also in the subcutaneous tissue of the fingers and in other parts of the skin. It is evident from the above that the nerve terminations are only stimulated through the intermediation of surrounding cells which may form quite elab- THE NERVOUS SYSTEM 435 Fig. 300. — Middle Third of Muscle Spindle from Striated Voluntary Muscle of Cat. (From Barker, after RufSni.) A, annular terminations; S, spiral termina- tions; F, arborescent terminations. Fig. 301. — Tendon of Muscle of Eye of Ox. (Ciarcio.) Two muscle-tendon organs of (iolgi, each showing ring-like and brush-like endings, i^/i, sheath of Henle; sr, node H ^ a cord the posterior horn expands into a head or ca/>M/, external to 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 base of the dorsal horn by a narrower neck or cervix. External to the gelatinous substance of Rolando is a thin zone containing a plexus of fine medullatcd fibres (Weigert stain) known as the marginal zone or zona spongiosa, and external to this, occupying the space 20 450 THE ORGANS between it and the periphery, is a zone composed of fine longitudinal meduUated fibres rather sparsely arranged and therefore staining more lightly with the Wei- gert method. This is the zona teryninalis or zone of Lissauer. It belongs ob- viously to the white matter of the cord (see page 452). The portion of the gray matter connecting the dorsal and ventral horns may be termed the intermediate or middle gray. Note the well-defined groups of large nerve cells in the anterior horns and the fibres passing out from the anterior horns to the surface of the cord, ventral {anterior) nerve roots. (Figs. 311 and 312.) White Matter. — Note the general appearance of the white matter and the disposition of the supporting strands of neuroglia tissue (light in the Weigert, usually darker in other stains). The neuroglia is seen to form a fairly thick layer just beneath the pia mater from which trabeculae pass in among the fibres, the broadest strand forming the posterior median septum. If the section has been cut through a dorsal {posterior) nerve root, a strong bundle of dorsal root fibres can be seen entering the white matter of the cord along the dorsal and mesial side of the posterior horn. Just ventral to the anterior gray commissure is a bundle of transversely-running meduUated fibres — the, anterior white commissure. (Fig. 312.) It is composed of the axones of various heteromeric column cells, and of decussating terminals of various fibres. In the dorsal part of the dorsal gray commissure are also a few fine transversely running meduUated nerve fibres — the dorsal white commissure. It consists of collaterals of fibres in dorsal funiculi and axones of heteromeric column cells. Cell Groupings. — The positions and groupings of the various cell bodies should now be studied. (Nissl, Hsematoxylin-Eosin, Cajal, Figs. 311 and 312.) (For convenience, their general arrangement throughout the cord is here given; also the course of their axones, though this is usually only seen in Golgi preparations.) (A) Cells of the Dorsal Horn. — (a) Marginal cells arranged tangentially to the border of the gelatinous substance of Rolando, (b) Cells in the gelatinous substance of Rolando, arranged radially. The Golgi method indicates that the axones of (a) and (b) principally enter the adjoining lateral column, (c) Large stellate cells in the apex of the caput, most of the axones of which go to the lateral columns, but some cross in the ventral white commissure, (d) A central group in the central part of the dorsal cornu, some of the axones of which may cross in the ventral commissure, (e) Basal cells in the base of the horn and in the pro- cessus reticularis, the axones of which usually go to the lateral column but may cross, (f) Dorsal (thoracic) nucleus or cells of darkens column in the mesial part of the base of the dorsal horn. These are tautomeric cells, the axones of which from the dorsal spino-cerebellar tract (see page 463). Clarke's column is mainly confined to the dorsal or thoracic cord, but is also present in the first lumbar segment. (B) Cells or the Intermediate Gray. — (a) Middle nucleus whose cells may send their axones across in the ventral commissure or uncrossed to the lateral column; (b) various small cells including the accessory nucleus near Clarke's column; (c) intermedio-lateral group which in parts of the cord forms a projection of the gray known as the lateral horn. These are probably root cells whose axones pass as pregangHonic fibres into the sympathetic system. This nucleus THE NERVOUS SYSTEM 451 is more conspicuous in the thoracic cord, but extends from the seventh or eighth cervical to about the third lumbar segment and is also present in the sacral cord (especially the third segment), (pp. 456, 457 and 458.) From the above it is seen that all the cells of the dorsal and intermediate gray, except the interniedio-lateral group, are column cells, either tautomeric, heteromeric or hecateromeric. ■X 0 C rt ^ -ji ■3 V 2 c3 ca 0 'so c 0 a t3 a ■Ji 6" (C) Cells of thf. Vlntral Worx.— These fall into two categories: (i) Column Cells. These may he tautomeric, sending axoncs to the adjoining while matter or heteromeric, the axones of which cross in the ventral commissure to the while matter of the opposite side. Among I he hitter may be a well-marked group in the dorso-mesial part of the horn (commissural nucleus). Hecateromeric cells may be present. (2) Root cells. Two main divisions may be distinguished: (a) Mesial group, present throughout the cord above the fifth sacral segment 452 THE ORGANS (Bruce). This group probably innervates the striated voluntary (somatic) muscles of the trunk. The mesial group is in part of the cord subdivided into a ventro-mesial and dorso-mesial group, the latter being present in the first, sixth and seventh cervical, thoracic (except first), and first lumbar segments, (b) Lateral group, present in the cervical, first thoracic, and lumbo-sacral re- gions. This group innervates the muscles of the extremities and exhibits the following subdivisions: An antero-lateral (C4 to C8, L2 to S2), a postero-lateral (C4 to C8, L2 to S3) and a post-postero-lateral (C8 to Thi, Si to S3). There is also a central group (L2 to S2) and a small anterior group (Li to L4). The exact muscle groups innervated by these cell groups, respectively, have not yet been definitely determined. Other special cell groups are tht phrenic group (C4), cen- trally located, cilio-spinal and other cells (C8 to Th2, to sympathetic ganglia which send fibres to dilator pupillae and blood-vessels of head), and the spinal accessory (Ci to C6). The latter is located laterally and innervates the sterno- mastoid and trapezius muscles. In the lumbo-sacral cord below the fourth lumbar there is also a medio-ventral splanchnic group which together with the lateral horn group of the sacral cord furnishes the preganglionic fibres emerging from the sacral cord. For the determination of the destination of the axones of the efferent root cells the method of studying the changes in the cell body (Nissl stain) in definite lesions of the peripheral fibres (axonal degeneration, see Chapter VI) is used. Arrangement of Fibres (Fig. 312). — With the low- and high-power objectives the course of the transverse (j.e., longitudinally cut) nerve fibres should be care- fully studied in Weigert and Cajal preparations. These fibres pass from gray to white matter or vice versa, and are in general (a) root fibres entering or leaving the cord; (b) either axones of column cells in the gray passing out into the white there to become longitudinal fibers by turning or splitting, or they are the col- laterals and terminals of the fibres of the white matter entering the gray to terminate there. The arrangement of these fibres should be carefully studied in all parts of the section (Weigert' and Cajal), taking one field at a time. In the dorsal part of the cord, the dorsal roots can be seen entering. From their lateral portion fine fibres detach themselves and enter the zone of Lissauer, the fibres of which are largely composed of their short ascending and descending arms. Most of the fibres of the root pass along the dorsal and mesial side of the dorsal horn, forming the zone of entry of the dorsal roots. By bifurcating (not visible in the preparation) they become the majority of the longitudinal fibres of the dorsal funiculus. From the entering root fibres and fibres of the dorsal funiculus, bundles of fine fibres (collaterals and terminals) pass radially through the gelatinous substance of Rolando or sweep around its mesial side and enter the gray. Some of these terminate in the gelatinous substance of Rolando (Golgi preparations), some form part of the dense plexus of fibres in the caput and ter- minate there, others can be traced to the intermediate gray and, in some cases, some ("direct reflex collaterals") can be traced to the ventral horn. It will be noted that not many come from the mesial part of the dorsal funiculus. Col- laterals from the zone of Lissauer enter the gelatinous substance of Rolando. In the middle part of the cord there is a similar interchange of fibres between THE NERVOUS SYSTEM 453 the plexus in the gray and the adjoining white matter. In the ventral part of the cord a similar interchange takes place, but here besides these fine fibres are seen the coarse fibres of the ventral roots gathered from various parts of the ventral horn to form bundles which leave the ventral side of the horn, pass through the white matter and emerge as the ventral root fibres. The larger bundles o£ fibres in the ventral horn separate the cell groups, but between individual cells are seen numerous fine medullated fibres (principally terminals of fibres from the white funiculi). Trace as far as possible the course of the fibres of the ventral and dorsal white commissures. Finer Structures.' — Study with the high power the general histological struc- ture of the gray and white matter. In the gray matter note (Cajal, Nissl, H.-E.) , besides the nerve cells and their processes, the neuroglia nuclei. Note also the structure and size of the medullated nerve fibres (Weigert). In the white matter Fig. 313. — I'rom Transv^erse Section of Elephant's Cord. Neuroglia Stain, b, c, d and i, Four types of neuroglia cells; k through several neuroglia cells; /, leucocyte. (Hardesty.) Benda's neuroglia fibre passing note the appearance of the cross-cut medullated nerve fibres in Weigert, Cajal and H.-E. preparations. With the neuroglia stains study carefully the neuroglia cells and neuroglia fibres, including the neuroglia zone forming the margin of the cord. (Fig. 313.) Note also the pia mater and the connective-tissue septa (usually perivascular) entering the cord from the pia accompanied usually by a denser aggregation of glia fibres. Note carefully the number of neuroglia nuclei in some small field. Increase in neuroglia is characteristic of many pathological conditions. Study the ependyma. (Weigert, Nissl, Cajal and glia stains). Study the internal structure of the nerve-cells of various sizes present, espe- cially the amount and arrangement of the chromophilic substance (Nissl). The smallest nerve cells of the cord have a limited amount of chromophilic substance, often either in the form of perinuclear cai)s or small bodies near the i)criphery * It may sometimes he afivantagcous to hi histological structure of the gray and while m;i architeclural arrangements of the ar ' ' ' ■al to have iho high power study of the gencru, _ ...alter of the cord precede the study of the ird here placed first. 454 THE ORGANS of the cell. In the medium cells more chromophilic bodies are present. The ceUs of Clarke's column have a considerable number of chromophilic bodies arranged near the periphery of the cell. The root cells are richest in chromo- philic substance (for further details see Chap. VI). In general it seems that the intermediate neurones belonging to efferent paths {i.e., acting on periph- eral motor neurones) have the definite, clear-cut. coarse chromophilic granules characteristic of the peripheral motor neurones; while the neurones forming parts of afferent paths have the fine, indefinitely grouped granules character- istic of the cerebro-spinal ganglionic neurones (Jacobsohn and Malone) . Blood-vessels (Fig. 3 14. — )Study the arrangement and structure of the blood- vessels of the cord and pia. There are three principal longitudinal arteries, the anterior spinal artery given off from the vertebral arteries near their union Posterior spinal artery- Column cells Region sup- plied by sulco- commissural artery Root cells Anterior spinal artery (giving off a sulco-commissural artery) Fig. 314. — Schematic transverse section of Cord, Showing General Distribution of Blood-vessels (left) and Nerve-cells (right) (Bing). Root-cells; i, postero-lateral group; 2, antero-lateral group; 3, antero-medial group; 4, central group; 5, postero-medial group. The broken black lines on the surface of the cord are portions of the vascular network in the pia mater. into the basilar artery, and two posterior spinal arteries, given off also from the vertebral arteries. These arteries are reenforced by small arteries passing to the cord along the dorsal and ventral roots and form an arterial network in the pia mater. From the network terminal {i.e., non-anastomosing) branches enter the cord, supplying all parts except the ventral horn and column of Clarke. The latter are supplied by branches from the anterior spinal artery which pass dorsally in the ventral sulcus {sulco-commissural arteries) and enter the cord alternately to right and left. They then break up into a rich capillary network in the ventral horn, supplying also a branch to the column of Clarke. The veins of the cord also form a plexus in the pia mater. Larger posterior median and an- terior median veins can be distinguished. Portions of the above vessels can be seen, cut in various planes in the pia and in the cord. The general appearance and structure of the blood-vessels, including capillaries, should be noted in the various methods of staining. THE NERVOUS SYSTEM 455 Variations in Structure at Different Levels 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. These variations are due to: (i) Variations in the size of the nerves entering and leaving, which cause correspond- ing variations in the gray matter which receives the afferent fibres and contains the cells of origin of the efferent fibres. Thus the larger nerves of the extremities cause the increase in size of the gray matter of the cervical and lumbo-sacral cord with which they are con- nected, and also an increase in the dorsal funiculi. (2) A gradual increase in the white matter of the cord, as higher levels are reached, due to an increase in the number of long ascending and descending fibres to and from the brain. PRACTICAL STUDY Section through the Twelfth Thoracic Segment (Fig. 316). — 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 seen the group of cells known as Clarke's column (p. 450). MeduUated fibres can be seen passing from the dorsal funiculus into Clarke's column, where they interlace among the nerve cells. These fibres are collaterals of the dorsal root fibres terminating in the nucleus. From the nucleus coarser fibres can be seen gathering at its ventral side and thence passing outward to the periphery of the cord where they bend upward forming ihe beginning of the dorsal spino-cercbellar tract fsee p. 46.3). Section through the Mid-thoracic Region (Fig. 316). — Compare with the lumbar sections. Xote the change in shape and size; that the cord is more nearly round and smaller; that while the reduction in size affects both gray matter and white matter it is the former that shows the greater decrease. The horns are even more slender than in the twelfth thoracic section, and the anterior horn contains still fewer cells. Clarke's column is present, but not so large. Section through the Cervical Enlargement (Fig. 315). — Note the marked in- crease in Jiizc of the cord, which affects both gray matter and white matter. De- pending upon the exact level at which the section is taken, the cord may be nearly round or flattened donso-vcntrally. The posterior horns remain slender while the anterior are much broader than the posterior horns. The relkidar process is more prominent than in any of the previous sections. As in the lumbar cord, the cell groups of the anterior horn arc numerous and well defined. .'\ more or less ^56 THE ORGANS C.Il ./^ 0^\ !»•' cm \ C. V C. VI C. VII C. VIII Fig. 315. — Transverse Sections through the Cervical (II-VIII) Segments of the Cord. Weigert preparations. (Rauber-Kopsch.) THE NERVOUS SYSTEM 457 Th. Ill Th. IV Th. V Th. XII Fig. 316. — Transverse Sections ihnjUKh the Thoracic (f-XII) Segmcnls u{ llic C ord. Weigcrt preparations. (Kaubcr-Koitsch.) 458 THE ORGANS L.I L.III L.IV L. V S.I S.III S.IV S. V Fig. 317. — Transverse Sections through the Lumbar (I-V) and Sacral (I-V) Segments of the Cord. Weigert preparations. Rauber-Kopsch.) THE NERVOUS SYSTEM 459 definite septum divides the posterior column into an inner part, the column oJGoll, and an outer part, the column of Burdach. For further variations and differences between the segments of the cord, compare Figs. 315, 316 and 317. Fibre Tracts of the Cord The determination of the fibre tracts of the cord has been accom- phshed principally by two methods: (i) The myelogenetic method, which is based upon the fact that the fibres of different systems ac- quire their myelin sheaths at different periods of embryonic develop- ment. Thus by examining cords from embryos of various ages and young specimens it is possible, using a myelin stain (e.g., Weigert), to distinguish different tracts by the presence or absence of myeliniza- tion of their fibres. (2) The method of secondary or axonal degenera- tion, based upon the fact that a fibre separated from its cell under- goes degenerative changes and ultimately disappears and that the cell body also usually shows certain changes (see page 140). The fibres distal to the injury can be distinguished during active degenera- tion'by applying the Marchi stain (page 34). After their disappear- ance, however, a negative picture is obtained by staining the sur- rounding normal fibres (Weigert) . The changes in the cell bodies whose axones are injured are distinguished by applying the Nissl stain (p. 38). Thus if the cord is cut at some particular level, at any level above the cut all fibres present which originate from cells below the cut will show degeneration ("ascending" degeneration), while the cell bodies of the cut fibres will show the axonal degenera- tion changes. On the other hand, at any level below the cut, fibres which originate from cells above the cut will show degenera- tion ("descending" degeneration), while the cell bodies of these fibres, located above the cut, will exhibit axonal degeneration. The indication this gives as to the direction of conduction is evident when it is remembered that the impulse passes from neu- rone body along the axone to its termination. (3) Atrophy (von Gudden's method). This method is based upon the fact that extirf)ation of some part of the nervous system in a young animal is followed by an atrophy of parts in intimate relation therewith. This method only demonstrates grosser changes than the pre- ceding, but on the other hand whole conduction paths involving more than one neurone rehiy may show changes. Other methods are the method of comparative anatomy, i.e., study of the simi)ler nervous 460 THE ORGANS THE NERVOUS SYSTEM 461 systems of lower forms and the correlated development or absence of related parts of the nervous system, and the method of physiology, i.e., study of the physiological eflfects of stimulation or extirpation of various portions of the nervous system thereby indirectly demon- strating anatomical pathways. Ascending Tracts. It will be recalled that various groups or systems of neurones (nuclei and tracts) are linked together to form conduction paths (p. 421). In general an afferent conduction path consists of (i) a primary system (afferent ganglionic) whose central processes (afferent root fibres) end in its terminal nucleus; (2) a secondary system whose bodies constitute the terminal nucleus of (i) and whose axones form a second ary tract and end in a secondary terminal nucleus; (3) the conduction path may continue through tertiary nuclei and tracts, etc. A . Tracts forming parts of afferent pallial paths. I. Long Ascending Arms of Dorsal Root Fibres {Posterior Funiculi). — The origin of these tracts — central processes of the cells of the spinal ganglia — has been described (page 436). The distribu- tion of the posterior root fibres to the gray matter of the cord was noted in connection with the study of the lumbar enlargement sec- tion Tpage 452). The general arrangement of these fibres in the dorsal funiculi remains to be noted. Each successive dorsal root sends its fibres into the cord next to the dorsal horn and therefore lateral to the ascending fibres from the next root below. Thus the fibres of the lower roots as they ascend the cord are gradually pushed toward the median line until they finally occupy that part of the posterior column lying near the posterior sep- tum. The separation of the posterior column by a connective-tissue septum into the column of GoU and the column of Burdach occurs Fig. 318. — Diagram of the Tracts of the Cord (Cervical Region). Ascending tracts are shown on the left side, and descending tracts on the right. It will be noticed that the tracts of the cord arc roughly divisible into three concentric zones: (i) A zone oc- cupying most of the posterior columns and the perijjheral i)art of the lateral columns. This zone comprises the principal long ascending tracts (beginnings of alTerent supra- segmcntal pathsj. (2) The second zone lies immediately within the first in the lateral columns and also occupies the peripheral part of the anterior columns. It comprises the principal long descending tracts from various |)arts of the brain (terminal i)ortions of ef- ferent suprasegmcntal paths). (3) The third zone borders the gray matter and includes the ground or fundamental bundles of the cord (chiefly sjjinal intersegmental libres). In the figure, for nu, DarkschewUschi read neuclcus of medial longitudinal fasciculus. 462 THE ORGANS only in the cervical cord (Fig. 311 and 315). Here the most median fibres, i.e., those lying in the column of Goll, are the longest fibres of the posterior columns, having come from the lower spinal ganglia (lower thoracic, lumbar and sacral), while the column of Burdach (Fig. 311) consists of short and medium length fibres (upper thoracic and cervical dorsal root fibres) . The fibres of Goll's column end in the nucleus juniculi gracilis or nucleus of the column of Goll in the medulla (see p. 485 and Fig. 326). Those fibres of Burdach's which do not terminate in the spinal cord terminate in the medulla in the nucleus funiculi cuneati or nucleus of the column of Burdach (p. 485 and Fig. 326). The nucleus gracilis and nucleus cuneatus — ■ which will be seen in sections of the medulla (Fig. 326) — thus serve as terminal nuclei for the afferent root fibres in the columns of Goll and those of the columns of Burdach which do not terminate within the cord. Inasmuch as many of the ascending arms are short, it is evident that only a fraction of the dorsal root fibres are represented at the higher levels. Those long arms which reach the medulla con- stitute the beginning of one of the principal afferent cerebral or pallial pathways. The axones of the neurones, whose bodies are the nuclei of Goll and Burdach, cross and form the tract known as the medial fillet (or lemniscus), composing the second system of this path. The fillet terminates in the thalamus and the path is completed by a third system of thalamo-cortical neurones to the cortex palHi, (probably principally to the post-central area). This path is, in brief, as fol- lows: spinal ganglionic (long ascending arms of dorsal roots) -\- fillet -j- thalamo-cortical path, decussating in the medulla (Fig. 322). n. The Spino -thalamic Tract.— This arises from heteromeric cells lying probably principally in the dorsal horn (groups c and d, p. 450). Their axones cross in the ventral commissure and reach the opposite lateral funiculus where they ascend in a position mesial to the ventral spino-cerebellar tract (see below). This tract terminates in the thalamus whence the path is completed by thalamo-cortical neurones. The path is thus : spinal ganghon (short arms and collaterals of dorsal roots) -|- spinothalamic -|- thalamo-cortical neurones, three systems of neurones. Its decussation takes place in the cord at about the level of entry of the dorsal roots involved. Associated with this system may be some fibres to the superior colliculus (spino-collicular tract). (Figs. 318, 319 and 323). B. Tracts forming part of paths to the cerebellum. THE NERVOUS SYSTEM 463 III. The Dorsal Spino-cerebellar Tract {Tract of Flechsig, Direct or Uncrossed Cerebellar Tract). — This tract Hes along the dorsal -lateral periphery of the cord, being bounded internally by the crossed pyramidal tract (Fig. 311 and Fig. 318). The fibres of the direct cerebellar tract are the axones of the cells of Clarke's column (tautomeric column cells) (Figs. 318, 319 and 323). These axones cross the intervening gray matter and white matter of the same side and turn upward as the direct cerebellar tract. In the medulla they form part of the restiform body or inferior cere- bellar peduncle and pass 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 450). As already noted above, some fibres of the posterior root, or their collaterals, end in the column of Clarke. This path is composed then of two systems of neurones, spinal ganglion cells and Clarke's column cells, and is uncrossed (with the exception that some fibres are interrupted in the nucleus lateralis of the medulla and that some of the fibres from the nucleus lateralis cross). (Fig. 337.) rV. The Ventral Spino-cerebellar Tract.- — This tract lies along the periphery of the cord, extending from the anterior limit of the direct cerebellar to about the exit of the ventral roots (Fig. 31 1 and Fig. 318). It is probably ft^rmed by axones whose cell bodies arc scattered Fig. 319. — ^Diagram showing Beginnings of Principal Long Ascending Tracts of Cord and Termination of Lateral Pyramidal Tract. Each group of neurones is repre- sented by one or two neurones, d.s-c, Dorsal spino-cerebellar tract; p, lateral pyramidal tract; s-L, spino-thalamic tract; v.s-c, ventral spino-cerebellar tract; v.r., ventral root. 464 THE ORGANS through the intermediate gray matter, possibly group a (p. 450 and Fig. 311). Some fibres come from tautomeric, others from hetero- meric cells, the axones of the latter crossing in the ventral commis- sure. The tract first appears in the upper lumbar cord and natur- ally increases in size as it passes upward. The fibres of this tract also end in the vermis of the cerebellum. They reach their desti- nation in the cerebellum by a different route, ascending considerably farther than the dorsal spino-cerebellar fibres and then turning back along the outer side of the superior cerebellar peduncle to the ver- mis. This path is thus also a two-neurone path (spinal ganghon cells and spino-cerebellar neurones) and is partly crossed and partly uncrossed. The ventral spino-cerebellar and spino-thalamic tracts are sometimes referred to as Gower's tract. (Figs. 323 and 337.) It seems probable that muscle-tendon sense passes up the cord by tract I (uncrossed in the cord), while pain and temperature pass up by the spino- thalamic tract (crossed). The path pursued by touch is more doubtful but it may pass up partly by tract I and partly by ascending arms of varying lengths which end in the cord around heteromeric column cells (thus partly uncrossed and partly crossed in the cord). This path may join the fillet in the medulla. It would seem probable that the cerebellar tracts convey stimuli from muscle- tendon receptors. Ascending paths may also be formed by successive relays of shorter tracts in the ground bundles of the cord and the reticular formation of the brain. This has been especially claimed for the pain pathway. The afferent visceral path to the pallium is not known. Descending Tracts I. The Pyramidal Tracts {Tractus Cortico-spinalis, Cerehro- spinalis or PalUo-spinalis). — The cell bodies of the neurones whose axones make up this system are situated in the cerebral cortex anterior to the fissure of Rolando (precentral area, Fig. 357). Their axones converge in the corona radiata and pass downward through the internal capsule, pes peduncuH, 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 decussation. Here {a) most of the fibres of each tract cross to the opposite dorso-lateral region of the cord and continue downward as the crossed or lateral pyramidal tract. This lies in the dorsal part of the lateral column (Figs. 311 and 318). It extends to the lower- most part of the cord. In the cervical and dorsal region it is sepa- rated from the surface of the cord by the direct cerebellar tract. In THE XERVOUS SYSTEM 465 the lumbar region the latter tract is no longer present and the crossed pyramidal tract comes to the surface, (b) The minority of the fibres of the anterior pyramids, instead of decussating, remain on the same side to pass down the cord along the anterior median fissure as the direct or anterior pyramidal tract, occupying a small oval area adjacent to the anterior sulcus (Fig. 318). It does not usually extend below the middle or lower dorsal region of the cord. As the pyramidal tracts descend they decrease in size from loss of fibres which continually leave them to terminate in the ventral horn% The fibres of the crossed tract terminate mainly in the horn of thor same side, while most of the fibres of the direct tract probably croi through the anterior commissure to the opposite side of the corc'^^ These tracts are thus mainly crossed tracts, as the great majorit ' of their fibres cross to the opposite side of the cord. There are^^ however, some homolateral (uncrossed) fibres in the lateral pyram- idal tract. The tracts are apt to differ in size on the two sides of the cord, owing to the fact that the proportion of fibres which decus- sate is not constant. The axones terminate in arborizations around the motor cells of the ventral horns. The pyramidal tracts or pallio- spinal system together with the spinal efferent peripheral neurone system constitutes the pallio-spino- peripheral efferent conduction path. According to some authorities the pyramidal fibres terminate around cells in the intermediate gray matter whose axones, which form a part of the ground bundles, in turn terminate around the eft'erent root cells. Short axone (Golgi type II) cells might also of course be intercalated in this connection. (Figs. 322 and 323.) The pyramidal tracts convey to the cord the impulses which result in volun- tary movements, especiall)', probably, individual movements of parts of the limbs (foot, hand, finger, etc.). n. The Colliculo -spinal Tract (Tectospinal Tract) orginates in the colliculi of the midbrain roof, decussates and descends to the cord, where it lies near the ventral sulcus. Its presence in the cord has been disj)uted. TFig. 350.) III. The Tract from the Nucleus of the Posterior or Medial Longitudinal Fasciculus.— This nucleus is located in the reticular formation of the tegmentum of the midbrain cephalad to the nucleus of nerve III.^ The tract originating from it forms in the brain a part ' The fibres in question have been variously slated to originate from the nucleus of Darkschewitsch, the "nucleus of the posterior longitudinal fasciculus," and the nucleus of the posterior commissure. Whether any of these nuclei is the same as the inlerslitial nucleus of Cajal, or the nucleus of van (iehuchten in fishes, is uncertain. Hy the nucleus of the medial longitudinal fasciculus is here meant the interstitial nucleus of Cajal. 30 466 THE ORGANS of the posterior or medial longitudinal fasciculus. It is uncrossed and lies near the floor of the brain cavity and next the median line. In the cord it occupies a similar position near the ventral sulcus. Its fibres terminate in the ventral horn. Some fibres have been traced into the lumbar cord. (Figs. 318 and 350.) IV. The Rubro-spinal Tract {von Monakow's Tract). — This con- sists of axones of the red nucleus (nucleus ruber) located in the teg- ' mentum of the midbrain. These axones cross and descend to the fa . • ^rd, being joined by axones of the other cells in the reticular forma- , n in the region of the pons. In the cord the tract lies mingled with ntral to the lateral pyramidal tract. Its fibres terminate in the ceJ • rt of the ventral horn. This tract is a lower link in a three- oath from cerebellum to cord composed as follows: (a) L cells in cerebellar cortex to nucleus dentatus in cerebel- ; axones of cells in nucleus dentatus via superior cerebellar .-..cle to the nucleus ruber; (c) axones of nucleus ruber as the xuoro-spinal tract to {d) efferent peripheral neurones of cord. (Figs. 318, 323 and 337.) V. The Deitero-spinal Tract {V estihulo-spinal Tract). — This tract originates from Deiters' nucleus which lies in the medulla and receives fibres from the vestibular division of the acoustic nerve and also from the cerebellum (see below). It occupies the ven- tral and mesial periphery of the cord. The more lateral fibres are uncrossed, those near the ventral sulcus come from the nuclei of both sides. Descending with these fibres are probably axones of other vestibular terminal nuclei, and of other nuclei in the gray re- ticular formation (reticulo-spinal fibres) of the medulla. These fibres all terminate in the ventral horn. Some fibres have been traced to the sacral cord. Deiters' nucleus receives fibres from the cerebellum, and thus this tract is a segment of a second efferent cerebellar path- way: {a) Axones of cells in cerebellar cortex to nucleus fastigii in cerebellum; {h) Axones of cePs of nucleus fastigii as the fastigio- bulbar tract to Deiters' nucleus; (c) axones of cells of Deiters' nucleus as the Deitero-spinal tract to {d) efferent peripheral neurones of cord. According to some authorities some fibres proceed from cerebellum to cord without interraption in Deiters' nucleus. (Figs. 318 and 323.) All the fibres of V are sometimes collectively called the antero- lateral descending tract or marginal bundle of Lowenthal. Tract III and the mesial part of V constitute the major portion of THE NERVOUS SYSTEM 467 the descending fibres of an important bundle in the segmental brain known as the medial longitudinal fasciculus. VI. The Fasciculus of Thomas. — Besides the reticulo-spinal fibres already mentioned are fibres in the lateral column which originate in the reticular forma- tion of the medulla and terminate in the gray of the cervical cord. These are known as the tract of Thomas. (Fig. 318.) VII. 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. 318). The origin and destination of its fibres are not definitely known. Some of its fibres appear to originate in the cord and terminate in the inferior olivary nucleus in the medulla (spino-olivary fibres). Vin. The Septo-marginal Tract. — This is a small bundle of fibres lying next the posterior septum. It appears to change its location in different levels, e.g., in the sacral cord it occupies a small dorso-median triangle, in the lumbar region it forms an oval bundle (of Flechsig) at the middle of the septum and a superficial bundle, in the thoracic and cervical cord its fibres are more scattered. It is probably composed of descending axones of cells in the cord. (Fig. 318.) IX. The so-called "comma" tract of Schultze is a small comma-shaped bundle of descending fibres lying about the middle of the posterior column (Fig. 318). It is most prominent in the dorsal 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). In general the descending tracts fall into two categories: (i) Tracts which are descending or efferent suprasegmental paths, or are parts of such paths. These comprise (a) the efferent pallial path (tract I), (b) the efferent midbrain paths (tracts II and III) and (c) the efferent cerebellar paths (tracts IV and V). (2) Descending inter- segmental tracts. Of the latter some originate in nuclei lying in the segmental brain (nucleus of the medial longitudinal fasciculus, nucleus ruber, nucleus of Deiters) which receive efferent supraseg- mental fibres, and thus form links of descending suprasegmental paths. Other reticulo-spinal libres come from other cells in the gray reticular formation. Other still shorter tracts (spino-spinal), from cells in the cord, form the descending fibres in the ground bundles (see below). Tracts VIII and possibly IX are also in this category. The descending j)ath from the i)allium which terminates around the bodies of the splanchnic efferent neurones in the cord is stated to consist of two neurones, a pallio-bulbar neurone to the medulla and a bulbo-spinal neurone to the efferent perij)heral neurone in the cord. The axones of the latter pass out of the cord as preganglionic fibers to the sympathetic ganglion cells. In the cord the fibrt-s of this ]);Lth 468 THE ORGANS probably lie in the ventro-lateral columns, though some authorities place them in the dorsal columns. This pallio-bulbo-spino-sympathetic path is the path by which various psychic states (emotions, perceptions, memories) affect the splanchnic effectors, as seen in blushing, perspiring, erection, pupillary changes, etc. Many tracts contain fibres proceeding in a direction opposite to that of most of the fibres. Fundamental Columns or Ground Bundles (Shorter Inter- segmental Tracts of Cord or Spino-spinal Tracts) 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 Receptor Fig. 320. — Diagram illustrating a Two-neurone Spinal Reflex Arc. Groups of neu- rones are represented by one neurone, gg, Spinal ganglion. (Van Gehuchten.) the lateral side of the posterior horn between the latter and the crossed pyramidal tract (Figs. 311 and 318). 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 (spino-spinal or proprio-spinal) systems of the cord. The THE NERVOUS SYSTEM 469 fibres serve as longitudinal commissural fibres to bring the different segments of the cord into communication (Fig. 321). The shorter fibres lie nearest the gray matter and link together adjacent segments. The longer fibres He 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 (pp. 444 and 445)- The fact, alluded to above, that the shorter fibres lie nearest, or mingled with the gray is, in a general way, true through- out the central nervous system. A result of this in the cord is the superficial posi- tion of many of the longest tracts. Attention has already been called to the concept of neural arcs which may traverse the cord or both cord and suprasegmental structures (page 421). It must be kept in mind that there are prob- ably no isolated neural arcs and that every neural reaction involving any given arc always influences and is influenced by other parts of the nervous system. Fig. 321. — Diagram ilhistrat- ing Three-neurone Spinal Reflex Arcs of one segment and more than one segment. Groups of neurones are represented by one neurone. g, Spinal ganglion cells; h.c.c, hetcromeric column cell; Lc.c, tautomeric column cell; v.r., ventral root. From the neurones thus far studied and the tracts which their axones form, the following neural arcs may be con- structed: (i) A Two-neurone Spinal Reflex Arc (Fig. 320). — (a) Peripheral ajjerent neu- rones; their peripheral processes and receptors, the spinal ganglion cells, their central processes with collaterals terminating around motor cells of anterior horn; {h) peripheral efferent neurones, i.e., motor cells of anterior horn with axones passing to effectors. Such a two-neurone reflex arc is chiefly uncrossed and in most cases involves only one segment or closely adjacent segments. As it involves only one synapsis (see chapter VI) (in the ventral gray) it is sometimes termed a monosynaptic arc. (2) A Three-neurone Spinal Reflex Arc (Ing. 321). — {a) Peripheral aferenl neurones as in (1), but terminating around column cells of the cord, {b) Cord neurones (column cells) — axones in the fundamental 470 THE ORGANS columns with collaterals and terminals to anterior horn cells of different levels, (c) Peripheral efferent neurones as in the two-neu- rone reflex. Such a three-neurone or disynaptic reflex arc may involve segments above or below the segment of entrance of the stimulus and is uncrossed or crossed according as the cord neurones are tautomeric or heteromeric. The independence of the cord as a reflex mechanism is much diminished in man. (3) A Cerebellar Arc may be constituted as follows: (a) Peripheral afferent neurones to (h) column cells in cord {e.g., Clarke's column) via spino-cerebellar tracts to cerebellar cortex; (c) various associative cortical cerebellar neurones; {d) axones of cortical cells to {e) dentate nucleus the axones of which (superior peduncle) pass to (/") nucleus ruber via mbro-spinal tract to {g) efferent peripheral neurones in cord. Another arc would consist of {a), (b) and (c) the same, (d) cerebellar cortex to nucleus fastigii in cerebellum to (e) nucleus of Deiters to (/") efferent peripheral neurones to effectors (Figs. 323 and 337)- Fig. 322. — Diagram showing the Most Important Direct Paths which an Impulse follows in passing from a Receptor (S) to the Cerebral Cortex and from the latter back to an Effector {M) {e.g., muscle), also some of the cranial-nerve connections with the cere- bral cortex. Groups of neurones are represented by one or several individual neurones. A, Sensory cortex; B, motor cortex; C, level of third nerve nucleus; D, level of sixth and seventh nerve nuclei ; E, level of fiUet or sensory decussation ; F, level of pyramidal or motor decussation; G, spinal cord. From Periphery to Cortex. Neurone No. i. — The Peripheral afferent Neurone: i, Spinal, cell bodies in spinal ganglia; receptor, S, peripheral arm of spinal ganglion cell; central arm of spinal ganglion cell as fibre of dorsal root to column of Goll or of Burdach, thence to nucleus of one of these columns in the medulla. Vi, Cranial (example, fifth cranial nerve, trigeminus; ceU bodies in Gasserian ganglion); receptor; peripheral arm of Gasserian ganglion cell; central arm of Gasserian ganglion cell to medulla as afferent root of fifth nerve, thence to terminal nuclei in medulla. Neurone No. 2. — 2, Spinal connection — CeU body in nucleus of Goll or of Burdach; axone passing as fibre of fillet to thalamus. V2, Cranial nerve connection (trigeminal), cell body in one of trigeminal nuclei in medulla, axone as fibre of secondary trigeminal tract to thalamus. Neurone No. 3. — 3, Cell body in thalamus, axone passing through internal capsule to termination in cortex. (Various association neurones in cortex omitted.) From Cortex to Periphery Neurone No. 4. — 4, Cell body in motor cerebral cortex; axone through internal cap- sule and pes to (a) motor nuclei of cranial nerves (Jb) by means of pyramidal tracts to ventral gray of spinal cord. Neurone No. 5. — 5, Spinal, cell body in ventral gray of cord; axone as motor fibre of ventral root through mixed spinal nerve to effector (muscle). Neurone No. 5. — Cranial — Vi, Cell body in motor nucleus of trigeminus; axone passing to muscle as motor fibre of fifth nerve. Illh, Peripheral efferent neurone of third nerve — oculomotor. F/5, Peripheral efferent neurone of sixth nerve — abducens. Vlh, Peripheral efferent neurone of seventh nerve — facial. Xlli, Peripheral efferent neurone of twelfth nerve — hypo- glossal. THE NERVOUS SYSTEM 471 (4) A Cerebral or Pallial Arc: (a) Peripheral afferent neurones, via long ascending artns to (b) nucleus gracilis or cuneatus, thence as medial lemniscus to (c) thalamus to cortex pallii; (d) associative neurones of cortex; (e) cortical precentral neu- rones via pyramid to (f) efferent peripheral neurones to effectors. Another arc would in- volve the spino-thala- mic tract instead of the lemniscus . ( Figs . 322 and 323.) Similar arcs may in- clude efferent sympa- thetic neurones. (See pp. 465 and 436.) 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 shee'c 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 flat- ten the cord as it shrinks in hardening. With a sharp razor now cut the cord, but not the dura, into seg^nents about i cm. thick. Fix in Orth's fluid (p. 7). Pieces of the cord may i)e cut out as wanted and cm- bedded in cclloidin. Sec- tions shoulrl be tut ai;out 15/' in thickness. 472 THE ORGANS (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 (i) in haematoxylin-picro-acid-fuchsin (technic 3, p. 21) and another section through the same level in Weigert's hasmatoxylin (technic p. 32). Mount both in balsam. For Weigert staining, material fixed in formalin or in Orth's fluid should be further hardened in MiiUer's fluid for at least a month, changing the fluid frequently at first to remove the formalin. Mallory's glia stain should also be used with material fixed in Zenker's fluid (technic, p. 29). The silver method of Cajal (alcohol-fixation) should also be used (technic, p. 37) and that of Nissl. (3) From a cord prepared according to technic i, remove small segments from each of the following levels: (i) the twelfth dorsal, (2) the mid-dorsal, and (3) the cervical enlargement. The segments are embedded in celloidin, sections cut 15 to 20j« thick, stained by Weigert's method (page 32), and mounted in balsam. Mediillated sheaths alone are stained by this method and appear dark blue or black. (4) A human cord from a case in which death has occurred some time after fracture of the vertebras with resulting crushing of the cord, furnishes valuable but of course rarely available material. If death occur within a few weeks after the injury, the method of Marchi should be used ; if after several weeks, the method of Weigert (page 32). The picture in the cord is dependent upon the fact that axones cut off from their cells of origin degenerate and disappear. After a com- plete transverse lesion of the cord, therefore, all ascending tracts are found de- generated above the lesion, all descending tracts below the lesion. The method of Marchi gives a positive picture of osmic-acid-stained degenerated myelin in the affected tracts. The method of Weigert gives a negative picture, the neu- roglia tissue which has replaced the degenerated tracts being unstained in con- trast with the normal tracts, the myelin sheaths of whose fibres stain, as usual, dark blue or black. (5) Human cords from cases which have lived some time after the destruc- tion 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. (6) 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 (4). The most satisfactoiy 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 complications being much less than after complete section. (7) 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 pyramidal tracts are at this age non-medullated and are consequently unstained in Weigert preparations. The cord of an infant of about one month is also ex- cellent. Here the majority of the pallio-spinal fibres are medullated but very thinly so that they are easily distinguishable by their lighter stain. The Wei- gert-Pal method gives the best results. (8) 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 THE NERVOUS SYSTEM 473 and treat material by the Marchi or the Weigert method, according to the time elapsed between the operation and the death of the animal. BRAIN General Structure The principal peculiarities of the brain as distinguished from the cord depend upon two factors; certain pecuKarities of the re- ceptors and effectors of the head and the development of higher coordinating apparatus in the central nervous system of the head. Besides the receptors of the general senses (p. 436), there are in the head the highly specialized receptors of smell, sight, hearing and posi- tion (semicircular canals), which are respectively concentrated into certain locaKties and form, together with certain accessory structures, the organs known as the nose, eye and ear. Each of these groups of receptors has its own special connection with the brain (nerves I, II, and VIII) and its own paths within the latter (see below). The special receptors of taste show a less degree of aggregation into an organ and, together with other visceral receptors, are innervated by afferent portions of a group of nerves (VII, IX and X) which have a common continuation within the brain. The remaining somatic receptors of the general senses, scattered over the anterior part of the head, are innervated by one nerve (V) having its own central con- tinuations. It has already been stated that all the afferent periph- eral neurones which innervate these receptors (except the mesen- cephalic V) follow the general law of having their bodies located out- side the neural tube. The central processes (root fibres) usually split on entering the tube, but the descending arms are the longer. Nerves I and II present certain special peculiarities. The splanchnic effectors of the head include the usual splanchnic efifectors — smooth muscle and glands — and also the branchial stri- ated voluntary muscles. The somatic effectors are the remain- ing Tmyotomic) striated voluntary muscles. The striated voluntary muscles of the head fall into three groups; those of the eye (somatic); of the jaw, face, pharynx and larynx (splanchnic, modified branchial musc:ulature); and of the tongue (somatic). The peripheral path to the smooth muscles and glands follows the same general law as in the body, i.e., neurone bodies in the central nervous system send pre- ganglionic root fibres to sympathetic ganglion cells, which in turn send their axones to the effectors (p. 439). The peripheral path to 474 THE ORGANS the splanchnic (branchial) striated voluntary muscles, on the other hand, follows the sam.e law as obtains for the somatic striated volun- tary muscles, i.e., neurone bodies in the central nervous system send their efferent root fibres uninterrupted to the muscle. These dis- tinctions are shown centrally by differences in grouping of the neu- rone bodies which supply respectively the somatic muscles, the splanchnic voluntary muscles, and the smooth muscles and glands via the sympathetic ganglia The higher coordinating apparatus' or suprasegmental structures (p. 420) of the brain are essentially expansions of the dorsal walls of parts of the brain, each expansion having manifold afferent and efferent connections with the rest of the nervous system and having the endings and beginnings in it of its afferent and efferent paths complexly interrelated by enormous numbers of association neurones. The presence of these latter has probably necessitated the extensive layers of externally placed neurone bodies (cortex) characteristic of suprasegmental structures. The structure of the basal part of the 'brain, connected with the cranial nerves (segmental brain, p. 420), is affected by both the pecu- liarities of peripheral structures mentioned above and by the presence of bundles of fibres and masses of gray forming portions of paths to and from suprasegmental structures (see below). The following summary embodies the resulting general structural features of the brain: Segmental Brain and Nerves Afferent Peripheral (Segmental) Neurones. — (i) Splanchnic Group comprising nerves VII (geniculate ganglion), IX (superior and petrosal ganglia) and X (jugular and nodose gangha). The nerves of taste probably belong entirely to this group. The peripheral arms of these ganglia innervate visceral receptors and the central arms form a descending tract in the medulla, the fasciculus solitarius, which has its terminal nucleus giving rise to secondary tracts. (2) General Somatic Group (common or general sensibiHty). — Semilunar ganglion of V. The peripheral arms of the semilunar ganglion cells pass to the skin of the anterior part of the head, to the mouth and meninges. The central arms of the ganglion cells form a descending tract in the medulla, the radix spinalis V. The terminal nucleus of this tract is the continuation in the medulla of the dorsal THE NERVOUS SYSTEM 475 horn. The axones of the terminal nucleus form a secondary tract to the thalamus and thence a tertiary thalamo-cortical tract passes to the central cortex cerebri. Axones of the mesencephalic nucleus of nerve V innervate the muscle-tendon receptors of the jaw muscles and nerves III, IV and VI probably contain fibres to the muscle- tendon receptors of the eye muscles. (3) V estibulo-Semicircular Canal Group. — GangHon of Scarpa of VIII. The peripheral arms pass to vestibule and semicircular canals. The central arms constitute the vestibular portion of VIII, forming descending tracts in medulla and terminating in several vestibular terminal nuclei (including Deiters' nucleus). (4) Acoustic Group. — Ganglion spirale of VIII. The peripheral arms terminate in the organ of Corti of the cochlea. The central arms form the cochlear part of VIII and terminate in the medulla in various nuclei which originate the secondary tract (lateral fillet) to the midbrain, and medial geniculate body in thalamus. A third (or fourth?) system of thalamo-cortical neurones passes to the temporal cortex cerebri (also p. 582). (5) Visual Group. — Ganghon in retina. The second neurone system begins in the retina and forms the secondary tract (optic "nerve") to the lateral geniculate body in thalamus. The third thalamo-cortical neurone system passes to the calcarine cortex cerebri (also p. 561). (6) Olfactory Group. — " Ganglion" cells in olfactory mucous mem- brane form the olfactory nerve (fila olfactoria) which terminates in the olfactory bulb. Secondary tracts from the olfactory bulb (and tertiary tracts) proceed to diencephalon and hippocampus (also p. 585). Efferent Peripheral (Segmental) Neurones. — (i) Splanch- nic, (a) Lateral nuclei in the gray matter of the hindbrain. Their axones pass to the striated voluntary muscles of the jaw (V), face (VII), pharynx and larynx (IX and X). (b) Nuclei more deeply placed in the gray of mid- and hindbrain. Their axones pass as preganglionic fibres to sympathetic ganglia of head and body. (c) Sympathetic. 'J'heir bodies compose the sympathetic ganglia of the head (ciliary, sphenoi)alatine, submaxillary and otic). These ganglia receive the above preganglionic fibres and are thus connected with cranial nerves III, V, VII, IX, and X. (2) Somatic. — Medial nuclei located near the median line in the gray matter of hindbrain (XII and VI), and midbrain (IV and III), to 476 THE ORGANS muscles of tongue (XII) and eye (VI, IV and III) . Nerve III also contains splanchnic neurones whose axones pass to sympathetic ganglia (ciliary). Nerves III, IV and VI probably also contain afferent nerve fibres (p. 474, (2)). Intrasegmental and Intersegmental Neurones. — These are represented principally by the gray reticular formation of the hindbrain and midbrain and long descending tracts external to it. The gray reticular formation is composed of neurone bodies and short intersegmental tracts intermingled. Among the former are certain well differentiated nuclei {e.g., nucleus ruber, nucleus of Deiters, and nucleus of the medial longitudinal fasciculus) the axones of which form long, principally descending, intersegmental tracts external to the gray reticular formation. These may also be links in efferent suprasegmental paths. (See also p. 477B, XIV, XVI, XVII and XVIII.) Other cells in the gray reticular formation form the shorter tracts within it. The reticular formation may also con- tain motor nuclei of the cranial nerves and is traversed by various fibres passing to tracts and by terminals from tracts. Apeerent and Efferent Suprasegmental Paths These paths are paths from receptors of body and head to cere- bellum, midbrain roof (colliculi) and pallium and paths from cere- bellum, midbrain roof and pallium to effectors of head and body. There are also paths connecting pallium, mid-brain roof and cerebellum. Afferent Suprasegmental Paths A . A -ff event Pallial I. General Somatic Sensation: Spinal and trigeminal ganglionic -|- spino- thalamic and bulbo-thalamic (medial fillet) crossed -|- thalamo-pallial (to post- central cortex) neurones. (Fig. 322.) II. Visceral Sensation, including Taste. This important path is not well known. The gustatory path enters by ganghonic neurones of nerves V (?), IX and X (fasciculus solitarius) . Its secondary tract may lie partly in the medial fiUet. III. Hearing (cochlear): Spiral ganglionic + lateral fillet (crossed) and brachium of inferior coUiculus -f- geniculo-pallial (to temporal cortex) neurones (Fig. 330). IV. Vestibular: A somewhat doubtful path except perhaps to the pallium through the cerebellum (paths X -f VII). THE NERVOUS SYSTE]\r 477 V. Sight: Retinal bipolar ganglionic + retino-geniculate (optic nerve and tract, crossed and uncrossed in chiasma) + geniculo-pallial (to calcarine cortex) neurones. (Fig. 350.) \'I. Smell: Olfactory ganglionic + bulbo-rhinencephalic-pallial (crossed and uncrossed to hippocampal cortex) neurones. (Fig. 351.) VII. Cerebello-pallial: Cerebellar cortical + dentato-rubral and dentato- thalamic (superior cerebellar peduncle, crossed) + rubro-thalamo-pallial neurones. (Fig. 337.) B. Afferent Mesencephalic VIII. A comparatively unimportant path composed of spinal ganglionic + spino-collicular (crossed) and bulbo-collicular (? medial fillet, crossed) neurones. C. Afferent Cerebellar IX. The spinal ganglionic (innervating proprioceptors of body) + spino- cerebellar path to vermis of cerebellum. The dorsal spino-cerebellar and ventral spino-cerebellar tracts of this path pursue somewhat different routes. (Figs. 323 and 337.) This path may receive accessions from the columns of GoU and Burdach and their nuclei in the bulb. X. The vestibular ganglionic (Scarpa) -f Deitero-cerebellar path. The vestibular ganglionic neurones may send axones to the cerebellum without inter- ruption. (Figs. 323, 331 and 337.) Paths via other cranial nerves, especially nerves V and II, may also pass to the cerebellum. Olivo-cerebellar neurones form part of another, not well known, afferent cerebellar path. Efferent pallial path XIII is obviously also an afferent cerebellar path. Erf erent Suprasegmental Paths A . Efferent Pallial XI. \'oluntary Motor: Pallio (precentral cortex) — spinal (crossed) + spinal peripheral somatic motor neurones, also paUio-tegmental and pallio-bulbar (crossed and uncrossed) + cranial peripheral somatic motor and branchiomotor neurones. A short intersegmental system probably is intercalated between the pallial and peripheral neurone systems. (Figs. 322, 323.) XII. Splanchnic Efferent (except branchiomotor): This is the pallio-bulbo- spino-sympathctic path mentioned on page 467 and comprises also descending pallial fibres which act upon neurones in the brain whose axones pass out as cranial preganglionic fibres. XIII. Pallio-cerebcJlar: I'allio-{)ontile + ponto-cerebelhir (middle cere- bellar j>eduncle, crossed) neurones. (Figs. 323, 337.) XIV. Pallio-rubral -|- rubro-spinal (crossed) + peripheral motor neurones (p. 512). This path may [)arlly supplement, physiologically, i)ath XI. Other less important paths are mentioned on p. 523. 478 THE ORGANS B. Efferent Mesencephalic XV. Colliculo-bulbar and coUiculo-spinal (crossed) + cranial (especially VII for squint reflex) and spinal peripheral motor neurones. (Fig. 350.) XVI. CoUiculo-medial longitudinal fasciculus: This path probably consists of coUicular neurones which pass to the nucleus of the medial longitudinal fascic- ulus + the latter nucleus and its descending axones in the medial longitudinal fasciculus + peripheral motor neurones of brain (especially oculomotor) and cord. (Fig. 350.) Inasmuch as the superior coUiculus receives optic fibres, paths XV and XVI are probably largely concerned in optic reflexes. ' C. Efferent Cerebellar XVII. Cerebellar cortico (cortex of cerebellar hemispheres) — dentate + dentato-rubral (superior cerebellar peduncle, crossed) + rubro-spinal (crossed) + peripheral motor neurones. (Figs. 323, 337.) XVIII. Cerebellar cortico (cortex of vermis)— fastigial + fastigio — Deiters (mesial part of inferior cerebellar peduncle) + Deitero-bulbar and Deitero-spinal (crossed and uncrossed) + peripheral oculomotor and spinal motor neurones. (Figs. 323, 331 •) Inasmuch as Deiters' nucleus also receives directly vestibular nerve fibres, there exists the important vestibulo-Deiters + Deitero-bulbar and Deitero-spinal + peripheral motor neurones reflex path whereby the vestibulo- semicircular canal receptors directly influence the position of eyes and body. Afferent pallial path VII is obviously also an efferent cerebellar path. (Figs. 323, 337-) The large efferent pallial paths XI and XIII markedly affect the configura- tion of the brain. These two paths are added ventrally to the segmental and intersegmental apparatus and form the pes pedunculi or crusta (added ventrally to the tegmentum of the midbrain), the pons Varolii (added ventrally to the teg- mentum of midbrain, isthmus and hindbrain) and the pyramids (added ventrally to the hindbrain). Arising from the neopallium (p. 532), they are to be regarded as largely more recent acquisitions by the vertebrate nervous system. SUPRASEGMENTAL STRUCTURES These are the pallium or cerebral hemispheres, the inferior and superior colHculi or corpora quadrigemina and the cerebellum. They consist essentially of the endings and beginnings of their re- spective afferent and efferent paths and of their own association neurones, the bodies of which lie in their respective cortices. The corpora quadrigemina are relatively of much less importance in the human brain. In accordance with the above there are usually to be distinguished in transverse sections of the brain at various levels the following: THE NERVOUS SYSTEM 479 A. Peripheral {segmental) neurones, (i) Efferent ("motor" nuclei and root fibres) . (2) Central continuations of afferent neurones (afferent roots). (a) Those entering at and therefore belonging to the segment involved, (b) Those entering above or below the segment and represented in the segment by descending or ascending (overlapping) tracts. B. Terminal nuclei of (2) and the secondary tracts originating from them. These may fall under category C or D (below) . C. Intrasegmental and Intersegmental nuclei and tracts of the seg- mental brain, consisting principally of the gray reticular formation and long descending tracts (arrangement much modified in forebrain). D. Nuclei and tracts forming parts of afferent and efferent supra- segmental paths. The aft'erent paths include some of the nuclei and tracts under B, and their continuations, and the efferent include some of the longer systems under C, together with efferent suprasegmental tracts to them. E. Suprasegmental structures (not present in many transections). The general histology of the brain is similar to that of the cord. Cells of the motor cranial nuclei have an arrangement of chromophihc substance similar to their analogs in the cord, while cells of afferent cranial gangha present a chromophihc picture similar to their cord analogs. Certain cells whose axones act directly upon motor cells in cord and brain {e.g., cells in motor cortex and certain cells in reticular formation) resemble motor cells, in arrangement of chromo- phihc substance, while certain cells in close connection with afferent peripheral neurones resemble the latter. The neuroglia cells and fibres also present the same general characteristics as those in the cord, with variations peculiar to certain localities {e.g., parts of the cerebellum) . Hindbrain or Rhombencephalon This includes the medulla, cerebellum and part of the tegmentum and pons. Its peripheral nerves are the V, VI, VII, VIII, IX, X, andXII.i The Medulla Oblongata or Bulb is the continuation upward of the spinal cord and extends from the lower Umit of the pyramidal decussation below to the lower margin of the pons above. ^ ' It is better probably to reckon the so-called medullary or hulhur p.irl of iheXI with theX. * Jt would be better to include in the term medulla ohlonj^'aLa wlial. here falls under pontile tej^mentum of the hindbrain. 480 THE ORGANS N.VEST. N.cocri. RAO. ANT. Fig. 323. THE NERVOUS SYSTEM 481 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 antero-lateral surface of the medulla is also marked by the exit of the fifth to the twelfth (inclusive) cranial nerves. The VI and XII (somatic motor) emerge near the mid- ventral line, the others, including the splanchnomotor portions of the V, VII, IX and X, emerge more laterally. The posterior surface shows two prominences on either side. The more median of these, known as the dava, is caused by the nucleus gracilis, or nucleus of the column of Goll; the other, the cuneus, lying just to the outer side of the clava, is due to the nucleus cuneatus or nucleus of the column of Burdach. Lateral to this is a third eminence, the tuherculum cinereum, due in part to the descending root of the V and its underlying terminal nucleus, the continuation of the dorsal gray column of the cord. This eminence merges anteriorly with the eminence of the restiform body. The central canal of the cord continues into the me- dulla, where it gradually approaches the dorsal surface and opens into the ca\dty of the fourth ventricle. The floor of the fourth ventricle exhibits a medial eminence {trigonum hypoglossi) occupied caudally by the nucleus hypoglossi. Lateral to this is a triangular area, the ala cinerea {trigonum vagi), surrounded by furrows. This is partly occupied by nuclei of the vagus. Cephalad and laterally a broader Fig. 323. — Principal afferent and efferent suprasegmental pathways (excepting the rhinopallial connections, the efferent connections of the midbrain roof and the olivo- cerebellar connections). EfTerent peripheral neurones of cranial nerves are omitted. Each neurone group (nucleus and fasciculus) is indicated by one or several individual neurones. Decussations of tracts arc indicated by an X. ac. Acoustic radiation, from medial geniculate body to temporal lobe; hr. conj, brachium conjunctivum (superior cerebellar peduncle); br. ponlis, brachium pontis, from pons to cerebellum; h.q.i, brachium quadrigeminuminfcrius; c.^'./, lateral or external geniculate body; c.g.m, medial or internal geniculate body; c.qiiad, corpora quadrigemina; /.cor/. -5/), pallio-spinal fascic- ulus (pyramidal tract); f.c.-p.f, frontal pallio-pontile fasciculus (from frontal lobe); f.c.-p.t, temporal pallio-ponlilc fasciculus (from temporal lobe); f.c.-p.o, occipital pallio-pontile fasciculus (from occipital lobe); /.am, fasciculus cuneatus (column of Hurdach;; l.f.-b, fastigio-bulbar tract;/, grar, fasciculus gracilis (column of Goll); f.s.-t, >pino-thalamic fasciculus; f.sp.-c.d, dorsal spino-ccrebellar fasciculus (tract of Flcchsig) ; f.sp.-c.v, ventral spino-cerebellar fasciculus; km. lat, lateral lemniscus or lateral lillet; Irm. tnrii, medial lemniscus or fillet; n. cocli, cochlear nerve; n.cun, (terminal) nucleus of the column of Hurdach; n.d, nucleus of Dcitcrs; n.dcnl, nucleus dentatus; n.grac, nucleus of the column of (ioli; n.opt, optic nerve; w.r, nucleusuber; n.l, nucleus tecti (or fastigii); tt.lri^, trigeminal nerve; n.vest, vestibular nerve; prs.pcd, j)es pcdunculi (crusta); piilv thai, [)ulvinar thalami; pyr, pyramid; rod. anl, ventral spinal root; rad. post, dorsal sjiinal root; rad. opt, of)tic radiation (from lateral geniculate body to calcarine region); iow ff5. I>undles from thalamus to jjostcentral region of neo|)allium; .s/>. i^ang, spinal ganglion; l.f.-b., traclus fastigio-bulbaris; thai, thalamu.s; l.n.d, tract from the nucleus of Deiters; /. rub.-sp, rubro-spinal tract (von Monakow). (Lateral view of brain.) 482 THE ORGANS triangular area with an angle directed into the lateral recess marks the area occupied by the nuclei of the acoustic nerve {area acustica) . Still further cephalad near the median line are eminences indicating the positions of the nucleus abducentis and genu facialis. The roof of the fourth ventricle is formed by the thin plexus chorioideus and the cerebellum. (Fig. 324.) The Pons is a mass of fibres and gray matter extending across the ventral surface of portions of mid- and hindbrain. The term is often Corp. mamillaria Corp. pineale Colliculus sup. Colliculus inf. Etninentia med. Olive (in A) Area acustica (in B) Eminentia med. Ala cinerea Clava Dec. pyramids 1 . Tub. cuneatum J Tub. cinereum Fig. 349 Fig. 348 Fig. 326 Fig. 32s Fig. 324. — Ventral (A) and dorsal (B) views of part of Brain Stem (cerebellum removed) . Structures named at the left are indicated by their reference lines running to an X. On the right are named the figures showing transverse sections through the brain at the levels indicated by the reference lines. The level for Fig. 349 is not quite accurately indicated. used to include the whole of the basal part of the brain thus covered by the pons. It is better, however, to restrict it to the pons itself. The part of the hindbrain dorsal to the pons, which is the continua- tion forward of the medulla, may be included in the term tegmentum of the hindbrain. The Cerebellum is described on p. 507. TECHNIC The technic of the medulla (and the rest of the segmental brain) is the same as that of the cord (page 471). Transverse sections should be cut through the following typical levels, stained by Weigert's method (page 32), and mounted in balsam: THE NERVOUS SYSTEM 483 1. Through the pyramidal decussation. 2. Through the sensory decussation. 3. Through the lower part of the olivary nucleus. 4. Through the middle of the olivary nucleus. 5. Through the entrance of the cochlear nerve. 6. Through the entrance of the vestibular nerve. 7. Through the roots of the sixth and seventh cranial nerves. 8. Through the roots of the fifth cranial nerve. The methods of Nissl, Cajal and glia stains should also be used when practic- able. PRACTICAL STUDY I. Transverse Section of the Medulla through the Decussation of the Pyram- idal Tracts (Motor Decussation) (Figs. 324 and 325) The most conspicuous features of this section are the decussation of the pyra- mids, the larger size of the dorsal horn and the beginning of the gray reticular formation. Surrounding the centra' canal is the central gray. Efferent Peripheral Nevirones. — Nuclei of first cervical spinal nerve in ventral gray, and root fibers passing out to emerge on ventral aspect. Nuclei of XI, in mesial position in central gray, or in ventral gray. Axones pass out laterally from latter and emerge on the lateral surface. The mesial or deep nuclei are best reckoned with nerve X. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Some afferent fibres of the first cervical spinal nerve are still entering at this level. Ascending afferent roots: The dorsal funiculus comprising the fasciculus cuneatus and fasciculus gracilis remain as in the cord. Collaterals and terminals from them can be seen entering the subjacent gray. Descending afferent roots: Some of the fine fibres between the enlarged dorsal horn and the periphery, occupying the position of the zone of Lissauer in the cord, are descending afferent root fibres of the V-cranial nerve or tractus spinalis trigemini {spinal V). Collaterals and terminals from these fibres terminate in the gelatinous substance of Rolando and also traverse it to form a plexus of medullated fibres in its inner side very similar to the cord. The axones of the dorsal horn cells (or terminal nucleus of the V) form the secondary tracts of the V which cannot be distinguished (p. 506 and Fig. 336). Secondary tracts, forming parts of afferent suprascgmental paths: These form a mass of fibres along the lateral periphery of the medulla which consists of (a) the dorsal spino-Cirrebellar (b) the ventral spino-cerebellar and (c) the spino- thalamic tracts. Intersegmental Neurones. — The neurone bodies are, as in the cord, scattered throughout the gray. The continuation of the ventro-latcral intersegmental tracts of the cord (and the coiliculo-spinal tract) is the U-shaped mass of fibres around the ventral gray. This mass consists of the long descending, the shorter ascending and descending intersegmental tracts, and the coUiculo-spinal; i.e., (a) rubro-spinal (in lateral arm of U), (b) Deitero-spinal (lateral and mesial), 484 THE ORGANS ^ b •° s v^ w S2 >. O ■fe m'S. tn ^-> dti 2 PI O I* s P g^ ^^ ^^""^ s g CU„- U '. cn O m o +3 fe C 1) ^o cS "^ 'S CO *^ a C Q Id ^ ■' »-. g--d oH g^8 f^ THE NERVOUS SYSTEM 485 (c) tract from nucleus of medial longitudinal fasciculus (mesial), (d) colliculo- spinal (mesial), (e) shorter descending and ascending tracts which may be re- garded as the equivalent of the ground bundles of the cord comprising shorter reticulo-spinal and spino-reticular fibres. The shortest of these fibers, which in the cord were next the lateral gray, are now mingled with the gray, the combina- tion constituting the gray reticular formation. Other short intra- and interseg- mental tracts lie in and adjoining the dorsal horn, as in the cord. Descending suprasegmental paths include certain of the above long descend- ing intersegmental tracts as previously explained. Besides these there are the eferent suprasegmental neurones known as the pallio-spinal or pyramidal tracts and the colliculo-spinal tracts. Bundles of fibres are seen crossing {pyramidal decussation) from the anterior pyramid of one side to the opposite dorso-lateral column, where they turn downward as the crossed pyramidal tract. In their passage through the gray matter, they cut off the ventral horn from the rest of the gray matter. These fibres, as already noted in the cord, are descending axones from motor cells situated in the precentral cerebral cortex. In the pyram- idal decussation most of these fibres cross to the opposite dorso-lateral region to pass down the cord as the crossed pyramidal tract (p. 464, and Fig. 318; Fig. 322, F). The remaining fibres stay in their original anterior position and continue down the cord as the direct pyramidal tract (p. 465, and Fig. 318; Fig. 322, F). A few pass to the ventral tract in the same side, thus becoming uncrossed fibres in the lateral tract. The bundles of fibres do not cross in a trans- verse 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. 324). 2. Transverse Section of the MeduUa through the Decussation of the Fillet or Lemniscus (Sensory Decussation) (Figs. 324 and 326) The most conspicuous features are the appearance of the nuclei cuneatus and gracilis, the decussation and formation of the medial lemniscus or fillet, and the increase of the gray reticular formation. Peripheral Efferent Neurones. — In the lateral part of the central gray is the dorsal nucleus of the X {nucleus alee cinerecB). In the ventral part of the central gray is the nucleus hypoglossi and, passing ventrally and emerging lateral to the pyramids, may be seen the axones of its cells — the root fibres of the XII. In tht nucleus XII can be distinguished (Weigert stain) coarse fibres which arc the root fibres, and fine fibres which arc terminals of other fil)res ending in the nucleus. Among these have been distinguished collaterals from secondary vagoglossopharyngeal and trigeminal tracts (three-neurone reflex, and from various parts of the reticular formation. Whether pyramidal fibres reach the nu(l'.-iis directly or via int(T( alntcd neurones is uncertain. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Entering afferent root fibres are usually not present. The funiculi or fasciculi cuneatus and gracilis have diminished, and internal to them have appeared large masses of gray. These are the nuclei of the columns, 486 THE ORGANS Pi S- ^ S o ^ in mi F " orN. > .2 C! aj cd >-j ;-i rf H M 1^ D. -d ■u M i-i rf) tu bO d L*" ^ p4 O THE NERVOUS SYSTEM 487 and are known, respectively, as the nucleus of the column of Goll or the nucleus gracilis, and the nucleus of the column of Burdach or the nucleus cuneatus. In the higher sensory decussation levels there is usually an accessory cuneate nucleus. These nuclei serve as nuclei of termination for the fibres of the posterior funiculi. Their termination in these nuclei is the ending of that system of fibres which has been traced upward from their origin in the cells of the spinal ganglia; the completion of the course of the spinal peripheral afferent neurones. As the fibres of the posterior columns are constantly terminating in these nuclei, there is, in passing from below upward, a constant increase in the size of the nuclei and a corresponding decrease in the size of the posterior columns, until, just below the olive, the whole of the column of GoU and most of the column of Burdach are replaced by their respective nuclei. (Pp. 461, 462.) Study the plexus of fine fibres in these nuclei, formed by the terminals of the column fibres, also the coarser fibres (axones of the cells of the nuclei) gathered in the ventral part of the nuclei, whence they emerge and curve^ around the cen- tral gray, cross to the opposite side ventral to it and dorsal to the pj^ramids, and then turn brainward forming the bundle of fibres known as the medial lemniscus or medial fillet. The spinal V has increased and also its terminal nucleus, the dorsal horn. The spino-cerebellar and spino-thalamic tracts occupy about the same positions. In the centra] gray dorsal to the central canal is a nucleus representing a union of the caudal ends of the terminal nuclei of the fasciculi solitarii (see next section) — the nucleus commissuralis. Fibres of the fasciculi solitarii also de- cussate here. Intersegmental Neurones. — The rubro-spinal and Deitero-spinal tracts and the tract from the nucleus of the medial longitudinal fasciculus occupy about the same positions. The reticular formation has increased, the whole of the ventral horn and intermediate gray containing bundles of longitudinal fibres. The formation is also traversed by transverse fibres, representing the beginnings or terminations of various longitudinal fibres. Efferent Suprasegmental Neurones. — The decussation of the pyramids has now nearly or entirely ceased. The lateral pyramidal tracts are no longer in the lateral columns but are parts of the anterior pyramidal tracts which form two large masses of fibres one on each side of the ventral sulcus. The coUiculo- spinal tract occupies the same position. 3. Transverse Section of the Medulla through the Lower Part of the Inferior Olivary Nucleus (Figs. 324 and 327) The central canal has opened into the fourth ventricle, the central gray (in- cluding the central gelatinous substance) now being spread out on its floor. The roof of the ventricle is formed by its chorioid plexus. The most conspicuous new feature is the olive. Efferent Peripheral Neurones— The nucleus of the XII is large and occupies * Fibres having a transverse curved or arched course are in general termed arcuate fibres. If they are (ieei)ly located, they are internal arcuate fibres, if near the j)eriphcry, they are superficial or external arcuate fibres. Obviously the same fibre may be, in different parts of its course, internal arcuate, external arcuate, and longitudinal. 488 THE ORGANS H f^ THE NERVOUS SYSTEM 489 the swelling in the floor of the ventricle each side of the median line known as the eminentia or trigonum hypoglossi. The root fibres of the XII pass lateral to the medial lemniscus, between the olive and pyramid, and then emerge at the groove between olive and pyramid. The dorsal nucleus of the X occupies a sweUing lateral to the preceding and known as the ala cinerea. Some of the root fibres of the X are axones from this nucleus. They probably innervate (via sympathetic neurones) some, at least, of the smooth muscles, heart (and glands ?), innervated by the vagus (X). The dorsal nucleus appears to be relatively deficient in termi- nals. What it does receive ap- pears to come from the secondary vago-glossopharyngeal and tri- geminal tracts. The bodies of another group of peripheral efferent neurones form the nucleus ambiguus, often difficult to distinguish, in the reticular formation. Their ax- ones pass obliquely dorsally and mesially, join the other root fibres of the X, and then bending ab- ruptly, pass with them to the lateral surface of the medulla. Some pass across the median line and [leave by the root of the op- posite side. They probably in- nervate the striated muscles of the pharynx, larynx (and oesoph- agus ?). The nucleus receives various terminals, some at least appearing to come from the secondary trigeminal tracts and from the lateral part of the retic- ular formation. (Fig. 328.) Afferent Peripheral Roots, their Terminal Nuclei and Secondary Tracts.^ Other root fibres are the afferent fibres of the X which form a common root with the preceding. Sometimes they can be seen joining the fasciculus solitarius of which they form a part. Some fibres or collaterals may enter the adjacent gray {terminal nucleus of the X) (see Fig. 328). If the roots of the X do not show well in the section, defer their study until the following .section where the IX shows similar relations. The spinal Vis partly [lierced anrl partly covered by transverse fibres, princi- pally olivo-cerebellar fibres (see below). Its terminal nucleus is less conspicuous. Two new bundles of descending root fibres have appeared; one is iho fasciculus solitarius composed of the afTerent root fibres of the X, IX (including gustatory Fig. 328. — Diagram of Origin of Cranial Nerves X and XII. (Schafer.) />jyT, Pyramid; 0, olivary nucleus; r, restiform body; d.V, spinal root of fifth nerve; n.XII, nucleus of hypo- glossal; XII, hypoglossal nerve; d.n.X.XI, dorsal nucleus of vagus; n.amb, nucleus am- biguus;/.5., 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.n, cell of nucleus of solitary fasciculus sending axone as internal arcuate fibre to opposite side of cord (secondary vagus and glossopharyngeal tract.) This course of the secondary tract is doubtful. 490 THE ORGANS fibres), and higher up, of the VII. A small mass of gray of a gelatinous appear- ance near it is its terminal nucleus. The course of the secondary tract cannot be made out and is not accurately known. The other bundle is the descending vestibular root. It lies lateral to the fasciculus solitarius. Accompanying it are cells which constitute its terminal nucleus. Occupying the floor of the ventricle lateral to the dorsal nucleus of the X is another terminal nucleus of the vestibular nerve, the nucleus medialis (triangular or chief nucleus.) Internal arcuate fibres emerging from these regions ma}^ represent secondary tracts (probably reflex) from these nuclei. (P. 493; Fig. 331.) The nucleus gracilis has disappeared. The nucleus cuneatus may be pre- sent, much diminished, and give rise to some internal arcuate fibres to the medial lemniscus. The lemniscus is now a tract which has become built up on each side of the median line. This latter is known as the raphe (i.e., "seam," stitched by the decussating fibres). The ventral spino-cerebellar tract and spino-thalamic tract are in about th,e same lateral position, but the dorsal spino-cerebellar tract has moved dorsally and together with olivo-cerebellar fibres (see below) begins to form the restiform body (see below). In the lateral part of the reticular formation, between spinal V and olive are seen the nuclei laterales. In these nuclei some of the spino-cerebellar fibres end. The axones of these nuclei partly enter the restiform body on the same side and partly cross to the opposite restiform body (p. 507; Fig. 337). They form some of the ventral external arcuate fibres seen in the section. The lateral nuclei are thus partial interruptions in the spino-cerebellar path. The nuclei arcuati are well marked. Other Afferent Cerebellar Neurones. — A new and important convoluted mass of gray is the inferior olivary nucleus, forming the bulge of the lateral surface of the medulla known as the olive. Near it are the dorsal and medial accessory olives. The axones of the olivary cells are the olivo-cerebellar fibres. They cross through the fillets, pass through or around the opposite olivary nucleus, thence proceed dorso-laterally, being gathered into more compact bundles, traverse or surround the spinal V and dorsal to it bend longitudinally, forming a great part of the restiform body. The latter produces an eminence on the dorso-lateral surface of the medulla. The restiform body, thus formed by these spino-cere- bellar and olivo-cerebellar fibres, together with certain others, passes into the cerebellum higher up, forming the major part of the inferior cerebellar arm or peduncle. According to many authorities fibres from the columns and nuclei of Goll and Burdach of the same side (dorsal external arcuate fibres) and opposite side (ventral external arcuate fibres) may join the restiform body. (Comp. P- 507-) Fibres appearing on the external surface of the olivary nucleus are the term- ination of a large tract descending to the olivary nucleus, the central tegmental tract. Its origin in higher levels is not accurately known. Intersegmental Neurones. — The reticular formation is now still more extensive. The original U-shaped mass of intersegmental tracts (and the coUiculo-spinal tract) has now become widely separated into two parts. The lateral part, con- THE NERVOUS SYSTEM 491 sisting principally of the rubro-spinal tract and uncrossed Deitero-spinal fibres, lies mesial to, or partly mingled with, the spino-thalamic and ventral spino-cere- bellar tracts. The mesial part of the U, consisting principally of crossed and uncrossed Deitero-spinal fibres and fibres from other nuclei in the reticular formation, and of fibres from the nucleus of the medial longitudinal fasciculus, now forms the medial longitudinal fasciculus dorsal to the fillet. Near this bundle, or united with it, is the colliculo-spinal tract {predorsal tract). When these tracts have passed down to below the formation of the fillet and the olives, they assume the positions noted in the lower levels of the medulla. Efferent Suprasegmental Neurones. — The pyramids are the same. Small bundles of more lightly stained fibres present in the fillet here and in higher levels (Weigert stain, not indicated in the figures) are efferent pallial fibres detached from pes or pyramids. They are aberrant fibres which rejoin the pyra- mids or are fibres innervating motor cranial nuclei. The colliculo-spinal tract (see above). 4. Transverse Section of the Medulla through the Middle of the Olivary Nucleus Such a section is so similar to 3 and 5 that its detailed description may be omit- ted. The nucleus cuneatus has disappeared; the fillet increased somewhat; fasciculus solitarius and descending vestibular root have increased; also their terminal nuclei. The olivary nucleus, olivo^cerebellar fibres, and the restiform body have greatly increased. The formatio reticularis has increased in extent. 5. Transverse Section of the Medulla through the Entrance of the Cochlear Root of Nerve VIII (Figs. 324 and 329) Efferent Peripheral Neurones. — The dorsal vagus nucleus is not present, but the nucleus ambiguus is usually present and probably sends some axones to nerve IX, passing out with the afTerent fibres (see below). The nucleus XII has disappeared and also its root fibres. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Usually the afferent root fibres of nerve IX are present. They enter on the lateral aspect of the medulla ventral to the restiform body, traverse the spinal V, and pass to the fasciculus solitarius or its terminal nucleus. The fasciculus solitarius is smaller, and just above the entrance of the IX consists of only a comparatively few descending afferent root fibres of the VII. The fibres of the cochlear nerve enter the extreme lateral angle of the medulla, where many or, according to some, all of them terminate in two masses of cells enveloping externally the restiform body and known as the ventral (or accessory) and dorsal (or lateral) cochlear nuclei. Most of the axones of the dorsal nucleus pass across in the floor of the ventricle {strice medullares) to form a part of the opposite secondary tract (lateral lemniscus) of the cochlear nerve. The axones of the ventral nucleus also decussate, but by a more ventral route, (trapezius), and also form a part of the lateral lemniscus. This latter decussation takes place at a higher level (sec next section). 492 THE ORGANS tn m U. Vh o > M a H 1 Pi M a rl' o THE NERVOUS SYSTEM 493 The auditory nerve is divided into two parts: the cochlear nerve (gangHon spirale) and the vestibular nerve (ganghon of Scarpa). The fibres of the coch- lear root enter at a lower level than those of the vestibular. Some of them enter the ventral cochlear nucleus; the remainder pass dorsalward to the dorsal cochlear nucleus, or nucleus of the acoustic tubercle. According to some author- ities, some root fibres pass to the superior olivary and trapezoid nuclei. The axones of the cells of the ventral and dorsal nuclei form the secondary cochlear tract (lateral fillet). These fibres decussate and send collaterals to, or are partially interrupted in, the nucleus ohvaris superior, trapezoideus, nucleus of lateral fillet and inferior coUiculus (posterior corpus quadrigeminum). According to some authorities all the fibres of the lateral lemniscus terminate in the inferior coUiculus. From the inferior colliculus the path is formed by the arm or brachium of the latter to the medial geniculate body and thence to the temporal cortex cerebri. It is thus not possible to state definitely how many neurone systems are involved, but the principal ones are: (i) ganglion spirale, (2) dorsal and ventral nuclei and (decussation) lateral lemniscus, (3) posterior corpus quadrigeminum and its brachium, (4) medial geniculate body of the thal- amus and geniculo-cortical fibres. If the lateral lemniscus fibres be regarded as simply passing by the posterior corpus quadrigeminum, giving collaterals to it (Cajal), the path would in part consist of three neurone systems analogous to those of the paths from the cord, trigeminus and eye. (Figs. 323, 330.) The fibres of the vestibular root enter higher and mesial to those of the coch- lear root, passing dorsally along the inner side of the restiform body to four terminal nuclei, which cannot all be clearly seen in any one section; (a) Deiters' nucleus (lateral vestibular nucleus) situated at the end of the main bundle of root fibres, just internal to the restiform body ; {b) von Bechterew's nucleus (supe- rior vestibular nucleus) situated somewhat dorsal to Deiters' nucleus in the lateral wall of the fourth ventricle; (c) the median or principal nucleus of the vestibular division — a large triangular nucleus, occupying a considerable part of the floor of the fourth ventricle; (d) the descending vestibular nucleus which accompanies the descending fibres of the vestibular root (spinal eighth). Fibres also pass to the cerebellum. The axones of the cells of the terminal vestibular nuclei form the secondary vestibular tracts, some axones going to (a) the cerebellum (?),(b) the midbrain, especially to the nuclei of nerves III and IV (via Deiters and von Bechterew, the former by the medial longitudinal fasciculus), (c) the medulla and cord, probably to various motor nuclei, via the medial longitudinal fasciculus, lateral tract from Deiters' nucleus and other tracts in the reticular formation. (Figs. 323, 331 and 337.) The descending vestibular root is large, as is also its terminal nucleus and the medial terminal vestibular nucleus, in the present section. The spinal \' is unchanged, its terminal nucleus being rather indistinct. Secondary trigeminal tracts cannot be distinguished — such fibres probably either join the medial lemniscus or form an inrlependent ascending tract in the reticular formation. 'J"he fillet is about the same. 'J'he venlral spino-cerebellarand si)ino- ihalaniii I rai ts arc- in 1 lie same posil ions. Other Afferent Cerebellar Neurones. The olives are still larger and send many bundles of olivo-cerebcUar fibres to the opposite restiform body which has 494 THE ORGANS Nu. Corf), trabe^oidei :—....... OlivOL sub^rior Fig. 330 THE NERVOUS SYSTEM 495 EXPLANATION OF FIG. 330 Fig. 330. — ^Diagram showing Connections of the Cochlear Portion of the Auditory (VIII) Nerve. A, Section at level of superior colliculi {Ant. cor p. quad.) and red nucleus; B, through level of inferior colliculi {Corpus, quad. ^05^.); C, through level of nucleus of lateral lemniscus {Nii. lem. lat.); D, through pons at level of VIII nerve. Spacing between the different levels is not proportionate. In B and C the basis pedunculi is omitted. Each neurone group is indicated by one or several individual neurones. Neurone No. i. — Cell bodies in spiral ganglion {gang, spiralis); peripheral processes end in organ of Corti; central processes terminate principall}' in ventral or accessory nucleus {N^u. accessorius) and lateral nucleus {Nu. lateralis) or tuberculum acusticum; some also terminate in superior olives {Oliva superior), and nuclei of trapezoid body {Nu. Corp. trapezoidei) of same and opposite sides. Neurone No. 2. (and 3 ?).• — Axones of cells in accessory nucleus, in superior olives, and in nuclei of trapezoid body, constitute a ventral path in the lower border of the tegmen- tum, and form the lateral part of the lateral lemniscus {Lemniscus lateralis) or lateral fillet on the opposite side. Axones of cells in the lateral nucleus traverse the floor of the fourth ventricle as the striae medullares, forming a dorsal pathway, decussate and then turn ventrally to a point dorsal to the superior oUve and join the lateral lemniscus as its mesial part. Some axones also of cells in the accessory and lateral nuclei pass dorsally, looping around the restiform body, and then proceed ventrally (bundle of Held) to join the opposite lemniscus. The lateral lemniscus passes upward to the inferior colliculus, some of the axones terminating en route in the nucleus of the lateral lemniscus. From cells in this nucleus some axones again join the lateral lemniscus, and a few decussate and then pass upward to the inferior coUiculius. The axones of the lateral lemniscus ter- minate in the inferior colliculus, or pass on to terminate in the internal geniculate body, merely giving off collaterals to the superior colliculus. Some fibres of the lateral lemniscus probably go to the superior colliculus. Neurone No. 3 (and 4?). — Axones of cells in the gray matter of the inferior col- liculus form its brachium {Brachium corp. quad, post.) and ascend to terminate in the internal or medial geniculate body {Corp. genie, inter.). Neurone No.ii, (and 5?). — Axones of cells in the internal geniculate body pass as a part of the thalamic radiation via the posterior part of the internal capsule to the cortex of the temporal lobe of the cerebrum. The axones which constitute the ventral path (Neurones i and 2) form a bundle of fibres known as the trapezoid body {Corpus trapezoideum) or trapezius. The decussa- tion of these is peculiar in that the dorsal axones of the bundle on one side become the ventral ones on the opposite side; this accounts for the convergence of the a.xones at the median raphe. Axones of ceils in the superior olive pass to the nucleus of VI nerve (reflex). There is possibly also a descending path from the lateral nucleus to the spinal cord (not indicated). 496 THE ORGANS Corp. guad. ii.^§^- ...Itet-l '^'t^Mr Fig. 33^- THE NERVOUS SYSTEM 497 EXPLANATION OF FIG. 331 Fig. 331. — Principal Connections of the Vestibular Portion of the Auditory (VIII) Nerve. .4, Section at level of oculomotor (III) nerve; B, section through pons and cere- bellum; C, through inferior olives; D, through spinal cord. Each neurone group is indicated by one or several individual neurones. Neurone No. i. — Cell bodies in ganglion of Scarpa; peripheral processes end in semi- circular canals; central processes bifurcate, and ascending arms go to Deiters' nucleus (A'«. lat. n. vestib.) (i a), to von Bechterew's nucleus {Nu. sup. n. vestib.) (i b), and to nuclei fastigii and cortex of vermis of cerebellum (i c); descenchng arms go to nucleus of descending root {Nu. n. vestib. desc.) (i d) and (collaterals?) to principal or median nucleus {Nu. med. n. vestib.) (i c). Neurone No. 2. — Axones of some cells in Deiters' nucleus descend {Tr. desc. nu. Deitersi) uncrossed to antero-lateral column of the cord, axones of other cells enter the posterior longitudinal fasciculus (Fasc. long, post., 2 b) of same side and descend to anterior column of the cord, others pass to the medial longitudinal fasciculus of opposite side whence some (2 c) descend to anterior column of the cord, occupying a position near the anterior median tissure, while some (2 d) ascend in the medial longitudinal fasciculus and terminate principally in the nuclei of VI, IV, and III nerves. Axones of cells in von Bechterew's nucleus ascend (2 e), joining lateral part of medial longitudinal fasciculus of same side, and terminate in nuclei of IV and III nerves. Axones of cells in the nucleus of the descending root probably pass in part to lateral part of reticular formation of same and opposite sides, ascending and descending (to other motor nuclei?). Axones of cells in the median nucleus probably pass largely into the reticular formation, possibly also to the medial longitudinal fasciculus (not indicated). Axones of cells in the nuclei fastigii of the cerebellum pass to von Bechterew's nucleus (2/) and to Deiters' nucleus (2 g). The cerebellar associations intercalated between these (2/, 2 g) and the vestibular fibres to the cerebellum (i c) are not known. (It is evident that impulses other than vestibular ones entering the cerebellum mcjy also by 2 /and 2g act indirectly upon the motor nuclei innervated by axones of the cells in Deiters' and von Bechterew's nuclei. Compare Figs. 323 and' 337.) 498 THE ORGANS md' /f- Nvm sra -"-Fa vin "'/ NVII Fig ^^2— Section through the Hindbrain at the Level of the Junction of Ponsand Cerebdlum ;nd the Entrance of the Vestibular, Part of the Eighth ferve. Weigert nrenaration (Marburg.) Va, Radix spinalis tngemini (spmal root of the tittJij, V i, Servus abducins (external eye' muscle nerve); y//a, pars nuclearis nervi facialis (pars Srima crus of origin or ascending facial root); VIII, nervus acusticus (eighth, vestibular S;' TA/brachium pontis (middle cerebellar peduncle) ; 5rcibrachium conjunc ivum [superior cerebellar peduncle) ; cH, fasciculus tegmenti centralis; Q&, .^TJlTe £.'" fEssickVCrs^ corpus restiforme; Dca, decussatio cerebelh anterior; I^ec/ declive, J^mb, SSu '(nucleus e'^bohformis);/. P. fibr. p^ bulbaris uncinate bundle of Russell);//.^, pedunculus Ao^^-^^' /^^.^' ,f ^^^^f °^° ^"^ Lm, Lemniscus medialis (mesial fillet tract); NVII, nucleus facialis ^^^^^^J^''^^^ of facial nerve); NaB, nucleus angularis, or superior, vestibularis (Bechterew), Ndt Slleus dentatus cerebelli; Nod, Ldulus cerebeUi; iV.. nucleus oh^^ns supenor Nrl, nucleus reticularis lateralis (nucleus of the ^toal column) iVrt^,m^^^^^^^^^^ reticularis te-menti; Nt, nucleus tecti (nucleus fastigu); iVz-w, nucleus vestibularis rJaSoceTlularis or lateralis (Deiters) (large-celled nucleus of vestibular nerve); PlcU plSuschSeus lateralis; P., pons;pi pyramid ; S^nstraturn -termedium pedun^^^^^^^^^^^^ Tr, corpus trapezoides; vIV, ventriculus quartus; vNdt, vellus nuclei dentati cerebellx (fleece of the cerebellar olive)- THE NERVOUS SYSTEM 499 still further increased in size. External arcuate fibres may be present and prob- ably contain fibres to the restiform body and possibly fibres from the cerebellum, which end in the reticular formation. The arcuate nuclei are present. The central tegmental tract is larger. Intersegmental Neurones. — The reticular formation is very extended. Its composition (longitudinal and transverse fibres, and cells) should be examined carefuUy. The rubro-spinal tract is in the same position, but the lateral Deitero- spinal tract is now more internally located. Its fibres cannot usually be dis- tinguished, but are bending inward and toward its nucleus of origin (lo- cated somewhat higher). The medial longitudinal fasciculus may be partially separated from the medial lemniscus. It is a complex bundle and contains at va- rious levels (a) descending and ascending fibres from Deiters' nucleus and other cells scattered in the reticular formation, (b) descending fibres from the nucleus of the medial longitudinal fasciculus in the tegmentum of the midbrain. The fibres of this fasciculus probably terminate in many nuclei, especially those of eye-muscle nerves (III, IV and VI) (comp. Figs. 331, 337 and 350). Efferent Suprasegmental Neurones, — The pyramids and coUiculo-spinal tracts are in the same positions. The aberrant efferent pallial fibres already noted (p. 491) may be seen in the lemniscus. 6. Section through the Hindbrain at Level of Junction of Pons and Cerebellum and Entrance of Vestibular Nerve (Figs. 324 and 332) The most conspicuous features are the nuclei and fibres of the pons, added ventrally to the preceding structures, which are now collectively known as the tegmentum; the cerebellum, enclosing dorsally the fourth ventricle; and the connec- tions {inferior and middle peduncles, arms, or brachia) of the cerebellum with the rest of the brain. The greater part (rtstiform body) of the inferior peduncle represents ascending cerebellar connections from all parts below this level, the middle peduncle (from the pons) is the second link in the descending connection from the pallium to the cerebellum (pallio-cerebellar path). The superior ped- uncle (efiferent cerebellar) is not fully formed at this level. (Comp. p. 508.) Efferent Peripheral Neurones. — In this level and that of the next section are present the nuclei and root fibres of nerves VII and VI. The nucleus of nerve VII, or nucleus facialis is seen occupying a lateral posi- tion in the reticular formation similar to that of the nucleus ambiguus. In it may be made out the usual plexus of fine terminals and the coarser root fibres which proceed dorso-mesially to the floor of the ventricle, where they partially envelop the nucleus of the VI (usually not present in this level). They then turn cephalad, forming a compact longitudinal bundle (next section) and finally turn vcntro-laterally and caudally to emerge on the lateral aspect at the caudal border of the pons. This latter part is the second part as distinguished from the first part of the connection. The bend is known as the genu facialis. Ac- cording to some authorities, some of the fibres cross and pass out in the root of the opposite side. Four groups of cells composing the nucleus facialis have been distinguished: three ventral groups which, jiassing from the most mesial to the most lateral group, innervate respectively the mu.sclcs of tympanum, of pinna and of mouth 500 THE ORGANS ventr /i^ and face. The dorsal group (to superior branch of facial) innervates the frontalis, corrugator supercilii and orbicularis palpebrarum. Among the terminals in the nucleus have been distinguished collaterals (reflex) from the lateral part of the reticidar formation, from the secondary acoustic and trigeminal tracts and other adjacent fibres; also terminals of the colliculo-spinal tract. Whether the nucleus receives direct terminals from the pyramids or whether fibres of the latter are only connected with it via intercalated neurones is uncertain. Some of the root fibres of the VI are usually seen in the ventral border of the tegmentum. For nucleus see next section. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The afferent vestibular root fibres enter at the caudal border of the pons and pass lateral to the spinal V, mesial to the ventral cochlear nucleus and restiform body, and enter the field previously occupied by the descending vestibular root, the fibres of which are a continua- tion of the root. Scattered large cells in this region form the nucleus of Deiters. Dorso-mesial to this is still the medial vestibular terminal nucleus and dorsal to Deiters' at the external angle of the fourth ventricle is the superior vestibular terminal nucleus (von Bechterew). Fibres seen passing from the vestibular region to the cerebellum and lying near the ventricle are partly vestibular root fibres to the cerebellum (especially to the nucleus tecti, or fastigii, see below), and partly descend- ^///t Fig. 333. — Diagram of Origin of Sixth and Seventh Cranial Nerves. (Schafer.) pyr, Pyramid; cr, restiform body; dV , spinal root of fifth nerve; Venlr. IV, fourth ventricle; VIII. v, vestibular root of eighth nerve; w.F/, chief nucleus of sixth nerve; n'VI , accessory nucleus of sixth nerve; F/, sixth nerve; n.VII, nucleus of seventh nerve, from which the axones pass dorso- mesially to the floor of the ventricle, where they turn brainward, appearing as a bundle of transversely cut filsres, aVII , and ascend to the "genu." g, where they turn and pass ventro-laterally and somewhat caudally to the surface as the seventh nerve, VII. ing fibres from cerebellar nuclei (especially from the nuclei fastigii, forming fasti gio-bulbar fibres) to Deiters' nucleus, other vestibular nuclei, and other cells in the reticular formation. It is thus evident that such nuclei as Deiters' may act as parts of vestibular bulbo-spinal reflex arcs and also as parts of efferent and possibly afferent cerebellar paths. Internal arcuate fibres from the vestibular area are probably principally fibres (secondary tracts) from the various vestibular nuclei to the medial longitudinal fasciculus and other tracts in the reticular formation. (Comp. pp. 493, SOI, 507, Figs. 331, 337.) The nucleus olivaris superior lies ventral to the nucleus facialis and lateral to the central tegmental tract. This nucleus together with several other small nuclei in its immediate vicinity (preolivary nucleus, semilunar nucleus, trapezoid nucleus) is one of the nuclei intercalated in the cochlear path (Fig. 330) which provides reflex connections {e.g., with the \T and VII motor nuclei). Lateral THE NERVOUS SYSTEIM 501 to it is seen a mass of fibres which pass by it toward the median line through the medial lemniscus, and decussate, finally turning longitudinally dorso-lateral to the opposite superior olive. These are fibres of the trapezius, and together with the more dorsal secondary cochlear fibres (p. 493) form the lateral lemniscus (See Fig. 330 and page 493). The lateral lemniscus is thus one of the links in the cochlear or auditory pathway. Fibres pass from superior olive to nucleus of nerve VI (reflex). In some cases the slender afferent root of VII {portio inter- media or nerve of Wrisberg) from the ganglion geniculi may be seen entering between the vestibular and main facial roots. Its fibres proceed to a gray mass which may be regarded as a continuation of the nucleus fasciculi solitarii and there probably partly terminate and partly send descending arms to join the fasciculus solitarius. The spinal V occupies the same position though separated from the surface by the pontile fibres; internal to it is its terminal nucleus. Note the change in the shape of the lemniscus. The ventral spino-cerebellar and spino-thalamic tracts are in the same position though herv separated from the surface by the added pontile fibres. Other Afferent Cerebellar Nexxrones. — The inferior olives are not present, and the olivo-cerebellar fibres are here entering the cerebellum as a part of the restiform body. The central tegmental tract (to the olives) occupies the ventral part of the reticular formation. The restiform body is entering the white matter of the cerebellum. It has been seen to be composed of the dorsal spino-cerebellar tract, olivo-cerebellar fibres and fibres from the lateral and possibly other nuclei in the reticular formation. The dorsal spino-cerebellar tract terminates in the cortex of the vermis or middle lobe of the cerebellum, the olivo-cerebellar fibres terminate in all parts of the cerebellar cortex. The fibres mesial to the restiform body, consisting of ascending vestibular fibres to the cerebellum and descending fibres to vestibular and other nuclei (see cerebellum) , are sometimes called the in- ternal or jiixta-restiform body. This and the restiform body proper constitute the inferior cerebellar peduncle. The pons consists of gray matter— the pontile nuclei — and of transverse a.nd,longitudinal fibres. The longitudinal fibres include the pyramids which pass through to the medulla and cord, and other fibres from the pallium (pallio-pontile or cerebro-pontile) which terminate in the pontile nuclei. The axones of the latter form the transverse pontile fibres (ponto-ccrebellar fibres) which cross and pass to the cortex of the opposite cerebellar hemisphere. They constitute the middle cerebellar peduncle or brachium pontis. (Comp. p. 508.) The pallio-pontile and ponto-ccrebellar neurones constitute the pallio-ponto- cerebellar path connecting one cerebral with the opposite cerebellar hemisphere (p. 477, path XllI). There are probably also transverse fibres in the pons connecting cerebellum and reticular formation. Fibres passing vertically in the raphe from pons to reticular formation [perpendicular fibres of pons) may be in part continuations of these and in ijart efferent pallial fibres from pes to teg- mentum. The latter arc cither aberrant fibres or fibres innervating directly or indirectly motor cranial nuclei. Intersegmental Neurones. -The reticular formation is extensive. In it there may be distinguished, besides the nuclei already mentioned, various mc^re 502 THE ORGANS or less well-defined reticular nuclei (see Fig. 332). The fibres of the lateral Deitero-spinal tract (not distinguishable) are here emerging from Deiters' nucleus. The medial longitudinal fasciculus occupies the same position, but is here well separated from the fillet. Some of the internal arcuate fibres in the dorsal part of the reticular formation may be fibers from Deiters' nucleus to the medial longitudinal fasciculus. They may be crossed or uncrossed, and may descend in it as already mentioned (pp. 466 and 499,. or ascend (see Fig. 331). Other internal arcuate fibres here, as elsewhere, pass from the various terminal nuclei to form secondary tracts. Other transverse fibres are axones of cells of reticular nuclei or collaterals and terminals ending in them. Efferent Suprasegmental Neurones. — The colliculo-spinal tract lies ventral to the medial longitudinal fasciculus. The pyramids are in the same position, but are partly surrounded by pontile fibres and nuclei. (See also pons, above.) The Cerebellum. — The gray matter consists of the external gray or cortex, and internal nuclei forming interruptions or relays in paths from the cortex. The white matter consists of the fibres of various afferent and efferent cerebellar paths and possibly association fibres of the cerebellum. The cortex is studied elsewhere. The internal nuclei can usually be distinguished. They are the nucleus dentatus cerebelli {corpus dentatum), a convoluted mass of gray resembling the inferior olives (and sometimes called the cerebellar olives), and mesial to this the nucleus glohosus. nucleus emboliformis and the nucleus tecti or fastigii. The nucleus fastigii receives fibres from various parts of the cerebellar cortex and also vestibular root fibres (p. 497). Its axones, in part at least, pass to Deiters' nucleus and other nuclei in the reticular formation {fastigio-bulbar tract). This forms a link of the cortico-fastigio-Deitero-spinal path (p. 478, XVIII). The nucleus dentatus, nucleus globosus, and nucleus emboliformis also receive fibres from the cerebellar cortex. Their axones form the superior cerebellar peduncle {bruchium conjunctivum), cross and pass to the red nucleus, reticular formation, nucleus of nerve III, and thalamus. At the level of the section the superior pe- duncle is not yet fully formed. This forms links in both the cortico-dentato-rubro- spinal and the cerebello-pallial paths (p. 478, XVII and VII. See also p. 507.) Note where possible the structure of the plexus chorioideus of the fourth ventricle. It consists of a layer of cuboidal epithelial cells next the ventricle which are ectodermic, and an outer mesodermic part consisting of connective tissue and blood-vessels. 7. Transverse Section of the Hindbrain through the Roots of Nerves VI (Ab- ducens and VII (Facial) (Figs. 324 and 334) Efferent Peripheral Neurones. — The nucleus facialis is usually not present, but various portions of the root fibres may be present (see preceding section), especially the longitudinal part. The nucleus of the VI or nucleus abducentis is present in about the middle of the floor of the ventricle and just beneath the central gray or partly within it. Its fibres, the root fibres of the abducens, are seen passing ventrally. The nucleus receives collaterals from the axones of Deiters' nucleus (secondary vestibular) on the same and opposite sides and collaterals or terminals from the superior THE NERVOUS SYSTEM 503 L\^- J '-'V; a 3 2 iv > c c m u .s-a rV -■ s. V S 3 c c '^ ' ^ . <« > o i--a c u c CX . ,-S-° o alis brospi spinot erebell C E o o Pi a 1 •-3 • U 0) XI 'A o ecun cis n Tra SC. S] G H M:s tt fn ^ (^ 504 THE ORGANS olive (secondary cochlear). The exact mode of connection with the pyramids is not well known. Afferent Roots, Their Terminal Nuclei and Secondary Tracts. — The lateral (Deiters') and medial vestibular nuclei are usually still present, also possibly fas- tigio-bulbar iibres. The ventral cochlear nucleus has disappeared, but other cochlear nuclei (superior olivary and trapezoid) are usually present. Often fibres can be seen passing from the superior olive to the nucleus VI. Fibres of the secondary cochlear tract (corpus trapezoideum) are still traversing the medial lemniscus, and decussating. The tract they are forming (lateral lemniscus) is not yet very distinct. The spinal V is in the same position, but it and its terminal nucleus tend to separate into groups of fibres and cells, and to change their relative positions. The medial lemniscus is more flattened in cross section, extending transversely instead of dor so-ventr ally. The ventral spino-cerebellar tract and spino- thalamic tract are in the same positions in the external part of the tegmentum ventral to the spinal V and external to the superior olive. Other Afferent Cerebellar Connections. — The restiform body has now merged with the white matter of the cerebellum. The nuclei and transverse fibres of the pons (ponto-cerebellar neurones) have increased. The longitudinal fibres in the pons at this level are principally the pyramids, but some are pallio- pontile fibres which terminate in the nuclei pontis. Perpendicular fibres are present. Intersegmental Neurones. — The reticular formation is practically unchanged. One of its nuclei {nucleus reticularis tegmenti) 'ca.n be seen as a lighter area (Weigert) in the medial part, dorsal to the medial lemniscus. The rubro-spinal tract is in the same position near or mingled with the spino-thalamic and ventral spino-cerebellar tracts. These fibres are not easily distinguished among the various fibres of the cochlear tract which cross them. The medial longitudinal fasciculus is now a well-marked tract occupying the same position. From now on, it contains ascending fibres from Deiters' nucleus and perhaps other reticular nuclei besides the descending fibres from the nucleus of the medial longitudinal fasciculus. Efferent Suprasegmental Neurones. — ^The pyramids and coUiculo-spinal tract (predorsal fasciculus) occupy the same positions. The fastigio-bulbar fibres have been mentioned. The superior cerebellar peduncle is now more distinct as it is being formed by fibres from the dentate nucleus. It lies near the dorso-lateral part of the ventricle. 8. Transverse Section of the Hindbrain Through the Roots of Nerve V (Trigem- inus) (Figs. 324 and 335) Efferent Perip' eral Neurones. — Motor nucleus of V. This is mesial to the terminal nucleus of the V and its coarse efferent root fibres may be seen, in favor- able levels, passing out just internal and somewhat cephalad to the entering afferent fibres. It is probable some of the fibres cross and pass out in the opposite motor root. Some of the finer terminal fibres present in the nucleus are afferent root fibres of the V (two-neurone arc) and collaterals of secondary THE NERVOUS SYSTEM 505 506 THE ORGANS tracts of V (three-neurone arc). The nature of its connections with efferent pallial fibres is not known. Many collaterals are also received from the mesencephalic root. (Fig. 336.) Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The afferent fibres of the V pass through the pons and enter the tegmentum where they divide into short ascending and long descending arms. The former, together with collaterals, terminate in the cephalic end of the terminal nucleus of the V. This is broken up into groups of cells which lie dorso-lateral to the entering fibres and is sometimes known as the "principal sensory" nucleus of the V. The long descending arms pass down to the cord as the spinal V, giving off collat- erals and terminals to the nucleus en route. (Fig. 336.) A third source of Fig. 336. — Diagram of Origin of Fifth Cranial Nerve. (Schafer.) . G, Gasserian ganglion; a, b, c, the three divisions of the nerve; m.n.V, principal motor nucleus; p.s.n.V, principal terminal "sensory" nucleus; d.s.n.V, terminal nucleus of spinal root; d.s.V, descending or spinal root; c.V and c'.V, secondary trigeminal tracts (axones of cells in terminal nuclei); r, median raphe; m'.n.V, mesencephalic nucleus. fibres of the V is a series of cells extending upward into the roof of the mesen- cephalon. The axones of these cells form the mesencephalic root of the V. There is reason to suppose, from their peculiar location and for other reasons, that these are afferent peripheral neurones which have remained within the neural tube. From the region of the terminal nucleus of the V, a transverse bundle passes to the opposite side in the floor of the fourth ventricle. This is considered a secondary decussating trigeminal tract which forms an ascending tract in the dorsal part of the reticular formation. Fibres of secondary tracts give off collaterals to various efferent nuclei and probably axones of some cells of the terminal nuclei become intersegmental fibres in the reticular formation. Second- 2 "5 O o , o S (/) c rt o _ r- " 1 — J) U u 3 — ts£) O O " o c c o u-;^ .2^ .2 N •" C w « fc 1 o c^ [l< (O •s=j||.g|u Ill's- a; |jj-|SQ|o^^||a a^^g-S-i III sails m^ilf iiiislrs i¥l-g-°srs= h^kl i g " a a . a ° THE NERVOUS SYSTEM 507 ary tracts to the thalamus (via fillet and also in reticular formation?) form part of the trigeminal afTerent pallial path. The superior terminal vestibular nucleus (of von Bechterew) may still be present. The superior olivary and trapezoid nuclei may be present. The secondary (and tertiary (?), see Fig. 330) cochlear tract or lateral lemniscus is now well formed and may be seen lying dorso-lateral to the superior olivary and trapezoid nuclei. The medial lemniscus is still more flattened. The spino-thalamic and ventral spino-cerebellar tracts occupy the same positions. Other Afferent Cerebellar Neurones. — The transverse pons fibres are the same, but the longitudinal fibres have increased owing to the presence of more pallio-pontile fibres. The perpendicular pontile fibres are seen passing dorsally in the raphe into the tegmentum. The central tegmental tract is in nearly the same position. Intersegmental Neurones. — The reticular formation is somewhat diminished. In it is the nucleus reticularis tegmenti. The rubro-spinal tract is in the same position, mingled with the spino-thalamic and ventral spino-cerebellar tracts. The medial longitudinal fasciculus is unchanged. Efferent Suprasegmental Neurones. — The pyramids now occupy the cen- tral part of the pons, and are broken up into a number of bundles. In the dcrsal part of the pons fibres pass obliquely dorsally. These are probably efferent pallial fibres which act directly or indirectly on some of the efferent nuclei of cranial nerves (motor path to cranial nerves). The pallio-pontile fibres have been mentioned. The superior peduncle is now a large bundle of fibres flattened against the ventricular surface of the dorso-lateral brain wall. The coUiculo-spinal tract is in the same position. Cerebellum The cerebellum, connected with the rest of the brain by its three peduncles, consists of two lateral lobes or hemispheres connected by a median lobe, the vermis. These are divided into various lobules, the surfaces of which are marked by parallel transverse- folds or laminae. When these are cut across it is seen that they give off secondary or tertiary laminae, the whole producing the appearance known as the arbor vitse. The surface of the cerebellum is composed of gray matter, the cortex, enveloping the white matter. Besides this there are masses of gray within, the internal nuclei of the cerebellum (dentatus, globosus emboliformis, and fastigii), embedded in its white matter. Fibres entering the cortex are the terminations of the fibres of the restiform body (dorsal spino-cerebellar tract to the cortex of the vermis, olivo-cerebellar fibres to the whole cortex, fibres from the lateral nucleus and possibly other nuclei in the reticular formation, also per- 508 THE ORGANS haps some fibres from the columns and nuclei of Goll and Burdach of the same and opposite sides), vestibular root fibres to the vermis, the ventral spino-cerebellar tract to the vermis, and the pontile fibres to the cortex of the hemispheres. The cortical cells do not send axones outside the cerebellum, all efferent fibres being interrupted in the internal nuclei. The dentate nucleus receives fibres from the cortex of the hemispheres; the globose and emboliform nuclei receive fibres from the cortex of the vermis; and the nucleus fastigii receives fibres from various parts. The axones of the first three form the superior Fig. 338. — Part of a Vertical Section through the Adult Human Cerebellar Cortex. Nissl Method. (Cajal.) A, Inner portion of the molecular layer; B, granular layer; C, body of a Purkinje cell; a, stellate cell of the molecular layer; h, nuclei of the epithelial- like neurogha cells (cells of the fibres of Bergmann); c, stellate cell with marginal chromo- philic substance; d, fibrillar mass corresponding to the baskets; e, nuclei of the granule cells; /, islands or glomeruli in the granular layer; g,/z, Golgi cells in the granular layer; ?', nuclei of neuroglia cells. peduncle; the axones of the nucleus fastigii are fastigio-bulbar fibres, principally crossed, to vestibular nuclei and possibly other reticular formation nuclei. There may be some efferent fibres in the middle peduncle to reticular formation nuclei, but the major part, at least, of this peduncle consists of the ponto-cerebellar fibres already de- scribed. The inferior and middle peduncles are thus largely afferent and the superior peduncle is efferent, to red nucleus, thal- amus and nucleus of nerve III. (Fig. 323, p. 546 and Fig. 337.) See also pp. 475 and 476. THE NERVOUS SYSTEM 509 In the cortex can be distinguished an outer or molecular layer with few cells and few medullated fibres, an inner, granular or nuclear layer, and between the two a single row of large flask-shaped cells, the cells o] Purkinje (Figs. 338, 339). These latter give off several main dendrites, which enter the molecular layer and form a remarkably rich arborization extending to the surface. The Golgi method shows the larger and medium branches smooth, but the terminal branches thickly beset w4th "gemmules." The dendritic arborization is flattened, extending at right angles to the laminae. The axone is Fir,. 339. — Purkinje Cell of .\dull Human Cerebellum. Golgi preparation. (Cajal.) a, .\xone; b, recurrent collateral; d, spaces occupied by basket cells; c, spaces occupied by blfK^d-vesscls. given ofi' from the end opposite to the dendrites and passes through the granular layer into the white matter, either to one of the internal cerebellar nuclei where it terminates, or to some other part of the cortex. The I'urkinje cells are almost the only cells the axones of which enter the white matter. It is evident that all intracortical connections must ultimately converge on these cells to reach the efferent cerebellar i)aths. The axones of the J'urkinje cells give off collaterals not far from their (origin, which ])ass into the molecular layer and ai)pear to terminate there in end "buttons" upon the 510 THE ORGANS bodies of adjacent Purkinje cells. The medullated axones form the coarser fibres traversing the granular layer. The cell body is fairly well filled with small chromophilic bodies of uniform size, often show- ing a slightly concentric arrangement (Fig. 3.38). The cells in the molecular layer (stellate cells) are either superficial stellate cells with irregular branching dendrites and a short axone or deep stellate {basket) cells. These latter are cells the axones of which (apparently non-medullated) have a narrow neck and unusual thicken- ing beyond the neck. They extend at right angles to the laminae Fig. 340. — Section of Adult Human Cerebellum. Silver Method of Cajal. (Cajal.) A, B, Cells in the granular layer enveloped by basket fibres; C, cell of Purkinje. The axone of one of the ceUs of Purkinje is shown. for a distance of several Purkinje cells, giving off to each Purkinje cell one or more collaterals which pass toward the granular layer and envelop, with their terminal arborizations, the body and proximal, non-medullated portion of the axone of the Purkinje cell. Collaterals of other basket cell axones may terminate around the same Purkinje cell, forming the "basket" (Fig. 340). The dendrites of the basket cells rarnify throughout the molecular layer. Besides the cells con- tained in it, and the dendritic arborizations of the Purkinje cells, the molecular layer contains the axones of the granule cells and the ter- minations of the chmbing fibres (see below) . In ordinary stains it pre- sents a general punctate appearance, with the scattered nuclei of the THE NERVOUS SYSTEM 511 Fig. 341. — Diagram of Longitudinal Section of Cerebellar Lamin^. Golgi method. (Kolliker.) gr, Cell of the granular layer; n, axone of granule cell; n', the same in molec- ular layer where it branches and runs in long axis of lamina; p, Purkinje cell showing how much less extensively its dendrites (/>') branch in long axis of lamina. (Compare Fig. 343-) ':;^>fi>^-^ Fig. 342. — Granule Cells and Mossy Fibres in the CenliLllimi of Adult ('at. Silver method of Cajal. (Cajal.^ A, (Jranulc cell; li, (Joigi cell; n, dLiidrilic arborization of granule cell; b, mossy fibres j)assing by (joigi cell; c, mossy fibre; d, termination oi a mossy'ftbre; e, terminal processes given ofT from a thickening in a mossy fibre. 512 THE ORGANS short axone and basket cells, and the coarser dendrites of the Purkinje cells distinguishable (Fig. 338). The granular layer with ordinary stains presents the appearance of closely packed nuclei with clear spaces here and there {"islands^' or " glomeruW) and also a few larger cells (Fig. 338). Most of these nuclei belong to the granule cells, which are caryochrome cells. The granule cells are small and possess three to six dendrites which are Fig. 343. — Semi-diagrammatic transverse Section of a Cerebellar Lamina of a Mam- mal, as shown by the Golgi Method. (Cajal.) A, Molecular layer; B, granular layer; C, white matter; a, Purkinje cell, seen flat; h, basket cells of the molecular layer; d, their terminal arborizations which envelop the iDodies of the Purkinje cells; e, superficial stel- late cells;/, Golgi cell; g, granule cells with their axis-cylinders ascending and bifurcating at i; h, mossy fibres; j, neuroglia cell; m, neuroglia cell in granular layer; n, climbing fibres. comparatively short and terminate in the glomeruli with a compact arborization, each branch of which ends in a small varicosity. The axones of the granule cells, which are non-medullated, ascend into the molecular layer where each divides into two branches running longi- tudinally along the laminae and terminating in varicosities (Figs. 341, 343). These are the parallel fibres of the molecular layer. They thus run at right angles to and through the dendritic expansions of THE NERVOUS SYSTEM 513 the Purkinje cells and their cross sections together with the terminal dendritic arborizations of the Purkinje cells give the molecular layer its punctate appearance. The scattered larger cells in the granular layer are principally short axone or Golgi cells, whose main dendrites usually penetrate and branch within the molecular layer. Their axones often form very extensive and complicated arborizations in the granular layer, the terminations of which are concentrated in the glomeruli (Fig. 343,/). Dislocated cells of this type may have their cell bodies in the molecular layer. Fig. 344. — Cross Section of a Cerebellar Convolution Stained by Weigert's Method, f Ko.liker.) m, Molecular layer; K, granular layer; w, white matter; q, fine fibres passing from white matter into the molecular layer; Ir, dots represent longitudinal fibres of molec- ular layer among bodies of Purkinje cells. In the cortex there are also the terminations of the afferent cerebellar fibres already mentioned (p. 507). These are of two types, mossy fibres and climbing fibres. The mossy fibres, so called from the appearance of their terminations in embryos, are the coarsest fibres of the white matter. While in the latter they bifurcate, branches going to different laminae. These main branches give off secondary branches which enter the granular layer and there arborize. Uuring their course, and also at their terminations, these branches are thickened in j)Iaces and there give off short, thick, terminal 3.3 514 THE ORGANS branches which end in varicosities. These terminal branches are located within the glomeruU. The glomeruH thus contain the dendritic terminations of the granule cells, the axonal terminations of the Golgi cells, and the terminations of the mossy fibres. (Figs. 343 and 342.) The cHmbing fibres pass from the white matter, through the granular layer to the cells of Purkinje. Passing by the bodies of the latter they arborize into terminals which envelop the smooth dendritic branches of the Purkinje cells; i.e., all but the terminal dendritic arborizations. (Fig. 343, n). Three kinds of fibres thus terminate around the Purkinje cells; the granule axones, probably in contact with its terminal dendritic arborizations; the climbing fibres around its coarser dendritic branches; and the basket fibres around its body. The respective sources of the mossy and chmbing fibres are unknown. There is some evidence that the chmbing fibres are from the pons. It is evident from the above that all of the cells of the cerebellar cortex except the Purkinje cells are association cells of the cortex. The medullated fibres of the cerebellum (Fig. 344) pass from the white matter into the granular layer and ramify throughout the latter, forming quite a dense plexus separating groups of granule cells. Sometimes straight fibres can be seen passing through toward the molecular layer which are probably either the climbing fibres or axones of the Purkinje cells. Beneath and between the bodies of the Purkinje cells is a plexus of fibres extending into the deeper part of the molecular layer, the remainder of this layer containing few or no medullated fibres. These fibres in the vicinity of the Purkinje cells are probably principally formed by the recurrent collaterals of the Purkinje cells already mentioned. The remaining fibres of the granular plexus would apparently consist of the arborizations of mossy fibres and of the Golgi cells. Whether the former are medul- lated is, however, somewhat uncertain. Most of the neuroglia cells in the cerebellum are of the same general type as seen elsewhere, but in the Purkinje cell layer are apparently epithehal-like cells which send vertical processes to the periphery. Some of these processes, as seen in the Golgi method, are rough and branched, others are smooth. In ordinary stains these processes are sometimes visible and are known as the fibres of Bergmann. (Figs. 338, 343.) THE NERVOUS SYSTEM 515 Isthmus PRACTICAL STUDY 9. Transverse Section through the Isthmus at the Exit of Nerve IV (Trochlearis) (Figs. 324 and 345) In this there are to be distinguished three parts, the thin roof (superior medul- lary velum), the tegmentum and, ventral to the latter, the pons. The tegmentum consists essentially of the reticular formation, efferent cerebellar and midbrain connections, and externally the afferent pallial connections. The pons contains the eff'erent pallial paths to the cerebellum and to parts of the nervous system caudal to it. The cavity is the iter or agnceductus Sylvii. Next to this is the central gray of the brain wall. Efferent Peripheral Neurones. — The root fibres of the IV are seen in the roof. They originate from nuclei lying further forward in the ventral part of the central gray. The fibres pass from the nuclei dorsally and caudally in the outer part of the central gray and finally decussate in the roof and emerge. It is only the latter part of this course which is seen in this level. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The mesen- cephalic root of the V lies in the lateral part of the central gray. Mingled with its fibres may be seen the rounded cells, the axones of which form these fibres. The lateral lemniscus occupies part of the lateral swelling on the surface of the tegmentum, forming the major part of the external structure known as the trigo- num lemnisci. Groups of cells among its fibres constitute the dorsal nucleus of the lateral lemniscus. The medial lemniscus is now still more flattened. The spino-thalamic tract is in about the same position, between the two lemnisci. Thus at this level, the principal afferent suprasegmental paths form an L-shaped mass, enveloping the rest of the tegmentum and representing general bodily sensation and hearing. There are also cranial nerve ascending paths lying probably within the reticular formation and fillet (secondary vago-glossopharyn- geal and trigeminal tracts, representing visceral, taste, and general head sen- sation). These cannot be distinguished in the section. The ventral spino-cerebellar tract is on the surface, and now comes to lie external to the superior cerebellar peduncle. At about this point it turns caudally, and passes back into the cerebellum, accompanying the superior peduncle. Otl er Afferent Cerebellar Connections. — 'J'hc central tegmental tract oc- cupies the same position. (For the pallio-cercbelhir connection sec "Efferent Suprasegmental Neurones" below.) Intersegmental Neurones.— 'J'hc reticular formation is diminished in extent. One of its nuclei, the nucleus centralis superior, lies near the raphe. The rubro- spinal tract has moved somewhat mesially. It is dorsal to the medial lemniscus. The merlial longitudinal fasciculus is in the same position, and is a well-marked bundle lying at the bounflary bet ween the ventral part of the central gray and the reticular formal ir)n. Efferent Suprasegmental Neurones. —The f)yramids arc now broken up into bunrlles which may .show a tendency to gather in the ventral part of the pons 516 THE ORGANS THE NERVOUS SYSTEM 517 (distinguishable in one-month infant, where thej^ are meduUated while the pallio- pontile system is not). Bundles apparently forming lateral and mesial portions of the medial lemniscus (not indicated in the figure) are aberrant efferent pallial fibres. Such bundles have been seen passing from pons to tegmentum and also imbedded in the medial lemniscus in lower levels (pp. 501, 491). Some of these fibres are possibly fibres acting directly or indirectly on the efferent periph- eral neurones or motor nuclei of the cranial nerves (see p. 521). The pallio-pontile fibres are still more numerous. The gray matter in the pons (nuclei pontis) is very extensive. The transverse fibres of the pons no longer pass at this level into the cerebellum, but are collected at the sides of the p>ons to pass backward to the cerebellum (compare with an external view of the brain). The superior cerebellar peduncles or brachia conjunctiva are two large crescentric bundles of fibres in the lateral part of the reticular formation. Some of their fibres have begun to decussate in the ventral part of the reticular formation. The colliculo-spinal tract or predorsal fasciculus lies ventral to the medial longitudinal fasciculus. Midbrain or Mesencephalon The dorsal surface of the midbrain presents four rounded promi- nences, the two inferior and two superior colHculi (posterior and anterior corpora quadrigemina) . Ventrally are seen two diverging masses of longitudinal fibres, the pes peduncuh, separated by a deep groove or sulcus. In the midbrain are to be distinguished, (a) the expanded roof, the colliculi or corpora quadrigemina^ {h) the tegmentum containing the segmental (cranial nerves IV and III) and interseg- mental apparatus and the afferent suprasegmental paths, and (c) the basis pedunculi, ventral to the tegmentum and comprising the principal efferent paUial paths {pes pedunculi) and the substantia nigra. The cavity of the midbrain is the aqueduclus Sylvii or iter. PRACTICAL STUDY 10. Transverse Section through Midbrain at Level of Superior Colliculi (An- terior Corpora Quadrigemina) and Exit of Nerve III (Oculomotor) (Figs. .^24 and 347) Compared with the f)rcceding section, the following arc the most conspicuous changes: The roof has now enlarged into the superior colliculi; the tegmentum now contains the nuclei and roots of nerve III and the red nucleus; instead of the pons, the ventral part (jf the brain is now composed of the basis i)edunculi, consi.sting of a mass of elTerent |)allial iilires and I he subsfanlia nigra. I'he 518 THE ORGANS term crura cerebri or cerebral peduncles is loosely used to include all except the roof of the brain at this level, i.e., tegmentum and basis pedunculi. Efferent Peripl eral Neurones. — The nucleus of nerve III or oculomotor nucleus is located in the ventral part of the central gray in a V-shaped trough formed by the fibres of the medial longitudinal fasciculus. The nucleus is divided into large and small-celled groups. The large-celled groups are two lateral groups subdivided i^ni£S£"^y^g^^^v^^ Fig. 346. — The region of the aquseductus Sylvii seen from above. Schema showing the position of the nuclei of nerves III and TV and their subdivisions. (Edinger.) I. The small-celled nucleus (here represented as one on each side) a, its ciliary, b, its pupillary portion. 2, The portion of the large-celled nucleus sending uncrossed fibres to M. levator palpebrae; 3, portion sending uncrossed fibres to M. rectus superior; 4 and 5, portions sending crossed and uncrossed fibres to Mm. rectus internus and obliquus inferior; 6, portion sending crossed fibres to M. rectus inferior. The nucleus trochlearis sends crossed fibres to M. obliquus superior. into anterior and posterior dorso-lateral and anterior and posterior ventro-mesial, and a central or median group — nine in all. Between the cephalic or anterior groups are on each side a small-celled group known as the Edinger-Westphal nucleus, and still further forward are two small-celled anterior median nuclei. The connections of these groups with the extrinsic muscles of the eye innervated by nerve III (internal, superior and inferior recti, and inferior oblique) and the levator palpebrae superioris and intrinsic eye muscles (ciliary and sphincter THE NERVOUS SYSTEM 519 520 THE ORGANS pupillae, via ciliary sympathetic ganglion) are uncertain. From a priori grounds, the innervation of the intrinsic muscles by the small-celled groups and the other muscles by the large-celled groups would seem probable. Some of the fibres, usually stated to be from the posterior dorsal-lateral group, decussate. Recent observations (Cajal) would indicate that the decussating fibres come from the ventro-mesial lateral groups. What is perhaps the prevailing view as to these relations is shown in Fig. 346. The various fibres pass ventrally in a number of bundles, some passing mesial to, some traversing, and some passing lateral to the superior cerebellar peduncle and red nucleus. Ventral to these the root fibres come together and emerge on the ventral aspect of the midbrain (Figs. 347 and 348.) The nucleus of nerve III receives terminals and collaterals from the medial longitudinal fasciculus, i.e., from the ascending axones from Deiters' nucleus and the descending axones from the nucleus of the medial longitudinal fasciculus, possibly collaterals from the coUiculo-spinal tract, collaterals or terminals from the superior cerebellar peduncle and from the reticular formation. Its exact relation to efferent pallial fibres is not known. Fibres in the medial longitudinal fasciculus connect the nuclei of III, IV and VI (synergic movements of eye muscles) . Immediately caudal to the nucleus of nerve III (not in the plane of the section) is the nucleus of the nerve IV occupying a position similar to the lateral groups of the nerve III nucleus. The course of the root fibres of the IV has been mentioned (preceding section). It receives terminals similar to those received by III. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The mesen- cephalic trigeminal root is sometimes distinguishable on the lateral border of the central gray. The lateral lemniscus has partly or wholly terminated in the inferior coUiculus at a lower level. Fibres from the latter form its arm or brachium, representing another link of the cochlear path. In the present section the fibres of the brachium of the inferior coUiculus are seen entering the medial geniculate body (which is a part of the thalamus) . Axones of the cells of this body constitute the last relay of the cochlear path (III, p. 476) to the temporal cortex cerebri (not present in this section). As already stated, there is doubt as to how far this path is interrupted in various nuclei along its course such as the superior olives, trapezoid, lateral lemniscus nucleus and inferior coUiculus. It may be reducible to three principal neurone groups: (i) the spinal ganglion and its cochlear nerve, (2) the lateral lemniscus and (3) the geniculo-temporal system. The medial lemniscus is now a laterally placed, curved bundle, displaced laterally by the red nucleus. According to some authorities, some of its fibres terminate in the superior coUiculus. The spino-thalamic tract is plainly distin- guishable as a bundle dorsal to the dorsal edge of the medial lemniscus. At this level, then, the afferent paths from cord to pallium have practically united. In the superior coUiculus are terminations of the optic tract (the continuation past the optic chiasma of the so-called optic nerve) (see below). The central tegmental tract is displaced dorsally by the red nucleus. Intersegmental Neurones. — The reticular formation is smaller. The rubro- THE NERVOUS SYSTEM 521 spinal tract (not distinguishable) is emerging from its nucleus of origin, the nucleus ruber. Just below this level its fibres decussate {ventral decussation of Forel) and pass to the location they have been seen to occupy in preceding levels. The nucleus ruber or red nucleus is very conspicuous, occupying a large part of the reticular formation. It consists of a large-celled part which gives rise to the rubro-spinal tract and to rubro-bulbar fibres, and a smaller-celled portion. The latter sends fibres to the thalamus and thence to the pallium. The nucleus ruber, probably the first named part, also receives fibres from the pallium. According to this it will be seen that the nucleus ruber is a part of the descend- ing cerebello-rubro-bulbar and spinal path (XVII, p. 478), of the cerebello- pallial path (MI, p. 477), and of the pallio-rubro-bulbar and spinal path (XIV, p. 477). The red nucleus probably also receives collaterals from the coUiculo- spinal tract. The medial longitudinal fasciculus is diminished, some of its descending fibres having been formed from reticular formation nuclei below this level. Many of its fibres, both ascending and descending, send collaterals and terminals to the cells of the oculomotor nucleus. Efferent Suprasegmental Neurones. — The ventral part of the brain is com- posed of a mass of efferent pallial fibres, the pes pedunculi. The pyramidal fibres from the precentral areas of the cerebral hemisphere (pallio-spinal and some pallio-bulbar fibres) occupy about the middle three-fifths, but are also scattered through other parts of the pes, especially the mesial part. The leg fibres are probably more numerous laterally, the arm fibres in the middle, and the face fibres mesially. In the lateral part of the pes are pallio-pontile fibres from the occipital and temporal lobes of the cerebral hemispheres (occipito-temporal pallio-pontile fibres). In the mesial part are efferent pallial fibres from the frontal lobe, in part to the pons nuclei (frontal pallio-pontile) and in part possibly from the lower frontal region to the motor nuclei of cranial nerves VII and XII. Besides the above pallio-pontile and pallio-spinal fibres in the pes there are two other aberrant peduncular bundles (p. 517) which probably contain efferent pallial fibres which pass to the motor nuclei of nerves V, VII and XII. These two bundles, which may be termed the mesial and lateral pcdunculo-tegmental bundles ("medial accessory fillet" and "lateral peduncular fillet") detach them- selves from the pes higher up, and below this level come to lie in the vicinity of the medial lemniscus. Dorsal to the pes and constituting the remainder of the l^asis pedunculi is a mass of gray matter which, on account of the pigmentation of its cells, is known as the substantia nigra. The substantia nigra receives collaterals from the ad- joining pes fibres. These arc probably fibres from the motor cortex but according to some may be from the corpus striatum or corpus suhthalamicum. The axoncs of the cells of the substantia nigra enter the tegmentum. Their des- tination is unknown. The superior cerebellar peduncle has completed its decussation below this level and its fibers are seen surrounding or within the nucleus ruber which is one of their terminal nuclei. Other fibres of the superior cerebellar peduncle terminate in the thalamus and some are stated to terminate in the nucleus of nerve III. 522 THE ORGANS Internal arcuate fibres from the gray matter of the superior colliculus pass through the reticular formation, and form an oblique decussation. This decussation is the dorsal, or fountain-like decussation of Meynert. The fibres originate from cells in the superior colliculus (tectum opticum), and after decussation form the descending colliculo-hulbar and spinal tract (tecto-spinal or predorsal tract) (see also below). The Anterior Corpus Quadrigeminum or Superior CoUicixlus. — In this four principal layers may be distinguished besides the usual covering of neuroglia cells and fibres: (i) An outer white layer, stratum zonale. This consists of fine nerve fibres coming from the superior brachium, possibly fibres from the optic tract and cerebral cortex. Among them are small nerve cells, mostly horizontal and with tangential or centrally directed axones. (2) A gray layer, the stratum cinereum. This consists of radially arranged nerve cells with their larger den- drites proceeding outward, and their axones inward. The largest cells lie deep- est. In this layer the optic fibers principally terminate. (3) The stratum opticum consists principally of optic fibres which send their terminals mostly into the preceding layer, but also into the deeper layers. It also contains cells whose axones pass into the next layer. (4) Deep gray-white layer, or stratum lemnisci, because it is stated to contain fibres from the medial lemniscus which terminate in the superior colliculus (denied by some). This layer contains large and medium stellate cells whose axones, together with axones from cells in the more superficial layers, either pass across to the opposite colliculus or sweep ventrally around the central gray, decussate in the raphe and proceed caudally as the colliculo-bulbar and spinal tract. The above relations have been principally ascertained by the Golgi method. The superior colliculus also possibly receives fibres from the lateral lemniscus and spino-thalamic tract . It also receives fibres from the occipital and temporal cortex cerebri (pallio- collicular fibres). Belonging to the midbrain is the posterior commissure (not in the section) the fibres of which cross in the roof just anterior to the superior colliculus. Its fibres originate from coUicular cells (in turn receiving optic fibres), decussate and terminate in the nucleus of the medial longitudinal fasciculus and other nuclei in the reticular formation. The colliculus thus consists essentially of (o) afferent fibres from the retina (optic tract), the pallium and possibly other parts of the nervous system, and (&) efferent neurones to other parts of the brain and cord brought into various relations with each other in the colliculus either directly or by (c) the association cells of the colliculus, the axones of which do not leave the latter. Forebrain or Prosencephalon Inteebrain (diencephalon or thalamencephalon) In the interbrain or diencephalon, three parts may be distin- guished; the thalamus, epithalamus, and hypothalamus. The epithala- mus consists principally of the pineal body, the habenulae, and striae thalami. The hypothalamus consists mainly of the structures in THE NERVOUS SYSTEM 523 the ventral expansion of the interbrain, such as the corpora mamil- laria, tuber cinereum, infundibulum and posterior lobe of the hy- pophysis. The epithalamus and hypothalamus are principally con- nected with olfactory paths (see p. 530 and Fig. 351). Certain extensions forward of the tegmentum are also termed subthalamic (e.g., corpus subthalamicum or corpus Luysii). The thalamus comprises the great bulk of the interbrain. It con- sists of a number of nuclei forming links in afTerent and efferent pallial paths and of other nuclei connected with the corpora striata. There is much difference of opinion both as to the number of the nuclei and their connections. According to some authorities the thalamus may be regarded as divided into internal and external segments (usually separated by the lamina medullaris medialis). The in- ternal segment consists of an anterior nucleus, median nucleus, the "median center" or nucleus of Luys, and a nucleus arcuatus. The external segment consists of a dorso-lateral, an external ventro- lateral, an internal ventro-lateral, and a ventral nucleus. To the external segment should be added the pulvinar and lateral and medial geniculate bodies (metathalamus). The various nuclei of this external segment receive the fibres of the afferent pallial paths and complete the paths by sending fibres to the cortex palhi. These paths are (i) the medial lemniscus, spino-thalamic, and secondary trigeminal tracts (general sensory from body and face) to the ventro- lateral nuclei and thence to the cortex of the central region of the paUium; (2) the lateral fillet or brachium of inferior colliculus (hearing) to the medial geniculate body, and thence to the temporal region of the pallium; (3) the optic tract to the lateral geniculate body and thence to the occipital pallial cortex; (4) part of the superior cere- bellar peduncle (also said to be distributed to nuclei of inner segment). The visceral (including gustatory) and vestibular paths to the pallium are not definitely known. (See also p. 475.) As the olfactory nerve belongs to the endbrain, its path to the pallium does not traverse the thalamus. Besides giving rise to the above thalamo-cortical fibres, the external thalamic segment in all probability receives many de- scending fibres from the cortex i)allii. The various fibres connecting thalamus and cortex constitute the thalamic radiations. In general the anterifjr parts of the cortex are connected with the anterior part of the external thalamic segment, the middle with the middle, and the posterior with the posterior. It is also probable that the thalamo- cortical fibres from the various lateral nudei arc arranged dorso- 524 THE ORGANS ventrally, so that the fibres from the ventral parts pass to the ventral part of the central region of the pallium, dorsal to dorsal, etc. (E. Sachs.) The nuclei of the internal segment of the thalamus do not appear, according to some recent researches, to have direct connections with the cortex pallii. The anterior nucleus receives the bundle of Vicq d' Azyr (mamillo-thalamic tract) and probably sends fibres to the nucleus caudatus (see p. 533). It thus belongs to the olfactory apparatus. The median nucleus is also probably connected with the nucleus caudatus. The median center of Luys and the nucleus arcuatus send fibres to adjoining thalamic nuclei, especially the lateral nuclei, and appear to be thus association nuclei. Other authorities affirm that ascending tracts are received by some of these internal nuclei and that they have direct cortical connections. Descending tracts coming directly from the thalamus have not been definitely demonstrated. PRACTICAL STUDY 1 1 . Transverse Section through the Jvinction of Midbrain and Thalamus. (Figs. 324 and 348.) The most conspicuous change from the last section is the appearance, or increase, of the geniculate bodies and pulvinar, and the thalamic radiations. Efferent Peripheral Neurones. — The nuclei and root fibres of the III nerve are stUl present. Afferent Roots, their Terminal Nuclei, Secondary Tracts, and Tertiary Neurones. — The fibres of the optic tract (optic "nerve") are seen entering the ventral surface of one of their terminal nuclei, the lateral or external geniculate body. Other optic fibres (not entirely traceable) enter the pulvinar thalami and the superior coUiculus {Strd) (see also preceding section). On the dorso-lateral surface of the lateral geniculate body, a bundle of fibres accumulates which represents the beginning of the geniculo-calcarine tract to the occipital cortex, thereby completing the visual path. Internal to the lateral geniculate body is the medial (internal) geniculate body which now contains the terminals of the brachium of the inferior coUiculus. Fibres from its cells gather on its lateral surface. These represent the beginning of the geniculo-temporal tract to the temporal cortex, thereby completing the auditory path. Internal and ventral to the medial geniculate body are the medial lemniscus (bulbo-thalamic) and spino-thalamic tracts about to terminate in the ventro- lateral thalamic nuclei whose axones complete the general sensory path by passing to the central cortex. The central tegmental tract can hardly be distinguished. ' Intersegmental Neurones. — The nucleus ruber is still large, but the remainder THE NERVOUS SYSTEM 525 V be— -— "T C> c •- tn f 2;i5^^'3:^ E ? 3 3 5 D O S: . -^ >- O;^ ft; c aTi " 5 Ji «'S "1 •si o ^^ g ^ ^ - '-S o. ^ ^ c g i'o 3',_0 U~H '2 3 V) "G ^^ i'n -t; TJ '. '~-itC~rtrtr~"^'^ -Oi ■^ « s ^ "^.^ "S r: > i?) !tq -^j' pc !_. k< " ^* ,• ^ i3 q C3pj3-!2--^tf,3 e J ^ S S--y.H S-S ° 3 2:3 a.0 S 526 THE ORGANS of the reticular formation has nearly disappeared. The medial longitudinal fasciculus is much diminished as its ascending fibres terminate in the nucleus of nerve III and many of its descending fibres originate from cells below this level. Efferent Suprasegmental Neurones. — The pes pedunculi occupies the same position, and dorsal to it is the diminished substantia nigra. Along the ventro- mesial border of the pes a bundle of fibres can sometimes be distinguished (Fig. 348, Lmp), which at lower levels comes to lie mesial to the medial lemniscus (aberrant peduncular fibres, comp. p. 521). Descending pallial fibres (not distinguishable) also probably form part of the thalamic radiations (PP- 523, 53°)- Fibres of the superior cerebellar peduncle may be seen within and around the nucleus ruber. Some of these terminate in the latter, some pass further forward to end in the thalamus (compare pp. 507, 521). The Superior Colliculus is somewhat diminished. The posterior commissure passes across in the roof dorsal to the central gray (see preceding section) . The Thalamus (can hardly be included under the preceding structures). — The corpora geniculata have been mentioned. The puhinar thalami is a large gray mass dorsal to the medial geniculate body. Fibres passing laterally from it contribute to the retrolenticular portion of the internal capsule {Cirl) . These fibres are a part of the thalamic radiations. The nucleus caudatus (a portion of the corpus striatum of the endbrain) is present. 12. Section through the Interbrain at the Level of the Optic Chiasma. (Figs. 324 and 349.) Efferent Peripheral Neurones. — None present. Afferent Roots, their Terminal Nuclei, Secondary Tracts, and Tertiary Neurones. — Fibres of the optic nerve are seen decussating and forming the optic chiasma. The further continuation of the optic fibres to their termination is called the optic tract. Both nerve and tract constitute the secondary optic tract, the optic chiasma being analogous to the decussation of the medial lemniscus and of the lateral lemniscus (trapezius). (For further description of optic paths see Fig- 350.) The medial lemniscus, spino-thalamic, and secondary trigeminal tracts are Fig. 349. — Section through the Interbrain at the Level of the Optic Chiasma. (The chorioid plexus of the third ventricle has been removed.) Weigert preparation. (Mar- burg) Ce, capsula externa; Cex, capsula extrema; Chll, chiasma nervorum opticorum (or optic chiasma); Ci, capsula interna; CI, claustrum; Cml, ganglion laterale corp. mammillaris; Cmm, ganglion mediale corp. mammilL; Coa, commissura anterior; Cospm, commissura supramammillaris; Cstk, corpus subthalamicum; e, nucleus externus gangl. med. corpor. mammillaris; Fmp, fasciculus mammillaris princeps; Fo, fornix; Fp, fibr^e perforantes (pedunculi) ; /r/!/, fasciculus retroflexus (Meynert); Fsp, fasciculus subthal- amico-peduncularis; Fu, fasciculus uncinatus; Ghb, ganglion habenulae; glp, globus palli- dus; H, area tegmenti Forel; HI, pars dorsalis arese tegmenti; HII, pars ventralis areae tegmenti; I, insula Reillii; i, nucleus internus gangl. medial, corp. mammillaris; Lml, lamina meduUaris lateralis; Narc, nucleus arcuatus thalami; Nc, nucleus caudatus; NL, nucleus Luysii (nucleus centralis, or median centre, thalami;) Nl, nucleus lateralis thalami; Nlv, nucleus lateralis ventralis thalami; Ntg, nucleus ruber tegmenti; Fp, pes pedunculi; Pm, putamen; SnS, substantia nigra; St, stria cornea; Strz, stratum zonale thalami; Til, tractus opticus; Tbc, tuber cinereum;. Tt, taenia thalami; VIII, ven- triculus tertius; Zi, zona incerta. THE NERVOUS SYSTEM 527 Ife 528 THE ORGANS Cotter • --.fase. lonq.hoif'. , ,..'.". Nu.SlN." I >^-''^~~-=--<;^ Tk tecto-iuliaris et sbilJU.liS' Fig. 3SO. THE NERVOUS SYSTEM 529 EXPLANATION OF FIG. 350. Fig. 350. — Diagram of the Optic (II) Nerve and some of its Principal Connections. A, Level of nerves II and III; B, level of nerve IV; C, level of nerves VI and VII; D, spinal cord. Neurone groups are represented by one or several individual neurones. The rods and cones (receptors) and the bipolar cells ( = Neurone No. i) of the retina are not indicated. Neurone No. 2. — 2a, Axones of ganglion cells in temporal part of retina pass to pulvi- nar of thalamus of same side; 2 b, axones of ganglion cells in temporal retina pass to superior coUiculus of same side; 2 c, axones of ganglion cells in temporal retina pass to external geniculate body of same side; 2 e, axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to external geniculate body of opposite side; 2 /, axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to superior coUiculus of opposite side; 2 g, axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to pulvinar of thalamus of opposite side. Macular fibres are partly crossed and partly uncrossed. Neurotic No. 3. — ^a, Axones of cells in pulvinar to cortex of occipital lobe of cerebrum (this connection is disputed) ; 3 b, axones of cells in external geniculate body to cortex of occipital lobe of cerebrum; 3 a and 36 constitute the primary optic radiation; 3 c, s d and 3 e, axones of cells in deep (fourth) layer of superior coUiculus (see p. 522) decussate ventral to medial longitudinal fasciculus (dorsal tegmental decussation or decussa- tion of Meynert) and form the tractus coUiculo-bulbaris et spinalis or predorsal bundle. (Tr. tectobulb. el spin.) to bulb (medulla) and anterior column of cord, inner- vating by coUaterals and terminals, directly or indirectly, chiefly the nuclei of III, IV, VI, and VII cranial nerves and motor nuclei of spinal nerves. 3 / and 3 g (possibly another neurone intercalated between these and optic terminals), axones of cells in inter- stitial nucleus of Cajal (Nu. fasc. long, post.) form part of medial longitudinal fasciculus and descend on same side to anterior column of cord next to anterior median fissure, innervating nuclei of III, IV, and VI cranial nerves and motor nuclei of spinal nerves. Neurone No. 4. — Axones of cells in above-mentioned motor nuclei. Axones from cells in median nucleus of nerve III (Nu. med. N. Ill) and possibly in Edinger-Westphal nucleus, probably innervate the intrinsic muscle of eyeball (ciliary and pupillary reflex path). PaUio-tectal fibres, by means of which the coUicular reflex centre is brought under the control of the cerebral cortex, are not indicated. It is evident from the diagram that the cerebral pathway of the optic nerve is via the external geniculate body (and pulvinar of thalamus?), and the reflex pathway is via the superior colliculu s. 34 530 THE ORGANS now lost in the ventre- lateral thalamic nuclei, cells of which constitute the tertiary neurones of the various afferent pallial paths (see pp. 523 and 524). Intersegmental Neurones. — The cephalic end of the nucleus ruber is still present on the right {Ntg). On the left are seen some fibres in its place, lateral to which are two transverse bundles enclosing a strip of gray matter. These are known as the area tegmenti or field of Forel and represent a subthalamic forward extension of the tegmentum. The gray or zona incerta may be regarded as representing a continuation of part of the reticular formation. The ventral bundle of fibres {HII) are probably fibres from the nucleus lenticularis {glp and Pu, right) passing through the pes (or rather through the posterior part of its continuation — the internal capsule) as perforating fibres {fp on right) to the corpus subthalamicum {Csth, left) and other subthalamic regions, possibly also to the nucleus ruber. Some of these fibres are collaterals of pes fibres and may consequently come directly from the pallium. The dorsal bundle {HI) probably contains fibres connecting red nucleus and pallium. Other fibres in this region are probably fibres of the superior cerebellar peduncle which have passed by the nucleus ruber to the lateral nucleus and median center of the thalamus (see also p. 523) and possibly also fibres of the secondary trigeminal tract. The nucleus of the medial longitudinal fasciculus falls in the level between this and the preceding section. In general it seems probable that portions of the corpus striatum, the corpus subthalamicum and certain other subthalamic nuclei, the substantia nigra and part of the nucleus ruber represent certain phylogenetically old, rather obscure, efferent forebrain paths. Efferent Suprasegmental Neurones. — The pes pedunculi now lies partly between the thalamus and nucleus lenticularis (see p. 533) constituting the greater part of the internal capsule. The parts of the internal capsule as shown in horizontal sections of the hemispheres are shown in Fig. 352. The most dorsal part is here passing into the corona radiata (p. 533) of the cerebral hemi- spheres (not included in the section). The part present in this level is the most posterior part of the capsule (occipito-temporal pallio-pontile fibres (see p. 533). Dorsal to the mesial part of the pes is the corpus subthalamicum which has replaced the substantia nigra. It receives collaterals from the pes and is said to contribute fibres to the latter. It also appears to be connected by fibres with the nucleus lenticularis (see above). Superior cerebellar peduncle (see Inter- segmental Neurones above). Thalamus. — At this level the ventro-lateral nucleus, the nucleus arcuatus, and the median center of Luys can usually be distinguished. At the outer border of the thalamus, fibres accumulate forming the lateral medullary lamina. These fibres continue outward as thalamic radiations, entering the internal capsule which they may follow a distance, or cross obliquely and enter the corona radiata. Epithalamic and Hypothalamic Structures and their Connections. — The ganglia habenulce are two small masses of gray matter occupying eminences on the mesial walls of the thalamus. A bundle of fibres near each is the stria medul- laris (near the tcenia thalami) consisting of fibres from the olfactory bulb and trigonum and representing afferent olfactory connections (p. 532). The ganglion habenulse contains a mesial small-celled and a lateral large-celled nucleus. Their THE XERVOUS SYSTEM 531 axones form ihe fasciculus retroflexus of Meynert to the interpeduncular ganglion situated more caudally (Fig. 351). There is also a commissura habcnulari's connectmg the two ganglia. The stria terminalis (stria cornea), another olfactory connection, lies in the groove between the ventricular surfaces of nucleus caudatus and thalamus. The tuber cinereum is seen projecting ventrally Dorsal to this are seen the corpora mammillaria containing lateral and mesial nuclei. The mammiUary body receives some fibres of the fornix (from the rhinopalhal cortex, see below) and also fibres from the medial fillet and other Fig. ^35'.— i>iaKram of Olfactory Paths (von Bcchtercvv.j A', Root fibres of vacus- ^a, commissura anlerior; cm, corpus mammillare; cp, fibres fn,m nucleus halenurt^ postenor comm,ssure;/6-, tract from corpus mammillare to (uuiden's nucLs j " Scu- us mammillo-thalamicLis; y/, fasciculus lonKitudinalis medialis;/;-, fornix //f, res of o n.x loHKUs; ;;/; nu.leus habcnuhe; ,/, Kan^lion intcrpcduncularc A'A, Kyr is wri ormis i'J r?'"' 'ned.al.s ,« fibres from (;,Kiden's nucleus lo substai ia rc^ticul SS na nucleus anterior thalam.; nG, (iuddcn's nucleus; ;,/, nucleus icKmenti {v ( ud IcnV ^n:::^:^^::^ "Tk"''-';' ^"''■' "T'--"'- -n>oris mamminaris from lillct; ^ ! corpora quadrij,'emma; r, fibres from nucleus IcKmenti (v. (uhKIcii) lo nuclei of cr' n -d nerves; re, radix lateralis tractus olfaclorii; ./, fibres of\ractus olfiior , i«oi m torius i Un! .h ' '"■' """'f "f.^T"""! olfactorium; //,, ihalanuis; /;-., tniclus olfac- lorius, «, ticnia Ihalami; x, fasciculus retroflexus. 532 THE ORGANS ascending tracts in the reticular formation. It gives rise to the bundle of Vicq d'Azyr (mammillo-thalamic tract) to the dorsal nucleus of the thalamus, and mammillo-tegmental fibres to the nucleus of Gudden (Fig. 351) and red nucleus. The fibres entering the mammillary body from the fillet and other ascending tracts, together with the mammillo-tegmental tract, constitute its peduncle. It is not improbable that the mammillary body is a link in the afferent gustatory path (p. 476, II). (See also Endbrain.) On the right is seen the posterior part of the nucleus lenticularis of the corpus striatum. The Endbrain or Telencephalon. The endbrain consists of pallium (dorsal expanded part), corpus striatum, and rhinencephalon. Two principal parts of the pallium may be distinguished; the olfactory palhum or rhinopallium (archi- pallium), including principally the cornu ammonis and gyrus denta- tus; and the neopallium including the greater part of the cerebral hemispheres. The rhinencephalon^ includes the olfactory nerves and bulb, the trigonum olfactorium, the tuberculum olf actorium or anterior perfor- ated space, and the gyrus hippocampi, in part at least (pyriform lobe). The olfactory nerve is composed of axones of cells in the olfactory mucous membrane which terminate in the olfactory bulb. They there form synapses with the dendrites of the mitral cells, the axones of which constitute the secondary tract, part of which decussates in the pars olfactoria of the anterior cerebral commissure. A secondary tract ("lateral root") proceeds, with tertiary tracts, to the cortex of the gyrus hippocampi and thence to the cornu ammonis. Efferent axones of cornu ammonis cells are collected in the fimbria and descend by the fornix to the mammillary body, the further caudal connec- tions of which have been described (p. 531 j. Fibres of the fimbria also cross, forming the commissure of the fornix (olfactory pallial commissure) . Secondary olfactory tracts also pass to the trigonum, whence tertiary neurones pass as the stria medullaris to the ganglion habenulae (see p. 530 and Fig. 351). The principal commissure of the rhinencephalon is the anterior cerebral commissure. The corpus striatum consists of the nucleus caudatus and nucleus lenticularis, the connections and significance of which are obscure. They receive collaterals from the descending pallial fibres which pass by them and also apparently send out fibres to join the latter. They also have connections with rhinencephalon and thalamus. 1 The term rhinencephalon is often used to include also the olfactory pallium. THE NERVOUS SYSTEM 533 The pallium consists of an extensive external convoluted sheet of gray matter {cortex pallii or cortex cerebri) and of white matter underlying the gray. In the white matter may be distinguished the corona radiata composed of the afferent and efferent pallial fibres connecting the pallium mth other parts of the brain {projec- tion fibres). The remaining fibres of the white matter are association fibres of the pallium and are either crossed or com- missural, connecting the two hemispheres {corpus callosum and fornix commissure), or are uncrossed. The term association fibres is often restricted to the uncrossed fibres. The aft'erent connections of the neopal- lium (p. 523) and rhinopallium (p. 532) have been summarized and also the efferent connections of the rhinopallium (p. 532). The following are the principal descend- ing or efferent connections of the neopallium: (i) The pyramidal or pallio-spinal tract. This is composed of the axones of the giant cells (of Betz) of the arm, body, and legpre- central motor areas. They descend in the corona radiata, the posterior limb of the internal capsule, middle part of the pes, and thence through pons and medulla to the cord. Their decussation and further course has been described. (2) The de- scending tracts to the motor nuclei of the cranial nerves originate from precentral cells of the various areas controlling the muscles in question and pass down in the vicinity of the genu of the internal capsule. Their path is not so well known but they appar- ently do not pass down in the pes through- out their course (pp. 524, 521, etc.). (3) The pallio-pontile system to the j)ons (continuation to opposite cerebellar hemisi)here). This originates in various parts of the cortex. The fibres from the occipital (?) and temporal regions pass down in the extreme posterior part of the internal capsule and lateral Fig. 352. — Scheme of General Arrangement of fibres in Internal Capsule, (von Bechterew.) /, //, ///, The three parts of the lenticular nucleus; nc, nu- cleus caudatus;^/;, thalamus; gp, globus pallidus; pt, put- amen; i, fibres of anterior thalamic ])cduncle; 2, fibres of medial (frontal) pons system (frontal paUio-pon- tile fibres); 3, efferent pal- lial fibres to motor nuclei of cranial nerves; 4, pyramidal fibres (efferent pallial fibres to motor nuclei of spinal nerves); 5, pyramidal fibres mingled with those of the afferent (sensory) path; 6, fibres of the lateral jions system (occipilio-temporal palUo-pontilc fibres). The various systems arc not sharply marked off as indi- cated, but are more or less intermingled. 534 THE ORGANS PiTh Coa X Vli ti Rop Fli Fig. 353. — Transverse Section through the Cerebral Hemispheres, Corpora Striata and Thalamus. Weigert preparation. (Dejerine.) II, Tractus opticus; Alent, ansa lenticularis; c, sulcus centralis (Rolandicus) ; Ca, gyrus centralis anterior; Cell, corpus caUosum; Ce, capsula externa; CFo, columna fornicis; Cia, crus anterior capsul. int.; Cig, genu capsul. int.; CI, claustrum; dim, sulcus callosomarginalis; Cng, cingulum; Coa, commissura anterior; Cp, gyrus centralis posterior (ascending parietal convolution) ; CR, corona radiata; Far, fasciculus arcuatus; Fli, fasciculus longitudinalis inferior; Frn, gyrus fornicatus; fs, sulcus frontalis superior; Fs, gyrus frontalis superior; Fu, fasciculus uncinatus; Fus, gyrus fusiformis; glp, globus pallidus (inner segment); glp' , globus pallidus (outer segment); I, insula; Ime, lamina m.edullaris externa nuclei lentic- ularis; Ime' , supplementary lamina of the outer segment of the globus paUidus; Imi, THE NERVOUS SYSTEM 535 part of the pes, those from the frontal region pass down in the anterior Hmb of the internal capsule and mesial part of the pes. (4) Pallio- tectal fibres to the midbrain roof. (5) Fibres to the substantia nigra and corpus subthalamicum. (6) Fibres to the red nucleus. (7) Pallio-thalamic fibres (see p. 523). (8) Fibres, or collaterals, to the corpora striata. (Fig. 352.) The crossed association fibres of the neopallium (corpus callosum) connect principally corresponding parts of the hemispheres. The long uncrossed association fibres (furthest from the gray matter) form certain more or less well-defined bundles among which are the following: (i) The cingulum, a longitudinal bundle near the corpus callosum; also contains projection fibres and belongs to the olfactory part of the brain as well as to the neopallium. (2) The superior longitudinal fasciculus or fasciculus arcuatus; connects frontal with occipital and part of temporal lobes. (3) The inferior longitudinal bundle connecting temporal and occipital lobes. It may, however, be a projection bundle. (4) The uncinate fasciculus connecting frontal and temporal lobes. (5) The perpendicular fasciculus of Wernicke connecting inferior parietal and fusiform lobules. Pro- jection fibres may form portions of these bundles. Besides the above long association fibres there are shorter association fibres nearer the gray matter which connect adjoining convolutions (fibrae propriee of Meynert). PRACTICAL STUDY 1.3. Transverse Section through the Cerebral Hemispheres, Corpora Striata and Thalamus (Fig. 353) First distinguish in general (i) the pallium, its cortex and white matter, (2) the corpus slrialum and its divisions, i.e., the caudate nucleus and the lenticular nucleus, the latter being subdivided into the pulamen and f!,Iol)us pallidus and (3) the thalamus and other structures of the interbrain. lamina mcdullaris interna nuclei lenticularis; mp and nis, sulcus tircularis (Reili); NA, nucleus amy^flaliformis; A'c, nucleus caudalus; OjiR, ()[)crculum; oti, sulcus occijjitotcm- poralis inferior; pCK, pes coronx' rarliata;; I'iTh, jjcdunculus inferior thalami; prs, sulcus prajccntralis; ]'u, putamen; rcc, stratum reticulum coronie radiatac;7?(>/', radiatio optica; .V, fissura Sylvii (jio.-.tcrior hranchj; Sgc, substantia ^risca centralis; sM, sulcus Monro!; Sge, substantia grisea subei)en(lymalis; ssc, stratum subcallosum: SLrz, stratum zonalc thalami; Tbc, tuber cinercum; Th, thalamus o[)ticus; ti, sulcus temporalis in- ferior; Ti, gyrus temporalis inferior; Im, sulcus temporalis mcdius; Tm, gyrus temi)oralis mcdius; Is, sulcus tcm|)oralis superior; Ts, gyrus temporalis superior; Tic, taenia tccta; U , uncus; VI, ventriculus lateralis; Vti, ventriculus lateralis (cornu inferius); vsl, }>edunculus anterior thalami; -Y, |)edunculus putaminis; CM, Commissure of Meynert. 536 THE ORGANS Afferent Roots, their Terminal Nuclei, Secondary Tracts, and Tertiary Neurones. — The optic tract here forms a part of the ventral surface of the brain. The geniculo-cortical portion of the optic path forming a part of the optic radiation may be seen. Other afferent pallial connections are hardly distin- guishable among the fibres connecting thalamus and pallium. Efferent Suprasegmental Neurones, — A great part of the pes has now entered the corona radiata. The part now about to enter the corona is the anterior limb of the internal capsule (Fig. 352). The ansa lenticularis is, according to some authorities, composed of fibres or collaterals from the pes to the lenticular nucleus. The fibres of the anterior peduncle of the thalamus are evident. They are con- sidered by some as composed of cortico-thalamic fibres. The anterior pillars of the fornix (efferent rhinopallial) are shown cut through twice, the upper section shows its earlier course emerging from the fimbria, the lower section is near its termination in the mammillary body. For other structures of thalamus, epithalamus, and hypothalamus see Fig. 353- The Pallium. — In the white matter distinguish as far as possible the corona radiata, the corpus callosum, and the long association bundles {inferior longitudinal fasciculus, fasciculus uncinatus, a.nd fasciculus arctiattis) (seep. 535). Note the nucleus amy gdaliformis , the anterior perforated space and the anterior commissure, belonging to the rhinencephalon. Other details shown in Fig. 353 should be studied. The Cerebral Cortex. — The following types of cells are found in the cerebral cortex: (i) Pyramidal cells. This is the prevaihng type and is characterized by a long apical dendrite usually directed toward the surface of the brain. This dendrite gives off branches, and usually reaches the outer cortical layer, there to break up into a number of branches. From the cell body are also given off a number of basal dendrites. By the Golgi and EhrHch methods, gemmules can be demonstrated on the dendrites. The axone proceeds from the base of the cell (opposite to the apical dendrite) and usually passes into the white matter. It gives off several collaterals on its way to the white matter. (2) Stellate cells. These have dendrites passing in various directions. Many, especially the smaller (granules), may have short axones (Golgi's second type). (3) Polymorphous cells of a triangular or spindle shape are usually found in the deepest layers of the cortex and send their axones into the white matter. (4) Hori- zontal cells (of Cajal), found in the outer layer, with long horizontal dendrites and axones confined to the outer layer. (5) Inverted pyramidal cells (of Martinotti) with axones directed toward the surface. (Fig. 355.) The largest cells of the cortex (giant cells of Betz) are very rich in chromophilic substance arranged similarly to that in the efferent THE NERVOUS SYSTEM 537 root cells of cord and brain. The medium and small cells have fewer chromophilic bodies which are often in the shape of irregular masses, either near the periphery of the cell or around the nucleus. The neurofibrils vary in their arrangement according to the shape of the cell. .'.■V -V^;,!- ;i.N-'«-ll lit '7/ a.' --1 .';* '■..'■'■A' .\' r-v -f i. I • Fig. 354. — Vertical Sections of Calcarine Area of Adult Human Cortex. Left, Weigcrt preparation sfiowing fibre arrangement. Right, Arrangement of Cells. (Cami>bell.) 6", Line ol Gennari; K, radiary layer; S, supraradiary layer; Z, Layer of superficial tangential filjres in molecular layer; i, molecular laj^er; 2, external granular (small |>yramidj layer; 3, j)yramid layer; 4, (large granules) and 5 (small granules), internal granular layer; C, ganglionic layer (containing solitary cells of Meynert); 7, multiform layer. 'I'ho neuroglia cells and fibres are in general similar to those in other parts of the nervous system. The cells of the cortex are arranged in layers which have the same general character throughout, but in various regions exhibit variations 538 THE ORGANS II such as suppression, diminu- tion, enlargement, or subdi- vision of certain layers. The cell layers of the cortex are: (i) Molecular layer (zonal layer, plexiform layer of Cajal). This contains the horizontal cells, other cells with short axones, and also receives the axones of the Martinotti cells. Besides this, it contains the terminal branches of the apical den- Fig- 355- Fig. 355. — Vertical section of Calcarine Area of Cortex of an In- fant 15-20 days old. (Cajal, com- bined from three, /, II and ///, somewhat overlapping figures. The multiform layer is not included). Golgi's method. /. — A, Molecular layer; B, ex- ternal granular layer (of small pyra- mids); C, pyramidal layer (of me- dium pyramids); a, descending ax- ones; h, ascending collaterals; c, apical dendrites of large pyramidal cells in ganglionic layer. //.• — A , Sublayer of large stellate cells; B, sublayer of small stellate cells; C, outer part of ganglionic layer; a, crescentic stellate cells; b, and /, horizontal, spindle-shaped stellate cells; c, medium-sized pyram- idal cells; e, stellate cells with arched axones; g, triangular stellate cells with stout, arched collaterals; //, pyramidal cells with arched axones. ///. — A, Part of internal granu- lar layer; B, sublayer of small pyram- idal cells with arched ascending axones; C, sublayer of large pyra- mids; a, large pyramidal cell; b, g medium-sized pyramidal cell with 2 long descending axone; c, small pyr- gi amidal cell with arched ascending ^ axone; d, pyramidal cell with axone split into two arched ascending branches; e, pyramidal cells whose axone sends out various ascending branches; f,g,h, stellate cells with as- cending axones which branch in B and in the sublayer of small stel- late cells; i,j,k, pyramidal cells with arched ascending axones which send branches into the ganglionic layer. THE NERVOUS SYSTEM 539 drites of the pyramidal cells. (2) External granular layer, very often termed the layer of small pyramids. The dendrites of the cells of this layer mostly enter the first layer, their axones pass downward into the white matter. (3) Pyramidal layer, often called the layer of superficial medium and large pyramids. This is com- posed principally of typical pyramids sending dendritic branches into the first layer and axones into the white matter. The larger cells are in the deeper part (sublayer of large pyramids). This layer also contains many granule cells with short axones and cells of Marti- notti, (4) Internal granular layer. Here the predominating elements are stellate cells, the larger usually sending their axones into the white matter. Among these are many short axone granules, the axones of which end in the same layer or in more superficial layers. (5) Ganglionic layer or deep layer of large and medium sized pyramids. These send their axones into the white matter. Mingled with them are short axone and Martinotti cells. (6) Multiform layer or layer of polymorphous cells. These usually send their axones into the white matter. Mingled with them are short axone and Martinotti cells. (Figs. 354, 355 and 356.) The cells of the cortex obviously fall into two classes: efferent pro- jection cells and association cells. Which cells are projection cells is not definitely known, except in the case of the precentral motor cor- tex where it has been established that these cells are the cells of Betz, the axones of which form the pyramidal tract and the fibres to motor cranial nerve nuclei. An examination of this area shows that the association cells must enormously outnumber the efferent pro- jection cells in the cortex. The association cells comprise the short axone cells and cells the fibres of which enter the white matter, but terminate in some other part of the cortex, forming the association fibres of the white matter. (Compare p. 533.) It is thus evident that every part of the cortex contains termina- tions of association fibres. The areas containing the terminations of afferent projection fibres are those which receive the thalamocorti- cal continuations of the afferent pallial paths and the continuations of the olfactory paths. From observations made with the Golgi method it seems probable that the afferent projection fibres arc coarse fibres which may ramify throughout the greater part of the thickness of the cortex, but are confined mainly to the third and fourth layers. The principal areas of the cortex receiving the af- ferent projection fibres are the hippocampal area (olfactory), the cal- THE ORGANS .1. i ^r^l-:-, i^ h ': ■ ,^i-:. i i >i ; • - ■ ]5 v\: V v.- /. ■r ii i Itt! f iJ^l^i r\ i^-'^'^ ''■'A » i>i/ i^- > ^ i' .•>:v « :;-^ Fig. 3 s6.— Vertical Sections of Precentral or Motor Area of Adult Human Cortex. Left Weigert preparation showing fibre arrangement. Right, Arrangement of cells. (Campbell.) B, Position of line of Baillarger, its position obscured by surrounding wealth of fibres; R, radiary layer; S, supraradiary layer; Z, layer of superficial tangential fibres in molecular layer, dense and weU defined; i, molecular layer; 2, external granular layer (small pyramids), 3 (medium-sized) and 4 (large), pyramid layer; s, mternal granular layer, indistinct and with scattered granule or stellate cells; 6, ganglionic layer (large deep pyramids); 7, multiform layer. THE NERVOUS SY§TE:M 541 carine area (visual, fibres from lateral geniculate body), the trans- verse temporal gyri of Heschl (auditory, fibres from medial geniculate body), and the pre- and postcentral areas (postcentral only, according to some authorities, area of general sensation from body and head, fibres from ventrolateral thalamic nuclei). (Fig. 357.) The medullated fibres of the cortex consist of radially, obliquely, and tangentially running fibres. The radial fibres enter the cortex from the white matter in bundles known as the radiations oj Meynert which extend a variable distance toward the periphery, diminishing until they end usually in the third layer. They consist mainly of the axones of the adjoining cells passing to the white matter. Their fibres are of varying calibre, the coarsest originating from the largest cells. The oblique fibres form a dense plexus of coarse and fine fibres between the radial fibres, the interradiary plexus. Toward the surface fin the second and third cell layers) they form a delicate plexus of fine fibres. This latter plexus lies principally superficial to the radiations of Meynert and is the supraradiary plexus. A denser aggregation of irregular fibres constitute the line or stria of Baillarger located in the layer of superficial large pyramids. It is probable that this represents a layer especially rich in terminals of fibres from the white matter. Other stria? are also described. Besides represent- ing the terminals of fibres from the white matter the oblique fibres in general are also composed of medullated collaterals of axones of pyramids and possibly arborizations of short axone cells. A few coarser fibres ascending to the molecular layer are ascending fibres from Martinotti cells. The deep tangential fibres, most marked on the sides of the convolutions and in the sulci, are considered short association fibres belonging to the fibrae propria) of Meynert. In the molecular layer are the superficial tangential fibres consisting of the axones of the horizontal cells and the terminals of the axones of Martinotti cells. (Figs. 354 and 356.) The cortex is divided into various areas by various investigators, the areas being distinguished (a) by the time of medullation (myelo- genctic method of Flcchsig), (b) by the number and arrangement of the medullated fibres (myeloarchitccture), especially the number, thickness, and distinctness of the stria), such as that of Baillarger, formed by them, and (c) by the number and arrangement of the cells (cytoarchitecture). Many such areas have been thus distinguished by different investigators with results agreeing in many respects but differing in others. The areas rrk)st clearly defined and concerning 542 THE ORGANS ^irO> Precenhal fos^'* yisM-psj/chiB ^udito-sensoru XertiT" eensory II Fig. 357. — Diagram (orthogonal) showing Cortical Areas as determined by the Arrangement and Distribution of Fibres and Cells (A. W. Campbell). Large portions of important areas are concealed within fissures, e.g., the calcarine or visual {visuo-sen- sory, within the calcarine fissure) precentral (motor) and postcentral (within the fissure of Rolando) and especially the acoustic (audito-sensory) which is almost completely hidden within the Sylvian fissure. A, B and C, parts of the limbic lobe. THE NERVOUS SYSTEM 543 which there is perhaps the most general agreement are the vari- ous sensory (afferent projection) areas already enumerated (p. 539) and the motor (efferent projection) precentral area (Fig. 357). These areas myelinate first (at or soon after birth), next areas adjacent to them, and last areas occupying a considerable portion of the human pallium but much less extensive in other mammals. There is much diff'erence of opinion as to the extent to which these last myelinating areas are supplied mth projection fibres. According to some authorities the areas myelinating last have no projection fibres and are consequently entirely composed of association cells and fibres. Perhaps the two best-marked of the various areas are the motor and visual areas the structure of which is shown in figures 354.' 355 s-^d 356. The motor precentral cortex is characterized by the presence of the giant cells of Betz, by an almost complete absence of an internal granular layer and by a great wealth of fibres. The calcarine or visual area is characterized by a strongly marked line of Gennari ( = Baillarger) and by a double or triple internal granular layer containing large granules in place, apparently, of the superficial large pyramids. The line of Gennari is probably partly composed of the terminals of the fibres of the optic path (geniculo-calcarine fibres). TECHNIC (i) The general structure of the cerebellum is well brought out by staining sections of formalin-Miiller's fluid-fixed material with hgematoxylin-picro-acid- fuchsin (technic 3, p. 21), and mounting in balsam. (2) The arrangement of the cell layers of both cerebellum and cerebrum as well as certain details of internal structure of the cells, can be studied in sections of alcohol- or formalin-fixed material stained by the method of Nissl (technic, P- 38). (^.3) 'J'he distribution of the meduUated nerve fibres of either the cerebellar or cerebral cortex is best demonstrated by fixing material in jVIiiller's fluid (technic 4, p. 6) or in formalin-Miiller's fluid, and staining rather thick sections by the Weigert or Weigert-Pal method (technic, p. :i^). (4) The external morphology of the cerebellar and cerebral neurones and the relations of cell and fibre can be thoroughly understood only by means of sec- tions stained by one of the (iolgi methods (technic, pp. 35 and 36). Ksi)ecially in the case of the cerebellum, sections should be made both at right angles, and longitudinal to the long axis of the convolution. Golgi preparations from embryonic material and fnjrn the brains of lower animals furnish instructive pictures. C5) 'I'he silver method of ("ajal shouUl be used esj)ecially with alcohol fixa- 544 THE ORGANS tion (technic p. 38, No. 2), both for the neurofibrils and for the external mor- phology of the neurones. It is especially successful with the cerebellum. (6) Neuroglia stains should also be used. The Pituitary Body {See page 407.) The Pineal Body The pineal body originates as a fold of the wall of the primary brain vesicle. It lies upon the dorsal surface of the inter- and mid- brain, being connected with the former. The pineal body is appar- ently of the nature of a rudimentary sense organ, being sometimes referred to as the median or pineal eye. In man it is surrounded by a firm connective-tissue capsule, which is a continuation of the pia mater. This sends trabeculse into the organ, which anastomose and divide it into many small chambers. The latter contain tubules or alveoli lined with cuboidal epithelium. This may be simple or strati- fied, and frequently almost completely fills the tubules. 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-Miiller's fluid (technic 5, p. 7) and staining sections with haematoxylin-eosin (technic i, p. 20). General References for Further Study Bailey and Miller: A Text-book of Embryology, New York, 1909. Chaps. XVII and XVIII. Barker: The Nervous System and its Constituent Neurones, New York, 1899. Dejerine: Anatomic des centres nerveux, Paris, 1895. Edinger L. : Vorlesungen iiber den Bau der nervosen Zentralorgane des Menschen und der Tiere, Leipsig, 1908 and 191 1. Van Gehuchten: Anatomic du systeme nerveux de I'homme, Louvaine, 1906. Golgi: Untersuchungen iiber den feineren Bau des centralen und peripher- ischen Nervensystems, Jena, 1894. Johnston, J. B.: The Nervous System of Veterbrates, 1906. KoUiker: Handbuch der Gewebelehre des Menschen, Leipsic, 1896. Von Lenhossek: Der feinere Bau des Nervensystems im Lichte neuester Forschungen, Berlin, 1895. v£-B ■" • '-' U a c c ■^ "" u >- ■ >— .v o «- 3-5° ° g O S « M U o ^ ^ O u o t, C ft ft f- H O O tc n 1, y: CO O 3 U eu :S -"^ o 2o •2 -d -o c >• i^ w g M _ 4) ft o ? o q 1-. 2 ^ • ■" 3 c o > •:;! o 1= S 2 1-. n! o M nl M ca S.S 2 - o'p, o> • C w Ri-i .S -M ■52 ■« .2 a -^ .Sj- . „• « Gj 0) 3 cs d o n O o 3-ri >?«! 5> is- p-i* o fe;s- (UJ3 C rt'^ S o C ^, 5-^J5 g - o - o c_ _ • u "■"IS" c"o'S P o S ^ ^-^ 5 33-3 g) tn T3 3 XI 0 ft E 0 e o c ■5_C " M ° nJ S <; °:s a^ c— c O 0! — c c o a u O £ E ii " 2 <« S . c ^J3 3 3 M 0 2° iis (U-O bo M OT3 u a a a 0^ a cd ^ 2;B 2 0 0 .2 a • . ^s |i^ " * G " 35 S to "^ cS fi g ■a H 4) . . apezoi lemnis ticse emnisc .y kno pezo: oli ssing eralis i-.'B iH gs *-§o^ •i oi C g 3^3 ft- 01 M rpus ppo iae arm t pr 5-t^« n o o^^ o O w 2 O '^ (D'^rii S H nf O O "(i) fi 4^ 4J crt of (UJ3 t3 n o p. rt C ttP. rn w S rn rn o 3 S 1^ rt (fl Fi ^^►j ftcdls p-^ •o a 22 ^ 0) ^ 2 tn 2 ft„^S& . Jg fto >> a. 2 ■ca al o m o o o C r% u "i O iH . IH H 2 •-■^ 1-1^ (H^ O^ nj g3 ni " gJ Wi ir; (-1 i-; Ih In 1-, I- .!=( +J.iH -(J. in +J.r^ 4J. (U^ c jj nj 3 M m ft £ bi c3 bo tC C M 3"S be T-i +^ 5 j3_^ . o C nj C 5 R " " ti . £ wi . S r! P,+3 O+J R "^ w 0)+^ R O (« w ft ° a'^ "d jH .3 R .22 t2 _: »-i JD 4^ !3^ a o S RO- SftO ^.R^ .g,^ ^Sa^^a •VboO bo R R R J3 1- oi bo o o '5'-" a o iH R B O ., o^ '\ s s / ' " -'»* ^ /, Fig. 362. — Section of Human Cornea cut Tangential to Surface — X3S0 (technic 8, p. 97) — showing corneal cells and their anastomosing processes. limit of the nervous elements of the retina — see Retina) to the margin of the iris (see below). The ciliary processes (Fig. 363), from seventy to eighty in number, THE ORGANS OF SPECIAL SENSE 553 are meridionally-running folds of the chorioid from which are given off numerous irregular secondary folds. The processes begin low at the ora serrata, gradually increase in height to about i mm., 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 -Ins -Pars indica retiun Anterior chamber- F^ Spaces of FonUna |j| ii j ll\V\'¥^ ^"'•'^ ^ Cii Ciliary process gainentura pectinatuin iridis rciilar fibres of ciliary muscle Conjunctiva' Radial fibres of ciliary muscle Pars ciliaris retinae Perichorioidal lymph space. Orbiculus ciliaris Zonule of Zinn Fig. 363. — Vertical Section throu^l: Human Sclero-corneal Junction. (CunninKliam.) are covered by the vitreous membrane, and internal to the latter is a continuation forward of non-nervous elements of the retina — pars ciliaris retince (Fig. 363). This consists of two layers of columnar epithelial cells, the outer layer being pigmented, the inner non- pigmentcd. The ciliary muscle (Fig. 363) is a band of smooth muscle which encircles the iris. It lies in the odter anterior i)art 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 grouj) near the base of the iris— circular muscle of Miiller; (h) an 554 THE ORGANS ^ Sw outer meridional group lying next to the sclera and known as the tenser choricide^, 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 era serrata, the latter radiating fan-like to a broad insertion in the cihary body and processes. The ciliary body is closely attached to the sclero-corneal junction by the ligamentum pectinatum (Fig. 363), a continuation of the posterior elastic lamina of the cornea. Within the ligament are spaces {spaces of Fontana) lined with endothelium. These are ap- parently lymph spaces, and com- municate with each other, with similar spaces around the canal of Schlemm, and with the anterior chamber. The canal of Schlemm (Fig. 363) 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. 364).— This represents a further continuation forward of the chorioid. Its base is attached to the ciliary body and ligamentum pectinatum. From this point it extends forward as a diaphragm 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. (2) The stroma. (3) The vitreous membrane. (4) The pigmented epithelium. (i) The anterior endothelium is a single layer of pigmented cells continuous with the posterior endotheHum of the cornea (Fig. 364, a). J" Fig. 364. — Vertical Section through Iris. (Merkel-Henle.) a, Anterior en- dotheHum; h, stroma or substantia pro- pria; c, vitreous membrane; d, pigment layer; v, blood-vessel. THE ORGANS OF SPECIAL SENSE 555 (2) The stroma is divisible into two layers: an anterior reticular layer, containing many cells, seme of which are pigmented, and a vas- cular layer, the vessels of which are peculiar in that their walls con- tain almost no muscle, but have thick connective-tissue sheaths. In the posterior part of the stroma are bundles of smooth muscle. Those nearest the pupillary margin encircle the pupil, forming its sphincter muscle, while external are scattered radiating bundles forming the dilator muscle. (3) The vitreous mcmhrane is continuous with, and has the same structure as the membrane of Bruch. (4) The pigmented epithelium (Fig. 364, d) consists of several layers of cells and is continuous with the pas ciliaris retinae. Except in albinos, both layers are pigmented. The Retina. — The retina is the nervous tunic of the eye. It lines the entire eyeball, ending only at the pupillary margin of the iris. Its nervous elements, however, extend only to the ora serrata, which marks the outer limit of the ciliary body (Fig. 363). The nervous part of the retina is known as the pars optica retincB, the non-nervous extension over the ciliary processes as the pars ciliaris rttincB, its further continuation over the iris as the pars iridica retince. Modij&caticns of the optic portion of the retina are found in the region of the macula lutea and of the optic nerve entrance. The Pars Optica Retince. — This is the only part of the retina directly concerned in the reception of impulses, and may be regarded as the extremely complex sensory end-organ of the optic nerve. It is divisible into ten layers, which from without inward are as follows (Fig. 365): (i) Layer of pigmented epithelium. (2) Layer of rods and cones. (3) Outer limiting membrane. [ I^ayer of neuro-epithelium. (4) Outer nuclear layer. (5) Outer molecular layer. (6) Inner nuclear layer. (7) Inner molecular layer. (8) Layer of nerve cells. (9) Layer of nerve fibres. (10) Inner limiting membrane. Ganglionic layer. The layer oj pigmented epithelium (Fig. 365, B, i) consists of a single layer of regular hexagonal cells (Fig. 25, p. 76J. The nuclei 556 THE ORGANS lie in the outer part of the cell, while from the inner side thread-like projections extend down between the rods and cones of the layer next internal. The pigment has the form of rod-shaped granules. Its distribution seems to depend upon the amount of light being admitted to the retina. When little or no light is being admitted, the pigment is found in the body of the cell, the processes being wholly or almost Fig. 365. — A, Scheme of retina as shown by the Golgi method. B, Vertical section of retina to show layers as demonstrated by the haematoxylin-eosin stain. (Merkel- Henle.) B. — i, Layer of pigmented epithelium; 2, layer of rods and cones; 3, outer limiting layer; 4, outer nuclear layer; 5, outer molecular layer; 6, inner nuclear layer; 7, inner molecular layer; 8, layer of nerve cells; 9, layer of nerve fibres; 10, inner limiting layer. A. — i, Pigment layer; 2, processes of pigmented epithelial cells extending down between rods and cones; 3, rods; 4, rod-cell nuclei and rod fibres; 5, cones; 6, cone fibres; 7, bipolar cells of inner nuclear layer; 8, ganglion cells of nerve-cell layer; 9, larger gang- lion cells of nerve-cell layer; 10, fibres of optic nerve forming layer of nerve fibres; 11 and 12, types of horizontal cells; 13, 14, 15, and 16, types of cells the bodies of which lie in the inner nuclear layer; 17, efferent optic-nerve fibre ending around cell of inner nuclear layer; 18, neuroglia cells; 19, MuUer's fibre; 20, rod-bipolar cell of inner nuclear layer. free from pigment; when the retina is exposed to a bright light, some of the pigment granules pass down into the processes so that the pig- ment becomes more evenly distributed throughout the cell. The layer oj rods and cones and the outer nuclear layer (Fig. 365, J5, 2,4) are best considered as subdivisions of a single layer, the neuro- epithelial layer. This consists essentially of two forms of neuro- THE ORGANS OF SPECL\L SENSE 557 epithelial elements, rod visual cells and cone visual cells. These, with supporting connective tissue, constitute the layer of rods and cones and the outer nuclear layer, the separation into sub-layers being due to the sharp demarcation between the nucleated and non-nucleated parts of the cells, and the separation of the two parts by the per- forated outer limiting membrane. The rod visual cell (Fig. 365, A, 4) consists of rod, rod-fibre, and nucleus. The rod (Fig. 365, yl, 3) is a cyhnder from 30 to 40/^ in length and about 2/« in diameter. It is divisible into an outer clear portion, which contains the so-called "visual purple" and an inner granular portion. At the outer end of the latter is a fibrillated eUip- soidal body, much more distinct in some of the lower animals, the ellipsoid of Krause. At its inner end the rod tapers down to a line fibre, the rod fibre, which passes through a perforation in the outer limiting membrane into the outer nuclear layer, where it expands and contains the nucleus of the red visual cell. These nuclei are situated at various levels in the fibre and constitute the most conspicuous element of the outer nuclear layer (Fig. 365, 5, 4). The cone visual cell (Fig. 365, A, 5,6) consists of cone, cone-fibre, and nucleus. The cone (Fig. 365, yl, 5) is shorter and broader than the rod, and like the latter is divisible into two parts. The outer part is short, clear, and tapering, the inner part broad and granular, and like the rod contains a fibrillated elhpsoid body. The cone fibre (Fig. 365, y4, 6) is much broader than the rod fibre, passes completely through the outer nuclear layer and ends in an expansion at the mar- gin of the outer molecular layer. The nucleus of the cone cell usually lies just beneath the cuter limiting membrane. The remaining layers of the retina must be considered in relation on the one hand to the neuro-epitheUum, on the other to the optic nerve. The inner nuclear layer (Fig. 365, B, 6) and the layer of nerve cells (Fig. 365, B, 8) are composed largely of nerve-cell bodies, while the two molecular layers (Fig. 365, B, 5, 7) are formed mainly of the ramifications of the processes of these cells. In the inner nuclear layer are two kinds of nerve elements, rod bipolar cells and cone bipolar cells. The bodies of these cells with their large nuclei form the bulk of this layer. From the rod bipolars (Fig. 365, yl, 20) processes (dendrites) pass outward to ramify in the outer molecular layer around the terminations of the rod fibres. From the cone bipolars ("Fig. 3O5, A, 7) similar processes (dendrites) extend into the outer molecular layer where they ramify around the terminations of the 558 THE ORGANS cone cells. Two other forms of nerve cells occur in the inner nu- clear layer. One is known as the horizontal cell (Fig. 365, A, 12). Its processes ramify almost wholly in the outer molecular layer. The other lies along the inner margin of the inner nuclear layer and sends its dendrites into the inner molecular layer (Fig. 365, A, 13, 14, 15, and 16). Many of these latter cells appear to have no axone and are consequently known as amacrine cells. The outer molecular layer is thus seen to be formed mainly of terminations of the rod and cone visual cells, of the dendrites of the rod and cone bipolars, and of the processes of the horizontal cells. From the cone bipolar a process {axone) extends inward to ramify in the inner molecular layer, while from the rod bipolar a process (axone) passes inward through the inner molecular layer to terminate around the cells of the nerve- cell layer. The layer of nerve cells (Fig. 365, B, 8) consists for the most part of large ganglion cells whose dendrites ramify in the inner molecular layer, and whose axones pass into the layer of nerve fibres and thence into the optic nerve. Some small ganglion cells are also found in this layer, especially in the region of the macula lutea (see page 559) . The inner molecular layer is thus seen to be composed mainly of the processes {axones) of the rod and cone bipolars and of the den- drites of the ganglion cells of the nerve-cell layer. The layer oj nerve fibres (Fig. 365, 5, 9) consists mainly of the ax- ones of the just-described ganglion cells, although a few centrifugal axones of brain cells (Fig. 365, A, 17) are probably intermingled. The outer and inner limiting layers or membranes (Fig. 365, B, 3,10) are parts of the sustentacular apparatus of the retina, being connected with the cells or fibres of Muller (Fig. 365, ^,19 and Fig. 366). 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 and the pigment layer. The inner ends of the cells, which are conical and fibrillated, unite to form the inner limiting membrane (Fig. 366, 10). Through the inner molecular layer the cell takes the form of a narrow stalk with nu- merous fringe-like side fibrils (Fig. 366, 7). This widens in the inner nuclear layer, where cup-Hke depressions in the sides of the Muller 's cell are caused by the pressure of the surrounding nerve cells (Fig. 366, b). This wide portion of the cell in the inner nuclear layer con- tains the nucleus (Fig. 366, a). In the outer jnolecular layer the THE ORGANS OF SPECI.\L SENSE 559 cell again becomes narrow (Fig. 366, 5) and in the outer nuclear layer broadens out into a sponge-like reticulum (Fig. 366, 4), which sup- ports the rod and cone bipolars. At the inner margin of the layer of rods and cones the protoplasm of the Mliller's cells spreads out and unites to form the so-called outer hmiting membrane (Fig. 366, 3), from which delicate fibrils (fibre baskets) pass outward between the rods and cones. In addition to the Mlil- ler's cells, which are neuroglia elements, spider cells also occur in the retina (Fig. 365,^,18). The retina of the macula lutea presents certain peculiarities. Its name is derived from the yellow pigment which is dis- tributed diffusely through the inner layers, extending as far out as the outer mole- cular 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 re- daction in the number of rods, while the number of cones is correspondingly increased. In the centre of the macula is a depres- sion, the jovea 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 non-nervous ret- inal extension over the ciliary body {pars ciliaris relince) and over the iris {pars iridica relince) have been described in con- nection with the ciHary body and iris. The Optic Nerve. — The optic nerve (Fig. 367, d) is enclosed by two connective-tissue sheaths, both of which are extensions of the brain membranes. The outer or dural sheath, (Fig. 367, a) is continu- ous with the dura mater of the brain posteriorly, while anteriorly it blends with the sclera. The inner or pial sheath, (Fig. 367, b) is an ex- tension of the pia mater and is separated from the outer sheath by Fig. 366.— Two MuUer's Fibres from Retina of Ox showing Relation to Layers of Retina. (Ramon y Caj'al.) 3, Outer limiting layer; 4, outer nuclear layer; 5, outer molecular layer; 6, inner nu- clear layer; 7, inner molecular layer; 8, layer of nerve cells; 9, layer of nerve fibres; 10, in- ner limiting layer; a, nucleus; b, cup-like depression caused by pressure from surrounding cells. 560 THE ORGANS the subdural space (Fig. 367, 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 subarachnoid space. The optic nerve fibres, in passing through the sclera and chorioid, separate the connective-tissue bundles so that they form a lattice-work, the already mentioned lamina crihrosa (Fig. 367, h). The optic nerve fibres are medullated, but have no neurilemma. As they pass through the lamina cribrosa the medul- lary 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. 365, A, 3, 4, 5, and 6). By their expanded bases in the outer molecular layer, the rod and cone cells Fig. 367.-Section through Entrance of Optic communicate with the neu- Nerve into Eyeball. (Merkel-Henle.) a, Dural (-^--iii^i^^^" sheath; b, pial sheath, inner and outer layers; c, ^onc System No.' I. of the space between inner and outer layers of pia mater; ^-^„^f ^W^c rr,mr>ri<;pQ (/, optic nerve; e, central artery of retina; a', OptlC tract. i lllS COmpriSeS sclera; /, chorioid; , retina; h, lamina cribrosa. ^^-^ ^^^ neurones, (&) COne neurones, (c) horizontal neurones. Neurone System No. L— (a) Rod neurones. The cell bodies of these neurones (Fig. 365, A, 20) He in the inner nuclear layer. Their dendrites enter the outer molecular layer where they form net- works around the expanded bases of the rod cells. Their axones pass through the inner molecular layer and end in arborizations around the bodies and dendrites of cells of the nerve-cell layer (neurone system No. II.). {h) Cone neurones (Fig. 365, ^,7). These have their cell bodies in the inner nuclear layer. Their dendrites pass to the outer molecular layer where they form networks around the expanded bases THE ORGANS OF SPECIAL SENSE 561 of the cone cells. Their axones pass only into the inner molecular layer where they end in arborizations around the dendrites of neu- rones whose cell bodies are in the layer of nerve cells (neurone system Xo. II.). (c) Horizontal neurones (Fig. 365, A, 11 and 12). These serve as association neurones between the visual cells and may be divided into rod association neu- rones and cone association neu- rones. The cone association neurones are the smaller and more superficial, and both den- drites and axones end in the outer molecular layer around the terminal expansions of the cone visual cells (Fig. 365, A, 11). The rod association neurones are larger, more deeply seated, and behave in a similar manner to- ward the rod visual cells (Fig. 365, A, 12). Some of these cells send processes to the inner mole- cular layer. 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 neurone system 8, 9) are in the cells and are, as associated either Fig. 368. — Diagram showing Main Relations of Optic Tract. (Testut.) R, Retina; No, optic nerve; CM, optic decussation or chiasma; Tro, optic tract; Tho, thalamus; Cgl, lateral geniculate body; Qa, anterior corpus quadrigemi- num; Rd, fibre of optic tract passing directly to cortex; Sm, third neurone sys- tem of optic tract (excepting Rd) connect- ing thalamus, lateral geniculate body, and anterior corpus quadrigemnium with the cortex, Co. of the second (Fig. 365, A, , layer of nerve above noted, 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. 365, A, 10). The optic nerves (Fig. 368, No) unite at the base of the brain to form the optic decussation or chiasma (Fig. 368, CM). Here the axones from the mesial j)art of the retina cross to the optic tract of the opposite side, while those from the lateral part of the retina remain in the optic tract of the same side. The axones of the optic tract 36 562 THE ORGANS Uoffer ■ Fig. 3^ THE ORGANS OF SPECL\L SENSE 563 EXPLANATION OF FIG. 369. Fig. 369. — Diagram of the Optic Nerve (II) and some of its Principal Connections. A, Level of nerves II and III; B, level of nerve IV; C, level of nerves VI and VII; D, spinal cord. The rods and cones (sensory cells) and the bipolar cells ( = Neurone No. i) of the retina are not indicated. Neurone No. 2. — 2 a, Axones of ganglion cells in temporal part of retina pass to pul- vinar of thalamus of same side; 2 b, axones of ganglion cells in retina pass to anterior corpus quadrigeminum of same side; 2 c, axones of ganglion cells in retina pass to external geniculate bodj^ (Corp. gen. ext.) of same side; 2 e, axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to external geniculate body of opposite side; 2/, axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to anterior corpus quadrigeminum of opposite side; 2 g, axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to pulvinar of thalamus of opposite side. Neurone No. 3. — 3 a, Axones of cells in pulvinar to cortex of occipital lobe of cere- brum (this connection is disputed); 3 b, axones of cells in external geniculate body to cortex of occipital lobe of cerebrum; 3 a and 3 b constitute the primary optic radiation; 3 c, 2, d and 3 c, axones of cells in middle layer of tectum (roof) of anterior corpus quad- rigeminum decussate ventral to posterior longitudinal fasciculus (dorsal tegmental decussation or decussation of IMeynert) and form the tractus lecto-bulbarlis et spinalis (Tr. tecto-bulb. et spin.) to bulb (medulla) and anterior column of cord, innervating by collaterals and terminals, directly or indirectly, chief!)- the nuclei of III, I\', \T, and VII cranial nerves and motor nuclei of spinal nerves. 3 / and 3 g (possibly another neurone intercalated between these and optic terminals), axones of cells in nucleus of posterior longitudinal fasciculus (Nu. fasc. long, post.) (Nucleus of Darkschewitsch) form part of posterior longitudinal fasciculus and descend on same side to anterior column of cord next to anterior median fissure, innervating nuclei of III, IV, and VI cranial ner\'es and motor nuclei of spinal nerves. Neuronic No. 4. — -Axones of cells in above-mentioned motor nuclei. Axones from cells in median nucleus of III nerve (Nu. med. Ill N. ), and possibly in Edinger-West- phal nucleus, probably innervate the intrinsic muscles of eyeball (ciliary and pupillary reflex path). It is evident from the diagram that the cerebral pathway of the optic nerve is via the external geniculate body (and pulvinar of thalamus), and the reflex pathway in via the anterior colliculus (anterior corpus quadrigeminum). 564 THE ORGANS (Fig. 368, Tro) terminate in the thalamus, in the lateral geniculate body, and in the anterior corpus quadrigeminum (Fig. 368). Neurone System No. III.^ — The neurones of this system have their cell bodies in the thalamus, lateral geniculate body, and anterior corpus quadrigeminum (Fig. 368). The axones of the two former terminate in the cortical visual centers in the occipital lobe (Fig. 368, Co). The axones of the latter form descending reflex paths (see Anterior Corpora Quadrigemina, p. 522). Neurone system No. I is the analog of the primary afferent neu- j c a rones (the cerebro-spinal ganglion cells). Neurone system No. II is the analog of the secondary afferent neurone system which receives the afferent root fibres and originates the secondary decussating tracts Fig- 370. Fig. 371. Fig. 370. — From Longitudinal Section through Margin of Crystalline Lens, showing longitudinal sections of lens fibres and transition from epithelium of capsule to lens fibres. (Merkel-Henle.) a, Lens fibres; h, capsule; c, epithelium. Fig. 371. — From Cross Section of Crystalline Lens, showing transverse sections of lens fibres and surface epithehum. (Merkel-Henle.) a, Lens fibres; h, epithelium. to the thalamus, such as the medial and lateral fillets. Neu- rone system No. Ill is the tertiary afferent neurone system or thalamo-cortical system. It will thus be seen that the optic chiasma is the analog of the decussations of the medial and lateral fillets. (See also pp. 475, 476 A.) The Lens. — The lens is composed of lens fibres which are laid down in layers (Fig. 370, a). The lens fibre is a long hexagonal, flat- tened prism with serrated edges. Most of the lens fibres are nucle- THE ORGANS OF SPECI.AJL SENSE 565 ated, 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 backward 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. 370, b), a clear homogeneous membrane which is about i2« 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. 370, c), the lens epithelium. Attached to the capsule of the lens anteriorly and posteriorly are membrane-like structures which con- stitute the suspensory ligament of the lens. These pass outward and unite to form a delicate membrane, the zonula ciliaris or zonule of Zinn (Fig. 363). This bridges over the 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 direction is the so-called hyaloid or CloqueVs canal, the remains of the embry- onic hyaloid artery (page 570). Blood-vessels. — The blood-vessels of the eyeball are divisible 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 capil- laries which supply the retina, passing outward as far as the neuro- epithelial layer and anteriorly as far as the ora serrata. The smaller 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 570). The veins of the retina accompany the arteries. The cih'ary arteries are divisible into long ciUary arteries, short 566 • THE ORGANS ciliary arteries, and anterior ciliary arteries. The long ciliary arte- ries are two in number and pass one on each side between the cho- rioid 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 ciHary 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 terminate in the chorio-capillaris of the chorioid. The anterior ciliary arteries enter the sclera near the corneal margin and communicate with the chorio-capillaris and with the greater arterial circle of the iris. The anterior cihary 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 quadrant of the eyeball. At the sclero- corneal junction is a venous channel, the canal of Schlemm, which completely encircles the cornea (Fig. 363). 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 communi- cates by means of a narrow cleft between iris and lens with the posterior chamber; {h) the posterior chamber; (c) the lymph canal- icuH of the sclera and cornea and the canal of Petit. The posterior lymph paths include {a) the hyaloid canal (see above) ; (&) the sub- dural and intra-pial spaces, including the capsule of Tenon; {c) the perichorioidal space, and {d) 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 supra- chorioid layer. From these nerves, branches are given off as follows: (i) To the chorioid, where they are intermingled with ganglion cells. (2) To the ciHary 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 THE ORGANS OF SPECIAL SENSE 567 cornea first form a plexus in the sclera — the plexus annularis — which encircles the cornea. From this, branches pierce the substantia propria 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-epitheUal, 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 con- nected with end-plates. The Lacrymal Apparatus The lacrymal apparatus of each eye consists of the gland, its excretory 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 alveoH are fined 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 fined with a two-layered stratified or pseudo- stratified columnar epithefium resting upon a basement membrane. The stroma contains much diffuse lymphatic tissue. The nasal duct has walls similar in structure to those of the lacry- mal 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 glands. The EYELm The eyelid consists of an outer skin layer, an inner conjunctival layer, and a middle connective-tissue layer. The epidermis is thin and the papillx' of the derma are low. Small sebaceous glands, sweat glands, and fine hairs are present. 568 THE ORGANS The conjunctiva (Fig. 372, d) 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 gob- let cells. Although not always up6n 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 occur- rence. Small glands, similar to the lacrymal glands in structure, are usually present (Fig. 372, k). At the margin of the eye- lid where skin joins mucous membrane are several rows of large hairs, the eyelashes (Fig. 372, h). Connected with their follicles are the usual sebaceous glands (Fig. 372, g) and the glands of Mall, the latter probably representing modified sweat glands. The middle layer contains the tarsus (Fig. 372, e) and the muscular structure of the eyehd (Fig. 372, b). The tarsus is a plate of dense fibrous tissue which lies just beneath the conjunctiva and extends about two-thirds the height of the Hd. It contains the tarsal or Meibomian glands (Fig. 372, e). 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. 372,/), and internally into a number of branched Fig. 372. — Vertical Section through Upper Eyelid. (Waldeyer.) a, Skin; b, orbicularis muscle; b', ciliary bundle of muscle; c, in- voluntary muscle of eyelid; d, conjunctiva; e, tarsus _ contaning Meibomian glands; /, duct of Meibomian gland; g, sebaceous gland with duct lying near eyelashes; h, eyelashes; i, small hairs in outer skin;j, sweat glands; k, posterior tarsal glands. THE ORGANS OF SPECIAL SENSE 569 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 Hd. A second arch, the external tarsal arch, is formed along the upper margin of the tarsus. From these arches are given ofif capillary networks which supply the structures of the lid. Lymphatics. — These form two anastomosing plexuses, one ante- rior, the other posterior to the tarsus. Nerves. — The nerves form plexuses in the substance of the lid. From these, terminal fibrils pass to the various structures of the lid. Many of the fibres end freely in fine networks around the tarsal glands, upon the blood-vessels, and in the epithelium of the conjunc- tiva. Other fibres terminate in end-bulbs which are especially nu- merous at the margin of the Hd. Development of the Eye The eyes begin their development very early in embryonic life. As optic depressions they are visible even before the closure of the medullary groove. Fore-brain vesicle Lens area-S Optic vesicle ' Surface ectoderm Optic vesicle FiG- 373. — Section through head of chick of two days' incubation. (Duval.) The formation of the optic vesicle and stalk api)cars to be somewhat more advanced on the left than on the right. As a result of the closure of this groove, the optic dejjressions are transformed into the optic vesicles. The conned ion between vesicle and brain now becomes narrowed so that the two are connected only by the thin optic stalk. The surface of the optic vesicle becomes firndy adiicrciit to the epidermis and as a 570 THE ORGANS result of proliferation of ectodermic cells at this point is pushed inward (invaginated), forming the optic cup. The invagination of the optic vesicle extends also to the stalk, the sulcus in the latter being known as the chorioid Fore-brain Lens invagination - - Optic vesicle ' Lens invagination ' Optic vesicle Fig. 374. — Section through head of chick of three days' incubation. (Duval.) fissure. The latter serves for the introduction of mesenchyme, and the develop- ment of the hyaloid retinal artery. Three distinct parts may now be distin- guished in the developing eye, which at this stage is known as the secondary Fore-brain Lens vesicle Optic cup. Fig. 375. — Showing somewhat later stage in development of optic cup and lens than is shown in Fig. 374. (Duval.) optic vesicle: (a) The proliferating epidermis which is to form the lens; (b) the more superficial 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- THE ORGANS OF SPECL\L SENSE 571 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 hah" eye should now be placed in a dish of water and the structures shown in Fig. 358 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 chorioid from which it may be scraped and examined in water or mounted in glycerin. In removing the lens note the lens capsule and the sus- pensory 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 chorioid shows the iridescent membrane of Bruch. By placing a piece of the chorioid, membrane-of- Bruch-side down, over the tip of the finger and gently scraping with a knife in Conjunctival epithelium Vitreus Lens vesicle ' sir-i j—jij . - - p^^a ^-j: /-, , • ^ ,, Q.ffl /-*7 •- - - " - F^i /Cn fptic stalk Retina (inner layer of optic cup) Pigmented layer of retina . _ . (outer layer of optic cup) Fig. 376. — Diagram of developing lens and optic cup. (Duval.) The cells of the inner wall of the lens vesicle have begun to elongate to form lens fibres. The epithelium over the lens is the anlage of the corneal epithelium. The mesodermal tissue between the latter and the anterior wall of the lens vesicle is the anlage of the substantia propria comese. the direction of the larger vessels, the latter may be distinctly seen. By now staining the piece lightly with hajmatoxjdin and strongly with eosin, clearing in oil or origanum and mounting in balsam, the choriocapillaris and the layer of straight vessels become distinctly visible with the low-power lens. In removing the chorioid note the close attachment of the latter to the sclera, this being due to the intimate association of the fibres of the lamina suprachorioidea 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 finer 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-Miiller's fluid (technic 5, p. 7) 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 posterior half. Block in celloidin, cut the following sections, and stain with haematoxylin-eosin (technic i, p. 20). (a) Section through the sclero-corneal junction, including the ora serrata. 572 THE ORGANS 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. (b) Section through the postero-lateral portion of the eyeball to show struc- ture of sclera, chorioid, 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 35). (4) The connective-tissue cells and cell spaces of the cornea may be demon- strated by means of technics 8 and 9, page 97. (5) The different parts of the lacrymal apparatus may be studied by fixing material in formalin-Miiller's fluid and staining sections in haematoxylin-eosin. (6) The Eyelid. An upper eyelid, human if possible, should be carefully pinned out on cork, skin side down, and fixed in formalin-Miiller's fluid. Vertical sections should be stained with haematoxylin-eosin or with hsematoxylin-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 piitna consists of a framework of elastic cartilage embedded in connective tissue and covered by skin. The latter is thin and con- tains 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 568) and are probably modified sweat glands. Their cells contain numerous fat droplets and THE ORGANS OF SPECT.\L SENSE 573 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 periosteum. 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-tissue 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-papillated 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 i)lexuses of fme fibres. The Middle Ear The middle ear or lympanum is a small chamber separatel)lasts, 228, 241 of ("laudius, 581 596 INDEX Cells of oral glands, 221, 222 olfactory, 300 osteoblasts, 198, 235 osteoclasts, 193, 199, 235 ovum, 52, 3SS, 356 oxyntic, 249 oxyphile, 405 Paneth's, 260, 273 parietal, 249 peptic, 249, 272 phseochromoblasts, 412, 415 phagocytes, 107, 174 pigmented, 46, 76, 131, 384, 388, 554 pillar, 580 plasma, 85, 193 polykaryocytes, 193 prickle, 383, 387 primitive ova, 354, 378 Purkinje, 509 red blood, 103, 184 replacing, 73, 252, 259 reserve, 403 respiratory, 311 resting, 54, 212, 403 secreting, 213 serous, 221 Sertoli's, 335, 378 sex, 378 signet-ring, 88 simple tactile, 431 single primitive, 5 2 smooth muscle, 112, 381 spermatids, 337, 344 spermatoblasts, 345 spermatocytes, 337, 344 spermatogenic, 335 spermatogones, 336, 344 spider, 143 spleen, 184 supporting, 335 sustentacular, 284, 300, 335, 575 sympathoblasts, 415 tactile, 433 tautomeric, 445 . tendon, 92 thrombocytes, 108 wandering, 86, 259 white blood, 105, 184 Cementing glycerin mounts, 22 Cementoblasts, 235 Cementum, 233 development of, 242 Central canal, 416, 449 cells, 249 chromatolysis, 142 gelatinous substance, 449 gray, 483, 485, 487, 502 nervous system, 416 neurones, 418 spindle, 54 tegmental tract, 490, 501, 507, 515, 518, 526 vein, 289 Centriole, 49 Centro-acinar cells of Langerhans, 283 Centrosome, 49, 62 archoplasm, 49 attraction sphere, 49 centriole, 49 daughter, 53 of fertihzation, 62 Centro-tubular cells of Langerhans, 283 Cerebellar arc, 470 connections, 463, 464, 468, 499, 506 cortex, 507, 512 peduncles, 502 Cerebello-olivary fibres, 490, 491, 499 Cerebellum, 417, 502, 507 arbor vitae, 507 ascending paths to the, 462 association cells of, 514 basket ceUs of, 510 cells of, 509, 510, 511 climbing fibres of, 510 cortex of, 509, 514 dentate nucleus of, 500, 502, 507 descending tracts from, 464 development of, 417 fibres of, 510 climbing, 510, 513 mossy, 513 of Bergmann, 514 parallel, 512 general histology, 507 glomeruli of (islands), 512 granular layer, 507 gray matter of, 507 hemispheres, of, 502, 507 internal nuclei of, 502, 507 laminse of, 507 middle peduncle of, 508 molecular layer, 509 neuroglia of, 514 nuclear layer, 509 INDEX 597 Cerebellum, nucleus dentatus, 507 emboliformis, 507 globosus, 502, 507 tecti or fastigii, 502, 507, 538 peduncles of, 508 Purkinje cells of, 509 technic of, 543 vermis, 507 Cerebral arc, 471 cortex, 481 dura, 422 hemispheres, 417 development of, 417 membranes, 422 peduncles, 518 vesicle, 416 Cerebro-spinal ganglia, 126, 418, 426 central processes of, 436 Dogiel's classification of, 427 peripheral processes of, 429 technic of, 441 nervous system, 417 neurones, efferent peripheral, 441 Ceruminous glands, 572 Cervical enlargement of cord, 443 segments of cord, 443 Cervbc, 367 epithehum of, 368 external os, 368 ovula Nabothi, 368 plicae palmatae, 368 technic of, 379 Chain ganglia, 436 Cheeks, mucous membrane of, 220 Chemotaxis, 51 Chiasma, optic, 526, 561 Chief cells, 245, 249, 272, 405, 407 Chloride of gold for staining connective- tissue cells, 28 Chloroform as solvent, 13 Choledochus ductus, 295 Choriocapillaris, 551 Chorion, 371 Chorionic villi, 371 Chorioid, the, 550 choriocapillaris of, 551 fissure, 570 Ilallcr's layer of, 551 lamina citrea, 551 suprachorioidca, 551 perichorioidal lymph spaces of, 551 plexus, 417,482,487, 502 Chorioid, tapetum cellulosum of, 551 fibrosum of, 551 ven£e vorticosas of, 551 vitreous membrane of, 552 ChromafEn cells, 410 granules, 412 organs, 410 Rose, concerning chromaffin cells, 410 Chromatic element of intranuclear net- work, 48 Chromatin, 48 Chromatolysis, 142 Chromatolytic changes, 363 Chrome-silver method of Golgi, 29 Chromophihc bodies, 129 significance of, 130, 138 Chromosomes, 55 daughter, 55 Chyle vessels, 270 Cilia, 52, 71, 76, 340 Ciliary artery, 565 body, 552 blood-vessels of, 565 canal of Schlemm, 554 hgamentum pectinatum, 554 muscles of, 552 processes of, 552 spaces of Fontana, 554 vitreous membrane of, 553 ganglion, 436 movement, 52 technic for, 570 muscle, 553 plexus, 566 processes, 552 Cingulum, 535 Circulatory system, 151 blood- vessel system, 151 carotid gland, 410 coccygeal gland, 411 development of, 163 lymph-vessel system, 166 Circumferential lamclL-c, 191 Circumvallatc i)apillx, 225 Cirl, concerning fibres of internal capsule, 526 Clarke's columns, 450, 455, 463 Clasmatocytes, 85 Clasmocytes, 85 Claude, concerning development of pan- creas, 297 598 INDEX Claudius, cells of, 581 Clava, the, 481 Clearing specimens before mounting, 23 Clefts of Schmidt-Lantermann, 135 Climbing fibres, 510, 513 Clitoris, 376 Cloquet's canal, 565 Closed skein (spireme), 54 Clove -oil celloidin, 12 Coagulum sheath, 137 Coccygeal glands, 411 segments of spinal cord, 455, 488 Cochlea, 576 bony spiral lamina of, 577 cupola of, 576 cupula of, 576 hamulus cf, 577 helicotrema, 578 membranous spiral ligament of, 577 modiolus of, 576 scala tympani, 578 vestibuli, 578 spiral ligament of, 577 Cochlear duct, 578 basilar membrane of, 579 crista basilaris, 579 external spiral sulcus, 579 membrane of Reissner, 579 organ of Corti, 580 spiral prominence of, 579 stria vascularis, 579 zona pectinata, 580 tecta, 580 nerve, 475, 489, 491, 493 tracts, 491, 500, 504 Coelom, 165 Cohnheim's field, 116 Collagenous fibres, 81 Collaterals, 132, 447 Colliculi, 417, 517, 522 Colloid, 402, 405, 408 Colostrum corpuscles, 400 Column cells, 444 hecateromeric, 444 heteromeric, 444 tautomeric, 444 technic of, 445 of Burdach, 459, 462, 481, 487 of Goll, 459, 462, 481, 487 Columnse rectales. 266 Columns of Bertini, 320 of Sertoli, 335 Comma tract of Schultze, 467 Commercial form.aUn, 4 Commissura habenularis, 531 Commissural fibres, 532 Common dental geim, 238 senses, 436 Compact bone, 188 Compound tactile cells, 431 Conduction path, 421, 461 afferent pallial, 461, 476 afferent and efferent suprasegmental, 476 auditory, 493 descending suprasegmental, 483 efferent cerebellar, 499 pallial, 476 pallio-cerebellar, 499 pallio-spino-peripheral efferent, 465 to cerebellum, 462 trigeminal afferent pallial, 507 Cone association- neurones, 561 fibres, 557 neurones, 560 -visual cell, 557 Cones, layer of rods and, 556 Conjunctiva, 568 end bulbs of, 433, 569 Connective tissue, 80, 82 adipose or fat, 87 aponeurotic, 92 areolar, 87 bone, IDG cartilage, 97 cells of, 83 characteristics of, 80 chlorid-of-gold method for demon- strating cells of, 28 classification of, 81 dense fibrous, 87 elastic, 92 elastin of, 87, 95 embryonal, 81, 82, 197, 203 fat, 87 fibres of, 86 elastic, 86 fibrillated, 86 reticular, 94 white, 86 yellow, 86 fibrillar, 82 fibroblasts, 82 fixed cells of, 83 INDEX 599 Connective tissue, formed, 91 gelatin of, 86 gelatinous, 82 histogenesis of, 80 intralobular, 91, 215 interalveolar, 313 intercellular substance of, 86 interlobar, 215 interlobular, 89, 215 intrafascicular, 208, 425 ligaments, 91 loose, 87 Mallory's stain for, 29, 30 mast cells of, 83 mucous, 82 neuroglia, 142, 418, 453, 514 pigmented cells of, 87 plasma cells of, 83 reticular, 94 retinaculcc cutis, 382 staining cells of, 29 technic for, 95, 102 theories of development of fibres of, 81 wandering cells of, 83 Constrictions of Ranvier, 134 Contact theory of neurones, 138 Continuity theory of neurones, 139 Convoluted tubules, 320, 322, 323 Cord, spinal, 416; see Spinal Cord Corium, 380; see Derma Cornea, the, 548 anterior elastic membrane of, 549 epithelium of, 549 corpuscles of, 550 endothelium of Descemet of, 550 membrane of Bowman of, 548 of Descemet of, 550 posterior elastic membrane of, 550 substantia propria of, 550 Corneal corpuscles, 550 Cornu ammonis, 532 Cornua of cord, 448 Corona radiata of ovum, 356 of pallium, 533, 536, 537 Coronary arteries, 162 Corpora amylacea, 348 cavernosa, 349 lutea of pregnancy, 360 spuria, 360 vera, 360 mammillaria, 531 Corpora, quadrigemina, 417, 493, 517 anterior, 517, 522 development of, 417 posterior, 493, 517 striata, 417 Corpus albicans, 360 callosum, 536 dentatum, 502 haemorrhagicum, 359 Highmori, or mediastinum testis, 333 luteum, 359 theory of, 362 Luysii, 523 quadrigeminum, anterior, 517, 522 spongiosum, 349 striatum, 532, 535 caudate nucleus, 532 lenticular nucleus, 532 putamen, 535 subthalamicura, 521, 530 trapezoideum, 500, 504 Corpuscles, blood, 103 bone. Id colostrum, 400 corneal, 550 crescentic, 348 Golgi-Mazzoni, 394, 434 Grandry's, 431 Hassall's, 177 Meissner's, 350, 394, 432 Merkel's, 431 Pacinian, 196, 350, 433 renal, 320 Ruftini's, 394, 434 salivary, i8c splenic, 182 tactile, 432 Vater-Pacinian, 394 Wagner, 394 Cortex ccrebelli, 509, 514; see also Cere- bellum cerebri, 533, 536 areas of, 541 association fibres of, 543 barren or molecular layer of, 538 cells of, 536, 539 of Betz, 536 of Cajal, 537 of Gclgi, Tyjjc II., 536 of Martinotti, 536, 539 I)yramidal, 536 600 INDEX Cortex cerebri, commissural fibres, 533 corona radiata of, 533, 536 deep tangential fibres of, 541 external granular layer, 539 ganglionic layer, 539 horizontal cells of Cajal, 536 internal granular layer, 539 interradiary plexus, 541 inverted pyramidal cells of Marti- notti, 536 layer of polymorphous cells, 536 of pyramidal cells of, 539 line of Baillarger, 541 of Gennari, 543 molecular layer, 538 multiform layer, 539 plexiform layer of Cajal, 538 polymorphous cells of, 536 projection fibres, 543 radiations of Meynert, 541 stellate cells of, 536 superficial, tangential fibres of, 541 supraradiary plexus of, 541 Cortical labyrinths, 320 pyramids, 320; see also Kidney Corti's arches, 581 organ, 580; see also Organ oj Corti tunnel, 580 Cotyledons, 371 Cowper's glands, 348 technic of, 349 Cox-Golgi method of staining nerve tis- sue, 36 Cranial nerves, 474, 545, 546; see also Nerves, cranial Crenation of red blood cells, 104 Crescentic corpuscles, 348 Crescents of Gianuzzi, 222, 278, 302 Cretinism, 403 Cricoid cartilage, 301 Crista acustica, 576 basilaris, 579 Crossed pyramidal tracts, 464, 485 Crura cerebri, 518 Crusta (exoplasm), 46 Crypt of Lieberkiihn, 252, 260 Cumulus ovigerus, 356 Cuneus, 481, 487 Cupola, 576 Cupula, 576 Cutaneous sensation, 436 Cuticle, 382; see Epidermis Cuticula, 47, 71 dentis, 233 Cuticular membrane, 71, 240, 255 Cystic duct, 294, 297 Cytoarchitecture, 541 Cytoplasm, 46 of nerve cells, 128 Cytoreticulum, 45 Cytosomes, 213 Darksche-witsch, nucleus of, 465 Daughter cells, 57 centrosomes, 53 chromosomes, 55 stars, 56 Decalcification, 3, 9 Decalcifying, 10 fluids, 10 Decidua basalis, 370 capsularis, 370 graviditatis, 370 menstrualis, 369 placentalls subchorialias, 374 reflexa, 370 serotina, 370 subchorionic-placental, 374 vera, 370 Decidual cells, 370 Decolorizing fluid for Weigert's hema- toxylin, 33 Decussation of fillet, 485 motor, 483 optic, 561 of Forel, 522 of Meynert, 522 of pyramids, 464, 485, 487 sensory, 435, 485 Deep sensation, 436 Degenerating nerves, Marchi's method for stairung, 34, 140 Degeneration of neurones, 140 Wallerian law of, 141 Dehiscent glands, 217 Deiter's cells, 581 nucleus, 466, 476, 493, 499, 502 descending tract from, 466, 483, 487, 490, S02 Delafield's hematoxylin, 17 Delomorphous cells, 249 Demilunes of Heidenhain, 222 Dendrites, the, 126, 131, 418 INDEX 601 Dental germ, 238 groove, 238 papilla, 238 periosteum, 237 pulp, 230 layer of Weil of, 2 28 odontoblasts, 228 ridge, 239 sac, 239 sheath, Neumann's, 232 shelf, 238 Dentate nucleus, 458, 500, 502, 507 Dentinal canals, 230 development of, 241 fibres of, 230 interglobular spaces of, 232 lines of Schreger, 232 nerves of, 236 Neumann's dental sheath, 232 Tomes' granular layer, 232 Dentine, 228 chemical composition of, 228 secondary, 232 Derma, or corium, 380 corpuscles of Meissner, 350, 394, 432 muscle cells of, 381 papilla;, compound, 381 nerve, 381 simple, 381 vascular, 381 pars papillaris, 381 reticularis, 380 pigmentation of, 384 subcutaneous tissue of, 381 Descemet, endotheUum of, 550 membrane of, 550 Descending degeneration, 459 fibre tracts of the spinal cord, 464; see Fibre tracts of spinal cord (descending) Deutoplasm, 46, 357 Diapedesis, 107 Diaphysis of bone, 203 Diarthrosis, 204 articular cartilages, 204 glenoid ligaments, 205 interarticular cartilages, 205 joint capsule, 206 Diaster, 56, Go Diencephalon, 417, 522 Digestive system, 219 Digestive system, alimentary tract of, 219 development of, 238, 296 endgut, 263 foregut, 243 gall-bladder, 295 gastro-intestinal canal, 245 headgut, 220 large intestine, 263 larger glands of, 275 liver, 287 mesentery, 268 midgut, 255 mouth, 220 oesophagus, 243 omentum, 269 pancreas, 281 peritoneum, 267 pharynx, 242 rectum, 266 salivary glands, 276 small intestine, 255 stomach, 247 teeth, 227 tongue, 223 vermiform appendbc, 265 Direct cerebellar tract, 463 pyramidal tract, 464 Discus proligerus, 356 Dissociation of tissue elements, 4 Disynaptic arc, 470 Dogiel's end plates, 350, 567 theory of structure of spinal gan- glion, 427 Dorsal accessory olivary nucleus, 490 decussation of Meynert, 522 gray commissure, 449 root fibres of white matter, 450 spino-cerebcllar tract, 463 white commissure, 450 Duct systems of glands, 214 Ducts, aberrans Halleri, 342 alveolar, 310 Bartholini's, 277 Bellini's, 321, 324 bile, 290 cholcdochus, 295 cochlear, 578 common, 294 cystic, 294, 297 endolymphatic, 575 ejaculatory, 342 602 INDEX Ducts, excretory, 214, 276, 282, 329, 364 Fallopian tube, 364 Gartner's, 363 hepatic, 289 mesonephric, 377 MiiUerian, 348 nasal, 567 of MiiUer (embryonal), 342 of sweat glands, 384 oviduct, 364 pancreatic, 281 pronephric, 377 reuniens, 575 Santorini's, 282 secondary pancreatic, 281 seminal, 339 Stenoni's, 277 thoracic, 164 thyreo-glossal, 403 utriculo-saccular, 575 vas deferens, 340 epididymis, 339 vas efferentia, 339 Wharton's, 278 Wirsung's, 281 Wolffian, 377 Ductus aberrans Halleri, 342 reuniens, 575 Duodenum, 262 Brunner's glands, 262 technic of, 275 Dura mater, 422 blood-vessels of, 424 cerebral, 422 spinal, 422 technic of, 424 Dyes, acid aniline, 20 basic aniline, 19 nuclear, 17 plasma, 19 Dynamic centre of cell, 57 Ear, 572; see also Organ of Hearing blood-vessels of, 582 development of, 583 drum, 573 external, 572 auditory canal, 572 auricle, 572 blood-vessels of, 573 ceruminous glands of, 572 ear drum, 573 Ear, external, auditory canal, 572 lymphatics of, 573 nerves of, 573 pinna, 572 tympanic membrane, 573 internal, 574 ampuUa, 574 blood-vessels of, 581 canalis communis, 575 cochlea, 576 ducts of, 578 ductus reuniens, 575 endolymph of, 574 endolymphatic duct, 582 sac, 582 fenestra ovalis, 574 rotunda, 574 lymphatics of, 582 membrana tectoria, 582 membranous labyrinth, 574 nerves of, 582 organ of Corti, 580 osseous labyrinth of, 574 perilymph of, 582 saccule, 575 scala media, 578 semicircular canals, 575 utricle, 575 utriculo-saccular duct, 575 vestibule, 574 middle, or tympanum, 573 fenestra rotunda of, 574 ossicles of, 574 stapes, 574 wax, 573 Ebner's glands, 226 hydrochloric salt solution, 10 Ectoderm, 62 derivations from, 67, 126, 142, 395 Edinger-Westphal nucleus, 518 Effectors, 418, 424, 426 Efferent pallial paths, 461, 462, 464, 475, 477 peripheral neurones, 418, 441, 483, 485, 487, 491, 499, 502, 504, 515, 518, 524 root fibres, 418 suprasegmental neurones, 499, 502, 504, 507, 515, 521, 526, 530, 536 Egg cords, Pfluger's, 354 technic of, 366 j nests, 354 INDEX 603 Ehrlich, granules of, 107 Ejaculatory ducts, 341 Elastic cartilage, 99 fibres, 86 tissue, 92 Verhoeff's differential stain for, 28 Weigert's stain for, 28 Elastin, 87, 95 Eleidin, 383 Ellipsoid of Krause, 557 Embedding, 11 celloidin, 11 parafBn, 13 Emboliform nucleus, 507 Embryonal tissue, 82 fat tissue, 88 Eminentia h\T30glossi, 481, 489 medialis, 481 Emulsion theory of protoplasmic struc- ture, 44 Enamel, 233 cells, 237, 239 chemical composition of, 233 cuticula dentis of, 233 membrane of, 240 development of, 238 fibres, 233 organ, 239 prisms, 233 lines of Rctzius of, 233 Endbrain (telencephalon), 417, 532 corpus striatum, 532 pallium, 533 neorjallium, 532 olfactory pallium, 532 rhinencephalon, 532 anterior perforated space, 532 gyrus hippocampi, 532 olfactory bulb, 532, 595 nerve, 532 pyriform lobe, 532 trigonum olfactorium, 532 tubcrculum olfactorium, 532 End-buttons, 509 of Aucrbach, 38 -feet of Aucrbach, 38 -bulbs, 433 of Krause, 227, 350, 394 Endgut, 263 large intestine, 263 mesentery, 267 omentum, 267 Endgut, peritoneum, 267 rectum, 266 vermiform appendix, 265 Endocardium, 161 primitive, 164 Endochondral ossification, 199 Endolymph, 574 Endolj'mphatic duct, 575 sac, 575 Endomysium, 208 Endoneurium, 133, 425 Endoplasm, 46 Endosteum, 195 Endothelial tube, 164 Endothelium, 70 of Descemet, 550 Engelmann, showing ciliated epithelial cell, 76 Entoderm, 62 tissue derivations from, 67, 296, 315 Eosin, 19 -glycerin, 22 -haematoxylin stain, 20 Eosinophile granules, 106, 174 ■ Epiblast, 62 Epicardium, 161 Epicranium, 198 Epidermis (or cuticle), 382 eleidin, 383 keratin, 383 keratohyaline granules, 383 mitosis of cells of, 383 pareleidin, 384 pigmentation of, 384 prickle cells of, 383 stratum corneum of, 383 oj'lindricum cf, 383 germinativum of, 382 granulosum of, 383 lucidum of, 383 Malpighii of, 382 mucosum of, 382 spinosum of, 383 Epididymis, 339 cells of, 339 vas deferens, 340 vas epididymis of, 3,^0 vasa cffcrcntia of, 339 Epidural space, 422 Eiiiglottis, 301 Epimysium, 208 Epineurium, 425 604 INDEX Epiphyseal cartilage, 203 Epiphysis of bone, 203 Epithalamus, 522, 530 Epithelium, 70 basal membrane of, 70 ceUs of, 70 ciliated, 75 classification of, 71 cuboidal, 72 cuticular membrane of, 71 endothelium, 77 follicular, 355 general characteristics of, 70 germinal, 354, 378 glandular, 77, 212 histogenesis of, 70 intercellular bridges of, 70 lens, 564 membrana propria of, 70 mesothelium, 77 neuro, 77 pigmented, 76 pseudo-stratified, 73 replacing cells of, 73 respiratory, 311 simple, 71 columnar, 71 pseudo-stratified, 73 squamous, 71 stratified, 73 columnar, 74 squamous, 73 transitional, 74 surface, of mucous membranes, 218 syncytium, 373 tactile cells of, 431 technic of, 78 transitional, 74 Eponychium, 387 Epoophoron, 363 Erectile tissue, 349, 376 Ergastoplasm, 213, 272 Erythroblasts, 192 Erythrocytes, 103 Erythrosin, 20 Eustachian tube, 574 Excretory ducts, 214, 276, 282, 329, 364 substances in ceUs, 46, 213 Exoplasm, 46, 82 External arcuate fibres, 490, 499 ear, 572; see Ear, external geniculate bodies, 493, 520 External os, 368 spiral sulcus, 579 Extero-ceptors, 436 Extracellular network, 139 Eye, the, 548; see Organ oj vision blood-vessels of, 565 development of, 569 eyeball or bulbous oculi, 548 eyelid, 567 lacrymal apparatus, 567 lens, 564 lymphatics, 566 nerves of, 566 neurone systems of, 560 optic nerve, 559 technic of, 570 Eyeball (or bulbus oculi), 548 blood-vessels of, 565 chorioid of, 550 ciliary body of, 552 cornea of, 548 development of, 569 iris of, 554 lens, 564 lymphatics of, 566 nerves of, 555, 559, 565 retina of, 555 sclera of, 548 technic of, 570 Eyelashes, 567 Eyelid, the, 567 blood-vessels of, 569 conjunctiva of, 568 epidermis of, 567 glands of, 568 of Mall, 568 lymphatics of, 569 Meibomian glands of, 568 muscles of, 568 nerves of, 569 tarsus of, 568 technic of, 569 Facialis (nerve VII), 474, 47S, 544 Fallopian tube, 364; see Oviduct ampulla of, 364 blood-vessels of, 365 coats of, 364 development of, 376 fimbriated extremity of, 364 isthmus of, 364 lymphatics of, 365 INDEX 605 Fallopian tube, nerves of, 365 ovarian extremity, 364 technic of, 366 False corpora lutea, 360 Fascicles of muscle, 207 of nerves, 421, 425 Fasciculus arcuatus, 535, 536 of Thomas, 467 inferior longitudinal, 535 medial longitudinal, 467, 485, 491, 500, 504, 507, 515, 518 perpendicular of Wernicke, 535 predorsal, 504 retroflexus of Meynert, 53 1 solitarius, 487, 489 superior longitudinal, 535 uncinate, 535, 536 Fastigio-bulbar tract, 500, 502 Fat, absorption of, 273 technic of, 275 blood supply of, 91 development of, 90 technic of, 91 globules, 237 osmic-acid stain for, 31 subcutaneous, 382 tissue, 87 histogenesis of, 87 technic of, 97 Fat-droplets in cells, 46, 90 Fat-lobules, 88 Fauces, mucous membrane of, 220 Feltwork of fibres, 440 Female genital organs, 352 pronucleus, 59 Fenestra ovalis, 574 rotunda, 574 Fenestrated membrane, 94 Ferrein, pyramids of, 320 Fertilization of the ovum, 58, 358 Fibrae propriae of Meynert, 535, 541 Fibre baskets, 559 systems, 421 eflcrent, 424 main motor, 424 short, 445, 468 proprio-spinal, 468 spino-spinal, 468 tracts of cord, 459 methods of determining, 459 (ascending), 461 Fibre tracts of cord, ascending, direct cerebellar, 463 Gowers', 464 long ascending, 445 arms of dorsal root fibres, 461 of spinal cord, 459 posterior columns, 443, 448 spino-collicular, 462 spino-tectal, 465 spino-thalamic, 462, 483, 501, 504, 515, 526 tract of Flechsig, 463 tractus spino-cerebellaris dor- salis, 463, 483, 487, 490, 507 ventralis,463, 483, 487, 501,515 uncrossed cerebellar, 463, 487, 49o> 507 (descending), 464 anterior marginal bundle of Lowenthal, 466 anterior pyramids, 464, 483,487, 504, 507 antero-lateral, 466 cerebro-spinalis, 464 comma tract of Schultze, 467 crossed pyramidal, 464 descending tract from Deiter's nucleus, 466 direct pyramidal, 465 fasciculus of Thomas, 467 from the interstitial nucleus of Cajal, 465 fundamental, 445, 468 Helweg's, 467 marginal bundles of Lowenthal, 466 origin of tracts, 443 oval bundle of Flechsig, 463 pallio-spinalis, 464, 533 pyramidal, 464, 533 rubro-spinal, 465, 466, 483, 487, 490, 504, 515 septo-marginal, 466, 467 short, 445, 462, 468 tecto-spinal tract, 465 tractus cortico-spinalis, 464 vestibulo-spinal, 466 Von Monakow's tract, 466 Fibres, afferent nerve, 419; see also Nerve fibres arcuate, 487, 489 association of pallium, 421, 533, 535 606 INDEX Fibres, calcified, 197 cartilage, 99 commissural, 533 cone, 557 connective- tissue, 82 development of, 82 cortical, 388 dentinal, 228 efferent root, 418 enamel, 233 external arcuate, 490, 499 genioglossal, 224 heart muscle, 119 hyoglossal, 224 intergeminal, 587 internal arcuate, 487, 500, 502, 522 interzonal, 56 intrageniinal, 587 involuntary striated (heart) muscle, 119 lens, 510, 564 Mallory's method of staining con- nective-tissue, 29, 30 mantle, 54 Muller's, 558 nerve, 131; see also Nerve fibres meduUated, 133 non-medullated, 132 neuroglia, 143 of areolar tissue, 87 of bone, loi of developing muscle, 124 of formed connective tissue, 92 of Remak, 133 of Sharpey, 192, 234 olfactory, layer of, 584 perforating or arcuate, of cornea, 550 projection, 533, 535, 539, 543 radiate, 292 reticulo- spinal, 467 rod, 557 styloglossal, 224 superficial arcuate, 487, 490, 499 tendon, 91 tunnel, 583 voluntary muscle, 114, 117 Weigert's method for staining elastic, 28 method for staining nerve, 31 white or fibrillated, 86 yellow or elastic, 86 Fibrillar connective tissue, 86 Fibrillar theory of protoplasmic structure, 44 Fibroblasts, 82 Fibrous cartilage, 99 Field of Forel, 530 Fila olfactoria, 475 Filar mass, 45 Filiform papilla, 225 Fillet (or medial lemniscus), 462, 485, 487, 491, 493, 504 Filum terminale, 442 Fimbria, 533 Fissure, anterior median, 448 chorioid, 570 Fixation, 5 by injection, 6 in toto, 6 Fixatives, 6, 7, 8 Fixed cells, 551 Flechsig, oval bundle of, 467 myelogenetic method of, 459, 541 tract of, 463 Flemming concerning cell-division, 53 Flemming's fluid, 7 Foam theory of protoplasm structure, 44 Foetal cells, 312, 316 structures, appendix epididymidis, 342 of genital system, 342, 363 testis, 342 ductus, aberrans Halleri, 342 organ of Giraldes, 342 paradidymis, 342 Foliate papillae, 587 Follicle, Graafian, 354; see also Graafian follicle Follicles, agminated, 260 solitary, 253 Follicular cavity or antrum, 355 FoUiculi linguales, 180; see Tonsils Fontana, spaces of, 554 Foramen caecum lingui, 180 Forebrain (prosencephalon) , 417, 522 diencephalon {thalamencephalon), 4.17, 522 epithalamus, 523 hypothalamus, 523 thalamus, 523 interbrain, 522 section through junction of mid- brain and thalamus, 524 INDEX 607 Foregut, the, 243 general structure of walls of the gas- tro-intestinal canal, 245 oesophagus, 243 stomach, 247 Forel, decussation of, 521 field of, 530 Formaldehyde, as a fixative, 6 for macerating, 4 -bichromate method, 36 Formalin, commercial, 4 Formahn-Miiller's fluid (Orth's), 7 Formed connective tissue, qi Fornix, 531 anterior pillars of, 536 commissure, 533 Fossa navicularis, 352 Fountain-like decussation of Mej'nert, 522 Fourth ventricle, 481, 487, 493 Fovea centralis, 559 Fraenkel's theorj- of corpus luteum, 362 Free endings of sympathetic ner\ es, 441 Frozen sections, 15 Fuchsin, 19 Function of cells, 51 Fundamental columns of spinal cord, 445 Fundus, 248 glands, 249 Fungiform papillae, 225 Funiculus cuneatus, 462, 485 gracilis, 462, 485 posterior, 461 Fusiform lobules, 535 Gage, showing muscle fibres, 119 Gage's hiematoxylin, 17 Galea capitis, 343 Gall-bladder, 295 Galvanotaxis, 51 Ganglia, 418, 419 amphicytes, 427 cerebral, 426 cerebro-spinal, 418, 426 chain, 436 ciliary, 436 Gasserian, 506 habenularis, 530 of Corti, 583 of Scari^a of VIII., 475, 493 otic, 436 peripheral, 436 satellite cells, 427 Gangha, sphenopalatine, 436 spinal, 426 spiral, 583 spirale of VIII., 475, 493 structure of, 426 sympathetic, 436 submaxillary, 436 technic for, 441 terminal, 436 vertebral, 436 Ganglion cells, 419 capsule of, 419 cardiac, 162 development of, 418 Gartner's canal, 377 duct, 363 Gasserian ganglion, 506 Gastric crypts, 248 glands, 248, 249 pits, 248 Gastro-hepatic omentum, 268 Gastro-intestinal canal, general structure of the walls of, 245 Gelatin, 86 carmine for injecting, 25 Prussian blue, for injecting, 25 Gelatinous marrow, 195 substance of Rolando, 449 Gemmules, 509 Geniculate body, 493, 520, 524 gangUon of, VII., 474 Genio-glossal fibres, 224 Genital gland, 378 organs, female, 352 male, 333 ridge, 378 system, 333; see also Reproductive system development of, 376 rudimentary structures connected with development of, 342, 363 Genitourinary system; see Urinary sys- tem, 518, and Reproductive system, 333 Gennari, line of, 543 Gentian violet, 19 Genu-facialis, 499 Germ hill, 356 layers, 62 tissues derived from, 07 primitive, 62 Germinal epithelium, 354, 378 608 INDEX Germinal spot, 357 vesicle, 59, 357 Giant cells of Betz, 533, 536 Gianuzzi, crescents of, 222, 278, 302 Giraldes, organ of, 342, 377 Gland cells, 212, 213 Glands, 212 accessory thyreoid, 403 acini of, 217 adrenal, 412 alveolar saccular, 214 compound, 217 simple, 215, 217 alveoli of, 217 Bartholin's, 376 Bowman's, 300 Brunner's, 252, 272 cardiac, 252 carotid, 410 cells of, 212, 213 ceruminous, 572 classification of, 212 coccygeal, 411 compound, 214 corpus luteum, 359 Cowper's, 348 dehiscent, 217 development of, 215 duct, 215 ductless, 214, 217 Ebner's, 226 epithelium of, 213 excretory ducts of, 214 fundus, 249 gall-bladder, 295 gastric, 249 general structure of, 212 genital, 378 giant, 192 hsemolymph, 173 internal secreting, 215, 217 interstitial tissue of, 215 intraepithelial, 340 kidney, 318 lacrymal, 567 large, of digestive system, 275 Lieberkiihn's, 252, 260 lingual, 222 Littre's, 351, 352 liver, 287 lobes of, 214 lobules, 214 Glands, lymph, 167 Mall's, 568 mammary, 395 Meibomian, 217, 568 mixed, 221 mucous, 221 membranes of, 218 of internal secretion, 215, 217 of the oral mucosa, 220 ovary, 352 pancreas, 281 parathyreoids, 404 parenchyma of, 215, 277 parotid, 277 peptic, 249 pineal, 544 prehj^oid, 403 prostate, 347 pyloric, 249 racemose, 21,5 reticular, 217 saccular, 217 compound, 217 simple, 215 salivary, 276 sebaceous, 351, 384, 391, 568, 572 secreting portions of, 213 serous, 221 simple, 214 spleen, 181 sublingual, 277 submaxillary, 278 sudoriferous, 214 suprahyoid, 403 sweat, 384, 567 tarsal, 568 thymus, 175 thyreoid, 402 accessory, 403 tonsils, 1 78 tubular, 214, 215 compound, 216 simple branched, 216 simple coiled, 216 simple straight, 215 tubulo-alveolar, 214 tympanic, 409 Tyson's, 351 uterine, 367 Glandulse sudoriparse, 384 vestibulares majores, 376 mine res, 376 INDEX 609 Glandular epithelium, 77, 212 Glans penis, 350 Glenoid ligaments, 205 Glisson, capsule of, 2S7 Gliosis, 144 Globus major, 334 minor, 334 pallidus, 535 Glomerulus cf kidney, 320 blood-vessels of, 325 olfactory, 585 Glosso-pharyngeal (IX. nerve), 474, 491, 496 Glycerin for mounting specimens, 22 jelly, 22 Glycogen granules, 290 Goblet cells, 75, 212, 257, 272 Gold chlorid for staining connective tissue cells, 28 Gold- size for glycerin mounts, 22 Golgi cell, Tj-pe I., 130, 132 cell, Type 11., 131, 132, 446, 536 method, bichlorid, 36 chrome -silver, 29 Cox modification, 36 formalin bichromate, 36 mixed, 35 rapid, 35 silver, for nerve tissue, 35 slow, for nerve tissue, 35 muscle-tendon organs of, 434 net, 139 organs of, 434 Golgi-Mazzoni corpuscles, 394, 434 Goll, column of, 459, 462 nucleus of, 462, 481, 487 Gowers' tract, 464 Graafian follicles, 354 antrum of, 355 corona radiata, 356 cumulus ovigerus, 356 development of, 356, 376 discus proligerus, 356 egg nest, 354 epithelium of, 354 follicular cavity of, 355 germ hill of, 356 liquor folliculi, 355 nerves oT, 363 ovum of, 355 Pfliiger's egg cords, 354 primitive, 355 39 Graafian follicles, primitive ova, 354 rupture of, 358 stratum granulosum, 355 technic cf, 365 theca folliculi, 356 tunica fibrosa, 356 vasculosa, 356 Graded alcohols, 7 Grandry, corpuscles of, 431 Granule theory of protoplasmic struc- ture, 40 Gray matter, 420, 449 rami communicantes, 437 reticular formation, 476, 4S5 Greater omentum, 235 Ground bundles of spinal cord, 445, 468 Griibler's methylene blue, 31, 38 water-soluble eosin, 31 Gums, mucous membrane of, 220 Gustatory canal, 587 Gyrus dentatus, 532 hippocampi, 532 of Heschl, 541 Habenul^, 530 Hffimalum, Mayer's, 18 Hasmatein, 17, 104 Haematoidin, crystals of, 360 Haematokonia, 108 Haimatoxylin, 17 and eosin, for staining double, 20 and picro-acid fuchsin, 21 Delafield's, 17 Gage's, 17 Heidenhain's, 18 Mallory's stain, 29 Weigert's, 19, 32 Ha;moglobin, 104 Hajmolymph nodes, 173 blood sinuses of, 173 blood-vessels of, 175 cells of, 174 eosinophiles, 174 mast cells, 174 phagocytes, 174 development of, 175 function of, 175 hilum of, 173 marrow-lymi)h, 174 relation of, to lymphatic system, 175 spleno-Iymph, 174 technic of, 175 610 INDEX Haemolysis, 104 Hair, 387 arrector pili muscle of the, 391 blood-vessels of, 394 bulb, 387 -cells, 575, 581 cells of the, 389 connective-tissue follicle of, 389 cortex of, 388 cortical fibres of, 388 cuticle of, 388 development of the, 358, 395 excretory duct of, 391 eyelashes, 567 follicle, 387, 388 germ, 392 growth of the, 392 hyaline membrane, 389 inner root sheath, 388 cuticle of, 389 Henle's layer of, 389 Huxley's layer of, 389 lanugo, the, 388 lymphatics, 394 medulla of, 387 nerves of, 394, 430, 431, 440 outer root sheath, 388 papilla of, 387 prickle cells, 389 root of the, 387 root sheath, 388 sebaceous glands of the, 391 sebum of the, 392 shaft of, 387 shedding of the, 392 stratum cylindricum, 389 technic of the, 393 vitreous membrane, 389 Halleri, ductus aberrans, 342 Haller's layer, 551 Hamulus, 577 Hardening, 8 Hassal's corpuscles, 177 Haversian canals, 189 development of, 202 fringes, 205 lamellae, 190 spaces, 202 systems, 191 development of, 202 Hayem's fluid, 63 Head, sympathetic ganglia of, 436 Headgut, 220 mouth, 220 pharynx, 242 teeth, 227 tongue, 223 Hearing, organ of, 572; see Ear Heart, 161 annuli fibrosi, 162 auricular muscle, 161 auriculo-ventricular ring, 161 blood-vessels of, 162 coronary arteries of, 161 development of, 164 endocardium of, 161 epicardium of, 161 lymphatics of, 162 muscles of, 161 myocardium of, 140 nerves of, 142, 440 technic of, 142 valves of, 141 -muscle, 119, 161; see also Involuntary striated muscle Hecateromeres, 445 Heidenhain, concerning voluntary striated muscle, 117 demilunes of, 222 Heidenhain's haematoxyUn, 18 Heisterian valve, 295 HeHcotrema, 578 Heller's plexus, 268 Helweg, tract of, 467 Hemispheres of cerebellum, 500, 507 Hendrickson, concerning coats of liver ducts, 295 Henle, concerning ovum, ^Ss Henle's layer, 389 loop, 320, 322, 333 sheath, 136, 426 Hensen's cells, 581 line, 115 Hepatic artery, 289 cells, 291 cords, 291 cyUnders, 297 duct, 289 Hermann, showing centrosome, 49 Heschl, transverse temporal gyri of, 541 Heteromeres, 445 Hilum of liver, 287 of kidney, 318 Hindbrain, 417, 479 INDEX 611 Hindbrain {rhombencephalon), bulb, 479 cerebellum, 418, 499 medulla oblongata, 418, 479 section of, through, at level of junc- tion of pons and cerebellum, and entrance VIII, vestibubular, 499 through roots of VI, abducens, and VII facial nerves, 500 through roots of V, trigeminus nerve, 502 His, marginal veil of, 417 mj^elospongium of, 417 spongioblasts of, 417 Histogenesis, 67 Holmgren, showing trophospongium, 46 Horizontal cells, 538, 558 Howship's lacunae, 199 Huxley's layer, 389 Hyaline cartilage, 98 Hyaloid canal, 565 membrane, 565 Hyaloplasm, 44, 124 Hydatid of IMorgagni, 342 stalked, 342 Hydrochloric acid for decalcif3dng, 10 Hyoglossal fibres, 224 Hypoblast, 62 Hypoglossal (XII nerve), 481, 485, 487, 497 Hyponychium, 387 Hypophysis cerebri, 407; see aLo Pituilary body Hypothalamus, 522, 533 Incisures of Schmidt-Latncrmann, 135 Incremental lines of Schregtr, 232 Indirect cell division, 53; see Mitosis Inferior brachium quadrigeminum, 520 cerebellar peduncle; see Restijorm body colliculi, 417 Infundibula, 310 Injection, 25 apparatus, 25 double, 26 separate organs, 26 whole animals. 26 Innervation of muscles, 434, 441 Inokomma, 117 Interalveolar connective tissue, 313 Intcrarticular cartilages, 205 Interbrain, 416, 522 Interbrain, epithalamus, 523 hypothalamus, 523 thalamus, 523 Intercallated discs, 121 Intercellular bridges of epithelium, 70, 73 bridges of muscle tissue, 112 substance, 68 of connective tissue, 86 silver-nitrate method of staining, 28 Interfilar mass, 45 Intermediate cartilage, 199 lamellje, 191 neurones, 418 Internal arcuate fibres, 487, 500, 502, 522 capsule, 530 ear, 574; see also Ear, internal nuclei of cerebellum, 502, 507 Internode, 134 Intero-ceptors, 436 Interradiary plexus, 541 Intersegmental neurones, 421, 483, 487, 490, 499, SOI, 504, 507, 515, 520, 524, 530, 536 tracts, 468 Interstitial lamellae, 191 nucleus of Cajal, 465, 485, 491, 500, 504 Intestine; see Small intestine, 255; Large intestine, 263 Intestines, development of, 296 Intima, 154 of arteries, 154 of lymph vessels, 165 of veins, 158 Intracartilaginous ossification, 199 Intracellular canals, 46 Intrafascicular connective tissue, 208 Intramembranous ossification, 197 Intranuclear network of typical cell, 48 Inverse fernient, 273 Invertin, 273 Involuntary striated muscle (heart), 119 Cohnheim's field, 120 development of, 124 intercallated discs of, 121 McCallum's views, 120 membrane of Krause, 120 muscle columns of Kdlliker, 120 nerves of, 440 sarcoplasm of, 120 tcchnic of, 125 612 INDEX Involuntary smooth muscle, iii intercellular bridges of, 112 nerves of, 440 Iodine, to remove mercury, g Iris, the, 554 greater arterial circle, 566 layers of the, 554 lesser arterial circle, 566 muscles of the, 555 pigmentation of, 554 Irriiability of cells, 51 Islands, blood, 108, 163 of Langerhans, 285 Isolated smooth muscle cells, 113 technic of, 124 Isotropic line, 115 substance, 115 Isthmus, 417, 515 section through, at exit of IV, trochlearis, nerve, 515 Iter, 417, 464, 515, 517 Jenner's blood stain, 31 Joint capsule, 205 stratum fiibrosum, 205 synoviale, 205 synovial membrane, 205 Joints, 204; see Articulations Jugular ganglion of X, 474 Juxta-restiform body, 501 Karyolysis, 383 Karyoplasm, 46 Karyosomes, 48 Katabolism, 50 Keratin, 383 Keratohyaline granules, 383, 387 Eadney, the, 318 arteriae arciformes, 326 rectag, 328 blood-vessels of, 325 Bowman's capsule of, 320 capillaries of, 328 columns of Bertini, 320 convoluted tubules of, 320 cortex of, 318 cortical pyramids of, 320 development of, 321, 376 duct of BelUni, 321 epithelium of, 324 glomerulus of, 320 Henle's loop, 322, 323 Kidney, interlobar arteries of, 326 hilum, of, 318 labyrinths of, 320 lobulated, 318 location of tubules in, 324 lymphatics cf, 328 main excretory duct of, 329 Malpighian body, 320 pyramid, 320 medulla of, 318 medullary (or Malpighian) pyra- mid, 320 rays, 320 nerves of, 328 papillae of, 321 pelvis of, 319 pyramids of Ferrein, 320 renal artery. 318 corpuscle, 320 vein, 318 renculi or lobes cf, 318 septa renis, 320 single lobe of, 318 stellate veins of Verheyn, 328 technic of, 331 ureter, 318, 329 uriniferous tubule, 320; see also Uri- nijerous tubtde Kidney-pelvis, 329 calyces of, 329 development of, 376 technic of, 331 Kolliker, muscle columns of, 116 concerning bronchi, 304 showing Golgi ceU type II, 131 spleen cells, 184 Krause, ellipsoid, 557 end-bulbs, 227, 350, 394, 567 line of, 115 membrane of, 117, 120 Kupffer, cells of, 292 Labia minora, sebaceous glands of, 384 Labyrinth, membranous, 574 osseous, 574 Lacrymal apparatus, 567 canal, 567 gland, 567 blood-vessels of, 567 lymphatics of, 567 nerves of, 567 technic for, 570 INDEX 613 Lacrymal apparatus, nasal duct of, 567 sac, 567 Lacteals, 240 Lacuna;, ici origin of, 198 Laguesse, concerning development of pancreas, 297 concerning lung lobules, 310 Lamellae, circumferential, 191 ground, 191 Haversian. 19c intermediate, 191 interstitial, 191 of bone tissue, loi Lamina, bony spiral, 577 citrea, 551 cribrosa, 548, 560 fusca, 548 membranous spiral, 577 reticularis, 581 suprachorioidea, 551 Lamina; of cerebellum, 507 Langerhans, cell islands of, 285 centro-acinar cells of, 283 centro-tubular cells of, 283 Langley and Sewell ccncerning secretion, 272 Lanugo hairs, 387 Large intestine, 263 Auerbach's plexus, 264, 270 blood-vessels, 268 coats of, 263 development of, 296 gland tubules, 263 Heller's plexus of, 268 linea; coli, 264 lymphatics, 270 nerves, 270 plexus of Meissner, 263, 271 myentericus, 270 technic of, 275 Larynx, the, 301 blood-vessels of, 303 cartilages of, 301 arytenoid, 301 cricoid, 301 epiglottis, 301 Santorini's, 301 thyroid, 301 Wrisburg's, 301 cells of, 301 development of, 315 Larynx, epithelium of, 301 lymphatics, 304 nerves, 304 perichondrium, 301 technic, 304 vocal cords, 301 Law of Wallerian degeneration, 141 Layer of Weil, 228 Lecithin, 413 Lemniscus, bulbo-thalamic, 524 lateral, 491, 493, 499> 5oi. 504, 5^5 medial, 462, 485, 487, 491, 493, 504 Lenhossek, concerning ciliated epithelium, Lens, 565 epithelium, 565 fibres, 565 hyaloid membrane, 565 invagination, 570 suspensory ligament. 565 vesicle, 571 zonula ciliaris, 565 zonule, of Zinn, 565 Lenticular capsule, 565 nucleus, 532, 535 Leopold, concerning pregnant uterus, 370 Leucocytes, 105 acidophile, 106 basophile, 106 granular, 106 lymphocytes, 105 migratory, 259 mononuclear, 105 neutrophile, 106 of milk, 400 polymorphonuclear, 105 poly nuclear, 105 transitional, 105 Lewis, concerning shape of blocd-ccUs, 103 Lieberkiihn, crypts of, 252, 260 glands of, 252 Ligament, glenoid, 205 spiral, 577 structure of, 91 suspensory, 564 Ligamcntum nucha-, 93 ptctinatum, 554 Linca; coli, 264 Line of Gennari, 543 of Haillargcr, 54 Lines of Rclzius, 233 Lingual glands, 222 tonsils, 180 614 INDEX Lingualis, genio-glossus fibres of, 224 h^'poglossus fibres of, 224 longitudinal fibres of, 224 styloglossus fibres of, 224 transverse fibres of, 224 Linin, 48 Lipoid granules, 412 Liquor ferri sesquichlorati, 28 folliculi, 355 Lissauer, zone of, 443, 450 Littre, glands of, 351, 352 Liver, the, 287 bile duct of, 290 blood supply of, 288 capillary network, 289 capsule of Glisson, 287 cells of, 290 of Kupffer, 292 central vein of, 289 compared with other compound tubular glands, 293 connective tissue, 287 cords of liver cells, 291 development of, 297 ducts, 289 common, 294 cystic, 294 hepatic, 289, 294 glycogen granules, 290 Heisterian valve, 295 hepatic artery, 289 cords, 291 duct, 289 hilum, 287 intralobular secreting tubules, 290 lobes of, 287 lobules, 288 lymphatics, 294 main ducts, 289, 294 nerves, 294 portal canal, 290 vein, 289 radiate fibres, 292 reticulum, 292 septa, 287 sublobular vein, 289 technic of, 297 tubules of, 293 Lobulated kidney, 318 Lowenthal, anterior marginal bundle of, 466 Longitudinal cleavage, 55 Longitudinal fasciculus, 491 500, 504, 515, 526 Loops of Henle, 320, 322, 323 Loose (areolar) connective tissue, 87 Lugol's solution, 28 Lumbar enlargement of spinal cord, 443 segments of cord, 443 Lungs, the, 308 air cells, 310 sacs, 310 vesicles, 310 alveolar bronchi, 309 ducts, 310 passage, 311 sacs, 309 alveoli of, 311 blood-vessels of, 313 bronchial artery, 313 system, 313 capsule of. 308 cells of, 311 development of, 315 epithelium of, 310 foetal cells of, 311 infundibula of, 310 inter alveolar connective tissue of, 313 lobes of, 308 lobules of, 308 lymphatics of, 315 nerves of, 315 parietal pleura, 308 pulmonary artery, 308 lobvde,-3o8 pleura of, 308 respiratory bronchi, 310 cells, 311 epithelium, 311 septa of, 308 technic of, 317 terminal bronchi of, 310 Lunula, 387 Lutein cells, 359 granules, 359 Luteum, corpus, 359 Luys, nucleus of, 523 Lymph, capillaries, 165, 270 glands, 167; see Lymph nodes nodes, 167 blood-vessels of, 17c capsule of, 167 chains of, 167 connective tissue of, 168 INDEX 615 Lymph nodes, cords of, 169 cortex of, 169 development of, 171 germinal centre of, 169 Ij-mphatics of, 171 medulla of, 169 nen-es of, 171 nodules of, 169 reticular connective tissue of, 170 sinuses of, 169 technic of, 172 nodule, 95, 169, 242, 266 germinal centre of, 169 paths of the eye, 566 spaces, 165 pericellular, 165 perivascular, 159 vessel system, 164 capillaries of, 165 development of, 166 lymph capillaries, 165 spaces, 165 relation of, to ha;molymph node, 17s stomata of, 165 technic of, 165 vessels, coats of, 165 structure of, 164 Lymphatic organs, 167 development of, 171, i7S> i77, 180, 186 haemolymph nodes, 1 73 lymph nodes, 95, 167 spleen, 181 technic of. 172, 175, 178, rSi, 187 thymus, 175 tonsils, 178 tissue, 167, i68 Lymphocytes, 105 Lymphoid cells, 170 tissue, 170 Macerating fluids, 4 Maceration, 4 Macrocytcs, 103 Macro-nucleus, 57 Macula acustica, 57S lutca, 557 fovea centralis, 559 Male genital organs, 333 pronucleus, 54 Mall, glands of, 568 Mall, concerning development of fibrillar connective tissue, 82 Mallory's aniline blue stain for connective tissue, 30 phosphomolybdic acid haematoxyUn stain for connective tissue, 29 phosphotungstic acid haematoxylin stain for connective tissue, 30 Malpighian bodies, 182 body of kidney, 320 development of, 321, 376 pyramid, 320; see Kidney Mamillo-thalaraic tract, 524 Mammary gland, 395 active, 398 alveoli of active, 398 ampulla of, 397 blood-vessels of, 400 cells of, 398 colostrum corpuscles, 400 development of, 401 ducts of, 397 of nipple, 397 inactive, 397 interlobar septa of, 397 interlobular septa of, 397 lobular ducts of, 397 lymphatics of, 400 milk, 399 nerves of, 400 secretion of, 399 structure of, 395 technic of, 401 Mantle fibres, 54 Marchi's method for staining degen- erating nerves, 34 Busch's modification of, 35 Maresh's modification of Bielschowsky's stain for connective tissue, 31 Marginal bundle of Lowcnlluil, 466 veil of His, 417 Marrow, 192; see Bone marrow lymph nodes, 1 74 Martinotti, cells of, 536, 539 Mast cells, 85, 174, 192. 193 Matrix of nail, 385 Maturation, 58 of ovum, 58, 358 of spermatozoon, 58 Mayer's hiL-maium, 18 McCallum, concerning heart muscle, 120 Media of arteries, 155 616 INDEX Media of lymph vessels, 165 of veins, 158 Medial eminence, 481, 489 fillet, 462, 485, 487 Median center of Luys, 524, 530 lemniscus, 462, 485, 487, 526 raphe, 488, 490, 507 septum, posterior, 448 Mediastinum testis, 333 Medulla oblongata, 417, 479 accessory olivary nucleus, 490 olives, 490 afferent cerebellar neurones, 474, 477, 490, 493, SOI, 504, 507 roots, 483, 485, 489 secondary tracts of, 483, 485, 489, 491 terminal nuclei of, 483, 485, 489, 491 ^la cinerea, 481, 489 anterior fissure, 481 ground bundles, 483, 487, 489, 499 pyramid, 481, 499 arciform (arcuate) mucleus, 490 arcuate fibres, 487, 489 area acustica, 482 auditory nerve, 493 central canal, 481 gelatinous substance, 487 gray matter, 483, 486, 489 tegmental tract, 490, 499 cerebellar peduncles, 499 cerebello-olivary fibres, 490 chorioid plexus, 487 clava, 481 cochlear nerve, 475, 489, 491, 493 nuclei, 493 column of Burdach, 459, 462, 481, 487 of Goll, 459, 462, 481, 487 compared with spinal cord, 481 corpus restiforme, 481, 485, 489, 490, 499, 501, S04 crossed pyramidal tract, 464, 483 cranial nerves of, 479, 480, 495 cuneus, 481 decussation of fillet, 485 of pyramids, 481 Deiter's nucleus, 493, 499 tract, 487 descending root of fifth nerve, 483, 487 Medulla oblongata, descending or spinal root of vestibular portion of • eighth nerve, 493 suprasegmental paths, 485 tract from Deiter's nucleus, 483, 491, 499 from the vestibular nuclei, 490 development of, 416 direct cerebellar tract, 483 pyramidal tract, 483 funiculus, 483 horns, dorsal, 481 nucleus of ninth cranial nerve, 491 of tenth nerve, 481, 485, 489 dorsal external arcuate fibres, 490 spino-cerebellar tract, 483, 487, 490 efferent peripheral neurones, 483, 485, 487 suprasegmental neurones, 485, 489, 499 eminentia h3^oglossi, 489 external arcuate fibres, 487, 490, 499 fasciculus cuneatus, 483 gracilis, 483 solitarius, 489, 491 fillet or medial lemniscus, 462, 485, 487 formatio reticularis, 485, 489, 491 fourth ventricle, 481, 487, 493 funiculus cuneatus, 485 gracilis, 485 ' • gelatinous substance of Rolando, 483 general structure of, 479 genu facialis, 481 Gowers' tract, 483 gray reticular formation, 485, 487 internal arcuate fibres of, 487, 500, 502, 522 intersegmental neurones, 483, 487, 489, 499 lateral fillet, 491, 493 lemniscus, 491, 493 longitudinal fasciculus, 491, 500 median lemniscus, 462, 485, 487, 489, 491, 493 longitudinal fasciculus, 491, 499 raphe, 488, 490 nuclei arcuati, 490 of the floor of the ventricle, 481 laterales, 490 INDEX 617 Medulla oblongata, nuclei of posterior columns, 481, 495 nucleus, abducentis, 482 accessory cuneate, 487 alae cinereae, 485 ambiguus, 489 arcuatus, 490 commissuralis, 487 cuneatus, 481, 485, 487 gracilis, 481, 487 hjT)oglossi, 481, 485 of acoustic nerve, 482, 489, 493 of the column of Burdach, 462, 481, 487 of the column of Goll, 481, 487 of the fifth spinal nerve, 483, 485, 489 of origin of eleventh cranial {spinal- accessory) nerve, 483 of origin of twelfth cranial (hypo- glossal) nerve, 481, 485, 487 of vagus nerve, 482, 489 olives, 489 olivarj' nucleus, 487, 490, 491 olivo-rerebellar fibres, 49c, 493 pallio-spinal tract, 485 peduncles of, 499 peripheral neurones of, 479 plexus chorioideus, 487 pons Varolii, 482, 499 posterior columns of, 487 longitudinal fasciculus, 491 septum, 481 predorsal tract, 491 pyramidal decussation, 485, 487 tracts, 485, 487 raphe, 488, 490 restiform body, 481, 485, 489, 490 reticular formation, 485, 487, 489, 490, 491 root fibres of spinal V., 483 and nucleus of origin of sixth (abduccns) cranial nerve, 499 of seventh (Jacial) cranial nerve, 491 and nuclei of eighth (auditory) cranial nerve, 493 of ninth iglosso-pharyngeal) and tenth (vagus) cranial nerves, 491 of eleventh (spinal-accessory) cran- ial nerve, 483 Medulla oblongata, root of twelfth (hypo- glossal) cranial nerve, 485 rubro-spinal tract, 483, 487, 490 secondary cochlear tract, 493 vestibular tract, 493 sensory tract of fifth nerve, 483, 487 section through decussation of fillet, 48s entrance of cochlear branch of eighth, 491 lower part of inferior olivary nucleus, 487 middle of oli\-ary nucleus, 491 pyramidal decussation, 483 sensory decussation, 485 roots and motor nuclei of the fifth nerve, 499, 500, 502, 506 solitary fasciculus, 489, 491 spinal (descending) root of fifth cranial nerve, 483, 487 V, 475, 483, 4S5, 489, 493, SOI, 504, 506 spino-cercbellar tract, 483, 487, 489 -tectal tract, 483, 490 -thalamic tract, 489 stria; medullares, 491 technic of, 482 tecto-spinal tract, 483, 487 tegmentum, 499 terminal nucleus of the descending (sensory) root fibres of the fifth nerve, 483 tract of Gowers, 483 from interstitial nucleus of Cajal, 465 of Helwcg, 466, 467 of Lciwenthal, 466 tractus spinalis trigcmini, 483 trapezius, 491 trigonum hypoglossi, 481, 489 vagi, 481 tubcrculum cinereum, 481 ventral external arcuate fibres, 490 horn, 485, 487 si)ino-ccrelK;llar tract, 483 vestibular nerve, 475 vestil)ul()-spinal tract, 483 Medullary lamina, 530 pyramid (Maljjighian), 320 618 INDEX Medullary rays, 320 sheath, 134 MeduUated axones, 133 nerve fibres, Weigerts' stain for, 32 Megalocytes, 184 Meibomian glands, 217, 568 Meissner, corpuscles of, 350, 394, 432 plexus of, 254, 262, 263, 271, 436 Melanin, 131 Membrana chorii, 371 elastica externa, 156 interna, 154 Umitans olfactoria, 300 preformativa, 241 propria, 70 tectoria, 582 Membrane, basal, 70 cuticular, 71, 240 mucous, 218 of Bowman, 548 of Descemet, 550 of Krause, 117, 120 of Reissner, 579 peridental, 234 serous, 165 synovial, 205 Membranes of brain and cord, 422 Membranous cochlea, 578 labyrinth, 574 spiral lamina, S77 hgament, 577 Meninges, 422 Meniscus, tactile, 431 Menopause, 401 Menstrualis, decidua, 369 Menstruating uterus, 368 Menstruation, 369 Mercuric chlorid as a fixative, 8 Merkel's corpuscles, 431 Mesencephalic root of fifth {trigeminus) cranial nerve, 475 Mesencephalon, 417 Mesenchyme, 77 Mesentery, 268 Mesoappendix, 265 Mesobjast, 62 Mesoderm, 62 tissue derivations from, 68, 122, 296, 315,377 Mesonephros, 364 derivations from, 342, 364 Mesothelium, 70, 77 Metabolism of cells, 50, 213 Metanephroi, 377 Metaphase, 55 Metaplasm, 46 Metathalamus, 523 Methods for studying fibre tracts of the cord, 459 atrophy, 459 axonal degeneration, 459 comparative anatomy, 459 myelogenetic, 459 physiology, 461 secondary degeneration, 459 von Guddens, 459 Methyl blue, 19 green, 19 violet, 19 Methylene blue, 38 Meynert, decussation of, 522 fasciculus retroflexus of, 531 fibrse propriae of, 535 radiations of, 541 Micron (micromillimeter), 15 Microsomes, 44 Microtome, 14 Midbrain, 416, 517 anterior corpora quadrigemina of, S17, S22 aqueductus Sylvii, 517 basis pedunculi, 517 brachia conjunctiva, 520 cerebral peduncles, 517, 521 coUiculi, 417, 517 . corpora quadrigemina, 417, 522 cranial nerves III. and IV., 520 crura cerebri, 518 decussation of Forel, 521 of Meynert, 522 Edinger-Westphal nucleus, 518 fourth cranial nerve, 520 geniculate bodies of, 520 inferior brachium quadrigeminum, 520 coUiculi, 520 internal arcuate fibres, 522 iter, 517 lateral peduncular fillet, 521 lemniscus, 462, 520 medial accessory fillet, 521 mesencephalic root of fifth nerve, 475 520 optic nerve, 475 INDEX 619 Midbrain, pes pedunculi, 521 posterior commissure, 522 corpora quadrigemina, 517 longitudinal fasciculus, 521 red nucleus of, 517, 520, 521 reticular formation, 520 root fibres and nucleus of origin of third {ociiLomotor) cranial nerve, 517 section through exit of third (oculo- motor) cranial nerve, 517 spino-tectal tract, 522 substantia nigra, 517, 521 superior cerebellar peduncles of, 521 collie uU, 520, 522 tegmentum, 416, 518 Middle ear, 573; see Ear, middle Midgut, 255 small intestine, 255 Migrator}' leucocytes, 259 Milk, 399 cells of, 399 colostrum corpuscles of, 400 teeth, 239, 241 Minot, concerning endothelium and meso- thelium, 77 concerning the pregnant uterus, 370 Mitochondri a, 213 Miton, 45 Mitosis, 53 anaphase, 56 metaphase, 55 method of demonstrating by Flem- ming's fluid, 8 prophase, 53 technic for, 63 telophase, 56 Mitotic figure, 55 Mitral cells, 584 Mixed spinal nerve, 424 Modiolus, 576 Mollier, concerning splenic pulp, 185 Monaster, 54, 55 Mononuclear cells, 184 leucocytes, 105 Monophylctic theory of blood cells, 109 Monosynaptic arc, 469 Mordanting, 32 Morgagni, hydatid of, 342 Mossy cells, 143 Motion of cells, 51 Motor cells of anterior horns, 444 Motor decussation, 483 end plate, 442 nuclei, 441 peripheral nerves, 441 precentral area, 543 ISIounting, 22 celloidin specimens, 23 in balsam, 23 in glycerin, 22 paraffin sections, 23 Mouth, the, 220 blood-vessel of, 222 end bulbs in mucous membrane, 433 glands of, 220 lymphatics of, 222 mucous membrane of, 220 nerves of, 222, 226, 432, 587 technic of, 223 Mucin, 82, 221 Mucous glands, 221 membranes, 218 end bulbs in, 433 general structure of, 218 of alimentary tract, 219 tactile cells of, 431 corpuscles of, 432 tissue, 82 Mucus, 213, 221 Muller, cells of, 558 circular muscle of, 553 fibres of, 558 Miiller's fluid, 7 Miillerian ducts, 348 Multinuclear cells, 184 Multipolar nerve cells, 127, 444 Muscle, arrcctor pili, 391 auricular, 161 cells. III ciliary, 553 circular, of Muller, 553 columns of KoUikcr, 116, 117 discs, 115 fibrillae, 114 nuclei, 112 of sweat glands, 395 spindles, or neuro-muscular bundles, 434 tendon junction, 208, 434 organs of (Jolgi in, 434 peripheral nerve terminations in, 434 tissue, III 620 INDEX Muscle tissue, classification of, iii development of, 122 heart, 119 histogenesis, 119 intercellular bridges of, 112 involuntary smooth, in striated, 119 technic cf, 124 voluntary striated, 114 anisotropic substance, 115 Cohnheim's fields, 116 cross fibre nets of, 117 end bulbs of, 434 ergastoplasm, 114 Heidenheim's scheme of struc- ture of, 117 Hensen's Une, 115 inokomma, 117 isotropic substance, 115 Krause's line, 115 mesophragma, 117 muscle columns of Kolliker, 116, 117 discs, 115 spindles, 434 nerves, terminations in, 434 Pacinian corpuscles of, 434 Ruffini's theory of nerve termi- nations in, 434 sarcolemma, 114 sarcoplasm, 115 sarcous element of Bowman, 116 technic of, 124 telophragma, 117 ultimate fib rillas, 115 white and red fibres, 117 Muscles, voluntary, 207 capsule of, 207 endomysium, 208 epiraysium, 207 fascicles of, 207 growth of, 209 intrafascicular connective tissue of, 208 perifascicular sheath, 208 perimysium, 208 Muscular system, 207 blood-vessels of, 210 lymphatics of, 210 nerves of, 211 technic of, 211 tendons of, 91, 208 Muscular system, voluntary muscle, 207 Muscularis mucosae of mucous mem- branes, 218 Musculature of intestine, 113, 263, 264 Myelin, 134 Myeloarchitecture, 541 Myelocytes, 192 Myelogenetic method for determining fibre tracts of cord, 459, 541 Myeloplaxes, 193 Myelospongium of His, 417 Myentericus, plexus, 270 Myoblast, 122, 210 Myocardium, 161 primitive, 164 Myofibril, 117 Myotome, 122 Myxoedema, 403 Nabothi, ovula, 368 Nails, the, 385 cells of the, 387 development of, 396 eponychium of, 387 growth of, 387 hyponychium, 387 keratohyalin of, 387 lunula of, 387 matrix of, 385 prickle cells of, 387 structure of, 385 technic of, 387 Nail-bed, 385 root, 38s groove, 385 wall, 38s Nares, 299 accessory nasal sinuses, 299 cells of, 30c basal, 300 olfactory, 300 sustentacular, 300 development of, 315 glands of Bowman, 300 membrana limitans olfactoria, 300 structure of, 299 olfactory region, 299 respiratory region, 299 vestibular region, 299 technic of, 304 zone of oval nuclei, 300 of round nuclei, 300 INDEX 621 Nasal duct, 567 Nemileff, showing amitosis, 53 scheme of medullated nerve fibre, 137 Neopallium, 532, 533, 535 association fibres of, 535 fibrae of Meynert, 535 Ner\-e cells, 126; see also Neurone amacrine, 558, 586 amphicytes, 427 anterior horn, 444 association, 514, 539 basket, 222, 510 Betz', 533, 536, 539 bipolar, 127, 428 brush, 585 Cajal's, 539 carj^ochromes, 129, 512 cerebro-spinal ganglia, 486 column, 444, 451, 469 cone-bipolar, 557 cone- visual, 557 efferent projection, 539 ependymal, 418 extrinsic, 443 ganglion, 418, 426, 443, 539 giant, of Betz, 533, 536, 539 glia, 144 Golgi, Type I, 130, 132 Type II, 131, 132, 446, 536 hecateromeric, 444, 445 heteromeric, 445 horizontal, 538, 558 in gray matter of cord, 444 intrinsic, 443 inverted pyramidal, 536 large granule cells, 5 1 2 marginal, 450 Martinotti's, 536 mitral, 584 mossy, 143 motor, of the anterior horn, 1 29, 444 Miillcr's, 558 multipolar, 127, 444 neurilemma, 419 neuroblasts, 126, 142, 418 neuroglia, 143, 418, 453, 514 nucleus of, 127 of motor area of cerebral cortex, 539 outside the spinal cord, 444 peripheral motor, 441 sensory, 426 pigment in, 131 Nerve cells, polymorphous, 536, 530 Purkinje, 509 pyramidal, 536 inverted, 536 rod-bipolar, 557 rod- visual, 557 root, 441, 444, 451 satellite, 427 small granule, 512 somatochromes, 129 spider, 143 spinal ganglion, 426, 443 spongioblasts, 143, 417 stellate, 510, 536 sympathetic ganglion, 436 tautomeric, 445, 463 unipolar, 127, 428 endings, 429; see Peripheral nerve terminalions fibres, 131 afferent, 419 root, 420 association, 421, 533, 535 climbing, 510, 513 commissural, 533 cone, 557 deep tangential, 541 efferent root, 418 fastigio-bulbar, 500, 504 felt works of, 440 layer of, of retina, 558 lens, 564 medullated, 133 of cerebellum, 5 1 1 mossy, 513 motor end plates of, 442 neuroglia, 143 non-meduUated, 132 of Bergmann, 514 of Remak, 133 of trapezius, 501 origin of, of white matter of cord, 443 pallial, 533 pallio-pontile, 501, 504 ])alli<)-tcctal, 535 pallio-thalamic, 535 parallel, of the cerebellum, 512 |)crj)endicular |)ontile, 499, 501, 507 l)oslganglionic, 437 l)reKanglionic, 437 projection, 533 622 INDEX Nerve fibres, rod and cone, 557 superficial tangential, 541 terminations, 430 motor, 440 sensory, 430 tissue, 126 Golgi methods of staining, 35 neuroglia, 142 neurone, 126 axone, 132 cell body, 126 dendrites, 131 protoplasmic processes, 131 technic for, 38, 144 Nerves, cranial, table of, 545, 546 I (olfactorius), 475, 532 II (opticus), 473, 520, 524, 559 optic decussation of, 526, 561 chiasma, 526, 561 tract, 520, 526, 536 motor and sensory nuclei of, 421 III (oculomotor), 475, 517 oculomotor nucleus, 517 root fibres and nucleus of origin of third, 502, 517, 518 IV (trochlearis), 475, 515, 518, 520 root fibres and nucleus of origin of fourth, 515, 518, 520 V (trigeminus), 475, 485, 489, 493, 501, 504, 506 mesencephalic root of fifth, 475, 506, 515, 520 motor nucleus of fifth, 504 "principal sensory" nucleus, 506 semilunar ganglion of fifth, 474 sensory nucleus of fifth, 506 and motor root fibres of fifth, 506 spinal root of fifth, 483, 487 terminal nucleus of, 483 VII (abducens), 475 nucleus of origin of sixth, 499, 500, 502 abducentis, 499, 502 root fibres of sixth, 499, 500, 502 VII (facial), 474, 475, 544 ganglion geniculate, 474 nucleus facialis, 499, 502 of origin of seventh, 499 root fibres of seventh, 502 VIII (auditory), 475, 481,491,493, cochlear branch of eighth, 475, 489, 491, 493 Nerves, ganglion of Scarpa, 475, 493 spirale, 474, 475, 493 nuclei of eighth, 491, 504 Deiter's, 500, 501 von Bechterew's, 493, 507 root fibres of eighth, 493 vestibular branch of eighth, 475, 491, 493 IX (glosso-pharyngeal), 474, 491 descending or sensory root fibres of the ninth, 489 dorsal nucleus of ninth, 491 ganglia of, 474 motor nucleus of ninth, 491 root fibres of ninth, 491 X (vagus), 474, 489 descending or sensory root fibres of tenth, 489 dorsal nucleus of tenth, 481, 485, 489 ganglion jugular, 474 nodose, 474 motor nucleus of tenth, 489 root fibres of tenth, 489 XI (spinal accessory), 483 nucleus of origin of eleventh, 483 root fibres of eleventh, 483 XII (hypoglossal), 475, 481 nucleus of origin of twelfth, 481, 485, 487 root fibres of twelfth, 487, 489 mixed spinal, 424 olfactory, 532 peripheral, 424 spinal, anterior, motor or efferent roots of, 485 sensory, or afferent portions, 426 Nervous system, the, 416 cerebro-spinal, 416 connective tissue of, 142 development of, 416 general structure of, 416 sympathetic, 416 cerebrospinal, 416 afferent peripheral neurones, 424 brain, 473 cerebro-spinal ganglia, 424 cranial nerves, 473, 546, 547 development of, 416 efferent peripheral cerebrospinal neurones, 441 general structure of, 416 INDEX 623 Nervous system, histological development of, 416 membranes of brain and cord, 422 segmental part, 420, 474 spinal cord, 442 ner\-es, 424, 546, 547 suprasegmental part, 420, 478 sympathetic, 416 development of, 418 ganglia, 416, 436 Neumann's dental sheath, 232 Neural arc, 421, 469 cerebellar, 470 cerebral, 470 disynaptic, 470 mono-synaptic, 469 paUial, 470 three-neurone, spinal, 469, 485 two-neurone, spinal, 469, 504 canal, 126 fold, 416 groove, 416 plate, 416 tube, 416 Neuraxone, 126 Neurilemma, 132, 135 and axolemma, relation of, 136 -cells, 419 Neurite, 126 Neuroblasts, 126, 148, 418 Neuro-epitheUum, 77 cone bipolar cells, 557 visual cells, 557 rod bipolar cells, 557 visual cells, 557 Neurofibrils, 128 Cajal's method of staining, 37 importance of, in neurone, 138 Neuroglia, 142 mossy cells of, 143 Miiller's cells of, 558 neuroblasts, 126, 142, 418 of cerebellum, 514 spider cells, 143 spongioblasts of, 143 technic for, 144 Neurokeratin network, 135 Neurological staining methods, 32 Neuromuscular bundles, 434 Neurone, the, 126 axonc of, 132 canals in, 131 Neurone, caryochromes, 129 cell body, 126 chromophilic bodies, 129 contact theory of, 138 continuity theory of, 138 cytoplasm of, 12S degenerative changes in, 139 dendrites of, 131 development of, 126 extracellular network of, 139 functional centre of, 137 genetic centre of, 137 Golgi net, 139 long axone neurone, 132 neurofibrils of, 127 Nissl, special method of technic for, 38 nucleolus, 128 nucleus, 127 nutritive centre of, 137 pericellular network of, 139 perifibrillar substance, 128 physiological significance of, 137 pigment in, 131 protoplasmic processes, 131 retraction theory of, 138 short axone neurone, 132 somatochromes, 129 synapsis of, 138 technic for, 38, 144 theory, 138 trophic centre of, 137 Neurones, aS'ercnt peripheral, 419, 474 segmental, 421, 474 suprasegmental, 421 associative, 470 central, 418, 420 cone association, 561 cord, 469 cortical precentral, 469 efferent peripheral, segmental, 418, 441, 474 suprasegmental, 421, 485 peripheral segmental, 421, 474 intermediate, 418, 420 intersegmental, 421, 476 intrascgmental, 421, 476 pallio-pontile, 501 peripheral, 419 afferent, 420, 426, 469, 474 efferent, 418, 420, 468, 475 pontocerebellar, 501, 504 624 INDEX Neurones, rod-association, 561 somatic (peripheral), 419 474, 475 splanchnic (peripheral), 419, 474,475 suprasegmental associative, 422 thalamo-cortical 462 visceral (peripheral), 419 Neuroplasm, 134 Neutral carmine, 20 Neutrophile granules, 106 Nipple, 397 Nissl method for staining nerve cells, 38 pathological value of, 130 concerning chromophilic bodies, 130 Nitric acid for decalcifying, ic for dissociating muscle tissue, 5 Nodes, lymph, 167 of Ranvier, 134 Nodose ganglion of X nerve, 474 Non-medullated axones, 132 Normoblasts, 192 Notochord, anlage of, 62 Nuclear dyes, 17 alum carmine, 19 basic aniUn, 19 carmine, 17 combination of Gage's and Mayer's formulas, 18 Delafield's hsematoxylin, 17 Gage's hsematoxylin, 17 hsematoxylin, 17 Heidenhain's hsematoxylin, 18 Mayer's hasmalum, 18 Weigert's hsematoxylin, 19 eccentricity, 142 fluid, 49 groups, 373 membrane, 48 sap, 49 structures, method of demonstrating by Flemming's fluid, 7 Nuclein, 48 Nucleolus of typical cell, 48 false, 48 Nucleoplasm, 49 Nucleoreticulum, 48 Nucleus, the, 47 abducentis, 502 accessory olivary, 490 alse cinerese, 485 ambiguous, 489 amygdaliformis, 536 arciform, 499, 530 Nucleus, arcuatus, 499, 530 caudatus, 526, 532, 535 centralis superior, 515 chromatin of, 48 commissuralis, 487 cuneatus, 487 Deitei's, 466, 476, 483, 500, 502 dentate, 466, 500, 502, 507 dorsal cochlear, 493 Edinger-Westphal, 518 emboliformis, 502, 507 facialis, 499 fastigii, 466, SCO, 502, 507 function of, 47 funiculi cuneati, 462, 485 gracilis, 462, 485 globosus, 502, 507 hypoglossi, 485 interstitial, of Cajal, 465, 485, 491, 50O) 504 karyoplasm of, 49 lenticularis, 532, 535 linin of, 48 mediaUs, 490 membrane of, 48 network of, 48 nuclein of, 48 nucleoreticulum of, 48 nucleolus of, 48 oculomotor, 518 of acoustic tubercle, 493 of a typical cell, 47 of column of Burdach, 462, 481, 487 of Goll, 462, 481, 487 of Darkschewitsch, 465 of Luys, 523 of origin, 421 oculomotor, 518 olivary, 487, 490, 491, 500, 507 pontis, 504 preolivary, 500 pulposus, 204 red, 436, 466, 476, 5x7, 524 resting, 57 reticvdaris tegmenti, 504 ruber, 436, 466, 476, 517, 524 semilunar, 500 tecti, 466, 500, 502 terminal, 421, 490 trapezoid, 491, 500, 507 triangular, 490 ventro-lateral, 530 INDEX 625 Nucleus, vestibular, 490, 493, 500 von Bechterew's, 500, 507 Nuel's space, 5S1 Nutrient canal, 196 foramen, 196 vessels, of bone, 196 Nutritive center of neurone, 137 Oculomotor III nerve, 517, 518 nucleus, 5x7, 518 Odontoblasts, 228, 241 CEsophagus, the, 243 technic of, 215 Oil of origanum Cretici for clearing specimens, 23 Olfactorius (I nerve), 475, 532 Olfactory bulb, 532, 585 granule laj'er, 586 of longitudinal fibre bundles, 586 of mitral cells, 586 of olfactory fibres, 586 molecular layer, 586 olfactory glomeruli of, 586 path, 475, 532, 533, 539 pallial commissure, 533 group of segmental neurones, 475 nerve, 475, 532 organ, 584 pallium, 532 tract, 475, 539, 586 Ohvary nucleus, 481, 487, 493, 500, 507 Olives, 487, 489, 491, 493, 501 Omentum, 268 gastro-hepatic, 268 greater, 268 Opie, concerning the pancreas, 286 concerning the cell-islands of Langer- hans, 286 Oppel's method of staining intralobular connective tissue of liver, 296 Optic chiasma, 526, 561 cup, 570 decussation, 561 depressions, 569 nerve, 475, 520, 524, 559 arachnoid of, 560 dural sheath of, 559 pial sheath, 559 relation to retina and brain, 560 subarachnoid space, 5O0 subdural space, 5O0 technic of, 570 40 Optic stalk, 569 tract, S20, 524, 536 vesicle, 569, 570 Opticus (II nerve), 473 Ora serrata, 552, 555 Oral glands, cells of, 220 crescents of Gianuzzi, 222 demilunes of Heidenhain, 222 mixed glands, 221 mucous glands, 221 serous glands, 221 technic of, 223 Orange G, 20 Organ of Corti, 580 acoustic neurones to, 475 cells of Claudius of, 581 Corti's arches, 581 tunnel, 581 Deiter's cells, 581 hair or auditory cells, 581 Hcnscn's cells, 581 lamina reticularis of, 581 Nuel's space of, 581 phalangeal processes, 581 pillar cells, 580 of Giraldes (paradidymis), 342, 377 of hearing, 572; see also Ear blood-vessels of, 582 development of, 583 ear, external, 572 internal, 574 middle, 573 lymphatics, 582 nerves, 582 technic of, 584 of smell, 584; see Oljaclory organ olfactory bulb, -85 mucosa, 584 tract, 586 technic of, 586 of taste, 587 cells of, 587 foUatc pai)illa;, 587 gustatory canal, 587 intergeminal fil)rcs of, 587 intrageminal fibres of, 587 taste buds, 226, 227, 432, 433, 587 technic of, 587 of vision, 548 lilood-vesscis of, 565 development of, 5(19 eyeball or bubus oculi, 548 626 INDEX Organ of vision, eyelid, 567 lacrymal apparatus, 567 lens, 564 lymphatics of, 566 nerves of, 566 neurone systems of, 560 optic nerve, 473, 520, 524, 559 technic of, 570 of Zuckerkandl, 412 Organs, the, 149 circulatory system, 151 digestive system, 219 acoustic neurones to, 475 glands, duct, 212 ductless, 402 lymphatic organs, 167 muscular system, 207 nervous system, 416 reproductive system, 333 respiratory system, 299 skeletal system, 188 skin and its appendages, 380 special sense organs, 548 urinary organs, 318 of special sense, 548 organ of hearing, 572 of smell, 584 of taste, 587 of vision, 548 Organs of Golgi, peripheral nerve ter- minations in, 434 Orth's fluid (f or malin-M tiller's), 7 Osmic acid as a fixative, 8 action on fat, 8 on myelin, 8 stain for fat, 31 Osseous labyrinth, 574 Ossicles of middle ear, 574 Ossification, 82 centres, 197 endochondral, 199 intracartilaginous, 199 intramembrancus, 199 subperichondral, 199 subperiosteal, 199 Osteoblasts, 198, 235 Osteoclasts, 199, 235 Osteogenetic tissue, 197 Otic ganglion, 436 vesicle, 583 Otocyst, 583 Otolithic membrane, 575 Otoliths, 575 Oval bundle of Flechsig, 467 Ovary, the, 352 blood-vessels of, 363 corpora lutea, of pregnancy, 360 spuria, 360 vera, 360 corpus albicans, 360 haemorrhagicum, 359 luteum, 359 cortex of, 353 development of, 376 egg nest, 354 epoophoron, ^('S Fallopian tube, 353, 364 germinal epithelium of, 354 Graafian follicles, 354 haematoidin crystals, 360 hilum of, 353 lutein cells, 359 lymphatics of, 363 medulla of, 353 nerves of, 363 ovarian stroma, 353 oviduct, 353 ovum, 352, 3SS paroophoron, 363 Pfliiger's egg tubes or cords, 354 primitive ova, 354, 378 rudimentary structures connected with, 363 secretion of, 352 structure of, 353 technic of, 365 tunica albuginea, 353 zona vasculosa, 353 Oviduct, the, 364; see Fallopian tubes Ovula Nabothi, 368 Ovum, the, 52, 355, 356 atresia of follicle, 363 cells of, 357 deutoplasm granules, 357 development of, 356 fertilization of, 58, 358 germinal spot, 357 maturation of, 357 perivitelline space, 357 primitive, 378 segmentation of, 62 yolk granules of, 357 zona peUucida of, 357 Oxyhfemoglobin, 107 INDEX 627 Oxyntic cells, 249 Oxj-phile cells, 405 Pacchionian bodies, 424 Pacinian bodies, 196, 433 corpuscles, 350, 434 Palate, mucous membrane of, 220 Palatine tonsils, 178; see Tonsils Pallial connections, 461, 462, 464, 469, 470, 477, 522 Pallio-pontile fibres, 501, 504, 517 Pallio-ponto-cerebellar path, 501 Pallio-spino-peripheral efferent conduc- tion path, 465 Pallio-thalamic fibres, 523, 535 Pallio-tectal fibres, 535 Pallium, 417, 532, 536 cortical areas of, 536 fibres of, 533 Pancreas, the, 281 blood-vessels of, 286 cell-islands of Langerhans, 285 centro-acinar cells of Langerhans, 283 development of, 295 duct of Santorini, 282 of Wirsung, 281 ductus choledochus, 295 excretory ducts, 282 intracellular secretory tubules of, 285 lobules of, 281 lymphatics of, 286 nerves of, 286 Opie, concerning cell-islands, 286 secondary excretory duct of, 286 secretion of, 285 sustentacular cells of, 284 technic of, 287 terminal tubules of, 282 zymogen granules of, 282 Paneth, cells of, 260, 273 Panniculus adiposus, 383 Papilla;, circum vallate, 225, 587 compound, 381 filiform, 225 foliate, 587 fungiform, 225, 587 nerve, 381 of mouth, 220 pharynx, 242 simple, 381 vascular, 383 Paradidymis, or organ of f iirald^s, 342, 377 Paraffin embedding, 13 apparatus for, 14 oven, 13 section-cutting, 15 sections, staining and mounting of, 23 Paraganglia, 409 carotid gland, 410 chromaflin organs, 410 coccygeal gland, 411 organ of Zuckerkandl, 412 tympanic gland, 412 Paralinin, 48 Paramiton, 45 Paranuclein, 48 Paranucleus, 213 Paraplasm, 46 Parathyreoids, 404 chief or clear cells of, 405 development of, 406 function of, 406 oxyphile cells, 405 Pool's theory of, 406 technic of, 406 Pareleidin, 384 Parenchyma of glands, 215, 277 Parietal cells, 249 peritoneum, 267 pleura, 308 Paroophoron, 363 Parotid gland, 277 development of, 297 intercalated tubule of, 267 nerves of, 280 Stenoni's duct of, 277 technic of, 281 Pars ciliaris retina;, 555, 559 iridica retina;, 555, 559 optica retinae, 555 papillaris, 381 reticularis, 380 Pavlow, concerning secretion, 273 Peduncle, inferior, 499, 500 middle, 499 superior, 499, 500, 502, 507, 517 Pellicula, 47 Pcnicillus, 183 Penis, 349 arteries of, 350 cavernous sinuses, 350 corpora cavernosa of, 349 corpus spongiosum of, 349 erectile tissue, 349 628 INDEX Penis, glans, 350 glands of Tyson of, 351 lymphatics, 350 nerve endings of, 350 prepuce of, 351 sebaceous glands of, 351 technic of, 352 tunica albuginea of, 349 Pepsinogen granules, 272 Peptic cells, 249, 272 glands, 249 Perforated space, anterior, 536 Perforating fibres, 192 of cornea, 550 of Sharpey, 192 Perforatorium, 343 Periaxial sheath, 134 Pericardial cavity, 165 Pericellular network, 139 Perichondrium of bone, 199 of cartilage, 99 Perichoiioidal lymph spaces, 551 Pericranium, 198 Peridental membrane, 234 Perifascicular sheath, 208, 425 Perifibrillar substance, 128 Perilymph, 574 Perimysium, 208 Perineurium, 425 Periosteal buds, 200 Periosteum, 191, 199 Peripheral afferent neurones, 419, 426, 474 cerebro-spinal ganglia, 426 symphathetic ganglia, 436 efferent neurones, 420, 441, 475 motor neurone system, 441 nerves, 419, 424 afferent part of, 424 cranial, 424 efferent part of, 424 endoneurium of, 425 epineurium of, 425 fascicles of, 425 intrafascicular connective tissue of, 425 medullated fibres of, 133 motor or efferent, 424 motor nerve terminations, 441 non-meduUated fibres of, 132 perifascicular sheath of, 425 perineurium of, 425 sensory or afferent, 424 Peripheral nerves, sensory nerve termina- tions, 429 sheath of Henle, 426 spinal, 424 structure of, 424 technic of, 426 nerve terminations, 429 annular, 434 diffuse, 430 end-bulbs, 431, 433 free endings, 430 in penis, 350 golgi Mazzoni corpuscles, 394, 434 grandry, corpuscles, 431 in mucous membrane of mouth and conjunctiva, 432 in muscle-tendon junctions, 434 in skin, 394, 430 in smooth muscle, 430 in voluntary muscle, 434 Krause's end-bulbs in penis, 350 Meissner's corpuscles in papillae of penis, 350 muscle spindles, 434 muscle-tendon organs of Golgi, 434 neuromuscular bundles, 434 Pacinian bodies, 196, 433 corpuscles of penis, 350 nerve terminations, Ruf&ni's theory of, 434 spiral terminations, 434 tactile cells, 431 corpuscles, 432 meniscus, 431 taste buds, 432 Peripheral nervous system, 416 spinal nerves, 424 Peritoneal cavity, 165 Peritoneum, 267 parietal, 267 subserous tissue of, 268 visceral, 267 Perivitelline space, 357 Permanent teeth, 239, 242 Perpendicular fasciculus of Wernicke, 535 pontile fibres, 499, 501, 507 Pes pedunculi, 417, 517, 521, 526, 530, 536 Petit, canal of, 565 Petrosal ganglion of IX, 474 Peyer's patches, 260 Pfliiger's egg tubes or cords, 354 Phaeochrome granules, 412 Phaeochromoblasts, 415 INDEX 629 Phagocytes. 107, 174 Phagocytosis, 107 Phalangeal processes, 581 PharjTigeal tonsils, 180; see Tonsils Phan-nx, the, 242 blood-vessels of, 243 lymphatics of, 243 nerves of, 243 structure of, 242 technic of, 243 Phloroglucin, 10 Pia mater, 422 arachnoid, 422 blood-vessels of, 424 cereb rails, 422 Pacchionian bodies, 424 spinalis, 422 technic of, 424 Picric acid as a fixative, 8 as plasma dye, 20 Picro-acid-fuchsin, 20 Picro-carmine, 21 Pigment granules in cells, 46, 384 in connective tissue, 87 in epithelium, 76 hair, 388 in iris, 554 in nerve cells, 131 Pillar cells, 581 Pineal body, 544 brain sand of, 547 technic of, 547 Pineal eye, 544 Pinna, 572 Pituitary body, 407 Berkley, concerning posterior lobe, 408 function of, 409 relation of, to pregnancy, 409 technic of, 408 Placenta, 371 blood-vessels of, 374, 375 canalized fibrin, 373 cell patches, 373 chorionic villi, 371 cotyledons, 371 fastening villi, 371 foctalis, 371 free or floating villi of, 371 lymphatics of, 375 membrana chorii of, 371 nerves of, 37S Placenta, nuclear groups, 373 septa of, 374 subchorionic placental decidua, 374 syncytium of, 373 technic of, 379 uterina, 373 villi of, 371 Plasma cells, 193 Plasma dyes, 17, 19 acid aniline, 20 eosin, 19 neutral carmine, 20 picric acid, 20 Plasmosome, 48 Plastids, 46 Plastin, 44 Platelets, blood, 108 Pleura, parietal, 308 pulmonary, 308 Pleural cavity, 165 Pleuroperitoneal cleft, 165 Plexiform layer of Cajal, 538 Plexus annularis, 567 Auerbach's 264, 270, 436 ciliary, 566 chorioideus, 417, 482, 487, 502 Heller's 268 interradiary, 541 Meissner's, 254, 262, 263, 271, 436 myentericus, 270 prevertebral, 436 supraradiary, 541 Plicae palmata^, 368 Pncumogastricus (vagus nerve), 474, 489 Polar bodies, 59, 358 globules, 61 rays, 54 Polykaryocytes, 193 Polymorphonuclear leucocytes, 105 Polym()ri)h()us cells, 105 Polynuclear leucocytes, 105 Pons Varolii, 17, 482, 499, 501, 515 longiUidinal fibres of, 499, 501 pcr|)fncri fascicular, 208, 425 myelin, 134 Sherrington concerning receptors, 436 Short fibre tracts, 445 Signet-ring cell, 88 Silver nitrate method, (Jolgi, 35 of staining intercellular substance, 28 Skein, closed, 54 Skeletal system, 188 articulatioDfi, 204 bone-marrow, 192 bonc&, 188 Skeletal sj^stem, cartilages, 204 Skin and its appendages, 380 blood-vessels of, 399 color of, 384 corium, 380 corpuscles of Grandry, 431 of ]\Ieissner, 394, 432 of Ruffini, 394, 434 of Wagner, 394 cuticle, 382 derma, 380 development of, 395 eleidin of, 383 end-bulbs in, 430 epidermis of, 382 glands of, 384 glandula; sudoripara;, 384 Golgi-Mazzoni corpuscles of, 394, 434 hair follicles of, 387 junction of, with mucous membrane of mouth, 220, 384 kcratohyalin granules, 383, 387 Krause's end-bulbs, 394 lymphatics of, 394 mammary gland, 395 IMerkel's corpuscles of, 431 mitosis of cells of, 383 nails, 385 nerves of, 394, 430 of scrotum, 381 Pacinian bodies of, 434 panniculus adiposus of, 383 pai)ilUc of, 381 ])areleidin, 384 ])ars ])apillaris, 381 pars reticularis, 380 peripheral nerve teiminations in, 304, 430 prickle cells of, 383, 387 retinacula; cutis, 382 .sebaceous glands of, 384, 391 subcutaneous tissue of, 381 sweat glands (glandula- suboripara'), 384 pores of, 384 tactile cells of, 431 corpuscles, 433 Icchnic of, 384 for l)loo(l- vessels of, 305 Vater-I'acinian corpuscles of, 394 Small intestines, 255 agminated follicles, 260 634 INDEX Small intestines, Auerbach's plexus, 263 blood-vessels of, 268 Brunner's glands of, 262, 273 cells of, 257 chyle capillaries of, 270 coats of, 262 crypts of Lieberkiihn, 260 development of, 296 lacteals of, 275 lymphatics, 270 Meissner's plexus, 262 migratory leucocytes, 257 muscle of, 262 nerves of, 270 Paneth's cells, 260, 273 Peyer's patches of, 260 plexus myentericus, 270 replacing cells, 259 secreting cells, 257, 272 solitary follicles, 260 technic of, 275 valvulse conniventes of, 255 villi of, 256 Smell, organ of, 584 Smooth muscle, 119; see Involimlary muscle Sodium hydrate as a macerating fluid, 4 Solitary fasciculus, 487, 489 follicles, 253 Somatic receptors, 473 (peripheral) neurones, 420, 473 Somatochromes, 129 Spaces of Fontana, 554 Special dental germs, 238 senses, 436 Spermatids, 337, 344 Spermatoblast, 345 Spermatocytes, 337, 344 Spermatogenesis, 58, 344 technic of, 346 Spermatogones, 336 Spermatozoa, 58, 337, 343 acrosome, 337 apical body, 337 axial thread, 344 development of, 58, 344 diagram of, 343 galea capitis, 343 perforatorium, 343 structure of, 58, 343 technic of, 346 Sphenopalatine ganglion, 436 Spider cells, 143 Spinal accessory nerve, 483 Spinal cord, 416 anterior columns of, 448 funiculus, 448 horns of, 444, 448 marginal bundle of Lowenthal, 466 median fissure, 448 nerve roots of, 450, 459 pyramids, 465 white commissure of, 450 antero-lateral columns of, 448 fimiculus, 448 white column, 448 ascending tract, 463, 483, 487 descending tract, 466 arachnoid membrane of, 422 arrangement of fibres of, 452 arteries of, 454 ascending tracts of, 461 blood-vessels of, 454 cell-groupings of, 450 cells of dorsal horn, 450 cells of the intermediate gray matter, 450 of Golgi, Type II, 446 of ventral horn, 451 cells outside cord, with axones to white matter of cord, 444 central canal of, 416, 449 gelatinous substance, 449 cervical enlargement of, 446, 455 segments of, 446 Clarke's column of, 450, 455. 463 coccygeal segments of, 446 collaterals of, 447 column of Burdach, 459, 461 column of GoU, 459, 462 cells, 444, 461 hecateromeric, 444 heteromeric, 444 tautomeric, 444 comma tract of Schultze, 467 conduction paths of, 421, 461 cornua of, 448 crossed pyramidal tract, 464 descending paths from higher centres, 464 tract from Deiter's nucleus, 466 from vestibular nuclei, 466 diagram showing tracts of, 460 INDEX 635 Spinal cord, direct ascending paths to higher centres, 461 cerebellar tract, 463 pyranaidal tract. 464 reflex collaterals, 452 dorsal graj' columns, 448 commissure, 449 spino-cerebellar tract, 463 white columns, 448 dura mater of, 422 ependyma of, 453 fasciculus, medial longitudinal, 467 of Thomas, 467 fibre tracts of, 459 methods of determining, 459 filura terminale of, 442 finer tructure of, 453 fundamental columns of, 445, 468 ganglion cells of, 443 gelatinous substance of Rolando, 449 general topography of, 448 Gowers' tract, 463, 464, 465 gray matter of, 420, 449 ground bundles of, 445, 468 Helweg's tract, 464 interchange of fibres, 452 intermediate gray matter, 450 intermedio-lateral column, 450 lateral horn of, 450 long ascending arms of dorsal root fibres, 461 long ascending fibre tracts, 445 longitudinal section of six days' chick embryo, 447 lumbar enlargement of, 443, 448 segments of, 443 main motor fibre systems of, 418, 441 marginal bundle of Lowenthal, 466 marginal zone, 449 medial fillet (lemniscus), 462 medullated fibres of, 406, 443 membranes of, 422 arachnoid 422 blood-vessels of, 424 dura mater, 422 pia mater, 422 spinal dura, 422 tcchnic of, 424 mixed spinal nerve, 424 motor cells of anterior horn, 444 multipolar ganglion cells of, 112, 127 439. 444 Spinal cord, neiuroglia cells, 453 fibres, 453 tissue, 450 neurone systems of, 421 nucleus, Darkschewitsch's, 465 Deiter's, 466 funiculi cuneati, 462 gracilis, 462 origin of fibres of white matter, 443 of posterior columns of, 443 oval bundle of Flechsig, 463 peripheral motor or efferent neurone system, 418, 441 sensory or afferent neurone system, 419, 426 pia mater, spinaUs, 422, 448 plexus of fine fibres, 452 posterior columns, 443, 448 funiculus, 448, 461 horns, 445, 448 median septum, 448 ' nerve roots, 450 root fibres, 450 postcro-lateral grooves, 448 sulci, 448 pyramidal decussation, 464 tracts, 464 reflex arcs, 469 reticular process, 448, 455 root cells, 441, 444, 451 rubro-spinal tract, 466 sacral segments of, 446 scheme of neurone relations of, 460 section through cervical enlarge- ment of, 405 through lumbar enlargement, 448 through mid-thoracic region, 455 through six-day chick embryo, 447 through twelfth thoracic segment, 455 segments of, 443 septo-marginal tract, 467 shape of, 443 short fibre systems of, 445, 462, 468 shorter intersegmental ir.nK ,](>H size of, 443 sj)ino tcctal tract, 465 thalamic tract, 462 Iccto-spinal tract, 465 lechnic of, 446, 471 thoracic segments of, 443 636 INDEX Spinal cord, tract from interstitial nucleus of Cajal, 465 tractus cerebro-spinalis, 464 cortico-spinalis, 464 pallio-spinalis, 464 reticulo-spinalis, 467 spino-cerebeUaris dorsalis, 463 cerebellaris ventralis, 463 spinalis, 468 thalamis, 414 variations in structure at different levels, 455 veins of, 454 ventral gray columns, 448 commissure, 449 spinal cerebellar tract, 463 white columns, 448 vestibulo-spinal tract, 466 von Monakow's tract, 466 white commissure, 450 matter, 420, 446, 450 zona spongiosa, 449 terminalis, 450 zone of Lissauer, 443 Spinal ganglia, 426 amphicytes, 427 capsule of, 426 development of, 418 technic of, 446 ganglion cells, 436, 443 central processes of, 436 classification of, 427 collaterals from, 47 descending arms from central pro- cesses of, 436 development of, 418 Dogiel's classification, 427 ectodermic origin, 416 modes of termination of peripheral processes of, 429 peripheral processes of, 429 Rufifini's classification of termi- nations in muscle spindles, 434 satellite cells, 427 structiure of, 426 technic of, 441, 446 Spinal nerves, 424 Spindle, achromatic, 54 Spino-cerebellar tract (dorsal), 463 (ventral), 426, 463 Spino-collicular tract, 462 Spino-peripheral motor neurone system, 442 Spino-spinal tract, 468 Spino-tectal tract, 465, 483, 487, 490 Spino-thalamic tract, 462, 483, 501, 504, 515,526 Spiral ganglion, 475, 493 lamina, 577 ligament, 577 limb us, 582 organ, 579 prominence, 579 sulcus, external, 579 terminations, 434 Spireme, closed, 54 open, 55 Spireme-thread, 54 Splanchnic effectors, 473 Splanchnic (peripheral) neurones, 419 Spleen, 181 ampullae, 183 blood-vessels, 182 cavernous veins, 183 cells of, 184 central arteries of, 183 connective tissue framework, 181 cords of, 184 corpuscles of, 182 -development of, 186 function of, 186 development of, 186 germinal centres of, 182 lymphatics of, 186 Malpighian bodies, 182 MolHer, concerning splenic pulp, 18s nerves of, 187 Spleen, penicillus, 183 pulp of, 184 cords of, 184 spindles of, 183 technic of, 187 Spleen-sinus, 183 Splenic corpuscles, 182 pulp, 184 Spheno-lymph nodes, 174 Spongioblasts, 142, 413 of His, 417 Spongioplasm, 44 Spongy bone (cancellous), 188 primary, 202 INDEX 637 Staining, 17 differential, 3 double with h;ematoxyHn-eosin, 20 in bulk, 21 methods, special, 28 chloride of gold, 28 Golgi's chrome silver for secretorx- tubules, 29 Jenner's, for blood, 31 Mallor>''s aniline blue for connec- tive tissue, 30 phosphomolybdic acid ha?ma- toxylin stain for connective tissue, 29 phosphotungstic acid ha?matoxy- lin stain for connective tis- sue, 30 Alaresh's modification of Bielsch- owsky's stain for fine connective tissue fibrils, 31 osmic acid, for fat, 31 silver nitrate, for intercellular sub- stance, 28 Verhoeff's differential stain for elas- tic tissue, 28 Weigert's elastic-tissue stain, 28 paraffin sections, 23 sections, 20 double with hjematoxylin-eosin, 20 triple with haematoxylin-picro-acid- fuchsin, 21 with picro-acid-fuchsin, 20 mth picro-carmine, 21 selective, 3 special neurological methods, 32 * Cajal's methods for neurofibrils in nerve cells, 37 Cox-golgi method, 36 Golgi bichlorid method, 36 silver method, 35 Marchi's, for degenerating nerves, 34 Nissl's method, 38 Weigert's, for mcdullalcd nerve fibres, 32 Weigcrt-I'a! method, a Stains, nuclear dyes, 17 plasma dyes, 19 Stalked hydatid, 342 Supcs, 574 Stellate cells, 510, 536 Stenoni, duct of, 277 Stohr,, concerning muscle fibres 119 scheme of spleen, 183 Stomach, 247 acid cells of, 245, 249 adelomorphous cells of, 249 Auerbach's plexus, 270 blood-vessels of, 268 cardiac gland of, 252 chief cells of, 249 delomorphous cells of, 249 development of, 296 epithelium of, 248 fundus glands of, 249 gastric glands of, 249 pits of, 248 Heller's plexus, 268 lymphatics of, 270 ]Meissner"s plexus, 270 mucous membrane of 248 muscular coat of, 254 nerves of, 270 oxyntic cells, 249 parietal cells of, 249 peptic cells of, 249 glands of, 249 plexus myentcricus, 270 pyloric glands of, 252 replacing cells, 252 ruga; of, 248 secretion of, 272 solitary follicles of, 253 stroma of, 252 technic of, 254 tubules of, 250 Stomata, 165 Stratum cinereum, 522 corncum, 383 cylindricum, 382 fibrosum, 205 germinativum 382 granulosum, 355 lemnisci, 522 lucidum, 383 Malpighii, 383 mucosum, 382 0|)licum, 522 spinosum, 383 submuscosum, 366 su|)ravascularc 366 vasculare 366 zonule, 532 638 INDEX Stratum cinercum, synoviale, 205 Streaming of protoplasm, 52 Stria cornea, 531 medullaris, 530 of Baillarger, 541 terminalis, 531 vascularis, 579 Striae thalami, 522 Styloglossal j&bres, 224 Subarachnoid space, 422 Subchorionic placental decidua, 374 Subcutaneous tissue, 318 Subdural space, 422 Sublingual gland, 277 crescents of Gianuzzi of, 278 development of, of, 297 duct of Bartholin of, 278 nerves of, 280 technic of, 281 Sublingualis minor, 278 Submaxillary ganglion, 436 gland, 278 development of, 297 duct of Wharton of, 278 nerves of, 280 technic of, 278 Submucosa, 218 Subperichondrial ossification, 201 Subperiosteal ossification, 201 Substantia alba, 420 grisea, 420 nigra, 131, 517, 526 propria corneae, 550 Sulcus, external spiral, 582 Superficial sensation, 436 Superior cerebellar peduncles, 499, 500, 502, 507, 517 colliculus, 417, 517, 522 gangUon of IX, 474 longitudinal fasciculus, 491, 500, 504 S15 526 olive 493, 500, 502, 504, 507 Suprachorioidea 551 Supraradiary plexus 541 Suprasegmental arc 421 brain 478 cerebral hemispheres 420, 478 connections (afferent and efferent) 476 corpora quadrigemina, 420, 478 intersegmental nuclei and tracts of segmental brain, 479 Suprasegmental nuclei and tracts form- ing suprasegmental paths, 479 pallium, 420, 478 paths, 476 afferent, 476 efferent, 477 peripheral (segmental) neurones, 479 structures, 478 terminal nuclei, 479 . neurones, 421 afferent, 421 associative, 421 efferent, 421 Suspensory ligament, 565 Sustentacular cells, 284, 300, 335, 575 Sweat glands, 384 development of, 395 ducts of, 384 muscle tissue of, 395 pore, 384 Sympathetic ganglia, 418, 436 cells of, 439 chain ganglia, 436 development of, 418 in Auerbach's plexus, 436 in Meissner's plexus, 436 pigmentation of cells of, 439 prevertebral plexuses, 436 structure of, 436 technic of, 446 termination of nerves, 440 vertebral ganglia, 436 nervous system, 416 Synapsis of neurones, 138 Synarthrosis, 204 Synchondrosis, 204 Syncytial cells, 417 Syncytium, 68, 373 S5mdesmosis, 204 Synovial membrane, 205 villi, 205 System, a, 68 Szymonowizc, showing intercellular bridges, 74 showing meduUated nerve fibre, 136 Tactile cells, 431 corpuscles, 223, 432 of Meissner, 432 of Wagner, 394 ] meniscus, 432 INDEX 639 Taenia thalami, 530 Tapetum celliilosum, 551 fibrosum, 551 Tarsal glands, 568 Tarsus, 56S Taste buds, 226, 227, 432, 433, 587 organ of, 587; see Organ oj Taste Tautomeies, 445, 463 Teasing, 4 Technic, general. 3 Tecto-spinal tract, 465, 483, 487, 490 Teeth, 227 apical foramina, 228 blood-vessels of, 235 cementum of, 227, 233 crown of, 227 cuticula dentis. 233 dental germ, 238 groove, 238 papilla, 238 periosteum, 234 pulp, 228 sac, 239 dentinal canals, 230 fibres, 228 dentine of, 227, 228 development of, 238 common dental germ, 238 cuticular membrane, 240 dental papilla, 238 enamel organ, 239 special dental germ, 238 technic of, 242 Tomes' process, ;<.40 enamel of, 227, 233 cells, 237 fibres, 233 » organ, 239 prisms, 233 fang of, 227 interglobular spaces, 232 layer of Weil, 228 lines of Rctzius, 233 of Schreger, 232 lymphatics of, 236 milk, 339, 241 nerves of, 236, 430 Neumann's dental sheath, 232 odontoblasts of, 228 peridental membrane, 234 permanent, 239, 242 pulp cavity, 227 Teeth, root of, 227 root-canal of, 228 special dental germs, 23S technic of, 242 Tomes' granular layer, 232 process, 240 true molars, 242 Tegmentum, 417, 47S, 483, 499, 515, 517 central tegmental tract, 490, 499 development of, 417 fillet, 462 fourth cranial nerve, 515, 517 lateral lemniscus, 491, 493, 499, 501, 504, 515 nucleus ruber, 436, 466, 476, 517, 524 posterior longitudinal fasciculus, 491, 500, 504, 515, 526 reticular formation, 476, 485, 487, 489, 490, 491, 501, 504, 507 superior colliculi, 417, 517, 522 peduncles, 499, 500, 502, 507, 517 Telencephalon, 417, 532 Telophragma, 117 Telophase, 56 Tendon, structure of, 91 sheaths, 208 Tendon-muscle junction, 208 organs of (Jolgi in, 434 peripheral-nerve terminations in, 434 Tenon, capsule of, 566 Tensor chorioide^, 554 Terminal arborizations, 131, 429 bronchus, 310 nucleus, 421, 490 Terminations, nerve, 429 armular, 434 arborescent, 434 RufTini's theory of, 434 spiral, 434 Testis, 233 blood-vessels of, 342 corjius Ilighmori, 23;^ dcveloi)ment of, 376 ducts of, 334, 339, 342 cjaculatory, 341 epididymis of, 333 lobules of, 333 lym|)lialics of, 342 mediastinum, 333 nerves, 342 rctc, 339 secretion of, 343 640 INDEX Testis, semen, 343 seminal ducts of, 339 vesicles, 341 seminiferous tubules of, 334 spermatozoa, 337, 343 technic of, 346 tunica albiiginea of, 333 vaginalis, 333 vasculosa, 333 vas deferens, 334 Thalamencephalon, 522 Thalamo-cortical neurones, 462, 523 Thalamus, 417, 523, 526, 530 anterior peduncle of, 536 bundle of Vicq d'Azyr, 524 external segment of 5 23 geniculate bodies, 520 internal segment of, 523 mamillo-tlialamic tract, 524 metathalamus, 523 nuclei of, 523 nucleus of Luys, 5 23 pulvinar, 523, 524 Sachs, E., concerning the, 523, 524 thalamic radiations, 523, 526 Theca folliculi, 356 Theoharra and Bensley concerning secre- tion, 272 Thermostat, 13 Thermotaxis, 51 Thionin, 19 Thomas, fasciculus of, 467 Thoracic duct, 165 technic for, 166 Three-neurone afferent suprasegmental conduction path, 421 spinal reflex arc, 469, 506 Thrombocytes, 108 Thymus, 175 blood-vessels of, 177 development of, 177 Hassall's corpuscles, 176 lymphatics of, 177 nerves of, 177 technic, 178 Thyreoid, 402 absence of, 403 blood-vessels of, 403 cartilage, 301 cells of, 403 colloid of, 402 Thyreoid, development of, 403 isthmus of, 402 lymphatics of, 403 nerves of, 403 Timofeew, concerning nerve fibres in prostate gland, 348 Tissue, 68 -elements, dissociation of, 4 Tissues, 67 adipose, 87 areolar, 87 blood, 103 bone, 100 cartilage, 97 , classification of, 69 connective 80, 82 derivatives from ectoderm entoderm, mesoderm, 67, 68 ' dissociation of, 4 elastic, 92 embryonal, 82 endothelium, 77 epithelial, 70 erectile, 349, 376 examination of fresh, 4 histogenesis of, 67 lymphatic, 164 lymphoid, 170 mesothelium, 77 muscle. III nerve, 126 osseous, 100 osteogenetic, 197 reticular, 94, 170 subcutaneous, 381 subserous, 268 white fibroi)^, 82 Toluidin blue, 19 Toluol, as solvent, 13 Tomes' granular layer, 231 process, 240 Tongue, 223 blood-vessels of, 226 circumvallate papillae, 225 connective tissue of, 224 Ebner's glands, 226 end-bulbs of Krause, 227 fibres of, 224 filiform papillae, 225 fungiform papillae, 225 glands of, 222, 226 longitudinal fibres of, 224 INDEX 641 Tongue, lymph follicles of, 226 spaces of, 227 muscles of, 2 23 nerves of, 227 papilla? of, 225 septum lingua, 224 taste buds, 226, 227, 432, 5S7 tcchnic of, 227 transverse fibres of, 224 vertical fibres of, 224 Tonsils, 1/8 blood-vessels of, 180 cr)-pts of, 179 development of, 180 germ centre of, 179 lingual; fcUiculae linguales, 180 foramen caecum lingui of, 180 lymphatics of, 180 lymphoid infiltration of epithelium, 179 nerves of, 180 nodule of, 179 palatine or true, 178 pharyngeal, 180 adenoids of, 180 salivary corpuscles of, 180 technic of, 181 Trachea, 301 blood-vessels of, 303 cartilages of, 302 glands of, 302 lymphatics of, 304 ner\'es of, 304 technic of, 304 Tract, a, 421 antero - lateral ascending -ventral spino-cerebellar, 463 antero-latcral descending, 466 Burdach's, 459, 462 central tegmental, 490, 501, 507 cerebro-sfiinalis, 464 cochlear, 491, 500, 504 comma, of Schultze, 467 corlico-spinalis, 464 crossed [)yramidal, 464, 485 Dcilcro-spinal tract, 4()(>, 483, 487, 490, 502 descending from pallium to motor nuclei, 533 from interstitial nucleus of Cajal,46s 485, 487 41 Tract from nucleus of posterior longi- tudinal fasciculus, 465 from vestibular nuclei, 466 direct cerebellar, 463 pyramidal, 465 dorsal spino-cerebellar, 463, 483, 487, 490. 507 dorso-lateral ascending — dorsal spino- cerebellar, 463 fasciculus of Thomas, 467 fastigio-bulbar, 500, 502 fundamental or ground bundles, 445, 468 Flechsig's, 463 Goll's, 459, 462 Gower's, 464 Helweg's, 467 intersegmental (shorter) 46S Lissauer's, 443, 450 long ascending arms of dorsal root fibres, 461 mamillo-lhalamic, 524, 532 marginal bundle of Lowcnthal, 466 medial lemniscus, 462, 485, 487, 489 491, 493, 517 optic, 524 oval bundle of Flechsig, 463, 467 pallio-spinalis, 464, 533 -tectal, 535 posterior, 462 funiculi, 461 predorsal, 491 •fasciculus, 517 Iiyramidal, 464, 533 anterior, 465 rubro-spinal, 466, 483, 487, 490, 504, SIS secondary cochlear, 493, 504, 507 vagoglossopharyngeal and trigem- inal, 515, 526 vestibular, 493 scpto-marginal, 467 short fibre, 445, 4^>2. 4^8 spinalis trigemini, 485, 500 spino-(erei)cilar, ventral, 463, 483, 487, 501 -colliculo, 465, 4?,^, 4S7, 490, 502, 504, 507, S«7 -spinal, 468 -tectal, 465, 483, 487, 490 642 INDEX Tract, spinothalamic, 462, 483, 501, 504, 515,526 tecto-spinal, 465, 483, 487, 490 uncrossed cerebellar, 463 ventral spino-cerebellar, 463, 501, 504, 51 5 vestibulo-spinal, 466 Von Monakow's, 466 Tractus, see Tract Transitional leucocytes, 105 Transverse temporal gyri of Heschl, 541 Trapezoid nucleus, 493 Trapezium, 493 Trigeminus (V nerve), 475, 485, 489, 493, 501, 504, 506 Trigonum hypoglossi, 481, 489 olfactorium, 532 vagi, 481 Trochlearis (IV nerve), 475, 515, 518, 520 Trophic centre of neurone, 137 Trophospongium, 46 True corpora lutea, 360 tonsils, 178 vocal cords, 301 Tuberculum cinereum, 481, 531 olfactorium, 532 , Tubular glands, 215 Tubules arched, 320, 323 collecting 321, 324 distal, 320, 323 first or proximal, 320, 322 intercalated, 276 intermediate, 277 junctional, 377 salivary, 276 second or distal, 320, 323 secreting, 276 seminiferous, 334 serous, 222 straight, 321, 324 uriniferous, 320 Tubulo-alveolar gland, 282 Tunica albuginea, of ovary, 353 of penis, 349 of testis, 2)iZ dartos, 381 fibrosa, 356 vaginalis, 333 vasculosa, 333, 356 Two-neurone spinal reflex arc, 469, 504 Tympanic gland, 412 membrane, 573 Tympanum, 573; see also Ear, middle Tyson, glands of, 351 Ultimate fibrillas, 115 Uncinate fasciculus, 535 Unipolar nerve cells, 127, 428 Ureter, 329 development of, 376 technic of, 331 Urethra, female, 351 glands of Littre of, 351 male, 351 fossa navicularis, 352 glands of Littre of, 352 technic of, 352 Urinary bladder, 330 blood-vessels of, 331 development of, 376 system, 318 development of, 376 technic of, 331 kidney, 318 -pelvis, 329 ureter, 329 urinary bladder, 330 technic of, 331 Uriniferous tubule, 320 arched tubule of, 321, 323 ascending arm of Henle's loop, 320, 323 blood-vessels of, 325 Bowman's capsule, 320, 322 descending arm of Henle's loop, 320, 322 development of, 376 duct of Bellini, 321, 324 epithelium of, 3 24 first or proximal convoluted, 320, 322 foramina papillaria, 321 glomerulus, 320 Henle's loop, 320, 323 location in kidney of, 3 24 Malpighian body, 321 membrana propria of, 321 neck of, 322 renal corpuscle, 320 Ruble, concerning, 325 second or distal convoluted, 3 20, 3 23 straight or collecting, 321, 324 Uterus, 366 blood-vessels of, 375 INDEX 6-43 Uterus, cen-ix, 367 coats of, 366 decidua basalis, 370 capsularis, 370 graviditatus, 370 menstrualis, 369 reflexa, 370 serotina, 370 subchorionic placental, 374 vera, 370 decidual cells of, 370 development of, 376; see also Re- productive system, development of lymphatics of, 375 masculinus, 248 mucosa of menstruating, 368 of pregnant, 370 of resting, 367 stratum submucosum, 366 supravasculare, 366 vasculare, 366 nerves of, 375 placenta, 371 plicae palmatae, 368 pregnant, 370 theories concerning, 369 stage of menstruation proper, 369 of preparation, 368 of reparation, 369 technic of, 379 with placenta in situ, technic of, 379 Utricle, 575; see Sacctde and utricle Utericulo-saccular duct, 575 Utriculus prostaticus, 348 Uvula, mucous membrane of, 220 Vagina, 375 blood-vessels of, 376 coats of, 375 lymphatics of, 376 nerves of, 376 rugjc of, 376 technic of, 376 Vagus (X) nerve, 474, 489 Valve, Ilcisterian, 295 Valves of heart, 162 of veins, 158 Valvula; conniventes, 246, 255 Vas deferens, 340 technic of, 346 qiididymis, 340 Vasa effercntia, 339 vasorum, 159 Vascular papillae, 381 system, 151; see also Circulatory system Vater-Pacinian corpuscles, 394 Veins, 158 adventitia of, 158 arcuate, 328 central, 289 coats of, 15S development of, 163 intima of 158 lymph channels of, 159 media of, 158 musculature of, 158 nerves of, 158 perivascular lymph spaces of, 159 portal, 289 renal, 318 splenic, 182 stellate, of Verheyn, 328 sublobular, 289 technic of, 160 valves of, 158 vasa vasorum, 159 vena; vorticosae, 551 Venae vorticosae, 551 Ventral spino-cerebellar tract, 463 Ventricles of brain, 416, 417,481,487,499 of heart, 161 Verheyn, stellate veins of, 328 Verhorff's differential elastic-tissue stain, 28 Vermiform appendix, 265 lymph nodules of, 266 mcsoappendix, 265 technic of, 275 Vermis of cerebellum, 415, 449, 4SS Vertebral ganglia, 436 Vesicle, air, 310 brain. 416 germinal, 59 optic, 569 otic, 583 seminal, 342 Vesicula prostatica, 348 Vestibular ganglion, 425 nerve, 425, 44' nuclei, 441 descending tract from, 441 Vestibule, 575 644 INDEX Vestibule, ductus reuniens of, 575 endolymphatic duct, 575 sac, 575 saccule of, 575 utricle of, 575 utriculo-saccular duct of 575 Vestibulo-semicircular canal group of seg- mental neurones, 475 Vicq d'Azyr bundle of, 524, 532 Villi, 256 development of, 297 fastening, 371 floating, 371 free, 371 lacteals of, 275 synovial, 205 terminal, 371 Visceral neurones, 419, 475 peritoneum, 267 Visual areay 488 group of segmental neurones, 475 path, 475, 524 purple, 557 Vision, organ of, 548 ; see Organ of Vision Vital properties of cells, 50, 213 function, 51 irritability, 51 metabolism, 50, 213 motion, 51 reproduction, 52 Vitreous body of the eye, 565 Cloquet's canal, 565 hyaloid canal of, 565 membrane of chorioid, 553 of iris, 553 Vocal cords, 301 Volkmann's canal, 191, 196 Voluntary striated muscle, 114; see Mus- cle, striated, voluntary Von Bechterew's nucleus, 500, 507 Von Bibra, concerning chemical composi- tion of dentine, 228 of enamel, 233 Von Gudden, concerning method of deter- mining fibre tracts of cord, 459 Von Monakow's bundle, 466 Wagner, corpuscles of, 394 Wallerian degeneration, law of, 141 Wandering cells, 83 Washing after fixation, 9 Weigert's elastic-tissue stain, 28 hcematoxylin, 19 method of staining meduUated nerve fibres, 32 Weigert-Pal method, 33 Weil, layer of, 228 Wernicke, perpendicular fasciculus of, 535 Wharton's duct, 278 Wheeler, showing amitosis, 52 White blood cells (leucocytes), 105 or fibiillated fibres, 86 fibrous tissue, 82 matter, 420, 450 rami communicantes, 425, 436 Wilson, E. B., diagrams showing mitosis, 54, 56 Wirsung, duct of, 281 Wolffian body, 338, 377 Wrisburg, cartilage of, 301 nerve of, 501 Xylol and cajeput oil for clearing, 23 -balsam, 23 damar, 30 Xylol-paraffin for embedding, 13 Yellow or elastic fibres, 86 Yolk granules, 357 Zenker's fluid for decalcifying, 10 for fixation, 8 Zinn, zonule of, 565 Zona incerta, 530 pectinata, 580 pellucida, 61, 357 spongiosa, 449, 450 tecta, 580 Zone of Lissauer, 443 of oval nuclei, 300 of lound nuclei, 300 Zonula ciliaris, 565 Zonule of Zinn, 565 Zuckerkandl, organ of, 412 Zymogen granules, 282 technic of, 287 C£8l638)MSO