COf>V I Columbia (Bmtotttfftp in tlje €f tp of I^rtti gcrk College of ^fjpsictanfi anb burgeons; lUbrarp GIFT OF Frederick S. Lee Digitized by the Internet Archive in 2010 with funding from Columbia University Libraries http://www.archive.org/details/anatomyofnervouOOrans THE ANATOMY of the NERVOUS SYSTEM FROM THE STANDPOINT OF DEVELOPMENT AND FUNCTION By STEPHEN WALTER RANSON, M. D., Ph. D. Professor of Anatomy in Northwestern University Medical School, Chicago WITH 260 ILLUSTRATIONS SOME OF THEM IN COLORS PHILADELPHIA AND LONDON W. B. SAUNDERS COMPANY 1920 Copyright, 1920, by \V. B. Saunders Company (1 PRINTED IN AMERICA PRE88 OF W. B. SAUNDERS COMPANY PHILADELPHIA PREFACE In the pages which follow the anatomy of the nervous system has Itch pre- sented from the dynamic rather than the static point of view; that is to say, emphasis has been laid on the developmental and functional significance of >truc- ture. The student is led at the very beginning of his neurologic studies to think of the nervous system in its relation to the rest of the living organism. Struc- tural details, which when considered by themselves are dull and tiresome, become interesting when their functional significance is made obvious. This method of presentation makes more easy the correlation of the various neurologic courses in the medical curriculum. For physiologic and clinical neurology a knowledge of conduction pathways and functional localization is essential, and this informa- tion can best be acquired in connection with the course in anatomic neurology. In selecting the material to be included in this book the needs of the medical student have been kept constantly in mind, and emphasis has been placed on those phases of the subject which the student is most likely to find of value to him in his subsequent work. In many laboratories the head of the shark and the brain of the sheep have been used to supplement human material. The book has been so arranged as to facilitate such comparative studies without making it any the less well adapted to courses where only human material is used. During the past twenty years very considerable additions have been made to the science of neurology, and the more important of these have been included in the text. While a detailed presentation of the evidence concerning new or dis- puted points would be out of place in a book of this kind, whenever the state- ments made here differ from those found in other texts the authority has always been cited, the author's name and the date of his contribution being given in parentheses. A full list of these references to the literature has been included in a Bibliography at the end of the volume. The terminology adopted is that of the B. X. A., which has been used, for the most part, in its English form. But in the case of the fiber tracts the Ba>le 1 2 PREFACE terms arc often misleading, and wherever this is the case, other names have been substituted. An outline for a laboratory course in neuro-anatomy has been included, and this has been so arranged as to be easily adapted by the instructor to his par- ticular needs. Free use has been made of material gathered and arranged by others in the various handbooks, texts, and atlases that deal with the nervous system. The classification of the afferent paths and centers adopted here is based on the work of Sherrington. The terms which he introduced and which are now coming into general use have been employed. In the analysis of the cranial nerves the American conception of nerve components, so ably presented by Herrick, has been utilized. Illustrations have been borrowed from many sources, in each case duly accredited, and our indebtedness for permission to use them is gladly acknowl- edged. The majority of the figures have been made from drawings prepared for this purpose by Miss M. E. Bakehouse. The large number of illustrations and the excellent manner in which they have been reproduced is to be credited to the generous policy of the publishers, W. B. Saunders Co. My thanks are due to Dr. Olaf Larsell for reading the manuscript and for many valuable suggestions, and to Mr. Michael Mason for assistance in reading the proof. S. W. Ranson. Chicago, III., November, 1920. CONTENTS CHAPTER I Origin and Fi n< noN of mi Nervous System 17 The Diffuse Nervous System of ( loelenterates V) rhe Central Nervous System 20 CHAPTER II The Neural Tube and its Derivatives 24 The Brain of the Dogfish 26 I development of the Neural Tube in the Human Embryo ^1 CHAPTER III HlSTOGl NESIS OF THE NERVOUS SYSTEM J7 I Jevelopmenl of the Neuron 37 Development of the Spinal Nerves 40 Differentiation <>f the Spinal Cord 42 CHAPTER IV Neurons and Neuron-chains 43 Form and Structure of Neurons 43 Interrelation of Neurons 49 The Neuron as a Trophic Unit 51 The Neuron Concept 52 Neuron Chains 53 CHAPTER V The Spinal Nerves 56 Metamerism 58 Functional Classification of Nerve-fibers 60 The Spinal Ganglia 62 Somatic Sensory Fibers and Nerve Endings 66 CHAPTER VI The Spinal Cord 73 External Form and Topography 73 The Spinal Cord in Section 78 Microscopic Anatomy 85 The Spinal Reflex Mechanism 91 CHAPTER VII Fiber Tracts of the Spinal Cord 95 Intramedullary Course of the Dorsal Root Fibers 95 Afferent Paths in the Spinal Cord 98 Ascending and Descending Degeneration in the Spinal Cord 105 Long Descending Tracts of the Spinal Cord 108 CHAPTER VIII General Topography of the Brain 113 Anatomy of the Medulla Oblongata 118 Anatomy of the Pons 123 The Fourth Ventricle 125 The Mesencephalon 129 13 14 CONTENTS j CHAPTER IX Pace The Structure of the Medulla Oblongata 132 The Rearrangement Within the Medulla Oblongata of the Structures Continued Upward from the Spinal Cord 133 Decussation of the Pyramids 136 Nucleus Gracilis, Nucleus Cuneatus, and Medial Lemniscus 137 Olivary Nuclei 141 Restiform Body 143 Formatio Reticularis 144 CHAPTER X Internal Structure of the Pons 147 Basilar Part of the Pons 147 Dorsal Part of the Pons 149 CHAPTER XI Internal Structure of the Mesencephalon 158 Tegmentum 158 Basis Pedunculi 164 Corpora Quadrigemina 165 CHAPTER XII The Cranial Nerves and Their Nuclei 168 Somatic Efferent Column of Nuclei 1 70 Special Visceral Efferent Column of Nuclei 1 74 General Visceral Efferent Column of Nuclei 177 Visceral Afferent Column 180 General Somatic Afferent Nuclei 182 Special Somatic Afferent Nuclei 185 Summary of the Origin and Composition of the Cranial Nerves 190 CHAPTER XIII The Cerebellum 195 Development 195 Anatomy 196 Morphology 199 Nuclei of the Cerebellum ' 203 Cerebellar Peduncles 204 Histology of the Cerebellar Cortex 206 Efferent Cerebellar Tracts 211 CHAPTER XIV The Diencephalon and Optic Nerve 213 Thalamus 213 Epithalamus and Metathalamus 220 Hypothalamus 222 Third Ventricle 223 Visual Apparatus 225 CHAPTER XV External Configuration of the Cerebral Hemispheres 229 Development of the Cerebral Hemispheres 229 The Dorsolateral Surface 232 The Medial and Basal Surfaces 238 CONTENTS I - CHAPTER XVI Imikwi Configuration oi mi Cerebral Hemispheres 243 Corpus Callosum 243 Lateral Ventri< lea Basal < >anglia "I the Telencephalon 252 Intern. il ( lapsule _' ; 7 Connections of the Corpus Striatum and I halamus 262 ( II ATI ER XVII 'I'm Rhine mi e phali »w 265 Parts Seen on tin- B.i-.il Surface of the Hrain _", : I [ippocampus 269 Fornix 270 Anterior Commissure 11 ^ Strm tun- and Connections of the Several Parts of the Rhinencephalon 274 Olfactory Pathways 280 CHAPTER XVIII Thf. Cortex and Medullary Center ok the Cerebral Hemisphere Structure of the Cerebral Cortex 28.1 Cortical Areas 2X7 Localization of Cortical Functions 290 The Medullary Center of the Cerebral Hemisphere 296 CHAPTER XIX The Great Afferent Systems 302 Exteroceptive Pathways to the Cerebral Cortex 302 Spinal Path for Touch and Pressure 303 Spinal Path for Pain and Temperature Sensations 306 Secondary Trigeminal Paths 307 Neural Mechanism for Hearing 309 Neural Mechanism for Sight 310 Proprioceptive Pathways 311 Spinal Proprioceptive Paths (Muscle Sense) 311 Cerebellar Connections of Vestibular Nerve 314 CHAPTER XX Efferent Paths and Reflex Arcs 316 The Great Motor Path 517 The Cortico-ponto-cerebellar Path 325 The Cerebello-rubro-spinal Path 326 Important Reflex Arcs 327 CHAPTER XXI The Sympathetic Nervous System Fundamental Facts Concerning Visceral Innervation Structure of the Sympathetic Ganglia 541 Composition of Sympathetic Nerves and Plexuses J45 Architecture of the Sympathetic Nervous System 346 Important Conduction Paths Belonging to the Autonomic Nervous System 352 A Laboratory Outline of Neuroanatomy Bibliography Index 383 THE ANATOMY OF THE NERVOUS SYSTEM FROM THE STANDPOINT OF DEVELOP- MENT AND FUNCTION CHAPTER I THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM Irritability and conductivity, which, as every biological student knows, are two of the fundamental properties of protoplasm, reach their maximum development in the highly differentiated tissue of the nervous system. Indeed, it is in response to the need for increased sensitiveness to stimuli and for better transmission of the impulses aroused by them that the nervous system has developed and been perfected in, the long process of evolution which has cul- minated in man. When an ameba is touched with a pointed glass rod it moves away from the source of stimulation. Changes are initiated in the superficial protoplasm which are transmitted through the unicellular organism, resulting in a flowing out of pseudopodia on the opposite side. Through a continuation of this stream- ing motion the entire organism moves forward. Thus the relatively undif- ferentiated living substance of which it is composed receives the stimulus, transmits the resulting disturbance, and carries out the appropriate response. When in the place of unicellular organisms we stud}' simple metazoa, the sea-anemones for example, we find that considerable differentiation has occurred among the component cells. A cuticle has formed, designed to protect the subjacent parts from the action of the surrounding objects, while other cells have differentiated in the direction of contractile elements or muscle cells. Because the general body surface has been adapted to cope with the environ- ment it becomes necessary to have certain cells at the surface which are sensi- tive to environmental changes. These sensory elements are able to transmit the waves of activation developed in them directly to the subjacent muscle cells. But in higher animals, because of the large size of the body and the 2 17 lb THE NERVOUS SYSTEM complicated reactions required, long lines of communication have been estab- lished between peripheral sense organs and muscle-fibers in widely separated parts of the body. The sensor}- elements and the lines of communication constitute the nervous system and, together with the musculature, the neuromuscular mechanism. It is well to keep in mind the fact that the nervous system was developed for the purpose of enabling the musculature to react to changes in the environment of the organism. But in all higher animals the nervous system responds not only to stimuli from without but also to stimuli from within the body, and helps to in Fig. 1. — Stages in the differentiation of the neuromuscular mechanism: A to C, Hypothetic early stages: A, epithelial stage; B, muscle cell at the stage of the sponge; C, partially differen- tiated nerve-cell in proximity to fully differentiated muscle-cell; D, nerve- and muscle-cell of coelenterate stage; E, a type of receptor-effector system found in many parts of sea-anemones, in- cluding not only receptors, r, with their nerve-nets, and of muscle cells, m, but also of ganglion cells, g, in the nerve-net; F, section at right angles to the sphincter of the bell of a jellyfish (Rhizos- toma): e, epithelium of the subumbrellar surface; n, nervous layer; m, muscle layer. (Parker.) bring about an internal adjustment of part with part. Here again it acts as a sensitive mechanism for receiving stimuli and conducting them to the appro- priate organs of response. These organs through which the nervous system produces its effects are known as effectors. While muscles and glands are by far the most important effectors, we must also include certain pigmented cells u>r chromatophores) and electric and phosporescent organs under this heading Except for the reactions produced through such effectors the nervous system would be meaningless. We can best understand the significance of the nervous system if we trace its early history. This, as it has been interpreted by Parker (1919), makes an lilt: ORIGIN \M» FUNCTION OF mi NERVOUS SYSTEM M, interesting story. According to this author contractile tissue develops before any trace oi the nervous system appears. In sponges, which arc devoid of nervous elements, the oscula open and close in response to appropriate timuli. These movements are brought aboul by a contractile tissue no1 unlike mooth muscle. The active element or effector is thus the first to make it- appearance and at this stage is brought into action by direct stimulation. Nexl in the order of development is the sensory cell, derived from the epithelium in the neigh- borhood of an effector, and specially differentiated to receive stimuli and trans mit them to the underlying muscle (Fig. 1, I)). This stage of development i- reached by such ccelenterates as the sea-anemones. The advantage which these forms derive from the specialized sensory cells or receptors is -ten in the character of their responses, which arc more rapid than those of sponges. Such Cerebral ganglion v Esophageal connective Pharynx Ventral nerve cord Cerebral ganglion Pharynx Esophageal connective Ventral nerve cord A B Fig. 2. — Anterior portion of the nervous system of the earthworm: A, Lateral view; B, dorsal view. a sensory cell may be compared to a percussion cap through which a charge of powder is ignited. But ccelenterates usually present a more complex arrangement of receptor and effector elements than that indicated in Fig. 1 , D. Fine branches from the sensory cells anastomose with each other and form a nervous net within which are scattered nerve-cells. Such a nerve net is seen in many parts of sea-ane- mones (Fig 1, E) and is well developed in the jellyfish (Fig. 1, F). It seems capable of conveying nerve impulses coming from the sensory cells in all direc- tions through the bell-shaped body of the jellyfish and to muscle-fibers far dis- tant from the receptors involved. The conduction of nerve impulses from receptors to effectors seems to occur diffusely through the net — not in stated directions nor along fixed paths. In this respect the diffuse nervous system of the ccelenterates is in contrast with the more centralized system in the worms. 20 THE NERVOUS SYSTEM The sensory cells are not so directly connected with muscle-fibers in the worms as in the sea-anemones, for between receptor and effector there is here interposed a central nervous system. This system, as it appears in the earth- worm, is illustrated in Fig. 2. It consists of a cerebral ganglion dorsal to the buccal cavity and a row of ventrally placed ganglia bound together by a ventral nerve cord. The most anterior of the ventral series of ganglia is connected to the dorsal one by nerve strands on either side of the esophagus. The ganglia of the ventral cord are placed so that one occurs in each body segment, and from each three pairs of nerves run to the skin and muscles of that segment. The arrangement of the constituent elements can best be studied in transverse sections (Fig. 3). The sensory cells are located in the skin, and from each of them a fiber runs along one of the nerves into the ganglion, within which it branches, helping to form a network known as the neuropil. Within each Fig. 3. — Transverse section of the ventral chain and surrounding structures of an earthworm: cm, Circular muscles; ep, epidermis; tin, longitudinal muscles; mc, motor cell-body; mf, motor nerve-fiber; sc, sensory cell-body; sf, sensory nerve-fiber; vg, ventral ganglion. (Parker.) ganglion are found large nerve-cells from which fibers run through the nerves to the segmental musculature. Here we have the necessary parts for the sim- plest reflex arc. Stimulation of the sensory cell causes nerve impulses to travel through its fiber to the neuropil, thence to a motor cell, and finally along a proc- ess of the latter to the muscle. In other words, we have a receptor, conductor, center, another conductor, and finally an effector; and all this is for the purpose of bringing the muscle-fiber under the influence of such environmental changes as are able to stimulate the sensitive receptor. In addition to the primary sensory and motor elements just enumerated the ganglia contain nerve-cells the fibers of which run from one ganglion to another and serve to associate these in co-ordinated activity. These internuncial ele- ments serve to establish functional connections among the different parts of the ganglionated nerve cord that constitutes the central nervous apparatus; Mil ORIGIN AND FUNCTION 01 mi NERVOUS SYSTEM and they lie entirely within this central organ. The slow waves of contraction that pass from bead to tail as the worm creeps forward may be advanced from segment to segment by such internuncial or association elements. The nervous system of the earthworm differs from thai of the ccelenterate in many ways, but the fundamental difference is one of centralization. In the former the greater part of it has separated from the skin and become con centrated in a series of interconnected ganglia which serve as a central nervous system. These ganglia receive nerve fibers, coming from the sense organs, and give off others, going to the muscles; and the fibers are brought together and grouped into nerves for convenience of passage. The neuropil within a ganglion offers a variety of pathways to each incoming impulse which may accordingly find its way out along one or more of several motor fibers. The spreading of nerve impulses through the chain of ganglia is facilitated by the presence of the association fibers already mentioned. Nevertheless, conduction is not diffuse as in ti.e nerve net of the medusa, but occurs along definite and more or less restricted lines. This is well illustrated by the experiment cited by Parker: "If an earthworm that is creeping forward over a smooth surface is suddenly cut in two near the middle, the anterior portion will move onward without much disturbance, whereas the posterior part will wriggle as though in convulsions. This reaction, which can be repeatedly obtained on even fragments of worms, shows that a single cut involves a stimulation which in a posterior direction gives rise to a wholly different form of response to what it does anteriorly; in other words, transmission in the nerve cord of the worm is specialized as com- pared with transmission in the nervous net of the ccelenterate." In the gan- glionated cord of the earthworm, as here described, we find many of the features characteristic of the central nervous system of higher forms. The vertebrate nervous system has much in common with that of the earth- worm. The central nervous system of the annelid is split off from the ectoderm by a process of delamination, as will be seen by comparing the ventral nervous cord of the marine worm, Sigalion, with that of the earthworm (Figs. 3, 4). Through a comparable process of infolding of the ectoderm to form a neural tube there is developed the central nervous system of the vertebrate (Fig. 6). The dorsal position of the neural tube in vertebrates as compared with the ventral position of the solid nerve cord of the annelid offers some difficulty and has led to ingenious theories in explanation of their phylogenetic relationship, theories which we need not consider here (Gaskell, 1908). In primitive chor- dates, such as the amphioxus, we already have a simple, dorsally placed, neural 22 THE NERVOUS SYSTEM tube associated with segmental nerves. In true vertebrates the anterior end of the neural tube becomes irregularly enlarged to form the brain, while the pos- terior end remains less highly but more uniformly developed and forms the spinal cord. The primary motor nerve-cells of vertebrates resemble very closely those of invertebrates in being located within the central nervous system and in send- ing motor nerve-fibers to the muscles (Fig. 31). The primary sensory cells lie outside the central system, as in invertebrates. Those for smell are located in the olfactory epithelium. But all others have migrated centrally along the sensory fibers, and now send one process toward the periphery and another into Fig. 4. — Transverie section of the ventral nervous cord of Sigalion: bm, Basement mem- brane; c, cuticula; e, epidermis; gc, ganglion-cells; n, nerve-fibers and neuropil; s, space occupied by vacuolated supporting tissue. (Parker, Hatschek.) the central system. The relative positions of these cells in the annelid, mollusc, and vertebrate are illustrated in Fig. 5. In the latter the sensory cells are aggre- gated into masses known as the cerebrospinal ganglia, which are associated with peripheral nerves and are usually placed near the point of origin of these nerves from the brain or spinal cord. A comparison of Figs. 3 and 31 will show a striking similarity between the simple reflex arc in the earthworm and in man. If space permitted we might trace the development of the central nervous sys- tem in some detail, but perhaps enough has been given to suggest that the nervous system of man represents the culmination of a long process of evolu- tion which began with a simple sensory mechanism like that of the sea-anemones. We shall be concerned with a study of the vertebrate nervous system, almost THE ORIGIN AND FUNI HON 01 Mil NERVOUS SYSTEM 23 exclusively with that of the mammal, and more particularly with that of man. In man we are so accustomed to think of the uervous system .1- the organ and agent of the mind that its true physiologic position is often forgotten. In this Introductor) chapter we have attempted to show that the primary function of the Dervous system is to receive stimuli arising from changes in the environment or within the organism, and to transmit these to effectors which bring about the adjustments uecessary for life. Biologically speaking, the nervous system is not to he regarded as an intelligence bureau, which gathers information for Fig. 5. — Peripheral sensory neurons of various animals: A, Oligochaetic worms (Lumbricus); B, polychaetic worms (Nereis); C, molluscs (Limax); D, vertebrates. The figure illustrates the gradual change in the position of the sensors- cells in the phylogenetic series: e, Epithelial cells of sensory surface; c, cuticula; sz, cell-body of peripheral sensory neuron; rm, rete Malpighii of epi- dermis; sn, axon; co, central nervous system. (Barker, Retzius.) a sovereign mind, enthroned within the brain, nor yet as a chief executive officer to carry out that sovereign's decrees. Sensory impulses from many source.- reach the brain, where they pass back and forth through a multitude of asso- ciation paths, augmenting or inhibiting each other before they finally break through into motor paths. Previous experience of the individual, having left its trace in the organization of the central nervous system, alters the character of the present reactions. It is in connection with the neural activity involved in these complex associational processes that consciousness appears — shall I say as a by-product? — at least as a parallel phenomenon. CHAPTER II THE NEURAL TUBE AND ITS DERIVATIVES Infolding of the Neural Tube. — The vertebrate nervous system develops from a thickened plate of ectoderm along the middorsal line of the embryo. By the infolding of this neural plate there is formed the neural groove, which becomes transformed into the neural tube (Fig. 6). The neural tube detaches itself from the superficial ectoderm and gives rise through a thickening of its walls to the brain and spinal cord. The latter is formed by a process of uniform Neural groove Neural plate Neural groove Neural plate Ectoderm Neural groove Neural tube Neural tube D Neural cavity Fig. 6. — Development of the neural tube in human embryos (Prentiss-Arey): A, An early embryo (Keibel) ; B, at 2 mm. (Graf Spee) ; C, at 2 mm. | Mall) ; D, at 2. 7 mm. CKollmann). thickening in the walls of the caudal portion of the tube. The derivatives of the rostral part are well illustrated in the accompanying diagram (Fig. 7). Brain Vesicles. — At an early stage in the development of any vertebrate embryo the rostral portion of the neural tube is distinguished from the caudal part by the more rapid development of the former, its walls bulging outward to form three bulb-like swellings or vesicles, which together represent the brain, and are named from before backward, the prosencephalon, mesencephalon, and 24 THE NEURAL IM;i WD Us DERIVATIVES 25 rhombencephalon (Fig. 7). The more rostral vesicle becomes subdivided by .1 oonstriction into the telencephalon and diencephalon (Fig. 7, B, (>. The rhom- bencephalon is less sharply subdivided into a rostral pari, which includes the cerebellum, and is known as the metencephalon t and a more caudal portion, the myelencephalon. The optic nerves and retinae, not illustrated in the figure, develop as paired evaginations from the prosencephalon. The Cerebral Hemispheres. The telencephalon includes a thickened portion of the ventrolateral wall loosely designated as the corpus striatum or. since there -,f6-*t Fig. 7. — Diagrams illustrating the development of the vertebrate brain: A, First stage, side view, the cavity indicated by dotted line; B, second stage; C, third stage, side view of a brain with- out cerebral hemispheres; D, the same in sagittal section; E, fourth stage, side view of a brain with cerebral hemispheres; F, the same in sagittal section; G, dorsal view of the same with the cavities exposed on the right side. Rkin., rhinoccele; Lat. Vent., lateral ventricle; Int. For., interventricu- lar foramen; Vent. Ill, third ventricle; Vent. 71', fourth ventricle. /, Prosencephalon; / a. Telen- cephalon; / a-r, Rhinencephalon; 1 a-p. Pallium; 1 a-lt, Lamina terminalis; / a-ch, Cerebral hemisphere; 1 a-cs, Corpus striatum; 1 b, Diencephalon; 1 b-t, Thalamus. 2, Mesencephalon; 2 c, Optic lobes; 2 d, Crura cerebri, j, Rhombencephalon; j a, Metencephalon; 3 a-c, Cerebellum; 3 b, Myelencephalon. is one of these on either side, the corpora striata (Fig. 7, D). Another part of the wall is relatively thin and is known as the pallium, while the part directly associated with the olfactory nerve belongs to the rhinencephalon. The most important factor in the evolution of the vertebrate brain is the progressive e\ -ag- ination of the lateral walls of the telencephalon to form paired masses, the cerebral hemispheres. In primitive forms like the cyclostomes only a part of the rhinencephalon has been evaginated, and in them the hemisphere consists only of an olfactorv bulb and olfactory lobe. This stage of development is roughly 26 THE NERVOUS SYSTEM indicated in Fig. 7, C, D. In the selachians, as illustrated in Figs. 8, 9, 10, and 11, the evagination has progressed further than in cyclostomes. Still further progress in this direction has been made by the amphibians, the cerebral hemi- spheres of which have reached about the stage of development indicated in Fig. 7, £, F, G. Here the entire lateral wall, including the pallium and corpus striatum, has been evaginated in the formation of the cerebral hemisphere. The Brain Ventricles. — The portions of the original cavity of the neural tube which are contained within the evaginated cerebral hemispheres are known as the lateral ventricles (Fig. 7, G). These paired ventricles communicate with the median prosencephalic cavity by openings known as the interventricular foram- ina. This median cavity, called the third ventricle, represents for the most part the cavity of the diencephalon, but its rostral part, bounded by the lamina terminalis, belongs to the telencephalon. It will be seen by a study of the accompanying diagrams that this lamina also belongs to the telencephalon and represents in a certain sense the rostral end of the brain. Its position should be' carefully noted in each of the diagrams. The cavity of the rhombencephalon is known as the fourth ventricle and that of the mesencephalon as the cerebral aqueduct. The latter connects the third and fourth ventricles. It will help us to understand the morphology of the vertebrate brain if we now consider the shape and arrangement of the various parts of a simple brain like that of the dogfish. THE BRAIN OF THE DOGFISH— SQUALUS ACANTHIAS The telencephalon of the selachian brain is evaginated to form a pair of laterally placed masses, the cerebral hemispheres, and in this respect is at a stage of development not far removed from that represented in diagrams E, F, and G of Fig. 7. The long axis of the brain is almost straight; and this freedom from ventrodorsal curvatures makes it especially easy to recognize the various funda- mental divisions already enumerated and to understand their relationship. The medulla oblongata, which together with the cerebellum forms the rhom- bencephalon, is continuous at the caudal extremity with the cylindric spinal cord, and within it the central canal of the spinal cord opens out into the fourth ventricle (Fig. 8). The medulla, which has somewhat the shape of a trun- cated cone, is considerably larger than the cord, but decreases in size as it is traced backward toward their point of junction. In the mammal a conspicuous transverse bundle of fibers, associated with the cerebellum, is found on the ventral and lateral aspects of that part of the medulla which belongs to the metencephalon and is known as the pons. But in the fish it is customary to MM \l l R \l. TUBE AM) ITS DERTVATIVES 2 7 consider the medulla oblongata as extending from the spinal cord to the tnesen cephalon. It forms the ventral and lateral walls of the fourth ventricle; and when the roof of this cavity has been removed these walls arc seen to surround a long and rather broad depression the fossa rhomboidea or floor of the fourth ventricle which tapers caudally like the point of a pen (Fig. 9). The cerebellum forms an elongated mass the rostral end of which overhangs the optic lobes, while the caudal extremity projects over the medulla oblongata /Nasal capsule Olfactory nrrve N. I / Rhinocaele , Lateral ventricle Olfactory bulb - - - Nervus terminates Olfactory trait Cerebral hemisphere Interventricular for ... .Epiphysis -*^m _^"" ~ ' Optic nerve_N. II -Thalamus--- Optic lobes M trochlear nerve N. Ill k::. ■ Cerebellum Lobus linece lateralis. Facial nerve N. VIL Acoustic nerve N. VIII Tuberculum acusticum. — Medulla oblongata- Glossopharyngeal nerve N. IX. Medial longitudinal fasc. — Visceral lobe Telencephalon -Vagus nerve N. X .--Spinal cord- — *\-Tkird ventricle ) Diencephalon Mesoceele V Mesencephalon I VMetaccele / Metencephalon Cerebellum (caudal part) Rhomboid fossa Myelencephalon Fig. 8. — The brain of the dogfish, Squalus acanthias, dorsal view. Fig. 9. — The brain of the dogfish, Squalus acanthias, with the ventricles opened, dorsal view. (Fig. 8). Its dorsal surface is grooved by a pair of sulci arranged in the form of a cross. It contains a cavity, a part of the original rhombencephalic vesicle, which communicates with the fourth ventricle proper through a rather wick opening (Fig. 11). Behind the cerebellum the fourth ventricle possesses a thin membranous roof which was torn away in the preparation from which Fig. 8 was drawn. 28 THE NERVOUS SYSTEM Mesencephalon. — The optic lobes on the dorsal aspect of the mesencephalon are a pair of rounded masses separated by a median sagittal sulcus. They represent the bulging roof of the mesencephalic cavity and are accordingly Cerebellum Optic lobe I Thalamus ; Cerebral hemisphere Olfactory bulb Vagus nerve N. X / / / \\\ Glossopharyngeal nerve N. IX ' / / \*A Acoustic nerve N. VIII / \ Abduccns nerve N . VI Olfactory trad Optic nerve N. II \ Inferior lobe Oculomotor nerve N. Ill _ . . , , , . , „ „ ., " ' Saccus vasculosus Trigeminal and facial nerves Nn. V, VII Trochlear nerve N. IV Fig. 10. — The brain of the dogfish, Squalus acanthias, lateral view. spoken of as the tectum mesencephali. Within this roof end the fibers which come from the retime through the optic nerves. The floor of the cavity is formed by the ventral part of the mesencephalon. This appears like a direct continua- tion of the medulla oblongata, and in the mammal bears the designation crura Optic lobe Epiphysis . Mesoceele Cerebellum Cerebral hemisphere Olfactory tract , /\ Olfactory bulb Paraphysis Metacode Tubcrciilum acusticum ■ Tela chorioidea ventricle rral lobe Preoptic recess Velum transversum \ Metcnccphalon Myclcncephalon Saccus vasculosus Mesencephalon Optic chiasma Third ventricle Fig. 11. — The brain of the dogfish, Squalus acanthias, medial sagittal section. cerebri. Emerging from the roof of the mesencephalon between the cerebellum and optic lobe is the fourth or trochlear nerve, and from the ventral aspect of this division of the brain arises the third or oculomotor nerve. The Diencephalon. — The thin roof of the diencephalon, which can easily THE NEURAL Mi:i WD IIS DERIVATIVES be torn away so as to expose the third ventre le I igs. 8, 9), is atta< bed bj its caudal margin to a ridge containing a pair of knob like thickenings the habe- nular nuclei and a commissure connecting tin- tw<> (Fig. 11). From a point just caudal to the middle of this commissure there projects forward over the membranous roof oi the ventricle a slender tube, the epiphysis cerebri or pineal body, which comes in contact with the roof of the >kull and ends in a slightly dilated extremity. The epiphysis and hahenular nuclei belong to the e pi thala- mus. 'The thalamus forms the thick lateral wall of the third ventricle and i- traversed by the optic tracts on their way to the optic lobes. The hypothalamus Nasal sac Epiphysis Superior oblique Trochlear nerve Medio! reel us Superior rectus Lateral rectus Vestibule Spiracle Semicircular canal Glossopharyngeal nerve Vagus Branchial cleft i uperficial ophthalmu \ '. 17/ Olfactory capsule Inferior oblique Maxillary V Mandibular V Palatim VII Spiracle Hyomandibular VII Glossopharyngeal i. Branchial cleft Vagus Spinal cord Lateral line branch of vagus Fig. 12. — Dissection of the brain and cranial nerves of the dogfish, Scyllium catulus. The eye is shown on the left side, but has been removed on the right. (Marshall and Hurst, Parker and Haswell.) is relatively large in the shark and presents, in addition to a pair of laterally placed oval masses, or inferior lobes, a thin walled vascular outgrowth, the saccus vasculosus. Closely related to the ventral aspect of the hypothalamus is a gland- ular mass, derived by a process of evagination from the oral epithelium, and known as the hypophysis. For a picture of this structure in the adult dogfish reference should be made to a paper on the subject by Baumgartner (1915 On the ventral surface of the hypothalamus the optic nerves meet and cross in the optic chiasma. The telencephalon includes all of the brain in front of the velum transversum, 3° THE NERVOUS SYSTEM a transverse fold projecting into the third ventricle from the membranous roof (Fig. 11), and consists of a median unpaired portion, and of the two cerebral hemispheres with their olfactory bulbs. The hemispheres are the evaginated portions of the telencephalon and are partially separated from each other by a r. ophthal. superfic. V r. ophthal. superfic. VII n. terminalis ,r. ophthal. profundus V \ Optic nerve (n. II) r. maxillaris V r. mandib. V Supra-orbital trunk Infra-orbital trunk Ganglion V r. palatinus VII Gang, geniculi VII Gang, later. VII r. prespirac. VII -Spiracle -r. hyomandib. VII n. IX n. X r. lateralis X r. branchialis X r. intestinalis X Fig. 13. — Diagram of the brain and sensory nerves of the smooth dogfish, Mustelus canis, from above. Natural size. The Roman numerals refer to the cranial nerves The olfactory part of the brain is dotted, the visual centers are shaded with oblique cross-hatching, the acoustico- lateral centers with horizontal lines, the visceral sensory area with vertical lines, and the general cutaneous area is left unshaded. On the right side the lateral line nerves are drawn in black, the other nerves are unshaded. (From Herrick's Introduction to Neurology.) median sagittal fissure, which has been to a large extent obliterated by the fusion of their median walls. The shape of the lateral ventricle and the position of the interventricular foramina are shown in Fig. 9. From the lateral side of the rostral end of the hemisphere there projects forward the long and slender olfactory tract with a terminal enlargement, the olfactory bulb. This lies in I III NEURAL l l BE WD I Is i > I i ; I \ STIVES contacl with the nasal sac to which it gives off a number of fine nerve bundles, which together constitute the olfactory or first cranial nerve. At the rostral end of the brain an additional nerve makes its exit from the hemisphere. Ii is known as the nervus terminalis and can be Followed forward over the olfactory tract and bulb to the nasal sac (Fig. 8). The roof of the selachian forebrain presents a number of st ructures of great morphologic interest, two of which have already been mentioned, namely, the epiphysis and velum transversum. The former is an outpocketing of the roof of the diencephalon; the latter is an infolding and marks the line of separation between the t wo divisions of the prosenceph- alon. Rostral to the velum the roof of the telencephalon is evaginated to form a thin-walled sac, the paraphysis. The velum and paraphysis arc readily identified in the mammalian embryo, hut become obscured in the course of later development. The morphology of this region has recently been studied in great detail by a number of American investigators: Minot (1901), Johnston (1909), Terry (1910), Warren (1911, 1917), and Bailey (1916). A good idea of the shape and connections of the various brain ventricles and of the relation of the various parts of the brain to each other can be obtained from a study of Figs. 9 and 11. In Fig. 13 there is indicated the location of the principal sensory areas of the brain of the smooth dogfish, and the relation of these areas to the corresponding peripheral nerves is apparent. The lateral line components of the seventh and tenth cranial nerves are indicated in black. DEVELOPMENT OF THE NEURAL TUBE IN THE HUMAN EMBRYO In its embryonic development the nervous system of man presents some- thing like a synopsis of the early chapters of its phyletic history. The neural groove is the most conspicuous part of an embryo of 2.4 mm. (Fig. 14). Xear the middle of the body it has closed to form the neural tube, and from this region the closure proceeds in both directions. The last points to close are situated at either end and are known as the neuropores. The rostral end of the groove shows enlargements which upon closure will form the brain vesicles. The longer portion, caudal to these enlargements, represents the future spinal cord. Except that it is flexed on itself, the brain of the human embryo of five weeks (Fig. 15) shows a marked resemblance to the diagram of a vertebrate brain without cerebral hemispheres (Fig. 7, C, D). The prosencephala vesicle is divided by a constriction into the telencephalon and diencephalon with freely intercommunicating cavities. The mesencephalon is well denned and presents a sharp bend, the cephalic flexure. The rhombencephalon shows signs of sepa- ration into the metencephalon and myelencephalon and is slightly bent dorsally at the pontine flexure. Another curvature which develops at the junction of 3 2 THE NERVOUS SYSTEM the brain and spinal cord is known as the cervical flexure (Fig. 16). From the walls of the prosencephalon there develop outpocke tings on either side, which form the optic cups and which are connected to the brain by the optic stalks. From the cup develops the retina and through the stalk grow the fibers of the optic nerve. These structures are, therefore, genetically parts of the brain. The Telencephalon of the Human Embryo. — By the time the embryo has reached a length of 13 mm. the brain has passed into the stage represented by Mesencephalon Rhombencephalon Myelenccphalon Amnion (cut) Mesodermal segment 14 Open neural groove Prosencephalon Stomodccum Amnion (cut) Yolk sac Bodv stalk Fig. 14. — Human embryo of 2.4 mm. showing the neural tube partially closed. (Kollmann.) diagrams E, F, G of Fig. 7. The lateral wall of the telencephalon, with the corpus striatum and olfactory brain or rliinenccphalon, has been evaginated on either side to form paired structures, the cerebral hemispheres (Fig. 16). Ex- cept for the corpus striatum and rhinencephalon the evaginated wall is relatively thin, develops into the cerebral cortex, and is known as the pallium. The lateral ventricles within the hemispheres represent portions of the original telen- cephalic cavity and communicate with the third ventricle through the inter- llll Ml K \l. I l BE \M) [TS Dl. kl\ ATIVES 33 ventricular foramina, which at this stage arc relatively large. The lamina terminalis, connecting the two hemispheres in front of the third ventri< le, repre- Dietu ephalon Pallium ephalon Cephalic flexure B Thalamus Pallium Optic cup Pontine flexure liyelencephalon — J . : fg Meten- i ephalon Corpus striatum Optic recess Hypothalamus Mesencephalon Isthmus Cerebellum Medulla obi Fig. 15. — Reconstructions of the brain of a 7 mm. embryo: A, Lateral view; B, in median sagittal section. (His, Prentiss-Arey.) sents in a certain sense the rostral end of the brain. Immediate]}- behind this lamina is a portion of the telencephalic cavity which forms the anterior part of Cerebral aqueduct Cerebral peduncle , _..__■_.., , Mesence bhalon Hypothalamus .v4j»5*^^^hfck- Epithalamus ■ _Vjf\^J| Rife, Mombenccphalic isthmus Thalamus. Diencephalon- Pallium Telencephalon-.. Cerebellum Metcnccphalon Rhomboid fossa Myelencephalon Rhincnccphalon | Corpus striatum Pons Lamina terminalis Spinal cord Fig. 16. — A median section of the brain of a 13.6 mm. human embryo: 1, Optic recess; 2, ridge formed by optic chiasma; 3, optic chiasma; 4, infundibular recess. (His, Sobotta.) the third ventricle. The further development of these structures is readily traced in Fig. 17, which represents the brain of a human fetus of the third 34 THE NERVOUS SYS1 I M month. The most striking feature of the brain at this stage is the great size attained by the cerebral hemispheres. The Diencephalon. — The three principal divisions of the diencephalon— the thalamus, epithalamus, and hypothalamus — faintly indicated in an embryo Thalamus i Pineal body {epithalamus) Diencephalon : Chorioid plexus Corpus striatum Telencephalon .;' Cerebral peduncle ! Cerebral aqueduct Mesencephalon Isthmus - Cerebellum ' Metcnccphalon Rhomboid fossa Myclcncephalon . 'tic Hypo- /chiasma physis Medulla Lamina terminalis / ^Hyp^ihaiamus oblon ^ ata Rhincnccphalon - Spinal cord Central canal Fig. 17. — The brain of a fetus of the third month in median sagittal section. (His, Sobotta.) of 13.6 mm., are well defined by the third month (Fig. 17). In transverse sections this division of the embryonic brain is seen to be composed of a pair of plates on either side, which with a roof and floor form the walls of the ventricle Roof plate (with chorioid plexus) ■ ^j^\.Al ar plate or Thalamus Sulcus limit ans Basal plate or Hypothalamus Mammillary recess Fig. 18. — Transverse section through the diencephalon of a 13.8 mm. embryo. (His, Prentiss- Arey.) (Fig. 18). The dorsal lamina is known as the alar plate, the ventral as the basal plate. On either side these meet at an angle, forming the sulcus limitans. These laminae and the sulcus limitans between them can be traced back through the I 111 Ml RAL I 1 BE AND I rS DERIVA1 1\ ES 35 mesencephalon and rhombencephalon into the spinal cord. The thalamus is produced by a thickening in the alar lamina and i> separated from the hypo- thalamus by the sulcus limitans, which can be traced as far as the optic re rostral to the ridge produced by the optic chiasma. The hypothalamus 1 represents the basal lamina and gives rise to the tuber Ctnereum, posterior lobe of the hypophysis, and the mammillary bodies. From the dorsal edge of the alar lamina. \\ here i his is attached t<> the thin roof plate, there is developed a thickened ridge, the epithalamus, which is transformed into the habenula and the pineal body. The root" plate of the diencephalon remains thin and forms the epithelial lining of the tela chorioidea or roof of the third ventricle. The Mesencephalon. — The basal plate of the mesencephalon thickens to form the cerebral peduncles (Fig. 17), the alar plate forms the lamina quad- rigemina in which are differentiated the quadrigeminal bodies; the cavity be- comes the cerebral aqueduct. Table Showing Subdivisions of the Neural Tube and Their Derivatives (Modified from a Table in Keibel and Mall, Hitman Embryology). Primary vehicles. Subdivisions. Derivatives. Lumen. f Prosencephalon. ... Telencephalon Cerebral cortex, Corpora striata, Rhinencephalon, Pars-optica hypo- thalami. Lateral ventricles. Rostral portion of the third ventricle. Brain 1 Diencephalon Epithalamus, Thalamus, Hypothalamus, Hypophysis, Tuber cinereum, Mammillary bodies, Metathalamus. The greater part of the third ventricle. Mesencephalon f Mesencephalon < Corpora quadri- gemina, Crura cerebri. Cerebral aqueduct. Rhombencephalon . Metencephalon < Myelencephalon Cerebellum, Pons, Medulla oblongata.] Fourth ventricle. Spinal cord Spinal cord. Central canal. x The pars optica hypothalami, including the optic chiasm, is, properly speaking, not a part of the hypothalamus at all, but belongs to the telencephalon (Johnston, 1909, Jour. Comp. ^eitr , vol. 19, and 1912, Jour. Comp. Neur., vol. 22). 36 THE NERVOUS SYSTEM The Rhombencephalon. — The ventral part of the rhombencephalon, includ- ing both alar and basal plates, thickens to form the pons and medulla oblongata (Fig. 17). Most of the roof of this division remains thin and forms the epithelial lining of the tela chorioidea of the fourth ventricle. But in the caudal portion of the myelencephalon the lumen of the neural tube becomes completely sur- rounded by thickened walls, forming the central canal of the closed portion of the medulla. The posterior edge of the alar plate in the metencephalon becomes greatly thickened and. fusing across the median line with the similar structure of the opposite side, forms the anlage of the cerebellum (Figs. 17. 137). Later we shall see that, in general, motor structures develop from the basal, and sen- sory parts from the alar, plate. The table on page 35 gives in brief the principal derivatives of the neural tube. CHAPTER III HISTOGENESIS OF THE NERVOUS SYSTEM Early Stages in the Differentiation of the Neural Tube. Hardesty (1904) has given a good account of the early development of the- spinal cord in the pig. At first the neural plate consists of a single layer of ectodermal cells I Fig. 19, A). These proliferate and lose their cell boundaries. When the neural tube has closed its wall is formed of several layers of fused cells — a syncytium — bounded by an external and an internal limiting membrane (Fig. 19, B, C). The syn- cytium now becomes more open and sponge-like in structure. The nuclei are so arranged that three layers may be differentiated: (1) an ependymal layer, (2) a mantle layer, with man)- nuclei, and (3) a marginal or non-nuclear layer. The ependymal layer is represented by a row of elongated nuclei, among which are found the large mitotic nuclei of the germinal cells. These germinal cells divide and give rise to ependymal cells, and to the indif- ferent cells of the mantle layer. Through division of the latter spongioblasts and neuroblasts are formed. From the former comes the neuroglia or supporting tissue of the nervous system, while from the latter are derived the nerve-cells and fibers. The Development of the Neuron. — A neuron may be defined as a nerve- cell with all its processes; and each is derived from a single neuroblast. From the pear-shaped neuroblast a single primary process grows out. and this be- comes the axis-cylinder of a nerve-fiber (Fig. 20). Other processes which de- velop later become the dendrites. The primary process, or axon, grows into the marginal layer, within which it may turn and run parallel to the long axis of the neural tube as an association fiber; or it may run out of the neural tube in a ven- trolateral direction as a motor axon. In this way the motor fibers of the cere- brospinal nerves are laid down. The axis-cylinder of each represents a process which has grown out from a neuroblast in the basal plate of the neural tube. Development of Afferent Neurons. — The sensory or afferent libers of the spinal nerves take origin from neuroblasts which are from the beginning out- side the neural tube. These neuroblasts are derived from the neural crest, a longitudinal ridge of ectodermal cells at the margin of the neural groove, where this becomes continuous with the superficial ectoderm. At first in contact with 37 38 THE NERVOUS SYSTEM the dorsal surface of the neural tube, the neural crest soon separates from it and comes to lie in the angle between it and the myotomes. In this position the neural crest gives rise to a series of sensory ganglia. From neuroblasts located in these ganglia arise the sensory fibers of the cerebrospinal nerves. Marginal layer Manlle layer Ependymal layer Germinal ell Marginal layer Ependymal layer Mesoderm Marginal layer 5" ..SfSfV--*,; ***£»*? -V.' Internal limiting membrane Ependymal layer Mm 'V £4 ^Germinal i ce// SpHfe External limiting membrane External limiting membrane Mantle layer Internal limiting membrane Germinal cell Internal limiting membrane Mesoderm Marginal layer * Mantle layer Ependymal layer Fig. 19. — Early stages in the differentiation of the neural tube: A, From a rabbit embryo before closure of neural tube; B, from a 5 mm. pig embryo after closure of tube; C, from a 7 mm. pig embryo; D, from a 10 mm. pig embryo. *, Boundary between nuclear and marginal layers. (Hardesty, Prentiss-Arey.) This last statement requires some qualification. The fibers of the olfactory nerve arise from cells in the olfactory mucous membrane. The fibers of the mesencephalic root of the trigeminal nerve, which in all probability are sensory, arise from cells located within the mesencephalon. The optic nerve is also an exception, but this is morphologically a fiber tract of the brain and not a true nerve. An ingenious theory, advanced by Schulte and Tilney (1915), attempts to bring this mesencephalic root and the optic nerve into more ob- HISTOGENESIS OF THE NERVOUS SYSTEM 39 vious relation with the other sensory nerves. They assume thai the pari of the neural i which lies rostral to the anlage Of the semilunar ganglion, fails tO separate from the neural tube. From this pari of the neural crest, retained within the brain, they would derive the mesencephalic nucleus of the trigeminal nerve and the optic vesicles. On I he ot her hand, there are observal ions which tend to show t hat some of t he cranial sensory ganglia arc derived at least in pari from other sources than the neural crest. This i> especially true of the acoustic ganglion (Strecter, 1912). According to Landacre (1910) many of the sensory ganglion nils ol the seventh, ninth, and tenth nerves are derived from Fig. 20. — A, Transverse section through the spinal cord of a chick embryo of the third day showing neuraxons (F) developing from neuroblasts of the neural tube and from the bipolar ganglion cells, d. B, Neuroblasts from the spinal cord of a seventy-two-hour chick. The three to the right show neurofibrils; C, incremental cone. (Cajal, Prentiss-Arey.) thickened patches of the superficial ectoderm, known as placodes, with which the ganglia of these nerves come in contact at an early stage in their embryonic development. The acoustic ganglion of the eighth nerve seems also to have a similar origin, i. c. from the cells of the otic vesicle which is formed by a process of invagination from the superficial ectoderm. The neuroblasts of these ganglia become bipolar through the development of a primary process at either end (Fig. 21). Originally bipolar, a majority of these sensory neurons in the mammal become unipolar through the fusion of the two primary processes for some distance into a single main stem. Beyond the point of fusion this divides like a T into two primary branches, one of which 4Q THE NERVOUS SYSTEM is directed centrally, the other peripherally. The centrally directed branch grows into the neural tube as a sensory root fiber (Fig. 20, A, d); the other grows peripherally as an afferent fiber of a cerebrospinal nerve. This general state- ment requires some qualification. It may be that some bipolar neuroblasts become unipolar by the absorption of one of the primary processes, while the remaining one divides dichotomously into central and peripheral branches (Streeter, 1912). It should also be noted that the cells of the sensory ganglia of the acoustic nerve remain bipolar throughout life. Development of the Spinal Nerves. — We have traced the development of the chief elements entering into the formation of the cerebrospinal nerves, and will now see how these are combined in a typical spinal nerve. The spinal ganglion, derived from the neural crest, contains bipolar neuroblasts, which are transformed into unipolar neurons. The axon of such a nerve-cell divides into a cen- tral branch, running through the dorsal root into the spinal cord, and a peripheral branch, running distally through the nerve to reach the skin or other sensitive portion of the body. Mingled with these afferent fibers in Fig. 21. — A section of a spinal ganglion from a 44 mm. fetus, showing stages in the trans- formation of bipolar neurons, A, into unipolar neurons, B. Golgi method. (Cajal.) the spinal nerves are efferent axons which have grown out from neuroblasts in the basal plate of the spinal cord, through the ventral root, and are distributed by way of the spinal nerve to muscles. So far we have dealt only with the origin of the axis-cylinders of the nerve- fibers. But these soon become surrounded by protective sJicaths which are also ectodermal in origin. In the path of the outgrowing axons there are seen nu- merous spindle-shaped ectodermal cells, which have migrated from the anlage of the spinal ganglia (Harrison, 1906), and perhaps also from the neural tube along the ventral roots (Held. 1909). These cells form such a prominent feat- ure in a developing nerve that some workers have thought the axons differen- tiate in situ from them. This theory, which has been known as the cell-chain HISTOGENESIS OP THE NERVOUS SYSTEM 4 i hypothesis, and gives to each axon a multicellular origin, has been supported by Schwann, Balfour, Dohrn, and Bethe, and in modified forms In other workers. There are good reasons, however, for believing that each axon arises as an out- growth from a single cell or neuroblast. 'I hi- idea, which i- in keeping with what is known of the structure and function of the neuron and whi< h forms an integral part of the now generally accepted neuron theory, was first developed in the embryologic publications of \\\>. Convincing experimental evidence has Keen furnished by Harrison (1906). Using amphibian larva', this author sho that if the neural crest and tube are removed no peripheral nerves develop. He further showed that isolated nerve-cells cultivated in clotted lymph will Roof pi ale Dorsal column Dorsal root Mantle laxcr Ventral column Dorsal funiculus Neural cavity Marginal layer Ependymal layer Floor plate Ventral median fissure Fig. 22. — Transverse section of the spinal cord of a 20 mm. human embryo. (Prentiss-Arey. I give rise to long axons in the course of a few hours. But the ectodermal cells, mentioned above, which migrate outward along the course of the developing nerve, take an important part in the differentiation of the fibers. From them is derived the nucleated sheath or neurilemma of the peripheral nerve-fiber. The myelin sheath is composed of a fatty substance of uncertain origin. It may be a product of the axon, of the neurilemma, or of both. The sympathetic ganglia consist of cells derived like those of the spinal ganglia from the neural crest, and. according to Kuntz (1910), also from the neural tube by migration along the course of the cerebrospinal nerves. These cells become aggregated in the ganglia of the sympathetic system and are asso- ciated with the innervation of smooth muscle and glands. 42 THE NERVOUS SYSTEM The spinal cord of a 20 mm. human embryo presents well-defined ependymal, marginal, and mantle layers. Figure 22 should be compared with the appear- ance presented by a cross-section of the spinal cord in the adult (Fig. 55). The mantle layer with its many nuclei differentiates into the gray matter of the spinal cord, which contains the nerve-cells and their dendritic processes. The mar- ginal layer develops into the white substance as a result of the growth into it of the axons from neuroblasts located within the mantle layer. These form association fibers which ascend or descend through the marginal layer and serve to connect one level of the neural tube with another. It is not until these longitudinally coursing axons develop myelin sheaths that the white substance acquires its characteristic coloration. The cavity of the neural tube is relatively large, and at the point marked "neural cavity" in Fig. 22 a groove is visible. This is the sulcus limitans. It separates the dorsal or alar plate from the ventral or basal plate. The mantle layer of the alar plate develops into the dorsal gray column which, like the other parts developed from this plate, is afferent in function. The afferent fibers, growing into the spinal cord from the spinal ganglia, either terminate in this dorsal column or ascend in the posterior part of the marginal zone to nuclei derived from the alar plate in the myelencephalon. Most of the association fibers which run in the marginal layer have grown out from neuroblasts located in the dorsal column. The mantle layer of the basal plate gives rise to the ventral gray column. From the neuroblasts in this region grow out the motor fibers of the ventral roots and spinal nerves. From what has been said it will be clear that the entire nervous system is ectodermal in origin. The nervous element proper or neurons are derived from the neuroblasts; the supporting tissue of the brain and spinal cord, the neuroglia, is derived from spongioblasts; while the neurilemma of the peripheral nerves is the product of sheath cells which have migrated out from the spinal ganglia and possibly also from the neural tube. CHAPTER IV NEURONS AND NEURON-CHAINS Tin oervous system is composed of highly irritable cellular units, or neurons, linked together to form conduction pathways. In the preceding chapter we have seen that each neuron is the product of a single embryonic cell or neuro- blast, and that, therefore, the nerve-cell with all its processes constitute- a gen- etic unit. In the present chapter, as we examine the form and internal struc- ture of the neurons and their relation to each other, we shall learn that they are also the structural and functional units of the nervous system. Form. — There is the widest possible variation in the shape of nerve-cell-, but all present - »me features in common. About the nucleus there is an accumu- lation of cytoplasm which together with the nucleus forms what is often called the cell body. A convenient term by which to designate the circumnuclear cytoplasmic mass is perikaryon. From the perikaryon cytoplasmic processes are given off. some of which may be of great length. The external form of the neuron depends on the shape of the perikaryon and on the number, shape, and ramification of these processes. Since the variety of forms is almost with- out limit, we will content ourselves with studying a few typical examples. The pyramidal cells of the cerebral cortex are good examples (Fig. 23). The perikarvon is triangular in form. One angle, that directed toward the surface of the cortex, is prolonged in the form of a long thick branching process, the apical dendrite. From the sides and other angles of the perikaryon arise shorter branching dendrites, while from the base or from one of the basal dendrite- uri-es a long slender process, the axon. The characteristic features of the den- drites are as follows: they branch repeatedly, rapidly decrea-e in size, and terminate not far from the cell body. Their contour is irregular and they are studded with short side branches, or gemmules. which give them a spiny appear- ance. Each neuron usually possesses several dendrites, but in some types oi nerve-cells they are absent altogether. The axon, on the other hand, is char- acterized by its uniform smooth contour, relatively small diameter, and in most instances by its great length and relative freedom from side branches. It may give off fine side branches, or collaterals, near its origin; and these arise at right 43 44 1111. M.k\ I 'I - SYS1 I.M Fig. 23. — A pyramidal cell from the cere- bral cortex of a mouse: a, Dendrites from the base of the cell; b, white substance of the hemisphere into which the axon, e, can be traced; c, collaterals from the first part of the axon; /, apical dendrite; p, its terminal branches near the surface of the cortex. Golgi method. (Cajal.) cerebrospinal ganglia (Fig. 40). angles to the parent stem. The axon terminates in a multitude of fine branches usually at a considerable distance and sometimes as much as a meter from its origin. The origin of the axon from the perikaryon is marked by an expansion known as the cone of origin or im- plantation cone. This cone, like the axon, differs somewhat in structure from the perikaryon. Such long axons as have just been described are character- istic of the cells of Golgi's Type I. That not all axons are long and relatively unbranched is seen from Fig. 24. which illustrates a cell of Gold's Type II. The axons of these cells are short, branch repeatedly, and end in the neighborhood of the cell body. Another good example is furnished by the primary motor neurons. Figure 25 illustrates such a cell from the anterior gray column of the spinal cord. This is a large nerve-cell with many rather long branching dendrites and an axon. which forms the axis-cylinder of a motor nerve-fiber and terminates by forming a motor ending in a muscle. As illus- trated in this figure, long axons tend to acquire myelin sheaths, and those which run in the cerebrospinal nerves are also covered by a nucleated mem- branous sheath — the neurilemma. Xerve-cells with many proce- such as have just been described, are called multipolar. Examples of unipo- lar and bipolar cells are furnished by the These cells, which will be described in more \i i RONS AND Ml RON < II \l\s 45 detail in another chapter, arc devoid of dendrites. The axon of such a unipolar cell divides dichotomously into a central and a peripheral branch, cadi po ing the characteristics of an axon. li is not uncommon to regard the peripheral branch of a sensory neuron as a dendrite, because like the dendrites ii conducts nerve impulses toward the cell body. But, since it possesses all the morphologic characteristics of an axon, and since any axon is able to con- duct nerve impulses throughout its length in either direction, and since these peripheral branches Of the sensory neurons actually convey impulses distally in the phenomenon ol Fig. 24. — Neurons with short axons (Type II of Golgi) from the cerebral cortex of a child: a, Axon. Golgi method. (Cajal.) antidromic conduction (Bayliss, General Physiology, p. 474), it seems best to consider both central and peripheral branches as divisions of a common axonic stem. (See Barker, The Nervous System, p. 361.) From what has been said it will be apparent that a neuron usually possesses several dendrites and a single axon, but some have only one process, which is then an axon. It may be added that some neurons have more than one axon. Nerve-fibers are axons naked or insheathed. Two myelinated peripheral nerve-fibers are shown in Fig. 26. The axon or axis-cylinder is composed of 46 THE NERVOUS SYSTEM delicate neurofibrils embedded in a semifluid neuroplasm. It is surrounded by a relatively thick myelin sheath and a nucleated membranous neurilemma sheath. Fig. 25. — Primary motor neuron (diagram- matic): ah, Implantation cone of axon; ax, axon; c, cytoplasm; d, dendrites; w, myelin sheath; m', striated muscle; n, nucleus; »', nucleolus; nR, node of Ranvier; sf, collateral; si, neurilemma; tel, motor end-plate. (Barker, Bailey.) Fig. 26. — Portions of two nerve-fibers stained with osmic acid (from a young rabbit). Diagrammatic. 425 diameters: RR, Nodes of Ranvier, with axis-cylinder passing through; a, neurilemma; c, opposite the middle of the seg- ment, indicates the nucleus and protoplasm ly- ing between the neurilemma and the medullary sheath. In A the nodes are wider, and the in- tersegmental substance more apparent than in B. (Schafer, in Quain's Anatomy.) The myelin sheath consists of a fatty substance, myelin, supported by a retic- ulum of neurokeratin. The latter, not seen in the living fiber, may be a coag- ulation product produced during fixation. The highly refractive myelin gives \l I RONS AMi \ll RON ' BAINS 47 to the myelinated fibers a whitish color. This sheath is interrupted al regular intervals by constrictions in the nerve fiber known as the nodes of Ranvier. The constrictions are produced by a dipping in of the neurilemma sheath toward the axon, which runs without interruption through the node. The part of a fiber between each node is an internoda] segment, and each such segmenl pos sesses a nucleus which is surrounded by a small amount of cytoplasm and lies jusl beneath the neurilemma. The latter is a thin membranous outer covering tor the fiber. Each segment of the neurilemma sheath, together with the cell which lies beneath, is the product of a single sheath cell of ectodermal origin. Fibers such as have just been described are found in the cerebrospinal nerves, and give these their white glistening appearance. The myelinated fibers of the brain and spinal cord are of somewhat different structure. There is no evidence of segmentation in the myelin sheath and neither the neurilemma nor its cells are present. This fact is of much im- portance in the phenomena of regeneration, as will be explained later. These are the fibers which give the characteristic color to the white matter of the brain and spinal cord. Unmyelinated fibers are of two kinds, namely, Remak's fibers and naked axons. The former possess nuclei which may be regarded as belonging to a thin neurilemma. They are found in great numbers in the sympathetic nervous system, and many of the fine afferent fibers of the cerebrospinal nerves also belong to this class (Ranson, 1911). Naked axons are especially numerous in the gray matter of the brain and spinal cord, and it may be added that every axon at its beginning from the nerve-cell, as well as at its terminal arborization, is devoid of covering. By way of summary we may enumerate four kinds of nerve-fibers: (1) myelin- ated fibers with a neurilemma, found in the peripheral nervous system, especially in the cerebrospinal nerves; (2) myelinated fibers without a neurilemma, found in the central nervous system; (3) unmyelinated fibers with nuclei (Remak's fibers), especially numerous in the sympathetic system, and (4) naked axons, abundant in the gray matter of the brain and spinal cord. Neuroglia cells and fibers will be considered in connection with the structure of the spinal cord. Structure of Neurons. — Like other cells, a neuron consists of a nucleus sur- rounded by cytoplasm, and these possess the fundamental characteristics which belong to nucleus and cytoplasm everywhere, but each presents certain features more or less characteristic of the nerve-cell. The nucleus is large and spheric; 48 THE NERVOUS SYSTEM and, because it contains little chromatin, it stains lightly with the basic dyes (Fig. 27, .1). It contains a large spheric nucleolus. The cytoplasm, enclosed in a cell membrane, is characterized by the presence of basophil granules and a fibrillar reticulum. The granules, which apparently are a product of the nucleus, are composed of nucleoprotein. They are grouped in dense clumps, known as Nissl bodies or tigroid masses, and stain deeply with methylene-blue. The size, shape, and arrangement of the Xissl bodies differ with the type of nerve-cell studied. They are much larger in motor than in sensory neurons (Malone, 1913). While they are found in the larger dendrites, the axon and its cone of origin are free from them. They are intimately concerned in the A xon Fig. 27. — Nerve-cells stained with toluidin blue: A, From anterior horn of spinal cord of the monkey, shows Xissl bodies in cytoplasm; B, from the facial nucleus of a dog, shows a partial disappearance of the Nissl bodies (chromatolysis) resulting from section of the facial nerve. (Schafer.) metabolic activity of the cell, increasing during rest and decreasing as a result of fatigue. They also undergo solution as a result of injury to the axon even at a great distance from the cell, the so-called axon-reaction or chromatolysis (Fig. 27, B). The neurofibrils were first brought forcefully to the attention of neurologists by Bethe (1903). These are delicate threads which run through the cytoplasm in every direction and extend into the axon and dendrites (Fig. 28). The appearance of the fibrillae differs according to the technic employed in preparing the tissue for microscopic examination. While in the preparations by Bethe's method the fibrils do not appear to branch or anastomose with each other, those seen in Cajal preparations divide, and by anastomosing with each other form M I R( INS WI> M i Rl IN ■ II \I\S 49 a true network. The fibrillar can be traced to the terminations oi the dendrites and axons. They have been looked upon by many as the chief elements in volved in the conduction of the nerve impulse; Other elements such as pigment granules may De present. Mitochondria have been described in nerve cells by Cowdrj 1 191 1 1 and Rasmussen 1 1919 Interrelation of Neurons. In the ccelenterates, as we have learned, a single nerve-cell may receive the stimulus and transmit it to the underlying muscle. But in vertebrates the transmission of a nerve impulse to an effector requires a chain of at least two neurons, the im- pulse parsing from one neuron to the next along the chain. One of the most im- portant problems in neurology, there- fore, is this: How are the neurons re- lated to each other so that the impulse may be propagated from one to the other? The place where two such units come into such functional relation is known as a synapse. In a synapse the axon of one neuron terminates on the cell body or dendrites of another. Func- tional connections are never established between the dendrites of one neuron and the cell body or dendrites of an- other. In Fig. 29 the axon of a basket cell of the cerebellum is seen giving off collaterals which terminate about and form synapses with the Purkinje cells. Another type of synapse is illustrated in denhain.) Fig. 70. The processes of one nerve-cell are not directly fused with those of others, but, on the contrary, each neuron appears to be a distinct anatomic unit. At least the most detailed study of Golgi and Cajal preparations, in which the finest ramifications of dendrites and axons are stained, has failed to demon- strate a structural continuity between neurons. In especially favorable material AW Fig. 28. — Neurofibrils in a cell from the anterior gray column of the human spinal cord: <;.v, Axon; lii, interfibrillar spaces nucleus; .r, neurofibrils passing from one dendrite to another; y, neurofibrils passing through the body of the cell. (Bet lie, llei- 5° iiiK xkrvoi s systkm Bartelmez (1915) has shown that an axon and dendrite, entering into the forma- tion of a synapse, are each surrounded by a distinct plasma membrane and that there is no direct protoplasmic continuity. It has been maintained by Bethe and others that at such points of contact the neurofibrils pass without interruption from one neuron to another, but this has been denied by Cajal. The relation between two neurons at a synapse appears to be one of contact, but not of continuity of substance. Nerve impulses pass across the synapse in one direction only, i. e., from the axon to the adjacent cell body or dendrite. As a corollary of this it is obvious that impulses must travel within the neuron from dendrites to perikaryon and then out along the axon, as indicated by the arrow in Fig. 30. This is known Fig. 29. — Basket cell from the cerebellar cortex of the white rat. The Purkinje cells are indicated in stipple. Golgi method. (Cajal.) as the law of dynamic polarity. The polarity is, however, not dependent upon anything within the neuron itself, but upon something in the nature of the synaptic interval which permits the impulses to travel across it in one direc- tion only. There are many lines of evidence which indicate that when once activated a nerve-fiber conducts equally well in either direction. When a motor fiber bifurcates, sending a branch to each of two separate muscles, stimulation of one branch will cause an impulse to ascend to the point of bifurcation, and then descend along the other branch to its motor ending (Fig. 30). This can often be demonstrated in regenerated nerves (Feiss, 1912). The phenomena of antidromic conduction and the axon reflex (Bayliss, 1915) are also explained by the assumption that impulses are able to travel along a nerve-fiber in either direction. M I RONS AND \i I RON < II \i\S 51 The Neuron as a Trophic Unit. All parts "I" a cell are interdependent, and a continuous interaction between the nucleus and cytoplasm is a necessan con dition for life. An\ part which is detached from the portion containing the nucleus will disintegrate. In this respect the nerve cell is no exception. When an axon is divided, that part which is separated from its cell of origin and therefore from its nucleus dies, while the part -till connected with the cell usually survives. The degeneration of the distal fragment of the axon extends to its finest ramifications, but does not pass the synapse nor involve the next neuron. It must not be supposed, however, that the part of the neuron containing the nucleus remains intact, for as a result of the division of an axon important Motor neuron Sensory neuron Fig. 30. — Diagram of a reflex arc to illustrate the law of dynamic polarity. The arrows indicate the direction of conduction. changes occur in the cell body. The Nissl bodies undergo solution, the cell becomes swollen, and the nucleus eccentric. This phenomenon is known as chromatolysis, or the axon reaction, and is illustrated in Fig. 27, B. If the changes have been very profound the entire neuron may completely disin- tegrate; but, as a rule, it is restored to normal again by reparative processes. The nucleus becomes more central, the Nissl bodies reform and usually become more abundant than before, while from the cut end of the axon new sprouts grow out to replace the part of the axon which has degenerated. From what has been said it will be apparent that the nucleus presides over the nutrition of the entire neuron, that the latter responds as a whole to an injury of even a distant part of its axon, that the changes produced by such a lesion are limited to the neuron directly involved, and that nerve-fibers are unable to maintain 52 THE NERVOUS SYSTEM a separate existence or to regenerate when their continuity with the cell body has been lost. This is what is meant by the statement that the neuron is the trophic unit of the nervous system. Degeneration and Regeneration of Nerve-fibers.— A- has already been stated, that portion of a divided fiber which has been separated from its cell of origin degenerates. The axon breaks up into granular fragments, the myelin under- goes chemical change and forms irregular fatty globules. Later the degenerated axon, and myelin are entirely absorbed. The neurilemma cells of a degener- ated peripheral nerve-fiber increase in number, their cytoplasm increases in quantity, and they become united end to end to form nucleated protoplasmic bands or band-fibers. These changes in the nerve-fiber are known as Wallerian degeneration. In regeneration new axons grow out from the old ones in the central unde- generated portion of the nerve. These grow into the distal degenerated stump and find their way along the nucleated protoplasmic bands, mentioned above, to the terminals of the degenerated nerve. These band-fibers serve as conduits for the growing axons and from them the new neurilemma sheaths are differ- entiated. Thus, while the neurilemma cells and the band-fibers derived from them appear to be incapable of developing new nerve-fibers by themselves in the peripheral stump, they play an important part in nerve regeneration in co-operation with the new axons from the central stump (Cajal, 1908; Ranson, 1912). It is important to note that the nerve-libers of the brain and spinal cord, which, as has been stated before, are devoid of neurilemma sheaths, are incapable of regeneration. The neuron concept, which is based on such facts as have been presented in the preceding paragraphs, was first clearly formulated by Waldeyer in 1891, who was also the first to use the name neuron for the elements under considera- tion. The neuron doctrine may be summarized as follows: 1. The neuron is the genetic unit of the nervous system — each being derived from a single embryonic cell, the neuroblast. 2. The neuron is the structural unit of the nervous system, a nerve-cell with all its processes. These cellular units remain anatomically separate, i. c. while they come into contact with each other at the synapses there is no continuitv of their substance. 3. The neurons are the functional units of the nervous system. Thev are conduction units and the conduction pathways are formed of chains of such units. NEURONS \\i> mi RON < II \l\s 53 4. The neuron is also a trophic unit, as is -ecu (a) in the degeneration of a portion of an axon severed from its cell of origin, (b) in the phenomenon of chromatolysis or axon reaction, and (c) in the regeneration of the degenerated portion ol the axon by an outgrowth from that part of the axon still In con tact with its cell of origin. 5. Neurons are the only elements concerned in the conduction of nerve impulses. The nervous system is composed of untold numbers of such unit- linked together in conduction systems. While a majority of neurologists now accept the neuron doctrine as pre- sented here, there are dissenters (Marui, 1918). In his very interesting book, Fig. 31. — Diagrammatic section through the spinal cord and a spinal nerve to illustrate a simple reflex arc: a, b, c, and d, Branches of sensory fibers of the dorsal roots; e, association neuron; /, commissural neuron. "Allgemeine Anatomie und Physiologie des Nervensystems," Bethe has vigor- ously controverted every one of the five cardinal points just presented. We will next examine some of the simpler chains of neurons to see how they enter into the formation of the conduction pathways. Neuron-chains.— The simplest functional combination of neurons is seen in the reflex arc, and this again in its simplest form is illustrated in Fig. 31. Such an arc may consist of but two neurons, one of which is afferent and conducts toward the spinal cord; the other is efferent and conducts the impulses to the organ of response. The arc consists of the following parts: (1) the receptor, the ramification of the sensory fiber in the skin or other sensory end organ; (2) the first conductor, which includes both branches of the axon of the spinal ganglion cell; (3) a center including the synapse; (4) the second conductor, which 54 Mil NERVOUS SYSTEM includes the entire motor neuron, with its cell body in the anterior gray column and its motor ending on the muscle, and (5) the effector or organ of response, which in this case is a muscle-fiber. A wave of activation, known as the nerve impulse, is developed in the sensitive receptor, travels over this arc, and on reaching the muscle causes it to contract. So simple a reflex is rare, but prob- ably the knee-jerk is an example (Jolly, 1911). A more common form of reflex arc involves a third, and purely central neuron, as illustrated on the right side of Fig. 31. Such central elements may be spoken of as association and com- missural neurons. Many of them serve to connect distant parts of the central Fig. 32. — Diagram representing some of the conduction paths through the mammalian central nervous system. An elaborate system of central or association neurons furnishes a number of alternative paths between the primary sensory and motor neurons. (Redrawn from Bayliss.) nervous system with each other (Fig. 68). It is to the multiplication of these central neurons that we owe the complicated pathways within the mammalian brain and spinal cord. Pathways Through Higher Centers. — A good idea of how the neurons of some of the centers in the brain are related to the primary motor and sensory spinal neurons is given by Fig. 32. It will be seen that many paths are open to an impulse entering the spinal cord by way of a dorsal root fiber: (1) It may pass by way of a collateral to a primary motor neuron in a two-neuron reflex arc. It may travel over an association neuron, belonging (2) to the same level of the M i !■' »NS AND \i i RON I II \I\s 55 spinal o>nl. or (3) to other levels, in reflex arcs of three or more neurons each; or I it may ascend to the brain along an ascending branch of a dorsal rool fiber. Here it may travel over one or more of a number of path-, each sisting of several neurons, and be anally returned to the spinal cord and make- it- exit by way of a primary motor neuron. The figure illustrates but a few of the possible paths, many of which we shall have occasion to consider in the subsequenl chapters. For an incoming impulse a variety of paths arc open, one or more of which may be taken according to the momentary resistance of each. There is reason to believe that the resistance to conduction across a synapse may vary from moment to moment, according to the physiologic state of the neuron- involved. It is therefore not necessary that every impulse entering by a given fiber shall travel the same path within the central nervous system nor produce the same result. The pathways themselves are, however, more or less fixed, and depend upon the structural relations established among the neurons. Many of these synaptic connections are formed before birth, follow an hereditary pattern, and are approximately the same for each individual of the species. In the child these are illustrated by the nervous mechanisms involved in breathing and swallowing, which are perfect at birth. The newly hatched chick is able to run about and pick up food, acts which are dependent on nervous connections al- ready established according to hereditary pattern. In man and to a less extent in other mammals the nervous system continues to develop long after birth. This postnatal development is influenced by the experience of the individual and is more or less individual in pattern. It is probable "that in certain parts of the nervous mechanism new connections can always be established through education" (Edinger, 1911). The neurons w r hich make up the nervous system of an adult man are there- fore arranged in a system the larger outlines of which follow an hereditary pat- tern, but many of the details of which have been shaped by the experiences of the individual. CHAPTER V THE SPINAL NERVES We have had a glance at the earliest beginnings of a nervous system in the animal series and learned something of its biologic significance. We have traced briefly its development in the mammalian embryo, and become familiar with its chief subdivisions. We have studied the microscopic units of which it is composed, learning something of their development, structure, and function. With this information we are prepared to take up a more detailed study of the various subdivisions of the system. Subdivisions of the Nervous System. — The most convenient and logical classification of the parts of the nervous system is that which emphasizes the distinction between the central organs and those peripheral portions which are concerned chiefly in conducting impulses to and from the central organs, as follows : The central nervous system : Brain. • Spinal cord. The peripheral nervous system : Cerebrospinal nerves: Cranial nerves. Spinal nerves. The sympathetic nervous system. The anatomic relationships of these subdivisions in man are illustrated in Figs. 33 and 34. The brain lies within and nearly fills the cranial cavity. It is continuous through the foramen magnum with the spinal cord, which occupies but does not fill the vertebral canal. From the brain arises a series of nerves usually enumerated as twelve pairs and known as cranial 01 cerebral nerves; while thirty-one pairs of segmentally arranged spinal nerves take origin from the spinal cord. Branches of the cerebrospinal nerves reach most parts of the body. They are composed of afferent fibers, which receive and carry to the central nervous system sensory impulses produced by external or internal stimuli, and of efferent fibers, which convey outgoing impulses to the organs of response. It is through 56 i in: spina i. NERVES 57 the centra] nervous system that the incoming impul e find their way into the proper outgoing paths. To bring about this shunting oi incoming impulses into the appropriate efferent paths requires the presence of untold numbers ( 'Mary ganglion Maxillary nerve Sphenopalatine ganglion Superior cerokal ganglion of sympathetic •P Cervical plexus Brachial plexus Greater Jffi splanchnic nerve ~~^K Lesser splanchnic nerve Lumbar plexus Sacral pie cus Pharyngeal plexus Middle cervical ganglion of sympathetic Inferior cervical gang, of sympathetic Recurrent nervt Bronchial plexus Cardiac plexus Esophageal plexus ^Coronary plexu i Left vagus nerve Gastric plexus Celiac plexus Superior mesenteric plexus —j- Aortic plexus ^—Inferior mesenteric plexus Hypogastric plexus Pelvic plexus Bladder Vesical plexus Fig. 33. Fig. 34. Fig. 33.— General view of the central nervous system, showing the brain and spinal cord in situ. (Bourgery, Schwalbe, van Gehuchten.) Fig. 34.— Diagram of the sympathetic nervous system and its connections with the cerebrospinal nerves. (Schwalbe, Herrick.) of central or association neurons, and it is of these that the central organs- brain and spinal cord — are chiefly composed. Many authors employ a classification which emphasizes the distinction be- 58 THE NERVOUS SYSTEM tween the cerebrospinal nervous system, composed of the brain and spinal cord with their associated nerves, and the sympathetic nervous system. But this usage has the disadvantage that it is likely to engender an entirely false notion of the independence of the sympathetic system. The spinal nerves take origin from the spinal cord within the vertebral canal and make their exit from this canal through the corresponding intervertebral foramina. As component parts of such a nerve there may be recognized a ventral and a dorsal ramus, a ventral and a dorsal root, and associated with the latter a spinal ganglion. The fibers of the ventral root have their cells of origin within the spinal cord and are distributed through both ventral and dorsal rami. Since they conduct impulses from the spinal cord they are known as efferent or motor fibers. The sensory or afferent fibers of the dorsal roots and spinal nerves arise from cells located in the spinal ganglia. These fibers are also distributed through both ventral and dorsal rami (Fig. 37). Metamerism. — That the spinal nerves are segmentally arranged, a pair for each metamere, is readily appreciated in the case of the typical body segments of the thoracic region. Here it is obvious that a nerve supplies the correspond- ing dermatome and myotome, or in the adult the skin and musculature of its own segment. While the thoracic nerves retain this primitive arrangement in the adult, the distribution of fibers from the other spinal nerves is complicated by the development of the limb buds and by the shifting of myotomes and dermatomes during the development of the embryo. Opposite the attachment of the limb buds the ventral rami of the correspond- ing nerves unite to form flattened plates, and from these plates the brachial and lumbosacral plexuses are developed. Within these plexuses the fibers derived from a number of ventral rami are intermingled in what appears at first to be hopeless confusion. Each nerve which extends from these plexuses into the limbs carries with it fibers from more than one spinal nerve. To determine the exact distribution of the fibers from each segmental nerve has been a very difficult problem, in the elucidation of which the work of clinical neurologists has been of the first importance. A study of the paralyses and areas of anes- thesia, resulting from lesions involving one or more nerve roots within the ver- tebral canal, has contributed much toward its solution. Sherrington (1894) attacked the problem of the distribution of the sensory fibers by experimental methods on cats and monkeys. He found that section of a single dorsal root did not cause complete anesthesia anywhere, and attributed this result to an overlapping of the areas of distribution of adjacent spinal nerves. Mil SPIN \l. M l:\ ES Next, selecting a particular dorsal root for study, he cut two or three roots both above and below it. The zone in which sensation -til! existed and which was surrounded by an area of anesthesia represented the cutaneous field of thai particular root. He found that each "sensory root field" overlapped tho adjacent roots Fig. 35). In the thoracic region each such field has the shape of a horizontal band wrapping half-way around the body from the middorsal to the midventral lines (Fig. 36). Sherrington also found that, although in the plexuses associated with the innervation of the extremities each segmental nerve contributes sensory fibers to two or more peripheral nerves, the cutaneous distribution of these fibers is not compiled of disjointed patches, but forms a continuous field running approxi- mately parallel to the long axis of the limb. The general arrangement of these sensory root fields in man is indicated on the right side of Fig. 36. On the /b Uh thoracic s< tuory skiri field. 4///////////4 Sd thoracic 5th thoracic. Fig. 35. — Diagram of the position of the nipple in the sensory skin fields of the fourth, third, and fifth thoracic spinal roots. The overlapping of the cutaneous areas is represented. (Sher- rington.) opposite side is indicated the distribution of the cutaneous nerves. It will be seen that in the extremities there is no correspondence between the areas sup- plied by these peripheral nerves and those supplied by the individual dorsal roots. It will also be evident that the fibers of a given dorsal root reach the corresponding sensory root field by way of more than one cutaneous nerve. A knowledge of the cutaneous distribution of the various nerve roots is of great importance in enabling the clinician to determine the level of a lesion of the spinal cord or nerve roots within the vertebral canal. In the same way the shifting of muscles during embryonic development has been accompanied by corresponding changes in the spacial distribution of the motor fibers. A familiar example is furnished by the diaphragm, the musculature of which is derived from the cervical myotomes and which in its descent carries with it the phrenic nerve. This explains the origin of the phrenic from the third, fourth, and fifth cervical nerves. 6o THE NERVOUS SYSTEM If, as seems probable, the musculature of the extremities has not developed along mctameric lines, there can be no true metamerism of the motor nerves to the limbs (Streeter, 1912). Yet the fibers from each ventral root are distributed in a very orderly manner. As is indicated in the table on page 77, almost every long muscle receives libers from two or more ventral roots. It will be apparent that the muscles of the trunk are innervated from the roots belonging to the Great auricular Cutaneous nerve of the neck Supraclavicular nerves Axillary Intercostobrachial Medial cutaneous of arm Posterior cutaneous of arm Medial cutaneous of forearm M useuloe utaneo us Radial Median Ulnar Genitofemoral Lateral cutaneous of the thigh Intermediate cutaneous rami Medial cutaneous rami Infrapatellar ramus Lateral sural Saphenous Superficial peroneal Sural Deep peroneal Fig. 36. — Sensory root fields on the right, contrasted with the areas of distribution of cutaneous nerves on the left. several metameres from the myotomes of which these muscles developed. The table shows in a general way the distribution of the fibers of the several ventral roots. Functional Classification of Nerve-fibers. — Many years ago Sir Charles Bell (1811, 1844) showed that the dorsal roots are sensory in function and the ventral roots motor; and this has been known since then as Bell's law. He recognized that sensory and motor fibers are distributed to the viscera as well as THE SPIN \i. NERVES (.1 to the rest of the body. But Gaskell (1886) was the first to make a detailed study of the nerve-fibers supplying the visceral and vascular \\ . now recognize in the spinal nerves elements belonging to lour functionally distinct varieties, namely, visceral afferent, visceral efferent, somatu afferent, and somatic efferent fibers (Fig. 37). Visceral Components.- The fibers which innervate the visceral and vascular systems, including all involuntary muscle and glandular tissue, possess, as Gaskell (1886) pointed out many years ago, certain distingirishing character- istics. They arc all fine myelinated fibers and end in sympathetic ganglia Somatic afferent fiber ) Dorsalroot ! Visceral afferent fiber! $\ — Spinal ganglion Dorsal ramus > V( nlral ramus Ramus communicans Sympathetic ganglion A> K . Visceral efferent fiber} ,. . , v s c «■ a i r) i I enlral root Somatic efferent fiber j Postganglionic fiber .Viscus Fig. 37. — Diagrammatic section through a spinal nerve and the spinal cord in the thoracic region to illustrate the chief functional types of peripheral nerve-fibers. from which the impulses are relayed to involuntary muscles and glands by a second set of neurons (Fig. 37). They are usually designated as visceral efferent fibers, and they run by way of the white rami to the sympathetic ganglia. It is usually stated that they are found only in the second thoracic to the second lumbar nerves inclusive, but Langley (1892) has shown that in the cat. dog, and rabbit they are present in all the thoracic and the first four lumbar nerves, and Miiller (1909) found white rami associated with the third and fourth lumbar nerves in man. There are also visceral afferent fibers distributed to the thoracic and ab- dominal viscera by way of the white rami from the thoracic and upper lumbar 62 THE NERVOUS SYSTEM nerves. These have their cells of origin in the spinal ganglia and are continued through the dorsal roots into the spinal cord (Fig. 37). We shall have much more to say about the visceral components of the spinal nerves in the chapter on the Sympathetic Nervous System. In the remaining pages of this chapter we will confine our attention to the somatic components, i. e., to those fibers which innervate the various parts of the body exclusive of the visceral and vascular systems. Somatic Efferent Components. — The skeletal muscles are innervated by myelinated fibers, which are, for the most part, of large caliber. The axis- cylinders of these fibers are the axons of cells located in the ventral part of the gray matter of the spinal cord, and they end on the muscle-fibers in special Fig. 38. — Nerve-ending in muscular fiber of a lizard (Lacerta viridis). Highly magnified: a. End-organ seen in profile; b, from the surface; s, s, sarcolemma; p, p, expansion of axis-cylinder. Beneath this is granular protoplasm containing a number of large clear nuclei and constituting the "bed" or "sole" of the end-organ. In b the expansion of the axis-cylinder appears as a clear network, branching from the divisions of the medullated fiber. (Kiihnc in Quain's Anatomy.) motor end- plates. Such a primary motor neuron is illustrated in Fig. 25. A motor fiber undergoes repeated division as it approaches its termination, but each branch retains its myelin sheath until in contact with the muscle-fiber. At this point this sheath terminates abruptly, and the neurilemma becomes continuous with the sarcolemma (Fig. 38). The terminal branches of the axon are short, thick, and irregular. They lie immediately under the sarcolemma in a bed of specialized sarcoplasm containing a number of large clear nuclei. The wave of activation, which travels down an axon as a nerve impulse, is transmitted through these motor nerve endings to the muscle and initiates a contraction. The Spinal Ganglia. — Since the afferent fibers in the spinal nerves take their nil SPINAL NERVES origin from the ganglia on the dorsal root- we will do well to interrupt for a moment our functional analyses of the spinal nerves and consider the struc- ture of these ganglia. The spinal ganglia arc rather simple structures so far as their fundan plan is concerned, but in recent year-, chiefly through the studies of Cajal (1906) and Dogiel (1908), we have learned to recognize in them main- complex histologic details, the significance of which is not yet underst 1. It has long been known that the typical cells of the mammalian spinal ganglion are uni- polar. The cell body is irregularly spheric. The axon. 1 which is attached to the perikaryon by an implantation cone, is coiled on itself in the neighborhood of the cell, forming what is known as a glomerulus (Fig. 39,/). It then runs into one of the central fiber bundles of the ganglion and divides in the form of a T or Y into two branches, of which one is directed toward the spinal cord in the dorsal root. The other and somewhat larger branch is directed distally in the spinal nerve. The cells vary greatly in size and the diameter of the axon varies with that of the cell from which it springs. An axon arising from a large cell usually forms a very pronounced glomerulus and soon becomes en- sheathed with myelin, and this myelin sheath is continued along both branches into which it divides. The branching occurs at a node of Ranvier. As was originally pointed out by Cajal (1906) and Dogiel (1908) and recently emphasized by Ranson (1911) the small cells of these ganglia give rise to fine unmyelinated fibers. These coil but little near the cell, or the glomerulus ma\- be entirely lacking (Fig. 39, a). They divide dichotomously, just as do the myelinated fibers, into finer central and coarser peripheral branches. At the point of bifurcation there is a triangular expansion in place of the constric- tion so characteristic of a dividing myelinated fiber. It has been shown by Hatai (1902) and Warrington and Griffith (1904) that the small cells are con- siderably more numerous than the large cells, though because of their small size they constitute a less conspicuous element. A few cells retain the bipolar form characteristic of all the spinal ganglion cells at an early stage of development (Figs. 21, 40, d). The spinal ganglion cells are each surrounded by a capsule or membranous sheath with nuclei on its inner surface (Fig. 39, d.f) which is continuous with the neurilemma sheath of the associated nerve-fiber. The cells forming the capsule are of ectodermal origin, being derived like the spinal ganglion cells themselves from the neural crest. 1 See fine print, page 45. 64 THE NERVOUS SYSTEM In good methylene-blue preparations and in sections stained by the newer silver methods it is possible to make out many additional details of structure. The axon may split into many branches, which subdivide and anastomose, forming a true network in the neighborhood of the cell (Fig. 39, b). From this network the axon is again assembled and passes on to a typical bifurcation. Or the axon may be assembled out of a similar plexus which, however, is con- I Fig. 39. — Neurons from the spinal ganglion of a dog: a, Small cells with unmyelinated axons; b, c, d, e, and/, large cells with myelinated axons; f, typical large spinal ganglion cell showing glomerulus and capsule. The arrow points toward the spinal cord. Pyridin-silver method. nected with the cell by several roots (Fig. 39. c). Some of the fibers give off collaterals terminating in spheric or pear-shaped end-bulbs. Such an end bulb may rest upon the surface of its own perikaryon (Fig. 39, d) or elsewhere in the ganglion. From the body of some cells short club-shaped dendrites arise, which, however, terminate beneath the capsule- which surround the cells. Based on such details as these Dogiel (1908) has arranged the spinal ganglion cells in groups and recognizes eleven different types. Two of his eleven types are of special interest. The cells of Type VIII resemble the typical spinal ganglion cell in all respects except that Mil. SPINAL NERVES 6^ the peripheral branch of the axon Wreaks up within the ganglion into numerous myelinated fibers, which after Losing their sheaths terminate in whal an- apparently sensory endings. The centra] branch runs apparently without division to the spinal cord. The cells of I vpe \ I possess, in addition to an axon, thai apparent ly runs wit houl division through the dorsal mot to the spinal cord, several processes thai resemble dendrites, in thai they divide re- peatedly within the ganglion, bul resemble axons in their appearance and in po i myelin sheaths (Fig. 40, b). These processes after repeated divisions become unmyelinated and end within the ganglion and dorsal root in what appear to be sensory endings. Ii would lead us too far afield If we should at tempi to summarise Dogiel's work. It should be pointed out, however, that he no longer believes in the existence of the cells whi< h he formerly de- scribed under the head of spinal ganglion cells of Type II and which find a cons£i< uous plat e in most text-books. He believes that what he formerly described as the branching fibers of these cells are, in reality, the dendrite -like branches of the cells of I vpe XI. Dorsa! root Dorsal ramus Ventral root ,*' Ramus communicans Ventral ramus Fig. 40. — Diagrammatic longitudinal section of a spinal ganglion and a spinal nerve (cervical or sacral) : a, Small cells with unmyelinated axons; b, cell of Dogiel's type XI ; c, large cell possessing a myelinated axon and surrounded by a pericellular plexus; d, bipolar cell. According to Dogiel every spinal ganglion cell is surrounded by a network of fine branching and anastomosing fibers; and he believes that these are formed by the ramifications of fine myelinated and unmyelinated fibers that have entered the spinal ganglion from the sympathetic nervous system through the rami communicantes. While the origin of these fibers is open to question, there can be no doubt that such pericellular networks exist on at least a considerable proportion of the cells and constitute an important element in the structure of the ganglion (Fig. 40, c). The fiber bundles of the ganglia are composed of both myelinated and un- 5 66 THE NERVOUS SYSTEM myelinated fibers representing the branches of the axons of the spinal ganglion cells. Both types of fibers can be followed through the dorsal roots into the spinal cord, as well as distally into the nerves. In the latter they mingle with the large myelinated fibers coming from the ventral roots (Fig. 40). When traced distally in the peripheral nerve the unmyelinated fibers are found to go in large part to the -kin. though a few run in the muscular branches (Ranson, 1911 and 1915). Classification of the Somatic Afferent Fibers According to Function. — Sherrington (1906) in an instructive book on "The Integrative Action of the Nervous System" has furnished us with a useful classification of the elements belonging to the afferent side of the nervous system. He designates those carrying impulses from the viscera as interoceptive, and subdivides the somatic afferent elements into exteroceptive and proprioceptive groups. The extero- ceptive fibers carry impulses from the surface of the body and from such sense organs, as the eye and ear, that are designed to receive stimuli from without. These fibers, therefore, are activated almost exclusively by external stimuli. The proprioceptive fibers, on the other hand, respond to stimuli arir-ing within the bodv itself and convey impulses from the muscles, joints, tendons, and the semicircular canals of the ear. Each group has receptors or sensory endings designed to respond to its appropriate set of stimuli, and for each there are special connections within the brain and spinal cord. Exteroceptive fibers and sensory endings are activated by changes in the environment, that is to say. they are stimulated by objects outside the body. The impulses, produced in this way and carried by these fibers to the spinal cord, call forth for the most part reactions of the body to its environment; and, when relayed to the cerebral cortex, they may be accompanied by sensa- tions of touch, heat, cold, or pain. The receptors are, for the most part, located in the skin ; yet it is convenient to include in the exteroceptive group the pressure receptors which are closely allied to those for touch, but which lie below the surface of the body. At this point it should be noted that sensibility to those forms of contact which include some slight pressure, such as the placing of a finger on the skin, is not abolished by the section of all of the cutaneous nerves going to the area in question, since the deeper nerves carry fibers capable of responding to such contacts (Head. 1905). This deep contact sensibility, which for lack of a better name we may call "pressure-touch," must not be overlooked in the analysis of cutaneous sensations. The balance of evidence is in favor of the assumption that each of the vari- 111! SPINAL NERVES eties of cutaneous sensation is mediated by a separate set of nerve fibers. Hut little progress has as yet been made toward identifying these various func- tional groups. We know that both myelinated and unmyelinated fibers are present in the cutaneous aerves (Ranson, 1915), hut arc not able to say with certainty which subserve each of the varieties el" cutaneous sensation. There arc many good reasons, however, for the belief that painful afferent impulses and possibly also those of temperature are carried by the unmyelinated fibers, and that those of the touch and pressure group are mediated by the myelinated libers. The evidence on which this statement is based has been briefly sum- marized on pages 102 104. Fig. 41. — Free nerve endings in the epidermis of a cat's paw: A, Stratum corneum; B, stratum germinativum Malpighii, and C, its deepest portion; a, large nerve trunk; b, collateral fibers; c, terminal branches; d, terminations among the epithelial cells. Golgi method. (Cajal.) All sensory nerve endings in the skin belong to the exteroceptive group, but it is not so easy to say which ones are responsible for each of the several varieties of cutaneous sensation, namely, touch, pain, heat, and cold. On structural grounds we may recognize three principal groups: (1) endings in hair- follicles, (2) encapsulated nerve endings, and (3) free terminations in the epi- dermis. Free Nerve Endings. — Some of the myelinated fibers as they approach their terminations divide repeatedly. At first the branches retain their sheaths, but after many divisions the myelin sheaths and finally the neurilemma are lost and only the naked axis-cylinders remain. These enter the epidermis, where, 68 THE NERVOUS SYSTEM after further divisions, they end among the epithelial cells (Fig. 41). This type of nerve ending is found in the skin, mucous membranes, and cornea. Similar endings are also found in the serous membranes and intermuscular connective tissue. We do not know what form the endings of the afferent unmyelinated fibers may take, but it is not unlikely that they also ramify in the epidermis like the terminal branches of the myelinated fibers just described. It seems certain that at least a part of the free nerve endings in the epidermis are pain receptors. In the central part of the cornea, the tympanic membrane, and the dentine and pulp of the teeth, such free nerve endings alone are present, and pain is the only sensation that can be appreciated. Some of the nerve-libers which enter the epidermis end in disk-like expansions in contact with specialized epithelial cells CFig. 42). These have been known Fig. 42. — Merkel's corpuscles or tactile disks from the skin of the pig's snout. The nerve- fiber, n, branches and each division ends in an expanded disk, m, which is attached to a modified cell of the epidermis, a; c, an unmodified epithelial cell. (Ranvier, Herrick.) as Merkel's touch-ails on the supposition that the endings in question are tactile receptors. Encapsulated Nerve Endings. — Among the encapsulated nerve endings are the corpuscles of Meissner. These have quite generally been regarded as tactile end organs and are located in the corium or subepidermal connective tissue of the hands and feet, forearm, lips, and certain other regions. They are of large size, oval, possess a thin connective-tissue capsule, and within each terminate one or more medullated fibers (Fig. 43). Within the capsule the fibers lose their myelin sheaths, make a variable number of spiral turns, and finally break up into many varicose branches, which form a complex network. To another type of encapsulated end organ belong those known as the end bulbs of Krause. One of these is illustrated in Fig. 44. They are found in the conjunctiva, edge of the cornea, lips, and some other localities. I III. SPINAL \i i 6 9 Fig. 43. — -Meissncr's tactile corpuscle. Fig. 44. — End-bulb of Krausc from con- Methylene-blue stain. (Dogicl, Bohm-David- junctiva of man. Methylene-blue stain. ofF-Huber.) (Dogiel, Bohm-Davidoff-Huber.) Fig. 45. — Pacinian corpuscles from mesorectum of kitten: A, Showing the fine branches of the central fiber; B, the network of fine nerve-fibers about the central fiber. Methylene-blue stain. (Sala, Bohm-Davidoff-Huber.) The Pacinian corpuscles, two of which are illustrated in Fig. 45, have a very wide distribution in the deeper parts of the dermis of the hands and feet, in the 7 o THE NERVOUS SYSTEM tendons, intermuscular septa, periosteum, peritoneum, pleura, and pericardium. They are also numerous in the neighborhood of the joints. According to Her- rick (1918) it is probable that "by these end organs relatively coarse pressure may be discriminated and localized (exteroceptive function), and movements of muscles and joints can be recognized (proprioceptive function)." They are ! — hst Fig. 46. — Nerves and nerve endings in the skin and hair-follicles: list, Stratum corneum; rm, stratum germinativum Malpighii; c, most superficial nerve-fiber plexus in the cutis; n, cutaneous nerve; is, inner root sheath of hair; as, outer root sheath; h, the hair itself; dr, glandulae sebaceae. (Retzius, Barker.) large oval corpuscles, made up in great part of concentric lamella? of connective tissue. The axis of the corpuscle is occupied by a core of semifluid substance containing the termination of a nerve-fiber. The fiber loses its myelin sheath as it enters the core, through which it passes from end to end. Its terminal branches end in irregular disks. Side branches are also given off within the core. Illi: SPIN \l. NERVES 71 Nerve Endings in the Hair-follicles. It has long been known thai the hairs arc delicate tactile organs. The hair-clad parts lose much of their responsive- Fig. 47. — Neuromuscular nerve end-organ from a dog. The figure shows the intrafusal muscle-fibers, the nerve-fibers and their terminations, but not the capsule nor the sheath of Henle. Methylene-blue stain. (Huber and De Witt.) ness to touch when the hair is removed. As would be expected on these ground-. the hair-follicles are richly supplied with nerve endings. Just below the open- ing of the sebaceous glancl into the follicle mvelinated nerve-fibers enter it, los- 7 2 THK NERVOUS SYS I I.M ing their myelin sheaths as they enter. They give off horizontal branches, which encircle the root of the hair, and from these ascending branches arise (Fig. 46). Some of these are connected with leaf-like expansions, associated with cells resembling Merkel's touch-cells. Practically nothing is known concerning the receptors for sensations of heat and cold. Proprioceptive Fibers and Sensory Nerve Endings.— To this group belong the afferent elements which receive and convey the impulses arising in the muscles, joints, and tendons. Changes in tension of muscles and tendons and movements of the joints are adequate stimuli for the receptors of this class and excite nerve impulses which, on reaching the central nervous system, give in- formation concerning tension of the muscles and the relative position of the various parts of the body. For the most part, however, these impulses do not rise into consciousness, but serve for the subconscious control of muscular activitv. The unsteady gait of a tabetic patient illustrates the lack of mus- cular control that results when these impulses are prevented from reaching the central nervous system. The proprioceptive fibers are myelinated and are associated with motor fibers in the nerves to the muscles. Some follow along the muscles to reach the tendons. Three types of end organs belong to this group. Pacinian cor- puscles, muscle spindles, and neurotendinous end organs. Many Pacinian corpuscles are found in the neighborhood of the joints. They have been de- scribed in a preceding paragraph. Neuromuscular End Organs. — The afferent fibers to the muscles end on small, spindle-shaped bundles of specialized muscle-fibers (Fig. 47). These muscle spindles are invested by connective- tissue capsules; and within each of them one or more large myelinated nerve-fibers terminate. Within the spindle the myelin sheath is lost and the branches of the axis-cylinders wind spirally about the specialized muscle-fibers, or they may end in irregular disks. Somewhat analogous structures are the neurotendinous end organs or tendon spindles where myelinated nerve-fibers end in relation to specialized tendon fasciculi. CHAPTER V] THE SPINAL CORD The spinal conl. or medulla spinalis, is a cylindric mass of nervous tissue occupying the vertebral canal. It is 40 to 45 cm. in Length, reaching from the foramen magnum, where it is continuous with the medulla oblongata, to the level of the first or second lumbar vertebra. Even above this level the vertebral canal is by no means fully occupied by the cord (Fig. 48), which, as shown in Fig' 49, is surrounded by protective membranes, while between these and the wall of the canal is a rather thick cushion of adipose tissue containing a plexus Extradural fat and venous plexus m spinal cord Subarachnoid space \ J |l / Cura mater S [) nuil nerve roots XY •\\J / Ligamentum denticulatum Fig. 48. — Diagram showing the relation of the spinal cord to the vertebral column. of veins. Immediately surrounding the cord and adherent to it is the delicate, highly vascular pia mater. This is separated from the thick, fibrous dura mater by a membrane having the tenuity of a spider web, the arachnoid, which sur- rounds the subarachnoid space. This space is broken up by subarachnoid trabecular and filled with cerebrospinal fluid. External Form. — The spinal cord is not a perfect cylinder, but is somewhat flattened ventrodorsally. especially in the cervical region. Its diameter is not uniform throughout, being less in the thoracic than in the cervical and lumbar portions. That is to say, the cord presents two swellings (Fig. 51). The cer- vical enlargement (intumescentia cervicalis) comprises all that portion of the cord 73 74 THE NERVOUS SYSTEM from which the nerves of the brachial plexus arise, that is. the fourth cervical to the second thoracic segments inclusive. The lumbar enlargement (intumes- centia lumbalis) is not quite so extensive and corresponds less accurately to the origin of the nerves innervating the lower extremity. At an early stage in the embryonic development of the spinal cord these enlargements are not present. In the time of their first appearance and in their subsequent growth they are directly related to the development of the limbs. Below the lumbar enlargement the spinal cord rapidly decreases in size and has a cone-shaped termination, the conns mcdullaris, from the end of which a slender filament, the filum tcrminalc. is prolonged to the posterior surface of the coccyx (Figs. 50. 51). This terminal filament descends in the middle line. surrounded by the roots of the lumbar and sacral nerves, to the caudal end of Septum posiicum Posterior spinal artery Ligamentum dentkulatum Subarachnoid trabecule ----- Pia mater - Epidural trabecule Anterior spinal artery — Dura mater *~« Subdural space A ra ditto id '-Nerve root Subarachnoid cavity Linca splendetis Fig. 49. — Diagram of the spinal cord and meninges. the dural sac at the level of the second sacral vertebra. Here it perforates the dura mater, from which it receives an investment, and then continues to the posterior surface of the coccyx. The last portion of the filament with its dural investment is often called the filum of the spinal dura mater (filum durae matris spinalis). The filum terminale is composed chiefly of pia mater; but in its rostral part it contains a prolongation of the central canal of the cord. The spinal cord shows an obscure segmentation, in that it gives origin to thirty-one pairs of metameric nerves. These segments may be somewhat arbitrarily marked ofl from each other by passing imaginary planes through the highest root filaments of each successive spinal nerve (Donaldson and Davis. 1903). The highest of these planes, being just above the origin of the first cer- vical nerve, marks the separation of the spinal cord from the medulla oblongata. THE SPIN \i. <()KD 75 This is again an arbitrary line of separation, since both as to external form and Interna] structure the cord passes over into the medulla oblongata by in Medulla oblongata v-T*T7" .V. cervicalis VIII A i:J — Ventral root of X. T. Ill Dorsal root of X . T. IV - Lateral funiculus Spinal dura mater N. thoricalis XII r Pom Medulla oblongata. --Anterior median fissure —Anterolalt ral sulcus -( 'ervical enlargement -Anterior funiculus -Thoracic portion of- spinal cord Lumbar enlargement ^> Rhomboid ; I'n tiritir median Mill US P • rior funic- ulus Posterior inter- mediate suit us Dorsal root A Cauda equina o^JZ- N. lumbal is V ■ ,■ Filum of spinal dura mater .Con us medullar is .-Filum lerminale — Cauda equina Fig. 50. Fig. 51. Fig. 52. Figs. 50-52. — Three views of the spinal cord and rhombencephalon: Fig. 50, Lateral view with spinal nerves attached; Fig. 51, ventral view with spinal nerves removed; Fig. 52, dorsal view with spinal nerves attached. (Modified from Spalteholz.) sensible gradations. According to this method of subdivision there are in the cervical portion of the cord eight segments, in the thoracic twelve, in the lumbar five, and in the sacral five, while there is but one coccygeal segment. 76 THE NERVOUS SYSTEM Several longitudinal furrows are seen upon the surface of the cord (Figs. 51, 52). Along the middle line of the ventral surface is the deep anteriniL median fissure (fissura mediana anterior). This extends into the cord to a depth amounting to nearly one-third of its anteroposterior diameter and contains a fold of pia mater. Along the middle line of the dorsal surface there is a shallow groove, the posterior median salens (sulcus medianus posterior; . As may be i en in cross-sections of the spinal cord, it i-. divided into approximately sym- metric lateral halves by the two furrows just described and by the posterior median septum (Figs. 55, 56, 57). On either side, corresponding to the line of origin of the ventral roots, i.-^ a broad, shallow, almost invisible groove, the anterolateral sulcus (sulcus lateralis anterior). And again on either side, cor- responding to the line of origin of the dorsal roots, is the narrower but deeper posterolateral sulcus (sulcus lateralis posterior). These six furrows extend the entire length of the spinal cord. In the cervical region an additional longi- tudinal groove may be seen on the dorsal surface between the posterior median and posterolateral sulci, but somewhat nearer the former. It is known as the posterior intermediate sulcus and extends into the thoracic cord, where it grad- ually disappears. Funiculi. By means of these furrows and the subjacent gray matter each lateral half of the cord is subdivided into columns of longitudinally coursing nerve-fibers known as the anterior, lateral, and posterior funiculi (funiculus .interior, funiculus lateralis et funiculus posterior). In the cervical and upper thoracic regions the posterior intermediate sulcus divides the posterior funiculus into a medial portion, the fasciculus gracilis, and a lateral portion, the fasciculus cuneatus. Nerve Roots. — From the lateral funiculus in the upper four to six cervical segments there emerge, a little in front of the dorsal roots of the spinal nerves, a series of root filaments which unite to form the spinal root of the accessory nerve (Fig. 125). This small nerve trunk ascends along the side of the cord, enters the cranial cavity through the foramen magnum, and carries to the accessory nerve the fibers for the innervation of the sternocleidomastoid and trapezius muscles. From the posterolateral sulcus throughout the entire length of the spina] cord emerge an almost uninterrupted series of root filaments (lila radicularia). Those from a given segment of the cord unite to form the dorsal root of the cor- responding spinal nerve. The filaments of the ventral roots emerge from the broad, indistinct anterolateral sulcus in groups, several appearing side by side, T11K SPIN a CI IRD 77 rather than in the accurate linear order characteristic of the dorsal roots. 1 hose [null a given segment unite with each other to form a ventral rout; and that in turn joins with the corresponding dorsal root just beyond the spinal ganglion to form the mixed nerve I l'"i,u r . 50). Relation of the Spinal Cord and Nerve Roots to the Vertebral Column. At an early fetal stage the spinal cord occupies the entire length of the vertebral Infrahyoid must les Diaphragm Muscles of shoulder, arm, and hand Abdominal musdes Flexors of hip Extensors of the knee and adductors of hip Other muscles of thigh, . and foot Perineal and anal mus- cles Cervical segments of spinal cord Thoracic segments of spinal i ord Lumbar segments of spinal cord Sacral and coccygeal segim i spinal cord Fig. 53.— Diagram showing the level of the various segments of the spinal cord with reference to the vertebrae,\vith a table showing the distribution of the fibers of the several ventral roots. canal and the spinal nerves pass horizontally lateralward to their exit through the intervertebral foramina. As development progresses the vertebral column increases in length more rapidly than the spinal cord, which, being firmly an- chored above by its attachment to the brain, is drawn upward along the canal, until in the adult it ends at about the lower border of the first lumbar vertebra. 78 THE NERVOUS SYSTEM At the same time the roots of the lumbar and sacral nerves become greatly elongated. They run in a caudal direction from their origin to the same inter- vertebral foramina through which they made their exit before the cord shifted its position. Since the thoracic portion of the cord has changed its relative position but little, and the cervical part even less, the cervical roots run almost directly lateralward, while those of the thoracic nerves incline but little in a caudal direction. Since the spinal cord ends opposite the first or second lumbar vertebra, the roots of the lumbar, sacral, and coccygeal nerves, in order to reach their proper intervertebral foramina, descend vertically in the canal around the conus medul- laris and filum terminale. In this way there is formed a large bundle, which is composed of the roots of all the spinal nerves below the first lumbar and has been given the very descriptive name cauda equina. The amount of relative shortening of the various segments of the cord differs in different individuals. In Fig. 53, where the quadrilateral areas represent the bodies of the vertebrae, we have indicated the average position of each segment of the spinal cord. This figure is based on data published by Reid (1889). It is obvious that the segments are longer in the thoracic than in the cervical and lumbar portions of the cord, while the sacral segments are even shorter (see also Fig. 59). We have been at some pains to explain the development of the cauda equina and the vertebral level of the various segments of the spinal cord because these are matters of much practical importance. In spinal puncture the needle is made to enter the subdural space caudal to the termination of the cord. In locating lesions of the spinal cord it is necessary to know the position of its various segments with reference to the vertebrae. It is particularly important to be able to distinguish between an injury to the lower part of the spinal cord and one which involves only the nerve roots in the cauda equina, since, although the symptoms in the two cases may be nearly identical, damage to the spinal cord is irreparable, while the nerve roots will regenerate. The Spinal Cord in Section. — When a section is made through any part of the brain or spinal cord one sees at once that they are composed of two kinds of tissue — the one whitish in color, the other gray, tinged with pink. The white substance consists chiefly of myelinated fibers, the gray is made up of nerve- cells, dendrites, unmyelinated and myelinated fibers, and many blood-vessels. Both have a supporting framework of neuroglia. The gray substance (substantia grisea) of the spinal cord is centrally placed THE SPIN \i. couij 79 and forms a continuous tinted column, which is everywhere enclosed bj the white matter (Fig. 54). In cross-section it has the form of a letter II (Fig. 55). rhere is a comma-shaped gray field in each lateral half of the cord, and these are united across the middle line by a transverse gray bar. The enlarged anterior end of the comma has been known as the Neu- tral horn, the tapering posterior end as the dorsal horn, and the transverse bar as the gray commissure. But, when it is remembered that the gray substance forms a .continuous mass throughout the length of the spinal cord, it will be seen that the term "column" is more appropriate than "horn." The long gray mass in either lateral half of the cord is convex medially and concave laterally. It projects in a dorsolateral direction as the posterior column (columna posterior). As seen in a cross-section of the cervical cord, the posterior column is rela- tively long and narrow and nearly reaches the dorsolateral sulcus (Fig. 55). Fig. 54. — Diagram of gray columns of spinal cord. Posterior intermediate sulcus and septum Posterior median sulcus and septum Fasciculus gracilis ] p , • I Fasciculus cuneatus j funiculus Collaterals from cuneale fuse. Substantia gelatinosa Posterolateral sulcus \ Posterior column [ ~ ' >ex . ' Cervix Reticular formation Posterior ...£* com. Anterior ._iisi gray com. A nterioi "7-- white com. A nterior' column Ventral root fibers ...:_^i_ Anterolateral sulcus Dorsal root Dorsolateral fasciculus ( Lissauer) Lateral funiculus tjGsfe ^ Hi m ' Central canal Anterior funiculus Anterior median fissure Fig. 55. — Section through seventh cervical segment of the spinal cord of a child. Pal-Weigert method. It presents a constricted portion known as the cervix, a pointed dorsal extrem- ity or apex, and between the two an expanded part sometimes called the caput. The apex consists largely of a special variety of gray substance, gelatinous in 80 THE NERVOUS SYSTEM appearance in the fresh condition and very difficult to stain by neurologic meth- ods, which in sections has a A -shaped outline. It is known as the substantia gelatinosa Rolandi. In the thoracic portion the posterior column, which is here very slender, does not come so close to the surface; and in the lumbosacral seg- ments it is much thicker (Figs. 56, 57). The anterior column is relatively short and thick and projects toward the anterolateral sulcus. It contains the cells of origin of the fibers of the ventral root. From its lateral aspect nearly opposite the gray commissure there pro- jects a triangular mass, known as the lateral column (columna lateralis). This is prominent in the thoracic and upper cervical segments; but it blends with the expanded anterior column in the cervical and lumbar enlargements (Fig. 56). Posterior median sulcus and septum Posterior funiculus Substantia gelatinosa | ; Dorsolateral fasciculus (Lissauer) Posterolateral sulcus /. ( , Dorsal root .... I ■/'.> Lateral funiculus Apex of posterior column - , >? i Nucleus dorsalis \ j / ' i-.-,. Lateral column '. -.:^vi*' ~ v ~\^i-\ •'' v -,*Sk Posterior commissure''' ^ Anterior 'white commissure " Anterior column' ' j' ':. ■ )'■■'-<* ~ Central canal ■ '' . ' -V-~,l ^'Anterior funiculus '- Anterior median fissure Fig. 56. — Section through the seventh thoracic segment of the spinal cord of a child. Pal-Weigert method. The reticular formation (formatio reticularis), situated just lateral to the cer- vix of the posterior column in the cervical region, is a mixture of gray and white matter (Fig. 55). Here a network of gray matter extends into the white substance, breaking it up into fine bundles of longitudinal fibers. The reticular formation is most evident in the cervical region, but traces of it appear at other levels. The gray commissure contains the central canal, and by it is divided into the posterior commissure (commissura posterior) and the anterior gray commissure (commissura anterior grisea). Ventral to the latter many medulla ted fibers cross the midline, constituting the anterior white commissure. The cavity of the neural tube persists as the central canal, which lies in the gray commissure throughout the entire length of the cord. The canal is so small as to be barely visible to the naked eye. It is lined with ependymal I ill SPINAL ( dkl) 8l epithelium and the lumen is often blocked with epithelial debris. The canal, which is narrowest in the thoracic region, expands within the lower pari of the eonus medullaris to form a fusiform dilatation, the ventriculus terminalis. Posterior median sulcus and septum Collaterals from fast it ulus cuneatus \ Posterior funiculus Dorsal root Substantia eelatinosa ' • Posterior column ]■'■ ( irvix Dorsolateral fasciculus (Lissauei Posterolateral mU us Lateral funiculus Posterior commissure ----^J**** Anterior urav ----:■"""" commissure llmm Anterior icliile com..--- •..-. Anterior column f ' Ventral root fibers _ Anterolateral side, Central canal \ A nterior median fissure 'Anterior funiculus Fig. 57. — Section through the fifth lumbar segment of the spinal cord of a child. Pal-Weigerl method. Dorsal roots of lumbar and sacral nerves ,/'/ Posterior funiculus Substantia gelatinosa Dorsolateral fasciculus Posterior column , Lateral funiculus ii ;■:.■■■* Anterior column Ventral roots of lumbar and sacral nerves Fig. 58.— Section of the third sacral segment of the human spinal cord and the lumbosacral nerve roots of the Cauda ecpiina. Pal-Weigert method. The White Substance.— The long myelinated fibers of the cord, arranged in parallel longitudinal bundles, constitute the white substance which forms a 82 THE NERVOUS SYSTEM thick mantle surrounding the gray columns. In each lateral half of the cord it is divided into the three great .strands or funiculi, which have been described While matter. Grey matter. — Enhire secrion. 100 • ao -"""^ .-'.. \ • 60 (l .— "^ \ 40 20 -.\ \ I 11 III IY Y \1 YUYin 1 II III IY Y YJ YII YJ1I IX X XI XII 1 D III IVY I IlUJKVl Fig. 59. — Curves showing the variations in sectional area of the gray matter, the white matter, and the entire cord in the various segments of the human spinal cord. (Donaldson and Davis.) on the surface of the cord. The anterior funiculus ('funiculus anterior) is bounded by the anterior median fissure, the anterior column, and the emergent fibers of the ventral roots. The lateral funiculus (funiculus lateralis) lies lateral to VII c- VI It c VI1IC -ID II D VII D IV S C Fig. 60. — Outline drawings of sections through representative segments of the human spinal cord. the gray substance between the anterolateral and posterolateral sulci, i. c., between the lines of exit of the ventral and dorsal roots. The posterior funiculus (funiculus posterior) is bounded by the posterolateral sulcus and posterior col- I in: SPINAL ' i U'i» ChARACTI RISTN I I Ml RES 01 TRANSVERSI Se< i m.n- \ I \ \i;imi - I i mi . i] mi SPINA! I ORD Level i m ii. il. 1 Imrucic. 1 umliar. ( )ui line ( )val, greatest di- ameter transverse ( )\ ,il to cir< nI. ii Nearly i irculai ( in ul. ii to quadrilateral Volume «>i graj matter 1 arge Sim. ill Relatively large Antei i,,r graj column Massive Slender Massive Massive Posterior graj column Relatively slender, but extends far posteriorly Slender Massive Ma-sue Lateral gray column Absorbed in the anterior except in tin' upper three cervical segments Well marked Absorbed in the anterior column Present Processus reticularis Well developed Poorly developed Absent Absent White matter In large amount Less than in the cervical region, but relatively a large amount in comparison to the gray matter Slightly less than in the thoracic re- gion; very little in comparison to the large volume of the gray Very little Sulcus interme- dius posterior Present throughout Present in upper seven thoracic segments Absent Absent uran on the one side, and the posterior median septum on the other. The sep- tum, just mentioned, completely separates the two posterior funiculi from each other. Incomplete septa project into the white substance from the enveloping pia mater. One of these, more regular than the others, enters along the line of the posterior intermediate sulcus. It is restricted to the cervical and upper thoracic segments, is known as the posterior intermediate septum, and divides the posterior funiculus into two bundles, the more medial of which is known as the fasciculus gracilis, while the other is called the fasciculus cuncatus. Characteristics of the Several Regions of the Spinal Cord — It will be ap- parent from Figs. 55-58 that the size and shape of the spinal cord, as seen in transverse section, varies greatly at the different levels and that the relative proportion of gray and white matter is equally variable. Two factors are 84 THE NERVOUS SYSTEM primarily responsible for these differences. One of these is the variation in the size of the nerve roots at the different levels; for where great numbers of nerve- fibers enter, they cause an increase in the size of the cord and particularly in the volume of the gray matter. It has already been pointed out that the cer- vical and lumbar enlargements are directly related to the large nerves supply- ing the extremities. The second factor is this: Since all levels of the cord are associated with the brain by bundles of long fibers, it is obvious that such long fibers must increase in number and the white matter increase in volume as we follow the cord from its caudal end toward the brain. All this is well illus- trated in a diagram by Donaldson and Davis reproduced in Fig. 59. The outline of a section of the spinal cord at the fourth sacral segment is some- what quadrilateral. The total area is small and the greater part is occupied by the thick gray columns (Fig. 60). The size of the cord is much greater at the level of tine first sacral and fifth lumbar segments, as might be expected from the large size of the associated nerves (Figs. 57, 60). There is both an absolute and a relative increase in the white substance, which here contains the long paths connecting the sacral portions of the spinal cord with the brain. Both the anterior and posterior columns are massive, and the anterior presents a prominent lateral angle. The large nerve-cells in the lateral part of the an- terior column give rise to the fibers which run to the muscles of the leg. At the level of the seventh thoracic segment (Figs. 56. 60) the cross-sectional area is less than in the lumbar enlargement. Corresponding to the small size of the tho- racic nerves the gray matter in this region is much reduced, both anterior and posterior columns being very slender. The apex of the latter is some distance from the surface and its cervix is thickened by a column of cells known as the nucleus dorsalis. The columna lateralis is prominent. The white matter is somewhat more abundant than in the lumbar region, and increases slightly in amount as we follow the cord rostrally through the thoracic region (Fig. 59). A transverse section at the level of the seventh cervical segment is elliptic in outline and has an area greater than that of any other level of the cord (Figs. 55, 60). The white matter is voluminous and contains the long fiber tracts connecting the brain with the more caudal portions of the cord. The gray matter is also abundant, as we might expect from the large size of the seventh cervical nerve. The ventral column is especially thick and presents a prominent lateral angle. The large laterally placed nerve-cells of the anterior column are associated with the innervation of the musculature of the arm. The posterior column is relativelv slender, but reaches nearlv to the dorsolateral sulcus. I III SPINAL < <)ki) 85 MICROSCOPIC ANATOMY Neuroglia. Occupying the Interstices among the true nervous elements of the centra] nervous system is a peculiar supporting tissue, the neuroglia, which is of ectodermal origin. In the chapter on Histogenesis we Learned thai from the original epithelium of the neural tube there are differentiated spongioblasts and neuroblasts, as well as a special epithelial lining for the tube, the ependyma. Fig. 61. — Ependyma and neuroglia in the region of the central canal of a child's spinal cord: A, Ependymal cells; B and D, spider cells in the white and gray matter, respectively; C, mossy cells. Golgi method. (Cajal.) The latter consists of long nucleated columnar cells which line the central canal of the spinal cord as well as the ventricles of the brain (Fig. 61). In fetal life their free ends bear cilia, which project into the lumen of the tube, and fine processes from the outer ends extend to the periphery of the cord. In the adult there are no cilia and the peripheral processes reach the surface only along the posterior median septum and in the anterior median fissure. 86 THE NERVOUS S VST EM The neuroglia cells are differentiated from the spongioblasts. These, when stained by the Golgi method, appear as small cells with many processes. Some have long slender processes, the spider cells or long rayed astrocytes; others have short thick varicose processes, the mossy cells or short rayed astrocytes (Tig. 61). Special neuroglia stains, like that of Weigert, show that an astrocyte is composed of a glia cell associated with many glia fibers. Some authors main- tain that the fibers run through the cytoplasm, while others assert that they merely pass along the surface of the cell. In any case the fibers are to be re- garded as products of these cells. Neuroglia cells and fibers are found every- where throughout the gray and white matter of the spinal cord, forming a sup- porting framework for the nervous elements. A special condensation of neu- f'/i % ;-- : {- : — Unmyelinated fibers " "■ - — ! — Myelinated fibers ■•■■■ '. ; ,-■''' .. ' ■ : Fig. 62. — From a cross-section through the spinal cord of a rabbit showing the structure of the white matter as revealed by the Cajal method. (Cajal.) roglia surrounds the central canal and is known as the substantia gelatinosa centralis. In addition to the neuroglia this contains some nerve-fibers and cells. Beneath the pia mater and closely investing the spinal cord externally is a thin stratum of neuroglia, the glial sheath, which dips into the cord along with the pial septa. The posterior median septum is composed of neuroglia and greatly elongated ependymal elements, and is in no part formed by the pia mater. White Substance. — The white matter of the spinal cord consists of longi- tudinally coursing bundles of nerve-fibers, bound together by a feltwork of neuroglia fibers in which are scattered neuroglia cells. A majority of the neu- roglia fibers run in a direction transverse to the long axis of the nerve-fibers. Blood-vessels enter the cord from the pia mater and are accompanied by con- THE SPINAL (OKI) 87 nective tissue from the pia and by the subpial aeuroglia. It has been generally supposed that the white Fascicles of the cord were composed almosl exclusively of myelinated Gibers; and it is true that these, parti) because of their size, are the most conspicuous elements. In cross-sections stained In the Weigert method the myelin sheaths alone are stained; and since the fibers are cut at right angles to their long axes, they appear as rings. Cajal (1009) has shown that there are also great numbers of unmyelinated fibers in the longitudinal fascicles of the cord (Fig. 62). The different Fascicles differ not only in the size of their myelinated fibers but also in the proportion of unmyelinated fibers which they contain. The fasciculus dorsolaterals or tract of Lissauer (Fig. 63) contains fine myelinated fibers with great numbers of unmyelinated axons. Fig. 63. — From a cross-section of the spinal cord of the cat; a narrow strip extending across the apex of the posterior gray column: a, Fasciculus cuneatus; b, fasciculus dorsolateralis (Lis- sauer); c, dorsal spinocerebellar tract. The unmyelinated fibers appear as black dots. Pyridin- silver method. Close to it lies the dorsal spinocerebellar tract which is composed almost ex- clusively of large myelinated fibers. Gray Substance. — The gray matter is composed of nerve-cells, including their dendrites, and of unmyelinated axons and smaller numbers of myelinated fibers — all supported by a neuroglia framework and richly supplied with capil- lary blood-vessels. The axons of the cells of Golgi's Type I are very long and run out into the white substance or into the ventral roots. Those of the cells of his Type II are short and end within the gray matter. In addition, great numbers of collaterals from the dorsal root fibers and from the longitudinal fibers of the cord, as well as terminal branches of these fibers, enter the gray substance and ramify extensively within it, entering into synaptic relations with the neurons which it contains. The branches of the myelinated fibers soon lose their sheaths, and it is this relative scarcity of myelin which gives to 88 THE NERVOUS SYSTEM this substance its gray appearance. The ramification of dendrites and unmy- elinated fibers forms a very intricate feltwork throughout the gray substance (Fig. 64). The nerve-cells of the spinal cord vary greatly in size. The largest are situated in the anterior column and may measure more than 100 micra. They are all multipolar, possess each a single axon, and may be classified in four groups: (1) Some of the cells, found in the posterior horn and particularly in the sub- stantia gelatinosa Rolandi, belong to Golgi's Type II. with short axons confined to the gray substance. These, however, are present in relatively small numbers in the spinal cord. (2) The motor cells, situated in the anterior column and >;■' Fig. 64.— From a section through the spinal cord of a monkey; showing part of the an- terior gray column including a multipolar nerve-cell and the surrounding neuropil. Pyridin- silver method. most numerous in the cervical and lumbar enlargements, are of large size and possess axons which leave the cord in the ventral roots. (3) Smaller cells are present in the lateral column in the thoracic region and give rise to the visceral efferent fibers of the ventral roots (Fig. 37). (4) Other cells of small or medium size, found chiefly in the posterior column, possess axons which pass into the white matter, where they bend sharply to become ascending or descending fibers, or divide dichotomously into ascending and descending branches (Fig. 68). Some of the ascending fibers reach the brain; the others merely connect the different levels of the spinal cord. The fibers of the latter group constitute the fasciculi proprii and vary greatly in length, some connecting adjacent, mi: SPINAL CORD others, more remote, segments. Their collateral and terminal branch) enter and ramify within the gray substance. Those which remain throughout in the same lateral half of the cord are called association fibers; while others, known as commissural fibers, cross the median plane chiefly in the white com- missure (Fig. 68). Some of the commissural fibers are -hurt and confined to a single level of the cord I Fig. 66). Cell-columns.— The nerve-cells are not uniformly distributed throughout the gray matter, for many of them are arranged in longitudinal cell-columns. In transverse sections each of these columns appears as a distinct group oi cells, somewhat separated from other similar groups within the gray matter Fig. 65). The large motor cells of the anterior column, which give origin to the ventral root fibers, form several subgroups. One of these, known as the anteromedian cell-column, occupies the medial part of the anterior column through- out almost its entire length, being absent only in the fifth lumbar and first sacral segments. Behind it is the posteromedian cell-column, which is. however, present only in the thoracic and first lumbar segments and for a short -tretch in the cervical region. The axons from these two medial groups of cells prob- ably supply the musculature of the trunk. In the cervical and lumbar enlarge- ments there are laterally placed groups of cells the axons of which supply the muscles of the limbs. These are: (1) the anterolateral cell-column, present in the fourth to the eighth cervical and in the second lumbar to the second sacral segments; (2) the posterolateral cell-column in the last live cervical, last four lumbar, and first three sacral segments; (3) the retro posterolateral cell-column in the eighth cervical, first thoracic, and first three sacral seg- ments, and (-T) the central cell-column in the second lumbar to the second sacral segments. The intcrmcdiolatcral cell-column is found in the lateral column in the tho- racic region of the cord and is prolonged downward into the upper lumbar seg- ments. It is composed of small cells, the axons of which run through the ven- tral roots, spinal nerves, and white rami communicantes into the sympathetic nervous system (Fig. 37). They have to do with the innervation of smooth and cardiac muscle and glandular tissue. The longitudinal extent of this column corresponds quite accurately to that of the spinal origin of the white rami. A group of cells, having a similar function, is also found in the third and fourth sacral segments. The cells of the posterior gray column are smaller, as a rule, than those of the ventral column; and except for the nucleus dorsalis they are not arranged in 9° THE NERVOUS SYSTEM definite groups. They are concerned with the reception and distribution of the impulses entering along the libers of the dorsal roots. S2/W S4- Fig. 65. — Outline sketches of ventral horn of left side of cord at different levels, showing the relative number and position of the chief cell-groups: G, G, Tk, etc., indicate the segments — e. g., first cervical, fourth cervical, sixth thoracic; G (b), lower part of eighth cervical. The following letters designate the cell-groups: v-m, Anteromedian; d-m, posteromedian; v-l, anterolateral; d-l, posterolateral; p. d-l, retroposterolateral; v in L^, L it ventral; c in L2, La, Si, central; /. c. in Tf,, Tn, intermediolateral; ace. in G, C 4 , accessorius; phr. in C\, phrenic; Cl.c. in T 6 , Tn, nucleus dorsalis. (Bruce, Quain's Anatomy.) The nucleus dorsalis, or column of Clarke, is a group of large cells in the medial part of the base of the posterior column. It extends from the last cer- THE SPINAL ( I IRD 91 vical or first thoracic to the second or third lumbar segments. It is a prom inent feature in cross-sections of the thoracic cord, appearing as a well denned oval area richly supplied with collaterals from the dorsal root.. The cells have an oval or pyriform shape; each has several dendritic processes and an axon which enters the lateral funiculus, within which it runs toward the cerebellum in the dorsal spinocerebellar tract. The Spinal Reflex Mechanism. — In the next chapter we will consider at length the long ascending and descending paths in the white substance of the cord by which afferent impulses from the spinal nerves reach the brain, and those through which the motor centers of the brain exert in return a controlling influence over the spinal motor apparatus. But fully as important as these are the purely intraspinal connections — the spinal reflex mechanism. Fig. 66. — Diagrammatic section through the spinal cord and a spinal nerve to illustrate a simple reflex arc: a, b, c, and d, Branches of sensory fibers of the dorsal roots; e, association neuron; /, commissural neuron. A reflex arc in its simplest form may be made up of only two neurons, the primary sensory and motor neurons wdth a synapse in the gray matter of the anterior column (Fig. 66). It consists of the following parts: (1) a receptor, the peripheral sensory endings; (2) a conductor, the afferent nerve-fiber; (3) a center, including the synapse in the anterior column; (4) a second conductor, the efferent nerve-fiber, and (5) an effector, the muscle-fiber. Usually, how- ever, there are interposed between the primary sensory and motor elements one or more intermediate neurons. These, when restricted to one side of the cord, are known as association neurons; when their axons cross the median plane, as many of them do through the anterior white commissure, they are called commissural neurons. When the circuit is complete within a single neural Q2 TIIK XKRVOUS SYSTEM segment it may be said to be intrasegmental (Fig. 66); if it extends through two or more such segments it is an intersegmental reflex arc. Intersegmental Reflex Arcs. — Impulses entering the spinal cord through a given dorsal root may be transmitted to the primary motor neurons of another segment in one of two ways: (1) by way of the ascending and descending branches of the dorsal root fibers, and (2) along the fibers of the fasciculi proprii (Fig. 67). A full account of these two pathways will be presented in the next chapter, but a word of explanation is required here. The fibers of the dorsal root divide, Fig. 67. — Diagram of the spinal cord, showing the elements concerned in a diffuse unilat- eral reflex: a, Spinal ganglion cell; b, motor cell in anterior column; c, association neuron. (Cajal.) soon after their entrance into the cord, into long ascending and shorter descend- ing branches, which together form the greater part of the posterior funiculus and give off many collaterals to the gray matter of the successive levels of the cord (Fig. 67). Many of the ascending branches reach the brain; but the others terminate, as do the descending branches and all the collaterals, in the gray matter of the cord (Fig. 68). The fasciculi proprii immediately surround the gray columns (Fig. 68) and consist of ascending and descending fibers, which arise and terminate within the gray substance of the cord. Most of these fibers remain on the same side as association fibers concerned in unilateral re- THE SPINAL « i 'li. flexes. Others cross in the anterior white commissure and are commi ural libers concerned in crossed reflexes. Afferent impulses ma) be transmitted along tin' cord in either direction b) the branches oJ the dorsal root fibers; or by means of synapses in the gray matter they may be transferred to the long eiation and commissural fibers and conveyed to the primary motor neurons oi the same or opposite side in more or less distant segments. The course of a nerve impulse in a unilateral intersegmental reflex is indicated on the Left side Dorsal root Ventral root Ascending branch of dorsal root fiber . I ssociation fibers -'-'"_ Descending branch of dorsal foot fiber Dorsal root -■Commissural fibers Ventral root Fig. 68.— Diagram of the spinal cord, showing the elements concerned in intersegmental reflexes. of Fig. 68, while on the right side of the same figure are shown the elements concerned in crossed reflexes. The observations of Coghill (1913 and 1914) and of Herrick and Coghill (1915) tend to show that the simple form of reflex arc illustrated in Fig. 66 is not the primitive type. In larval Amblystoma the first arcs to become functionally mature are composed ot chains of many neurons, so arranged that every cutaneous stimulus elicits the same complex response of the entire somatic musculature, i. e., the swimming movement. It is of particular interest to note that in this primitive reflex mechanism the sensory fibers arise from giant cells located within the spinal cord and that the ventral root fibers are collaterals from the central motor tract. In adult Amblystoma these sensory and motor elements are replaced by the usual type of primary sensory and motor neurons. 94 THE NERVOUS SYSTEM We may mention as an example of a reflex arc involving many segments of the cord the "scratch-reflex" of the dog, which has been very carefully investi- gated by Sherrington (1906). If, some time after transection of the spinal cord in the low cervical region, the skin covering the dorsal aspect of the thorax be stimulated by pulling lightly on a hair, the hind limb of the corresponding side begins a series of rhythmic scratching movements. By degeneration experi- ments it was shown that this reflex arc probably includes the following elements: (1) a primary sensory neuron from the skin to the spinal gray matter of the corresponding neural segment; (2) a long descending association neuron from the Fig. 69. — Diagram of the spinal arcs involved in the scratch-reflex: Ra and R3, Receptive paths from hairs in the dorsal skin of left side; Pa and Pft, association neurons; FC, motor fibers of ventral root. (Sherrington.) shoulder to the leg segments, and (3) a primary motor neuron to a flexor muscle of the leg (Fig. 69). A primary motor neuron seldom, if ever, belongs exclusively to one arc, but serves as the final channel to which many streams converge. Its perikaryon gives off wide-spread dendritic processes, through which it comes into relation with the ramifications of axons from many different sources. In this way impulses reach it from the dorsal roots, and from the fasciculi proprii of the spinal cord, as well as from a number of tracts which descend into the spinal cord from centers in the brain (the corticospinal, rubrospinal, tectospinal, and vestibulospinal tracts). The primary motor neuron is, as Sherrington has said, "the final common path." CHAPTER VII FIBER TRACTS OF THE SPINAL CORD The fibers composing the white substance of the spina] cord are not scat- tered and intermingled at random, but, on the contrary, those of a given func- tion are grouped together in more or less definite bundles. A bundle of fibers all of which have the same origin, termination, and function is known as a fiber tract . The funiculi of the spinal cord are composed of many such tracts of longitudinal fibers, which, while occupying fairly definite areas, blend more or less with each other, in the sense that there is considerable intermingling of the fibers of adjacent tracts. It is convenient to have a name for certain obvious subdivisions of the funiculi which contain fibers belonging to more than one tract. Such a mixed bundle is properly called a fasciculus. THE INTRAMEDULLARY COURSE OF THE DORSAL ROOT FIBERS The central end of a dorsal root breaks up into many rootlets or filaments (fila radicularia), which enter the spinal cord in linear order along the line of the posterior lateral sulcus. As it enters the cord each filament can be seen to separate into a larger medial and a much smaller lateral division. The fibers of the medial division are of relatively large caliber and run over the tip of the posterior column into the posterior funiculus (Fig. 72). Those of the lateral division are fine and enter a small fascicle which lies along the apex of the pos- terior column, the fasciculus dorsolateralis or tract of Lissauer. Very soon after their entrance into the cord each dorsal root fiber divides in the manner of a Y into a longer ascending and a shorter descending branch (Fig. 70). The ascending branches of the fibers of the medial division of the dorsal root run for considerable but varying distances in the posterior funiculus; some from each root reach the medulla oblongata, others terminate at different levels in the gray matter of the spinal cord. At the level of their entry into the cord these fibers occupy the lateral portion of the fasiculus cuneatus; but in their course cephalad, as each successive root adds its quota, those from the more caudal roots are displaced medianward. In this way the longer fibers come to occupy the medial portion of the posterior funiculus (Fig. 71). In the cervical region 95 9 6 THE NERVOUS SYSTEM the long ascending fibers from the sacral, lumbar, and lower thoracic roots constitute a well-defined medially placed bundle, the fasciculus gracilis, sepa- rated from the rest of the posterior funiculus by the posterior intermediate septum. Those of the long ascending fibers, which finally reach the brain, terminate in gray masses in the posterior funiculi of the medulla oblongata Fig. 70. — Bifurcation of the dorsal root fibers within the spinal cord into ascending and descending branches, which in turn give off collaterals; the termination of some of these col- laterals in synaptic relation to cells of the posterior gray column. (Cajal, Edinger.) (nucleus of the funiculus gracilis and nucleus of the funiculus cuneatus). Since the number of these long ascending branches must increase from below upward it is easy to understand the progressive increase in size of the posterior funiculus from the sacral to the cervical region (Fig. 60). The fasciculus gracilis and fasciculus cuneatus are composed for the most i'llil K I R \i fS 01 NIK SPINAL CORD 97 / i . gracilis I a i Utieatui part of these ascending branches of the dorsal root fibers, the former contain Ing those which have the Longest intramedullary course. The descending branches of the fibers of the medial division of the dorsal mot are all relatively short. The shortest terminate at once in the gray matter dt" the posterior column. Others descend in the fasciculus inter/ ascicularis, or comma tract of Schultze, which is situated near the center of the posterior fu- niculus; and still others run near the posterior median septum in the septomar- ginal fasciculus (Fig. 76). In both of these fas- cicles they arc intermingled with descending fillers, arising from cells within the gray matter of the spinal cord. Collaterals. — At intervals along both ascending and descending branches collaterals are given off which run ventrally to end in the gray matter (Fig. 70). They are much liner than the fibers from which they arise, and the total number arising from a given fiber is rather large. Some of them end in the ventral gray column; others, in the posterior gray column, including the substantia gelatinosa and the nucleus dorsalis; still others run through the dorsal com- missure to the opposite side of the cord, where they appear to end in the posterior columns (Fig. 72). In Fig. 70 there are illustrated the arborizations formed by some of these collaterals about cells of the posterior column. The terminals of the descending branches and of those ascending branches, which do not reach the brain, end as do the collaterals within the gray matter of the spinal cord. The fibers of the lateral division of the dorsal root are all very fine. The majority are unmyelinated and can be recognized only in preparations in which the axons are stained. A good account of their appearance in Golgi prepara- tions has been given by Barker (1899, pp. 466-468). In Weigert preparations we must look carefully to find the few myelinated fibers contained in this divi- sion. But in pyridin-silver preparations great numbers of fine unmyelinated fibers, accompanied by a few which are myelinated, can be seen to turn lateral- ward as the root filament enters the cord. These constitute the lateral division 7 Fig. 71. — Diagram to illustrate the arrangement of the ascending branches of the dorsal root fibers within the posterior funic- ulus of the spinal cord. 9 8 THE NERVOUS SYSTEM of the root and enter the dorsolateral fasciculus or tract of Lissauer (Fig. 72). The medial division, on the other hand, consists exclusively or almost exclu- sively of myelinated fibers. The fibers of the lateral division of the root divide into ascending and descending branches, both of which, however, are very short. The ascending branch, which is the longer of the two, does not extend at most more than the length of one or two segments in the long axis of the cord (Ranson, 1913, 1914). The dorsolateral fasciculus, or tract of Lissauer, lies between the apex of the posterior column and the periphery of the cord, and varies greatly in shape and size in the different levels of the cord (Figs. 55-58). It is composed of Medial division of dorsal root his cuncatus solalcral isciculus Dorsal spino- cerebellar tract Dorsal spinocc \i { V f^^ ^-^ >J Ventral spino- cerebellar tract Lateral spino- thalamic and spinotectal tracts Ventral spinothalamic Fig. 72. — Diagram of the spinal cord and dorsal root, showing the divisions of the dorsal root, the collaterals of the dorsal root fibers, and some of the connections which are established by them. unmyelinated and fine myelinated fibers, which are derived in part from the lateral division of the dorsal root and in part arise from cells in the neighboring gray matter (Fig. 63). AFFERENT PATHS IN THE SPINAL CORD We have been at some pains to make clear the course and distribution of the dorsal root fibers within the spinal cord because all afferent impulses which reach the cord are carried by them. Interoceptive fibers from the viscera, proprioceptive fibers from the muscles, tendons, and joints, as well as extero- ceptive fibers from the skin are included in these roots; and among the latter group are probably several subvarieties, mediating the afferent impulses out ETBEK TRACTS OF ill:: SPINAL CORD of which the sensations of touch, heatj cold, and pain arc elaborated. An important problem which in great measure awaits solution is this: How are the -fibers of the different functional varieties distributed in the spinal cord and along what paths are these various types of afferent impulses carried toward the brain? The proprioceptive fibers, which terminate at the periphery in neuromus cular and neurotendinous spindles and in Pacinian corpuscles, arc known to be myelinated. They must, therefore, pass through the well myelinated medial division of the dorsal root into the posterior funiculus. As shown by Brown Sequard in 1847 by a stud}- of patients with unilateral lesions of the spinal cord, sensations from the muscles, joints, and tendons reach the brain without undergoing a crossing in the spinal cord. This and other evidence points un- mistakably to the long ascending branches of the dorsal root fibers, which are continued uncrossed in the posterior funiculus to the medulla oblongata, as the conductors of this type of sensation. When these fibers are destroyed by a tumor or other lesion confined to the posterior funiculus, muscular sensibility and the recognition of posture are abolished, while touch, pain, and tempera- ture sensations remain intact (Dejerine, 1914). No better exposition of the proprioceptive functions could be furnished than by describing the sensory deficiencies found in cases of tabes dorsalis or loco- motor ataxia, a disease in which there is degeneration of the posterior funiculi. Lying in bed, with eyes closed, a tabetic may not be able to say in w r hat posi- tion his foot has been placed by an attendant because afferent impulses from the muscles, joints, and tendons fail to reach the cerebral cortex and arouse sensations of posture. Not only are the sensations of this variety lacking, but the unconscious reflex motor adjustments initiated by proprioceptive afferent impulses are also impaired. Standing with feet together and eyes closed, the patient loses his balance and sways from side to side. In walking his gait is uncertain and the movements of his limbs poorly coordinated. All of this motor incoordination is explained by a loss of the controlling afferent impulses from the muscles, joints, and tendons. The long ascending fibers of the posterior funiculus, which reach the brain and end in the nucleus gracilis and cuneatus, are for the most part proprio- ceptive in function (Fig. 235). The connections which they make there can best be considered in another chapter. Collaterals and many terminal branches end in the gray matter of the cord, entering into synaptic relations with the neu- rons of the spinocerebellar paths and with neurons belonging to spinal reflex arcs. IOO THE NERVOUS SYSTEM Proprioceptive Paths to the Cerebellum. — According to the researches of Marburg (1904) and Bing (1906) the spinocerebellar tracts are concerned with the transmission to the cerebellum of afferent impulses from the muscles, joints, and tendons, which remain, however, at a subconscious level (Dejerine, 1914). We may, therefore, appropriately consider these paths at this time. The dorsal spinocerebellar tract (fasciculus spinocerebellaris dorsalis, direct cerebellar tract of Flechsig, fasciculus cerebellospinalis) is a well-defined bundle at the surface of the lateral funiculus just ventral to the posterior lateral sul- cus (Figs. 72, 78). In cross-section it has the form of a flattened band, situated between the periphery of the cord and the lateral corticospinal tract. It begins in the upper lumbar segments and is prominent in the thoracic and cervical portions of the cord. It consists of uniformly large fibers, which take origin from the cells of the nucleus dorsalis of the same side. This nucleus forms a prominent feature of the sections through the thoracic portion of the cord, but is not found above the seventh cervical nor below the second lumbar seg- ments. A conspicuous bundle of myelinated collaterals from fibers of the fasciculus cuneatus run to this nucleus (Fig. 56) where their arborizations form baskets about the individual cells of the nucleus. The fibers arising from the cells of the nucleus dorsalis run laterally to the periphery of the lateral funiculus of the same side, where they turn rostrally and form the dorsal spinocerebellar tract. We will follow this tract into the brain in a later chapter. Here we need only say that it reaches the cerebellum by way of the restiform body (Fig. 235). The ventral spinocerebellar tract constitutes the more superficial portion of a large ascending bundle of fibers, known as the fasciculus anterolateralis super- ficialis or Gower's tract, which also includes the spinotectal and lateral spino- thalmic tracts (Fig. 72). It is situated at the periphery of the lateral funiculus ventral to the tract we have just considered. It is said to consist of fibers which arise from the cells of the posterior gray column and intermediate gray matter of the same and the opposite side (Page May, 1906; Dejerine, 1914). In a subsequent chapter we will trace these fibers by the way of the medulla, pons, and an- terior medullary velum to the cerebellum (Fig. 235). From what has been presented above it will be apparent that collaterals and terminal branches of dorsal root fibers, doubtless of the proprioceptive group, enter into synaptic relations with certain intraspinal neurons, the axons of which run to the cerebellum by way of the ventral and dorsal spinocerebellar tracts. The entire path from periphery to cerebellum therefore consists of two neurons with a synaptic interruption in the gray matter. I ir.i K l K At is OP i in; SPIN \i. CORD ioi Interoceptive fibers arc present in the thoracic and upper, lumbar doi I roots, hut are either absent or verj few in number in the others. We know practically nothing about their intraspinal course in mammals. The} will be considered In the chapter on the Sympathetic Nervous System. Exteroceptive fibers carry cutaneous afferent impulses, and probably are subdivided into several varieties. Most authors agree that there are separate fibers for the impulses aroused by tactile and thermal stimuli; and Sherrington 1906) lias presented evidence for the existence of a separate group of Gibers, whose end organs are responsive only to agents capable of inflicting injury, that is, to noxious or painful stimuli. Conduction of Tactile Impulses in the Spinal Cord. — The phenomena of sen- sory dissociation, characteristic of syringomyelia, show that the intraspinal path for the sensations of touch is rather widely separated from that for pain and temperature sensation (Fig. 73). In that disease a cavity is developed within the gray matter of the spinal cord; and sensations of pain and tem- perature may be abolished over a given cutaneous area which is still sensitive to touch. The separation of these two lines of conduction occurs at the place where the dorsal root fibers enter the cord. The fibers, mediating pain and temperature sensations, end almost at once in the gray matter, while those for touch ascend for some distance in the posterior funiculus of the same side (Head and Thompson, 1906; Dejerine, 1914). As these fibers ascend in the posterior funiculus they give off collaterals to the gray matter of the successive levels of the spinal cord through which they pass. The tactile impulses from a given root, therefore, do not enter the gray matter all at once, but filter forward through the collaterals and terminals of these dorsal root fibers to reach the posterior gray column in a considerable number of segments above that at which the root enters the cord. Within the posterior gray column at these successive levels the terminals and collaterals of the tactile fibers establish synaptic con- nections with neurons of the second order. The axons of these neurons form the ventral spinothalamic tract of the opposite side (Fig. 73). The ventral spinothalamic tract is an ascending bundle of fibers found in the anterior funiculus. It consists of fibers which take origin from cells in the pos- terior column of the opposite side, cross the median plane in the anterior white commissure, and ascend in the ventral funiculus to end within the thalamus (Fig. 73). It is possible that many of the fibers do not reach the thalamus directly, but terminate in the gray matter of the cord and medulla oblongata in rela- tion to other neurons, whose axons continue the course to the thalamus. If 102 THE NERVOUS SYSTEM this be so the path consists in part of relays of shorter neurons (Dejerine, 1914). The uncrossed path in the posterior funiculus for tactile impulses entering the cord through any given dorsal root overlaps by many segments the crossed path in the ventral funiculus (Fig. 230). Some of the uncrossed fibers even reach the nuclei of the funiculus gracilis and funiculus cuneatus in the medulla oblongata. This extensive overlapping of the uncrossed by the crossed paths accounts for the fact that lateral hemisection of the human spinal cord rarely causes marked disturbance of tactile sensibility below the lesion (Petren, 1902; Head and Thompson, 1906). ll Ascending branch of dorsal root fiber - Myelinated fiber of dorsal rooty Spina! ganglion Unmyelinated fiber of dorsal root Lateral spinothalamic tract (pain and temperature) Ventral spinothalamic tract (touch) Fig. 73. — Exteroceptive pathways in the spinal cord. Since it seems clear that the dorsal root fibers subserving tactile sensibility ascend for some distance in the posterior funiculus, they must be included among the myelinated fibers of the medial division of the dorsal root, because only myelinated fibers ascend in that funiculus. This conclusion is in keeping with the facts already mentioned concerning the termination of myelinated fibers in the supposedly tactile end organs, such as Meissner's corpuscles and Pacinian corpuscles. It is also in keeping with facts to be mentioned in a following paragraph concerning the structure of the median nerve. The Lateral Spinothalamic Tract. — It seems to be well established that the dorsal root fibers, which serve as pain conductors, terminate in the gray matter almost at once after entering the cord, and come into synaptic relations with neurons of the second order, whose axons run in the lateral spinothalamic tract. From cells in the posterior column fibers arise, which in man cross to the opposite side of the cord in the anterior white commissure and ascend in the lateral spino- thalamic tract to end in the thalamus (Figs. 73, 231). This is a tract of ascending FIBER TRACTS OF III!. SPINAL CORD 103 fibers situated in the 'lateral funiculus under cover of the ventral spinocerebellar tract. Together with the spinotectal and ventral spinocerebellar tracts it forms the fasciculus anterolateral superncialis (of Gowers). It mediat* pain and temperature sensations. Conduction of Painful Afferent Impulses iii the Spinal Cod. Nol all of the fibers of the lateral spinothalamic Iran reach the thalamus. According to May (1906), "Some of these fibers certainly pass directly to the thalamus, while others terminate in the inter- mediate gray matter, and thus, by means of a series of short chains, afford secondary paths to the same end station, which may supplement the direct path, or be made available after interruption of the direct path." It has been shown in many cases in man and animals that, after a complete hemisection of the spinal cord, the loss of sensibility to pain on the op- posite side of the body below the lesion was only temporary. In time there may occur a more or less perfect restoration of pain conduction, showing that the homolateral side of the cord is able to supplement or replace the heterolateral path. According to the researches of Karplus and Kreidl (1914) and Ranson and Billingsley (1916) these short chains, which are of secondary importance in man, are much better developed in the cat. In this animal pain conduction through the spinal cord is bilateral and is effected to a large extent through a series of short relays. According to Head and Thompson (1906) the path for pain in the spinal cord is the same whether the impulses arise in the skin or in the deeper parts, such as the muscles and joints. But Dejerine (1914) is of the opinion that painful impulses from the muscles may be trans- mitted in the posterior funiculus and remain uncrossed as far as the medulla oblongata. Until recently we possessed no information as to which dorsal root fibers served as pain conductors. But in the last few years evidence has been presented which points toward the unmyelinated fibers of the spinal nerves and dorsal roots as the pain fibers (Ranson, 1915). Space does not permit a detailed presentation of the evidence here. It should be noted, however, that the unmyelinated fibers of the lateral division of the dorsal root terminate in the gray matter almost immediately after their entrance into the spinal cord, and in this respect correspond to the known course of the fibers carrying painful impulses. The un- myelinated fibers are chiefly distributed in the cutaneous nerves, although a few run in the muscular branches. This coincides with the much greater sensitiveness to pain of the skin than of the deeper tissues. Furthermore, the median nerve at the wrist, a large nerve supplying a relatively small area of skin richly endowed with the sense of touch, contains relatively few unmyelinated fibers. On the other hand, nerves like the lateral cutaneous of the thigh and the medial cutaneous of the forearm, which supply relatively large cutaneous areas of low tactile sensibility, but not inferior to the fingers in sensitiveness to pain, are com- posed in large part of unmyelinated fibers. This difference between the composition of the median nerve and the medial cutaneous nerve of the forearm is just what should be expected if the touch fibers are myelinated and the pain fibers unmyelinated. Head and his co-workers (1905, 1906. 1908) have regarded the group of sensations (protopathic), to which according to their classification cutaneous pain belongs, as primitive in character and the first to appear in the phylogenetic series. It is well known that nerve-fibers in their earliest phylog' are unmyelinated. If our conception is correct, a great many of the afferent fibers of mam- mals remain in this primitive undifferentiated state and mediate a relatively primitive form of sensation. In this connection it is interesting to note that Dejerine 1914) believes that pain is conducted by the "sympathetic" fibers contained in the cutaneous and muscular nerves. He does not state the evidence on which this belief is based; but if by "sympathetic" he means to designate the unmyelinated fibers his view agrees perfectly with that presented in the preceding paragraphs. io4 THE NERVOUS SYSTEM The problem can be approached from the experimental standpoint. The seventh lum- bar dorsal root of the cat is especially adapted for such a test. This root as it approaches the cord breaks up into a number of filaments which spread out in a longitudinal direction and enter the cord along the posterolateral sulcus. Within each root filament, as it ap- proaches this sulcus, the unmyelinated separate out from among the myelinated fibers and take up a position around the circumference of the filament and along septa that divide it into smaller bundles. As the root enters the cord, these unmyelinated fibers turn laterally into the dorsolateral fasciculus, constituting together with a few fine myelinated fibers the lateral division of the root (Fig. 74). Almost all of the myelinated fibers run through the medial division of the root into the cuneate fasciculus. A slight cut in the direction of the Posterior [utuculus. Unmueli rated, [iters. Lissauers tract Dorsal , Fig. 74. — From a section of the seventh lumbar segment of the spinal cord of the cat, showing the unmyelinated fibers of the dorsal root entering the tract of Lissauer. arrow, which as shown by subsequent microscopic examination divided the lateral without injury to the medial division of the root, at once eliminated the pain reflexes obtainable from this root in the anesthetized cat, such as struggling, acceleration of respiration, and rise of blood-pressure. On the other hand, a long deep cut in the plane indicated by B, Fig. 74, which severed the medial division of the root as it entered the cord, had little or no effect on the pain reflexes. This series of experiments, the details of which are given else- where (Ranson and Billingslcy, 1916), furnishes strong evidence that painful afferent im- pulses are carried by the unmyelinated fibers of the lateral division of the dorsal root. These fibers probably terminate in the substantia gelatinosa Rolandi, and, if so, it is not unlikely that intermediate neurons are intercalated between them and the neurons whose axons run in the ventral spinothalamic tra.ct. FIBER TRACTS 0! THE SPINAL CORD 105 The Conduction of Sensations of Pain, of Heat, and of Cold. — It is well estab- lished IKI U BON IN W HII II I 111 S I > I < . I N I |< \ || Ascending iletrcneration. liny degeneration. Anterior funiculus Ventral spinothalamic tract Ventral corticospinal tract, Vestibulospinal tract, Tectospinal tract Lateral funiculus Dorsal spinocerebellar tract, Ventral spinocerebellar tract, Lateral spinothalamic tract, Spinotectal tract Lateral corticospinal tract, Rubrospinal tract, Bulbospinal tract, Tectospinal tract Posterior funiculus Ascending branches of the dorsal root fibers Fasciculus interfascicularis, Septomarginal tract The fasciculi proprii or ground bundles are composed of short ascending and descending fibers, which arise and terminate within the gray matter of the spinal cord and link together the various segments of the cord. These fascicles, one of which is present in each of the three funiculi, immediately surround the gray columns. After a transection of the spinal cord the fasciculi proprii undergo an incomplete degeneration for some distance both above and below the lesion (Figs. 75. 76). In cross-section the ground bundle of the posterior funiculus has the form of a narrow band upon the surface of the posterior column and posterior commissure, and was once called the cornu-commissural bundle (Fig. 78). In addition to this fascicle there are in the posterior funiculus two other tracts which in part belong to the same system — the septomarginal tract and the fasciculus interfascicularis, or comma tract of Schultze. These are both composed of descending fibers, in part of intraspinal origin and in part representing the descending branches of the dorsal root fibers. The septomar- ginal tract is situated along the dorsal periphery of the posterior funiculus in the thoracic region; it takes up a position along the septum in the lumbar segments (oval area of Flechsig); and in the sacral region it forms a triangular field at the dorsomedial angle of the posterior funiculus (triangle of Gombault and Philippe (Fig. 76). The fasciculus interfascicularis is best developed in the thoracic segments, where it occupies a position near the center of the posterior funiculus. io8 TILi: NERVOUS SYSTEM In the anterior funiculus, in addition to the fasciculus proprius which imme- diately surrounds the gray matter, there is a thin layer of similar fibers spread out along the border of the anterior fissure and known as the sulcomarginal fasciculus. This tract also contains the fibers which descend into the cord from the medial longitudinal bundle of the medulla oblongata. As a general rule the short fibers of the fasciculus proprius lie nearer the gray substance than the fibers of greater length; and the long tracts, which Fasciculus gracilis .. Spinocerebellar, spinotectal, and lateral _ spinothalamic tracts Cervical enlargement ascending degeneration I 'ppcr thoracic ascending degeneration '~M Middle thoracic .$ site of compression Lower thoracic descending degeneration Upper lumbar descending degeneration Lower lumbar descending degeneration Fig. 76. — Ascending and descending degeneration resulting from a compression of the thoracic spinal cord in man. March i method. (Hoche.) connect the spinal cord with the brain, occupy the most peripheral position. But the fact must not be overlooked that many fibers of the fasciculus proprius are intermingled with those of the long tracts. LONG DESCENDING TRACTS OF THE SPINAL CORD Fibers which arise from cells in various parts of the brain descend into the spinal cord, where they form several well-defined tracts. The most important Fasciculus intcrfascicularis «... Septomarginal fasciculus-... Lateral corticospinal lracl~.j '■I mm Septomarginal fasciculus, oval area of Flcchsig Lateral corticospinal tracU,^ I ir.iK TB \< is OF nil. SPINAL CORD and most conspicuous of these are the cerebrospinal fasciculi, which are more properly called the corticospinal trails. There are two in each lateral half of the cord, the lateral and the ventral corticospinal tracts. Their cob tituenl fibers take origin from the Large pyramidal cells of the precentral gyrus or motor region of the cerebral cortex and pass through the subjacent Levels of the brain to reach the spinal cord (Fig. 77). J list before they enter the spinal COrd they undergo an incomplete decussation in the medulla oblongata, giving ri>e to a ventral and a lateral corticospinal tract. The Lateral Corticospinal Tract (Crossed Pyramidal Trad, Fasciculus Cerebrospinalis Lateralis). — The majority of the pyramidal libers, after cross ing the median plane in the decussation of the pyramids, enter the lateral fu- Cerebral hemisphere Spinal cord Fig. 77. — Diagram of the corticospinal tracts. niculus of the spinal cord as the lateral corticospinal tract, which occupies a posi- tion between the dorsal spinocerebellar tract and the lateral fasciculus proprius (Fig. 78). In the lumbar and sacral regions, below the origin of the dorsal spinocerebellar tract, the lateral corticospinal tract is more superficial. It can be traced as a distinct strand as far as the fourth sacral segment; and as it descends in the spinal cord it gradually decreases in size. Throughout its course in the spinal cord it gives off collateral and terminal fibers which end in the gray matter. The ventral corticospinal tract (fasciculus cerebrospinalis anterior or direct pyramidal tract) is formed by the smaller part of the corticospinal fibers, which do not cross in the medulla, but pass directly into the ventral funiculus of the no THE NERVOUS SYSTEM same side of the cord. They form a tract of small size, which lies near the anterior median fissure and which can be traced as a distinct strand as far as the middle of the thoracic region of the spinal cord. Just before terminating these fibers cross in the anterior white commissure. They end like those of the lateral corticospinal tract, either directly or perhaps through an intercalated neuron, in relation to the motor cells in the anterior column. The crossing of these libers is only delayed, and it will be apparent that all of the corticospinal fibers arising in the right cerebral hemisphere terminate in the anterior column of the left side of the cord, and conversely, those from the left hemisphere end on the right side. It is along these fibers that impulses from the motor portion of the cerebral cortex reach the cord and bring the spinal motor apparatus under voluntarv control. Fasciculus septomarginalis Fasciculus gracilis Fasciculus inlerfasciciilaris Fascicillus proprius , Sensory fibers of the second order Lateral corticospinal _ tract Rubrospinal tract— Tectospinal tract - Fasciculus proprius- Bulbospinal trad — Vestibulospinal tract ,,-'Fasciculus cuneatus -Dorsolateral fasciculus _^s Dorsal spinocerebellar trad \ Fasciculus proprius Ventral spinocere- bellar trad '...Lateral spinothalamic tract " Spinotectal trad — Ventral root "" Ventral spinothalamic trad Sulcomarginal fasciculus Ventral corticospinal trad Fig. 78. — Diagram showing the location of the principal fiber tracts in the spinal cord of man. Ascending tracts on the right side, descending tracts on the left. It is stated by some authors, although on the basis of rather unsatisfactory evidence, that the fibers of the lateral corticospinal tract ramify in the formatio reticularis (Mona- kow. 1895) and the nucleus dorsalis (Schafer, 1899). The corticospinal path is from the standpoint of phylogenesis a relatively new system and varies a great deal in different mammals. It is found in the ventral funiculus in the mole, while in the rat it occupies the posterior funiculus. In the mole it is almost completely unmyelinated, in the rat largely so. It contains many unmyelinated fibers in the cat, fewer in the monkey (Linowiecki, 1914). In man it does not become fully myelinated before the second year. An uncrossed ventral corticospinal tract seems to be present only in man and the anthropoid apes, and this tract varies greatly in size in different individuals. The rubrospinal tract (tract of Monakow) is situated near the center of the lateral funiculus just ventral to the lateral corticospinal tract (Fig. 78). Its fibers come from the red nucleus of the mesencephalon, cross the median plane, ) ir.l R CRACTS OF Nil. SPDN \l. I ORD i i | and descend Into the spinal cord, within which some of them can be traced to the sacral region. Their collateral and terminal branches end within the an- terior column in rel ation to the primary motor neurons. Other Descending Tracts. The bulbospinal tract (olivospinal tract, trad of Helweg) is a small bundle of fibers found in the cervical region near the surface of the lateral funiculus opposite the anterior column. The fibers arise from cells in the medulla oblongata, possibly in the inferior olivary nucleus, and end somewhere in the gray matter of the spinal cord. The exact origin and u-r- Fasciculus cuneatus Fasciculus gracilis Lateral corticospinal tract Fasciculi proprii<\ Ventral corticospinal tract ' ' Dorsal spinocerebellar tract Oral area of Flcchsig D. Ill L. IV Fig. 80. Figs. 79 and 80. — Diagrams of the sixth cervical, third thoracic, and fourth lumbar segments of the spinal cord, showing the location of the different tracts as outlined by Flechsig on the basis of differences in time of myelination. (van Gehuchten.) ruination of the tract is unknown. The tectospinal tract, located in the ventral funiculus, is composed of fibers which take origin in the roof (tectum) of the mesencephalon, cross the median plane and descend into the anterior funiculus of the spinal cord, and end in the gray matter of the anterior column. The tract is concerned chiefly with optic reflexes. The vestibulospinal tract, also located in the anterior funiculus, arises from the lateral nucleus of the vestibular nerve 112 THE NERVOUS SYSTEM in the medulla oblongata and conveys impulses concerned in the maintenance of equilibrium. Some of its fibers can be traced as far as the lower lumbar segments. They end in the gray matter of the anterior column. Hemisection of the spinal cord in man produces a characteristic symptom complex known as the Brown-Sequard's syndrome — which the student is now in position to understand. Below the level of the lesion and on the same side there is found a paralysis of the muscles with a loss of sensation from the mus- cles, joints, and tendons; while on the opposite side of the body, beginning two or three segments below the level of the lesion, there is loss of sensations of pain and temperature. Tactile sensibility is everywhere retained (Dejerine, 1914). Order of Myelination. — The fiber tracts of the spinal cord do not all become myelinated at the same time. By a study of the fetal spinal cord at various developmental stages Flechsig was able to identify and trace many of these tracts because of the difference in the time of myelination. His results agree in general with those derived from a study of spinal cords showing ascending and descending degeneration (Figs. 79. 80). Myelination begins during the fifth month of intra-uterine life. The order in which the fibers of the spinal cord acquire their myelin sheaths is as follows: (1) afferent and efferent root fibers, (2) those of the fasciculi proprii. (3) the fasciculus cuneatus, (4) the fasciculus gracilis, (5) the dorsal spinocerebellar tract, (6) the ventral spinocerebellar fas- ciculus, (7) the corticospinal tracts. CHAPTER VIII THE GENERAL TOPOGRAPHY OF THE BRAIN. THE EXTERNAL FORM OF THE MEDULLA OBLONGATA, PONS, AND MESEN- CEPHALON The General Topography of the Brain. — The brain rests upon the floor of the cranial cavity, which presents three well-marked fossae. In the posterior cranial fossa are lodged the medulla oblongata, pons, and cerebellum, which together' constitute the rhombencephalon (Fig. 81). This fossa is roofed over by a partition of dura mater, called the tentorium cerebetti, that separates the cerebellum from the cerebral hemispheres. Through the notch in the ventral Calvaria Proscn- Telencephalon cephalon Diencephalon Frontal lobe of cerebral hemisphere in anterior cranial fossa Temporal lobe of cerebral hemisphere in middle cranial fossa Parietal lobe of cert hemisphere Mesencephalon Occipital lobe of cerebral hemisphere Tentorium cerebetti Posterior cranial fossa Hum Pons Medulla oblot Spinal cord Fig. 81. — Median sagittal section of the head showing the relation of the- brain to the cra- nium. The sphenoid bone is shown in transparency, and through it the temporal lobe may be seen. border of the tentorium projects the mesencephalon, connecting the rhomben- cephalon below with the prosencephalon above that partition. The cerebral hemispheres form the largest part of the prosencephalon, occupy the anterior and middle cranial fossa?, and extend to the occiput on the upper surface of the tentorium. The dorsal aspect of the human brain presents an ovoid figure, the large cerebral hemispheres, covering the other parts from view. In the sheep's brain the 8 "3 1 1.1 THE NERVOUS SYSTEM hemispheres are smaller and fail to hide the cerebellum and medulla oblongata (Fig. 82). The cerebral hemispheres, which are separated by a deep cleft called the longitudinal fissure of the cerebrum, together present a broad convex surface which lies in close relation to the internal aspect of the calvaria. From the latter it is separated only by the investing membranes or meninges of the brain. The thin convoluted layer of gray matter upon the surface of the hemispheres is known as the cerebral cortex. The ventral aspect or base of the brain presents an irregular surface adapted to the uneven floor of the cranial cavity (Figs. 83, 86). The medulla oblongata, Face and tongue Head and eyes Fore limb Hind limb Gyrus sylviacus (arcuatus) Gyrus lateralis Gyri mediates -^ Gvrus intemus _ Vermis cerebelli Hemisphcerium cerebelli Medulla oblongata Medulla spinalis Gyrus frontalis medial is Gyrus frontalis superior Sulcus coronalis Sulcus splenialis Fissura ansata (cruciala) Fissura lateralis (Sylvii) Fissura suprasylvia Fissura longitudinalis Sulcus lateralis Sulcus intermedins Sulcus medialis Flocculus - — Nervus accessorius Nervus spinalis I Fig. 82. — Dorsal view of the sheep's brain. The motor cortex is shaded on the left side. (Herrick and Crosby.) which is continuous through the foramen magnum with the spinal cord, lies on the ventral aspect of the cerebellum in the vallecula between the two cere- bellar hemispheres. Rostral to the medulla oblongata and separated from it only by a transverse groove is a broad elevated band of fibers, which plunges into the cerebellum on either side and is known as the pons. The cerebellum can be seen occupying a position dorsal to the pons and medulla oblongata, and can easily be recognized by its grayish color and many parallel fissures. A pair of large rope-like strands are seen to emerge from the rostral border of the pons and to diverge from each other as they run toward the under surface 111! GENERAL I' IPl IGB \l'll\ I 'I I II! BR W\ i IS of the cerebral hemispheres. These are the cerebral peduncles and they form the ventral part of the mesencephalon. At it- rostral extremit) each peduncle i> partially encircled by a flattened hand, known as the optic tract, which is con tinuous through the optic chiasma with the optic nerves. A lozenge-shaped depression, known as the interpeduncular fossa, i> outlined by the diverging cerebral peduncles and by the optic chiasma and tract-. Within the area thu> outlined and beginning at its caudal angle may he distinguished the following parts: the inter peduncular nucleus, which i- very large in the sheep and OCCU- Longitudinal fissure of cerebrum Optic lurve _ Optic chiasma Rkinal fissurt Insula--^ Lateral fissure Optic tract . Infundibulum -■ Manimillary body - Cerebral pedunch Interpeduncular fossa and nucleus Trigeminal nerve Abduccns nerve-- . ,. [Vestibular n Acoustie] [Cochlear n. Glossopharyngeal tier:' --" Vagus nerve'' Hypoglossal nerve ■' Anterior median fissurt \' Olfactory bulb edial olfactory gyrus interior perforated •uhstance Lateral olfactory stria ■—'Lateral olfactory gyrus -—Diagonal band Amygdaloid nucleus Pyriform Splenium Suprapineal recess Suprapi / .Superior colliculus / /Primary fissurr White center of v< rmis Infundib. ■Pons Aqueduct Olfactory bulb Medial olfaclor'y gyrus;,, ; ; Tu}d ^ ^ N > La ; nina ^ Anterior per], substance > , / < -. , . . ,. x \ p„ c ,„ •, *„„, Lamina terminalis'/ / f^assa intermedia \\^«JSZ Diagonal band 1 Optic cktasma ^Hypophysis Central canal , " Medulla \ Medial aperture of \ \ fourth ventricle \ \Tela chorioidca \ " Fourth ventricle x A nterinr medullary velum Preoptic recess Mammittary body Fig. 84. — Medial sagittal section of the sheep's brain. This is known as the rhinal fissure; and all that portion of the cerebral cortex which lies dorsal to it i> the new or non-olfactory cortex, the neopallium. In contrast to the older olfactory cortex or archi pallium, which includes the p\Ti- form area, the neopallium is of recent phyletic development. It first forms a prominent part of the brain in mammals and is by far the most highly developed in man. Interrelation of the Various Parts of the Brain.— An examination of a medial sagittal section of the brain will make clear the relation which the various parts bear to each other (Fig. 84). The medulla oblongata, pons, and cerebellum are seen surrounding the fourth ventricle, and are intimately connected with one THE GENERA] TOPOGRAPHS OF I UK BRAIN 117 another. The medulla oblongata is directly continuous with the pons, and on either side a large bundle of libers from the dorsal aspec t of the former runs into the cerebellum. These two strands, which are known as the restiform bodies or inferior cerebellar peduncles, constitute the chief avenues of communication between the spinal cord and medulla oblongata on the one band and the cere bellum on the other. The ventral prominence of the- pons is produced in large part by transverse bundles of libers, which when traced lateralward are seen to form a large strand, the brachium pontis or middle cerebellar peduncle, that enters the corresponding cerebellar hemisphere (Figs. 83, 86). The brachium conjunctivum or superior cerebellar peduncle can be traced ro^t rally from the cerebellum to the mesencephalon. The three peduncles are paired structures, symmetrically placed on the two sides of the brain (Figs. 87, 88). The Cerebrum. — The mesencephalon surrounds the cerebral aqueduct and consists of the ventrally placed cerebral peduncles, and a dorsal plate with four rounded elevations, the lamina and corpora quadrigemina (superior and inferior colliculi). The cerebral hemispheres form the most prominent part of the cerebrum and are separated from each other by the longitudinal fissure (Fig. 82), at the bottom of which is a broad commissural band, the corpus callosum, which joins the two hemispheres together (Fig. 85). Under cover of the cere- bral hemispheres and concealed by them, except on the ventral aspect of the brain, is the diencephalon. This includes most of the parts which help to form the walls of the third ventricle. These are from above downward, the c pi thal- amus, including the habenular trigone and pineal body near the roof of the ventricle; the thalamus, which forms most of the lateral wall of the ventricle, and is united with its fellow across the cavity by a short bar of gray substance, the massa intermedia; and the hypothalamus, including the mammillary bodies, infundibulum, and part of the hypophysis (Figs. 84, 85). The Brain Ventricles.— The central canal of the spinal cord is prolonged through the caudal portion of the medulla oblongata and finally opens out into the broad rhomboidal fourth ventricle of the rhombencephalon. At its pointed rostral extremity this ventricle is continuous with the cerebral aqueduct, the elongated slender cavity of the mesencephalon. This, in turn, opens into the third ventricle, which is a narrow vertical cleft between the two laterally sym- metric halves of the diencephalon. It is bridged by the massa intermedia. Near the dorsal part of the rostral border of the ventricle is a small opening in each lateral wall, the interventricular foramen or foramen of Monro. I his leads into the lateral ventricle, the cavity of the cerebral hemisphere. ii8 the nervous system THE ANATDMY OF THE MEDULLA OBLONGATA At its rostral end the spinal cord increases in size and goes over without sharp line of demarcation into the medulla oblongata, or myelencephalon, which, as we learned in Chapter II, is derived from the posterior part of the third brain vesicle. The medulla oblongata may be said to begin just rostral to the high- est rootlet of the first cervical nerve at about the level of the foramen magnum; Marginal fart of sulcus cinguli Sulcus of corpus callosum Splenium of corpus callosum \ Precuneus Sub parietal sulcus . Paricto-occi pita! fissure Lamina quadrigemina Cuneus Superior vermis % Calcarinc fissure''. Occipital pole Lingual gyrus Tra nsverse fissure ' Cerebellar hem Mid nllary subsja of vermis Inferior vermis Calamus scriptorius Central canal Spinal cord ' Tela chorioidea of fourth ventricle Fourth ventricle Medulla oblongata Anterior medullary velum Cerebral aqueduct Pons Posterior perforated substance Oculomotor nerve Central sulcus in paracentral lobule Pineal body Pineal recess Posterior commissure Tela chorioidea of third ventricle Massa intermedia Gyrus cinguli Thalamus Body of corpus callosum ■ Body of fornix Septum pcllucidum Sulcus cinguli Interventric. foramen Column of fornix ■Anterior commis- ;> Superior frontal gyrus 'Frontal pole Genu of corpus callosum ^Rostrum of cor p. callosum \\\\„^ Parol factory area and sulci \ \ \\ ^Subcallosal gyrus K \ \ \ "Hypothalamic sulcus \\ \ "Lamina tcrminalis \ \ \ ' Optic recess Optic nerve Optic chiasma Infundibulum Anterior lobe TT . . , Posterior lobe \ H yP°Phsis Mammillary body Fig. 85. — Medial sagittal section of the human brain. (Sobotta-McMurrich.) and at the opposite extremity it is separated from the pons by a horizontal groove (Figs. 81, 85). Its ventral surface rests upon the basilar portion of the occipital bone; while its dorsal surface is in large part covered by the cerebellum. The shape of the medulla oblongata is roughly that of a truncated cone, the smaller end of which is directed caudally and is continuous with the spinal cord. In man it measures about 3 cm., or a little more than 1 inch, in length (Fig. 86). Like the spinal cord, the medulla oblongata presents a number of more or A\ \Im\1\ 01 MM Ml M II. \ 0BL0NGA1 V less parallel longitudinal grooves. These arc the anterior and posterior median fissures, and a pair each <>i' anterior lateral and posterior lateral sulci I 89). By means of tin- fissures it i> divided symmetricallj into righl and left halves; while these, in turn, an- marked off by the sulci into ventral, lateral, ami dorsal areas, which as seen from the surface appear to be the direct upward con- tinuation of tin- anterior, lateral, and posterior funiculi of the spinal cord. But, as we shall sec in the following chapter, this continuity is not as perfeel a- it appears from the surface; because tin- tracts <>f the cord undergo a rear- rangement as they enter the medulla oblongata. The posterior median fissuM does not extend beyond the middle of the oblongata, at which point its lips separate to form the lateral boundaries of the caudal portion of tin- fourth ven- tricle. The caudal half of the medulla oblongata contains a canal, the dire< t continuation of the central canal of the spinal cord, and is known as the dosed portion of the medulla oblongata (Fig. 85). This canal opens out into the fourth ventricle in the rostral half, which helps to form the ventricular floor, and which is often spoken of as the open part of the medulla oblongata. Fissures and Sulci. — The posterior median fissure represents the continua- tion of the posterior median sulcus of the spinal cord and, as noted above, ends near the middle of the medulla oblongata. The anterior median fissure is con- tinued from the spinal cord to the border of the pons, where it ends abruptly in a pit known as the foramen ccBcum. Near the caudal extremity of the medulla oblongata this fissure is interrupted by interdigitating bundles of fibers which pass obliquely across the median plane. These are the fibers of the lateral corticospinal tract, which undergo a decussation on passing from the medulla oblongata into the spinal cord, known as the decussation of the pyramids. The anterior lateral sulcus also extends throughout the length of the medulla ob- longata and represents the upward continuation of a much more indefinite groove bearing the same name in the spinal cord. From it emerge the root filaments of the hypoglossal nerve. From the posterior lateral sulcus emerge the rootlets of the glossopharyngeal, vagus, and accessory nerves (Figs. 86, 88, 89). The ventral area of the medulla oblongata is included between the anterior median fissure and the anterior lateral sulcus, and has the false appearance of being a direct continuation of the anterior funiculus of the spinal cord. On either side of the anterior median fissure there is an elongated eminence, taper- ing toward the spinal cord, and known as the pyramid (pyramis — Fig. 86). It is formed by the fibers of the corticospinal or pyramidal tract. Just before the fibers of this tract enter the spinal cord they undergo a more or less complete 120 THE NERVOUS SYSTEM decussation, crossing the median plane in large obliquely interdigitating bundles, which nil up and almost obliterate the anterior median fissure in the caudal part of the medulla oblongata. This is known as the decussation of the pyra- mids (decussatio pyramidum). In the sheep these fibers pass into the opposite posterior funiculus of the spinal cord. In man the crossing is incomplete, a Infundibulum Orbital sulci of frontal lobe Orbital gyri of frontal lobe Hypophyi Temporal pole Anterior pcrfor. substance Oculomotor nerve -^ I Uncus -. '£ Mammillary body v f 1 Cerebral peduncle - ; v Pons rrn 1 ngcminal nerve . J / Temporal lobe ^Mgr^ Facial nerve Jb frontal pole olfartory sukus Olfactory bulb , Olfactory tract Optic nerve * >r i Optic chiasma ra^^- Lateral olfactory stria Fm .- Tuber cincrcum j^^^^k. Maxillary nerve Ophthalmic nerve Portio minor of trigem. nerve Mandibular nerve Semilunar ganglion L-- Trochlear nerve Ncrvus intermedins-' Acouslic nerve.' Flocculus of cerebellum ''^SjlR< Cerebellum ^5v Chorioid plexus of ventricle IV \ Glossopharyngeal nerve >' mi Vagus nerve -'^^M Hypoglossal nerve -^^BBHI Accessory nerve ' : Root filaments of cervical nerve I Decussation of pyramids /iter peduncular fossa Abducens nerve Olive Pyramid Medulla oblongata , \ Tonsil of cerebellum \ Occipital pole Spinal cord Vermis of cerebellum Fig. 86. — Ventral view of the human brain. (Sobotta-McMurrich.) majority of the fibers descending into the lateral funiculus of the opposite side, a minority into the anterior funiculus of the same side (Fig. 77). We are al- ready acquainted with these bundles in the spinal cord as the ventral and lateral corticospinal tracts (direct and crossed pyramidal tracts). In addition to the pyramid the ventral area of the medulla also contains a bundle of fibers, the W \1"\1Y 01 mi. Ml .Di l.l.\ OBLONGATA medial longitudinal fasciculus ; which is continuous with the anterior fasciculus proprius of the spinal cord. The lateral area of the medulla oblongata, included between the antero lateral and posterolateral sulci, appears as a dired continuation of the lateral funiculus of the spinal cord; hut, as a matter <>!' hut, many of the fibers of that funiculus find their way into the anterior area (as. lor example, the lateral cor- ticospinal tract) or into the posterior area (dorsal spinocerebellar tract). In the rostral part of the lateral area, between the root filament-, of the gli pharyngeal and vagus nerves, on the one hand, and those of the hypoglossal, on the other, i> an oval eminence, the olive (oliva, olivary body), which is pro- duced l>y a large Irregular mass of gray substance, the inferior olivary nucleus, located just beneath the surface (Figs. 87, 88). By a careful inspection of the surface of the medulla oblongata it is possible to distinguish numerous fine bundles of fibers, which emerge from the anterior median fissure or from the groove between the pyramid and the olive and run dorsally upon the surface of the medulla to enter the restiform bodies. These are the ventral external arcuate fibers and are most conspicuous on the surface of the olive (Fig. 88). In the sheep there are two superficial bands of fibers not seen in the human brain. Placed transversely near the caudal border of the pons is a belt-like elevation, known as the trapezoid body, through which emerge the roots of the abducens and facial nerves (Figs. 83, 87). In man the much larger pons cover.-, this band from view and the sixth and seventh nerves emerge from under the caudal border of the pons. Another bundle, beginning on the ventral sur- face of the trapezoid body near the seventh nerve, describes a graceful curve around the ventral border of the olive and becomes lost in the lateral area of the medulla oblongata. This has been called the fasciculus lateralis minor. The dorsal area of the medulla oblongata is bounded ventrally by the pos- terolateral sulcus and emergent root filaments of the glossopharyngeal, vagus, and accessory nerves. In the closed part of the medulla oblongata it extends to the posterior median fissure, while in the open part its dorsal boundary is formed by the lateral margin of the floor of the fourth ventricle. The caudal portion of this area is, in reality, as it appears, the direct continuation of the posterior funiculus of the spinal cord. On the dorsal aspect of the medulla oblongata the fasciculus cuneatus and fasciculus gracilis of the cord are con- tinued as the funiculus cuneatus and funiculus gracilis, which soon enlarge into elongated eminences, known respectively as the cuneate tubercle and the clava (Figs. 89, 91). These enlargements are produced by gray masses, the nucleus 122 THE NERVOUS SYSTEM gracilis and nucleus cunealus, within which end the fibers of the corresponding fasciculi of the spinal cord. 'J he clava and cuneate tubercle are displaced lat- erally by the caudal angle of the fourth ventricle. Somewhat rostral to the mid- dle of the medulla oblongata they gradually give place to the restiform body. More laterally, between the cuneate funiculus and tubercle on the one hand and the roots of the glossopharyngeal, vagus, and accessory nerves on the other, is a third longitudinal club-shaped elevation called the tuberculum cinereum. It is produced by a tract of descending fibers, derived from the sensory root of the trigeminal nerve, and by an elongated mass of substantia gelatinosa which Corona radiata -j Lentiform nucleus Lateral geniculate body Medial geniculate body •, Optic radiation •, Corona radiata , Pulvinar\ s Inferior quadrigeminal brachium\ Superior colliculus -, Trochlear nerve ». Inferior colliculus- Brachium pontis Brachium conjunctivum'* Restiform body Vestibular n Cochlear n — -^1 Dorsal cochlear nucleus Glossopharyngeal nerve ~" Vagus nerve and restiform body'' Accessory nerve — j__^ Clava'"" _- - Cuneate tubercle'"' Acoustic nerve A atcrior perforated substance /Optic tract 'Optic nerve ■Infundibulum ■ Mummillary body ,- Hypophysis ..^'Oculomotor nerve J - Transverse peduncular tract Cerebral peduncle Pons A bducens nerve f— Trigeminal nerve - Facial nerve v Trapezoid body ^ Olive *• Tractus lateralis minor *~ Hypoglossal nerve Fig. 87. — Lateral view of brain stem of the sheep. forms one of the nuclei of this nerve (Fig. 111). This bundle of fibers and the associated mass of gray matter are known as the spinal tract and nucleus of the spinal tract of the trigeminal nerve. The restiform body (corpus restiforme or inferior cerebellar peduncle) lies between the lateral border of the fourth ventricle and the roots of the vagus and glossopharyngeal nerves in the rostral part of the medulla oblongata (Figs. 87-89). There is no sharp line of demarcation between it and the more cau- dally placed clava and cuneate tubercle. It is produced by a large strand of nerve-fibers, which run along the lateral border of the fourth ventricle and then turn dorsally into the cerebellum. These fibers serve to connect the medulla AN \Im\IY OF III!. PONS oblongata and spinal cord on the one hand with the cerebellum on the other. By a careful inspection of the surface of the medulla it is possible to recognize the source of some of the fibers entering into the composition of the restiform body. The ventral external arcuate fibers can be seen entering it after crossing over the surface of the lateral area; and the dorsal spinocerebellar trad can also be traced into it Erom a position dorsal to the caudal extremity of the olive. At the point where the restiform body begins to turn dorsally toward the cerebellum, it is partly encircled by an elongated transversely placed elevation formed by the ventral and dorsal cochlear nuclei (Figs. 87, 88). This ridge is continuous on the one hand with the cochlear nerve, and on the other with several bundles of fibers which run medialward over the floor of the fourth ventricle .:nd are known as the stria mcdullares acusticcc (Fig. 89). The co< blear nuclei are more prominent in the sheep, while the medullary stria- are best seen in the human brain. Just caudal to this ridge there is sometimes seen another, running more obliquely across the restiform body, which is an outlying portion of the pons and has been described by Essick (1907) under the name corpus pontobulbare. Nerve Roots. — From the surface of the medulla oblongata there emerge in linear order along the posterior lateral sulcus a series of root filaments, which continues the line of the dorsal roots of the spinal nerves. These are the root- lets of the glossopJ/aryiigcal, vagus and accessory nerves. But unlike the dorsal roots, which are made up of afferent fibers, the spinal accessory nerve contains efferent fibers, while the vagus and glossopharyngeal are mixed nerves. The line of the ventral or motor roots of the spinal nerves is continued in the medulla oblongata by the root filaments of the hypoglossal nerve, which is also composed of motor fibers. The abducens. facial, and acoustic nerves make their exit along the caudal border of the pons in the order named from within outward. The abducens emerges between the pons and the pyramid, the acoustic far lateral- ward in line with the restiform body, and the facial with its sensory root, the ncrvus intermedins, near the acoustic nerve (Figs. 86-88). THE ANATOMY OF THE PONS The pons, which is differentiated from the ventral part of the metencephalon, is interposed between the medulla oblongata and the cerebral peduncles and lies ventral to the cerebellum. As seen from the ventral surface, it is formed by a broad transverse band of nerve-fibers, which on either side become aggre- gated into a large rounded strand, the brae hi urn ponds or middle cerebellar 124 THE NERVOUS SYSTEM peduncle, and finally enter the corresponding hemisphere of the cerebellum (Figs. 83, 86). This transverse band of fibers, which gives the bridge-like form from which this part derives its name, belongs to the basilar portion of the pons and is superimposed upon a deeper dorsal portion that may be regarded as a direct upward continuation of the medulla oblongata. The transverse fibers form a part of the pathway connecting the cerebral hemispheres with the opposite cerebellar hemispheres; and the size of the pons, therefore, varies with Anterior limb of ^ / internal capsule I Head of the can- S date nucleus Corona rod iota --' Tail of the caudate nucleus Lenticulotka- Itimic part .. Rttrolcnlicular '' part - Sublenticular part - Thalamus J ' Posterior limb of internal capsule A nlerior commissure'' Anterior perforated,- substance y' Optic nerve ,--' Basis pedunculi'' Pons'"' ,_ Ncrvus j portio minor'" _,,■ trigeminus \portio major''" Acoustic nerve Facial nerve Glossopharyngeal and vagus nerves •*--- ™ - ~ ' Olive Z2IJ Hypoglossal nerve \ \ Ventral external arcuate fibers 5 Pyramid Ventral root N. cerv. I -=-'-- Anterior lateral sulcus ''"_,— Ventral root N. cerv. 11"''-' Medial geniculate body '--Superior colliculus ""Inferior quadrigeminal brachium "^•Inferior colliculus ~~ -Trochlear nerve "-- Lateral lemniscus -- Brachium conjunctivum • Fila lateralia ponds - Dentate nucleus - Restiform body • Brachium pontis "' Dorsal cochlear nuc. ^ " Corpus pontobulbar "'Restiform body Tuber culum cinereum ~~' 'Accessory nerve ---Dorsal root N. cerv. II Fig. 88. — Lateral view of human brain stem. the size of these other structures. It is instructive to compare the brains of the shark, sheep, and man with this point in mind (Figs. 11, 84, 85). The ventral surface of the pons is convex from above downward and from side to side and rests upon the basilar portion of the occipital bone and upon the dorsum sellae (Fig. 81). A groove along the median line, the basilar sulcus, lodges the basilar artery (Fig. 86). The trigeminal nerve emerges from the ventral surface of the pons far lateral- ward at the point where its constituent transverse fibers are converging to form I III I PI A R I II \ IN I Kit II, the brachium pontis. In fact, it is customary to take the exit of this nerve as marking the point of junction of the pons with its brachium. The nen <• has two roots which lie close together: the larger is the sensory root, or portio major; the smaller is the motor root, or portio minor (Fig. 88). The posterior surface of the pons forms the rostral part of the floor of the fourth ventricle, along the lateral borders of which there are two prominent and rather large strands of nerve fibers, the brachia conjunctiva (Figs. 88, vn The brachia conjunctiva or superior cerebellar peduncles lie under cover of the cerebellum. As they emerge from the white center^ of the cerebellar hemi- spheres they curve rostrally and take up a position along the lateral border of the fourth ventricle. They converge as they ascend and disappear from view by sinking into the substance of the mesencephalon under cover of the inferior quadrigeminal bodies. Each consists of fibers which connect the cerebellum with the red nucleus, a large gray mass situated within the midbrain ventral to the superior colliculus of the corpora quadrigemina. The interval between the two brachia conjunctiva, where these form the lateral boundaries of the fourth ventricle, is occupied by a thin lamina of white matter, the anterior medullary velum (Fig. 85). This is stretched between the free dorsomedial borders of the two brachia and forms the roof of the rostral portion of the ventricle. Cauda 11 v it is continuous with the white center of the cerebellum. The fibers of the trochlear nerves decussate in the anterior medullary velum and emerge from its dorsal surface (Fig. 89). As they run through the velum the}' produce a raised white line which extends transversely from one brachium to the other. THE FOURTH VENTRICLE The lozenge-shaped cavity of the rhombencephalon is known as the fourth ventricle. It lies between the pons and medulla oblongata, ventrally, and the cerebellum dorsally, and is continuous with the central canal of the closed por- tion of the medulla, on the one hand, and with the cerebral aqueduct on the other (Fig. 84). On each side a narrow curved prolongation of the cavity ex- tends laterally on the dorsal surface of the restiform body. This is known as the lateral recess (Figs. 89. 90). It opens into the subarachnoid space near the flocculus of the cerebellum; and through this lateral aperture of the fourth ven- tricle (foramen of Luschka) protrudes a small portion of the chorioid plexus (Fig. 90). There is also a median aperture (foramen of Magendie) through the roof of the ventricle near the caudal extremity. By means of these three open- ings, one medial and two lateral, the cavity of the ventricle is in communica- 126 THE NERVOUS SYSTEM tion with the subarachnoid space, and cerebrospinal fluid may escape from the former into the latter. The floor of the fourth ventricle is known as the rhomboid fossa and is formed by the dorsal surfaces of the pons and open part of the medulla oblongata, which are continuous with each other without any line of demarcation and are irreg- ularly concave from side to side (Figs. 89, 91). The fossa is widest opposite the points where the restiform bodies turn dorsally into the cerebellum; and it gradually narrows toward its rostral and caudal angles. The lateral boundaries Thalamus Medial geniculate bod\ Inferior quadrigeminal __--- ~"\ brachium Frenulum veli Anterior medullary velum Brae liium eonjunetivum — Braeliiu m pontis~-~~- - Restiform body~-._ Superior fovea \T' Area acustica-=~'^. Inferior fovea Restiform body — - Ala cinerea--- Funieulus scparans—'' Area postrema-'' Obex-''' Funiculus gracilis-'' Funiculus cuneatus--' Pineal body - Superior colliculus Inferior colliculus — Cerebral peduncle Trochlear nerve Median sulcus Locus carulcus - - Facial colliculus i Medial eminence Sulcus limilans Lateral recess Stricr mcdullares Teen i 'a Trigonum hypoglossi ~"Cuneatc tubercle ~"~~Tubcrculum cinereum ""-Clava '--Posterior median fissure ~ ^Posterior intermediate sulcus ^Posterior lateral sulcus Fig. 89. — Dorsal view of human brain stem. of the fossa, which are raised some distance above the level of the floor, are formed by the following structures: the brachia conjunctiva, restiform bodies, cuncatc tubercles, and clavce. Of the four angles to the rhomboid fossa, two are laterally placed and correspond to the lateral recesses. At its caudal angle the ventricle is continuous with the central canal of the closed part of the me- dulla oblongata, and at its rostral angle with the cerebral aqueduct. Joining the two last named angles there is a median sulcus which divides the fossa into two symmetric lateral halves. The rhomboid fossa is arbitrarily divided into three parts. The superior I ill FOURTH \ l \ l RIl I i; part is triangular, with its apex directed rostrally and its base along an im an- line through the superior foveae. The inferior part is also triangular, bul with its apex directed caudally and its base at the level of the horizontal por- tions of the taeniae of the ventricle. Between these two triangular portions is the intermediate part of the fossa, which is prolonged outward into the lateral recesses. The Boor is covered with a thin lamina of gray matter continuous with that which lines the central canal and cerebral aqueduct. Crossing the fossa transversely in its intermediate portion are several strands of fibers known as the stria meduUares acustica. These arc subject to considerable variation in different specimens. Springing from the dorsal cochlear nuclei they wind around the restiform body in the lateral recess and run transversely across the fossa to disappear in the median sulcus. The inferior portion of the fossa bears some resemblance to the point of a pen and has been called the calamus scriptorius. It belongs to the medulla oblongata. In this part of the fossa there is on either side a small depression, the inferior fovea, shaped like an arrow-head, the point of which is directed toward the stria' medullares. From the basal angles of this triangle run diverging sulci: a medial groove toward the opening of the central canal and a lateral groove more nearly parallel to the median sulcus. By these sulci the inferior portion of the fossa is divided into three triangular areas. Of these the most medial is called the trigone of the hypoglossal nerve or trigonum nervi liypoglossi. Be- neath the medial part of this slightly elevated area is located the nucleus of the hypoglossal nerve. The area betw-een the two sulci, which diverge from the fovea inferior, is the ala cinerea or triangle of the vagus nerve. Both names are appropriate, the one, because of its gray color, and the other, because a nucleus of the vagus nerve lies subjacent to it. The third triangular field, placed more laterally, forms a part of the area acustica. The area acustica is, however, not restricted to the inferior portion of the fossa, but extends into the intermediate part as well. Here it form- a prominent elevation over which the stria? medullares run. Subjacent to this area lie the nuclei of the vestibular nerve. A part of the acoustic area and all of the ven- tricular floor rostral to it belong to the pons. Rostral to the stria? medullares there may be seen a shallow depression, the fovea superior, medial to which there is a rounded elevation, the facial colliciilus. Under cover of this eminence the fibers of the facial nerve bend around the abducens nucleus. Extending from the fovea superior to the cerebral aqueduct is a shallow groove, usually faint blue in color, the locus 121 THE NERVOUS SYSTEM car ulcus, beneath which lies the substantia ferruginea, composed of pigmented nerve-cells. Beginning at the cerebral aqueduct and extending through both the superior and inferior fovea? is a very important groove, the sulcus limitans, which repre- sents the line of separation between the parts derived from the alar plate and those which originate from the basal plate of the embryonic rhombencephalon. Lateral to this sulcus lie the sensory areas of the ventricular floor, including the area acustica, all of which are derived from the alar plate. Medial to this sulcus there is a prominent longitudinal elevation, known as the medial eminence, which includes two structures already described, namely, the facial colliculus and the trigone of the hypoglossal nerve. Beneath the medial part of this Tela chorioidea Chorioid plexus Median aperture of fourth ventricle Fig. 90. — Dorsal view of human rhombencephalon showing tela chorioidea and chorioid plexus of the fourth ventricle. trigone lies the nucleus of the hypoglossal nerve and beneath the lateral part is a group of cells designated as the nucleus inter calatus. One or two features remain to be mentioned. 'At the caudal end of the ala cinerea is a narrow translucent obliquely placed ridge of thickened ependyma, known as the funiculus scparans. Between this ridge and the clava is a small strip of the ventricular floor, called the area post rem a. which on microscopic examination is found to be rich in blood-vessels and neurogliar tissue. The roof of the fourth ventricle is formed by the anterior medullary velum, a small part of the white substance of the cerebellum, and by the tela chorioidea lined internally by cpendymal epithelium (Fig. 85). Caudal to the cerebellum the true roof of the cavity is very thin and consists only of a layer of ependymal epithelium, which is continuous with that lining the other walls of the ventricle. w \i"M\ OP mi. mi -i \< i I'll \i This is supported on its outer surface by a layer of pia mater, the tela chorio rich in blood vessels. From this layer vascular tufts, covered by epithelium, are invaginated Into the cavity and form the chorioid plexus of the fourth veil trick- (Fig. 90). The plexus is invaginated along two vertical lines close to the median plane and along two horizontal lines, which diverge at right angles from the vertical ones and run toward the lateral recesses. These righl and left halves are joined together at the angles so that the entire plexus has the shape of the letter T. the vertical limb of which . however, is double. After the tela chorioidea with its epithelial lining has been torn away to expose the lloor of the ventricle, there remains attached to the lateral bound- aries of the caudal part of the cavity the torn edges of this portion of the roof. These appear a- line-, the teenies of the fourth ventricle, which meet over the caudal angle of the cavity in a thin triangular lamina, the obex (Fig. 89). Ros- t rally each taenia turns lateralward over the restiform body and forms the caudal boundary of the corresponding lateral recess. THE MESENCEPHALON The midbrain or mesencephalon occupies the notch in the tentorium and connects the rhombencephalon, on the one side of that shelf-like process of dura, with the prosencephalon on the other (Fig. 81). It consists of a dorsal part, the corpora quadrigemina. and a larger ventral portion, the cerebral pe- duncles. It is tunneled by a canal of relatively small caliber, called the cerebral aqueduct, which connects the third and fourth ventricles and is placed nearer the dorsal than the ventral aspect of the midbrain (Fig. 84). The cerebral peduncles (pedunculi cerebri, crura cerebri), as seen on the ventral aspect of the brain, diverge like a pair of legs from the rostral border of the pons (Fig. 83). Just before they disappear from view by entering the ven- tral surface of the prosencephalon they enclose between them parts of the hypo- thalamus, and are encircled by the optic tracts. On section, each peduncle i- seen to be composed of a dorsal part, the tegmentum, and a ventral part, the basis pedunculi. Between the basis pedunculi and the tegmentum there inter- venes a strip of darker color, the substantia nigra (Fig. 113). By dissection it i> easy to show that the basis pedunculi is composed of longitudinally coursing fibers which can be traced rostrally to the internal capsule (Fig. 88). In the other direction some of these fibers can be followed into the corresponding pyra- mid of the medulla oblongata. On the surface two longitudinal sulci mark the plane of separation between the tegmentum and the basis pedunculi. The 13° THE NERVOUS SYSTEM groove on the medial aspect of the peduncle, through which emerge the fibers of the third nerve, is known as the sulcus of the oculomotor nerve, while that on the lateral aspect is called the lateral sulcus of the mesencephalon. Dorsal to this latter groove the tegmentum comes to the surface and is faintly marked by line bundles of fibers which curve dorsally toward the inferior colliculus of the corpora quadrigemina (Fig. 88). These fibers belong to the lateral lemniscus, the central tract associated with the cochlear nerve. The corpora quadrigemina form the dorsal portion of the mesencephalon, and consist of four rounded eminences, the quadrigeminal bodies or colliculi, Anterior limb of internal capsule.^ Stria terminalis Ilalu jiular commissurt y Habenular trigone, } Pineal body Posterior limb of internal capsule. Superior colliculus ^S? Optic radiation Attachment anterior ' medullary velum Inferior colliculus Superior fovea v Brachium conjunetivum ^ s . Brachium pontis Restiform body Dorsal cochlear nucleus Acoustic area Inferior fovea and restiform body -■'- Tania of fourth ventricle- Clava Cunealc tubercle Posterior lateral sulcus — Corona radiata -Head of caudate nucleus Stria medullaris of thalamus ,, Third ventricle , Thalamus ,, Tail of caudate nucleus . ■■ -Pulvinar \, Median sulcus y)S K Trochlear nerve '' /Facial colliculus ■'„'' Trigeminal nerve Sulcus limilaus ^-Medial eminence , A/a cinerea - Lateral recess of fourth ventricle Trigone of hypoglossal nerve Obex /-■Posterior median fissure Posterior intermediate sulcus -Funiculus gracilis - Funiculus cuneatus Fig. 91. — Dorsal view of brain stem of sheep. which arise from the dorsal aspect of a plate of mingled gray and white matter known as the quadrigeminal lamina (Figs. 89, 91). The superior colliculi are larger than the inferior, the disproportion being greater in the sheep than in man. A median longitudinal groove separates the colliculi on either side. In the rostral end of this groove rests the pineal body, while attached to its caudal end is a band which runs to the anterior medullary velum, and is known as the frenulum veli. A transverse groove runs between the superior and inferior collic- uli and extends on to the lateral aspect of the mesencephalon, where it inter- venes between the superior colliculus and the inferior quadrigeminal brachium (Figs. 87, 89). A\ \ i< i\l\ OP I Ml Mi SI v I I'll \l.<>\ The Brachia of the Corpora Quadrigemina. From each coUiculus there runs ventrally and rostrally on the lateral aspeel of the mesencephalon an arm or brachium ili.u r -. 87, 88). The interior quadrigeminal brachium is the more con SpiCUOUS and is the only one that can be readily identified in the sheep. It runs from the interior colliculus to the medial geniculate body. This ban oval eminence, belonging to the diencephalon, which has been displaced caudally so as to lie on the lateral aspeel of the mesencephalon. The superior quadrigeminal brachium runs from the superior colliculus toward the lateral geniculate body, passing between the pulvinar of the thalamus and the medial geniculate body. Some of the fibers can be traced beyond the lateral geniculate body into the Optic tract. CHAPTER IX THE STRUCTURE OF THE MEDULLA OBLONGATA The medulla oblongata contains the nerve-cells and fiber tracts associated with certain of the cranial nerves. These include the central mechanisms which control the reflex activities of the tongue, pharynx, and larynx, and in part those of the thoracic and abdominal viscera also. At the same time the ascending and descending fiber tracts, which unite the spinal cord with higher nerve centers, pass through the medulla oblongata. The central connections of the cranial nerves, except those of the first two pairs, are located in the medulla oblongata and in the tegmental portions of the pons and mesencephalon. In many respects they resemble the connections of the spinal nerves within the spinal cord. The following general statements on this topic, most of which are illustrated in Fig. 92, will help to elucidate the structure of the brain stem. 1. The cells of origin of the sensory fibers of the cranial nerves (Fig. 92, 1) are found in ganglia which lie outside the cerebrospinal axis and are homologous with the spinal ganglia. These are the semilunar ganglion of the trigeminal, the geniculate ganglion of the facial, the superior and petrous ganglia of the glossopharyngeal, the jugular and nodose ganglia of the vagus, the spiral gang- lion of the cochlear, and the vestibular ganglion of the vestibular nerve. 2. All of these sensory ganglia except the last two, the cells of which are bipolar, are formed by unipolar cells, the axons of which divide dichotomously into peripheral and central branches. The latter (or in the case of the acoustic nerve the central processes of the bipolar cells) form the sensory nerve roots, enter the brain stem and divide, each into a short ascending and a long descending branch. These branches give off numerous collaterals, which with the terminal branches end in gray masses known as sensory nuclei or nuclei of termination. It is the descending branches of the sensory fibers of the trigeminal nerve which form the spinal tract of that nerve illustrated in Figs. 92, 98, 99, 101. 3. The ascending branch may be entirely wanting, as in the case of the sen- sory fibers of the seventh, ninth, and tenth nerves, all of which bend caudally and form a descending tract in the medulla oblongata, known as the tractus soli- tarius (Figs. 92, 101, 103). 132 [HE STRUCTURE 01 nil Ml DULLA 0BL0NGA1 \ 4. The sensory nuclei (Fig. 92, 4), within which the afferenl fibers terminate, contain the cells of origin of the sensory fibers oj the second order I Some of these are short; others are long, and these may be cither direci 01 crossed. Many of them divide Into ascending and descending branches. They run in the reticular formation and some <)!' the ascending fibers reach the thal- amus. 5. These sensory fibers of the second order give off collaterals to the motor nuclei. Direct collaterals from the sensory fibers of the cranial nerves to the motor nuclei are few in number or entirely wanting. 6. The motor nuclei (Fig. 92, 5) are aggregations of multipolar cells which give origin to the motor fibers of the cranial nerves (Fig. 92, 3). - Main sensory nucleus of trigeminal nerve >ii fiber of ond order Tractus solitarius Nucleus of hypoglossal nerve Afferent fiber of second order Spinal Irarl of trigeminal nenc and ils nucleus Fig. 92. — Diagram of the tongue and rhombencephalon to illustrate the central connections and functional relationships of certain of the cranial nerves: 1, Sensory neurons of the first order of the trigeminal and glossopharyngeal nerves; 2, sensory neurons of the second order; 3, motor fibers of the hypoglossal nerve; 4, sensory nuclei; 5, motor nucleus of hypoglossal nerve. (Cajal.) The Rearrangement Within the Medulla Oblongata of the Structures Con- tinued Upward from the Spinal Cord.— At the level of the rostral border of the first cervical nerve the spinal cord goes over without a sharp line of demarcation into the medulla oblongata. The transition is gradual both as to external form and internal structure; but in the caudal part of the medulla there occurs a gradual rearrangement of the fiber tracts and alterations in the shape of the gray matter, until at the level of the olive, a section of the medulla bears no resemblance to one through the spinal cord. The realignment of the corticospinal tracts and the termination of the fibers of the posterior funiculi of the spinal cord are two of the most important factors 134 THE NERVOUS SYSTEM responsible for this gradual transformation. Traced rostrally from the spinal cord, the ventral corticospinal tracts are seen to enter the pyramids within the ventral area of the medulla oblongata, that is to say, they enter the medulla without realignment. But the fibers of the lateral corticospinal tracts on enter- ing the medulla swing ventromedially in coarse bundles, which run through the anterior gray columns and cut them off from the gray matter surrounding the central canal (Figs. 93, 95). After crossing the median plane in the decussa- tion of the pyramids these fibers join those of the opposite ventral corticospinal tracts and form the pyramids (Fig. 96). Thus fibers from the lateral funiculus come to lie ventral to the central canal and displace this dorsally; and at the same time a start is made toward breaking up the H -shaped gray figure characteristic of the spinal cord. Cerebral hemisphere ylkvP'M Spinal V Ss ^yjy cord Fig. 93. — Diagram of the corticospinal tracts. Shortly after entering the medulla oblongata the fibers of the posterior funiculi end in nuclear masses which invade the funiculus gracilis and funiculus cuneatus as expansions from the posterior gray columns and central mass of gray sub- stance (Figs. 95, 96). These are known as the nucleus gracilis and nucleus cu- neatus. They cause a considerable increase in the size of the posterior funiculi and a corresponding ventrolateral displacement of the posterior columns of gray matter. The fibers of the posterior funiculi end in these nuclei about cells, the axons of which run ventromedially as the axis-cylinders of internal arcuate fibers. These sweep in broad curves through the gray substance, and decus- sate ventral to the central canal in what is known as the decussation of the medial lemniscus. After crossing the median plane they turn rostrally between the TI1K STRUCT! RE OF llli: Ml in i.i.\ OBLONGA1 \ pyramids and the central gray matter to form on either side of the median plain- a broad hand of fibers known as the medial leninism I ; 96 ''7 Fascu ii/hs gracilis - I ast iculus cuneatus Dorsolateral fascA Lissauer) ^ Substantia gelatinosa Dorsal column " Lateral corticospinal tract " ( '< ntral canal Ventral column Ventral corticospinal trad ■ Funu iiln . grot ilis . A •.■ilis — 1- uni< iiln-. < uneatus Spinal trad of tl 1 spinal Ira I ' al column \ V Lateral < orlico pinal trait - ( '< ntral canal ~/~" Dei USSation of the pyramids Vi ntral i olunin Fie, 94. Fig. 95. Funiculus gracilis Nucleus gracilis Funiculus < uneatus Nucleus cuneatus Spinal tract of trigeminal n Nucleus of spinal tract of N.V Central way matter Internal arcuate fibers Central canal R< tit ular substam e Medial lemniscus Decussation of medial lemniscus Decussation of the pyramids Pyramid, corticospinal tract Fie. 96. -■ Fourth ventricle ..-Dorsal motor nucleus of vagus -■ Nucleus of hypoglossal nerve Tractus solitarius Nucleus of spinal tract of N. V Spinal tract of trigeminal nerve Fibers of hypoglossal >:■ Reticular substance Dorsal accessory olivary nucleus Media! lemniscus Inferior olivary nucleus Medial accessory olivary nucleus — Pyramid, corticospinal trad Fig. 97. Figs. 94-97. — Diagrammatic cross-sections to show the relation of the structures in the medulla oblongata to those in the spinal cord: Fig. 94, First cervical segment of spinal cord; Fig. 95, medulla oblongata, level of decussation of pyramids; Fig. 96, medulla oblongata, level of decussation of medial lemniscus; Fig. 97, medulla oblongata, level of olive. At the level of the middle of the olive most of the fibers of the funiculus cune- atus and funiculus gracilis have terminated in their respective nuclei; and the nuclei also disappear a short distance farther rostrally (Fig. 97). With the 136 THE NERVOUS SYSTEM disappearance of these fibers and nuclei there ceases to be any nervous sub- stance dorsal to the central canal, and this, which has been displaced dorsally by the accumulation of the corticospinal fibers and those of the lemniscus ven- tral to it, opens out as the floor of the fourth ventricle (Fig. 97). The outline of the gray matter in the most caudal portions of the medulla oblongata closely resembles that of the spinal cord. The anterior columns are first cut off by the decussation of the pyramids (Fig. 95). Then the posterior columns are displaced ventrolaterally due to the increased size of the posterior funiculi and the disappearance of the lateral corticospinal tracts from their ventral aspects. This rotation of the posterior column causes the apex of that column with its spinal tract and nucleus of the trigeminal nerve, which are continuous with the fasciculus dorsolateral and substantia gelatinosa of the spinal cord (Fig. 94), to lie almost directly lateralward from the central canal (Fig. 96). The shape of the gray figure is still further altered by the develop- ment of special nuclear masses, many of which are very conspicuous. These include the nucleus gracilis, nucleus cuneatus, inferior olivary nucleus, and the nuclei of the cranial nerves. The greater part of the gray substance now becomes broken up by nerve-fibers crossing in every direction, but especially by the internal arcuate fibers. This mixture of gray and white matter is known as the reticular substance. The central gray matter is pushed dorsad first by the pyra- mids and later by the medial lemniscus until it finally spreads out to form a thin gray covering for the floor of the fourth ventricle. The Pyramids and Their Decussation. — We have had occasion repeatedly to refer to the crossing of the lateral corticospinal tracts in this and preceding chapters, but there remain some details to be presented. The pyramids are large, somewhat rounded fascicles of longitudinal fibers, which lie on either side of the anterior median fissure 01 the medulla oblongata (Fig. 86). The constit- uent fibers take origin from the large pyramidal cells of the anterior central gyrus or motor cerebral cortex. The decussation of the pyramids or motor decussation occurs near the caudal extremity of the medulla oblongata (Fig. 93). Approximately the medial three-fourths of the corticospinal tract passes through the decussation into the lateral funiculus of the opposite side of the spinal cord, as the lateral corticospinal tract (fasciculus cerebrospinalis lateralis or lateral pyramidal tract); while the lateral one-fourth is continued without crossing into the ventral funiculus of the same side as the ventral corticospinal tract (fasciculus cerebrospinalis anterior or anterior pyramidal tract — Figs. 94, 95, 96, 98). The decussating fibers are grouped into relatively large bundles THE STRU< ll KK < >F Till MIDI l.l.\ OBLONGATA as tin s the median plane, the bundles from one side alternating with similar bundles from the other, and largely obliterating the anterior median fis- sure at this level. There is great individual variation as to the relative size of the ventral and lateral corticospinal tracts; and there may even be marked asymmetry due to a difference in the proportion of the decussating fibers on the two sides. The nucleus gracilis and nucleus cuneatus (nucleus funiculi ,L r ra< ili- and nucleus funiculi cuneati) are large masses of gray matter located in the pos- terior funiculi of the caudal portion of the medulla oblongata. They arc sur rounded by the libers of these funiculi except on their ventral aspects, where they are continuous with the remainder of the gray substance (Fig. 99). The libers Funiculus gracilis- Nucleus gracilis' of trigeminal j^'^V ^HSift^' '-''■' i "?•'"' 'V'.' Spinal tract nerve Nucleus of spinal tract of — l3v?\-~V- v '£>* ' Central canal Decussation of the pyramids Anterior column 5&2 Posterior median fissure Funiculus cuneatus Nucleus cuneatus Dorsal spinocerebellar tract Ventral spinocerebellar tract Ventral fasciculus proprius Bulbospinal tract Anterior median fissure Fig. 98. — Section through the medulla oblongata of a child at the level of the decussation of the pyramids. Pal-Weigert method. (XO.) of the gracile and cuneate fasciculi terminate in the corresponding nuclei; and their terminal arborizations are synaptically related to the neurons, whose cell bodies and dendrites are located there (Fig. 100). Accordingly, in sections through successive levels we see the fibers decreasing in number as the nuclei grow larger (Figs. 98. 99, 101). It is due to the presence of these nuclei that the funiculi become swollen to form the club-shaped prominences with which we are already familiar under the names clava and cuneate tubercle. At the level of the pyramidal decussation the gracile nucleus has the form of a rather thin and ill-defined plate, while the cuneate nucleus is represented by a slight projection from the dorsal surface of the posterior gray column (Fig. 98). At the level of the decussation of the lemniscus both have enlarged and the gracile nucleus has become sharply outlined (Fig. 99). As the central canal opens out into the 138 THE NERVOUS SYSTEM fourth ventricle the nuclei are displaced laterally and gradually come to an end as the restiform body becomes clearly defined (Fig. 101). As one would expect from the fact that there is no sharp line of separation between the spinal cord and medulla oblongata, some of the fibers of the cuneate fasciculus end in the substantia gelatinosa (here known as the nucleus of the spinal tract of the trigeminal nerve) and in the remnant of the head of the posterior gray column (Fig. 100). There are three smaller gray masses within the funiculus cuneatus: (1) the external round nucleus, an iso- lated portion of the substantia gelatinosa, near which it is situated; (2) the internal round nucleus, more variable in position; and (3) the accessory or lateral cuneate nucleus superficial to the main nuclear mass. Funiculus gracilis Nucleus gracilis Spinal tract of trigeminal nerve Nucleus of spina! tract ofN. V Dorsal motor nucleus of Jjy vagus Nucleus of hypoglossal nerve Decussation of medial lemniscus Lateral reticular nucleus Medial accessory olivary nucleus Ventral external arcuate fibers Funiculus cuneatus X ucl ens cuneatus Central canal Internal arcuate fibers Reticular substance Dorsal spinocerebellar tract '^iL___}'entra! spinocerebellar trad Ventral fasciculus proprius Hypoglossal nerve Pyramid, corticospinal 'trad Fig. 99. — Section through the medulla oblongata of a child at the level of the decussation of the medial lemniscus. (Pal-Weigert method.) (X 6.) The Medial Lemniscus and its Decussation. — The great majority of fibers which arise from the cells in the nucleus gracilis and nucleus cuneatus sweep ventromedially in broad concentric curves around the central gray substance toward the median raphe (Fig. 99). As has been stated on a preceding page, these are known as internal arcuate filers, and as they cross those from the opposite side in the raphe they form the decussation of the lemniscus (decussatio lemniscorum, sensory decussation). After crossing the median plane they turn rostrally in the medial lemniscus (fillet), and end in the thalamus (Fig. 235). These longitudinal fibers constitute a broad band which lies close to the median raphe, medial to the inferior olivary nucleus, and dorsal to the pyramids (Figs. 96, 97). By the accession of additional internal arcuate fibers this band in- creases in size and spreads out dorsally until at the level of the middle of the olive it is separated from the gray matter of the ventricular floor only by the nil STRU( 11 RE OP Mil Ml in n \ OBLONGA I \ fibers of the fasciculus longitudinalis medialis and the tectospinal trad 101). The decussation of the lemniscus begins at the upper border of the decussation of the pyramids, where the sensory fibers are grouped into coarse bundles arching around the central gray matter (Fig. 99), and extends as far as do the gracile and cuneate nuclei, that is, to about the middle of the olive. In sections through the lower half of the olive the internal arcuate fibers describe broad curves through the reticular formation and their decussation occupi considerable ventrodorsal extent of the raphe I Fig. 101). Nerve cell in the nucleus cuneatus Ramification of fibers front the fasciculus cuneatus Nucleus cum Substantia linosa Fig. 100. — From a transverse section through the medulla oblongata of a kitten, to illustrate the termination of the fibers of the fasciculus cuneatus, and at a the beginning of the internal arcuate fibers. (Combined from drawings by Cajal.) The arcuate fibers of the medulla oblongata may be separated into two groups: those which run through the reticular formation constitute the inter- nal arcuate fibers; and those which run over the surface of the medulla, the external arcuate libers. The internal arcuate fibers are of at least three kinds: (1) those described in the preceding paragraph, which arise in the gracile and cuneate nuclei and form the medial lemniscus; (2) sensory libers of the second order, arising in the sensory nuclei of the cranial nerves; and (3) olivocerebellar fibers, which will be considered in another paragraph. Our knowledge of the external arcuate fibers is less satisfactory. From the nuclei of the posterior funic- 140 THE NERVOUS SYSTEM uli and perhaps also from these funiculi themselves a group of dorsal external arcuate fibers make their way to the restiform body along the dorsal surface of the medulla (Fig. 101). According to Cajal these fibers are well developed in man, but absent in the cat and rabbit. The ventral external arcuate fibers are said to include a certain number which arise in the lateral reticular and arcuate nuclei and run dorsolaterally over the surface of the medulla to reach the cerebellum by way of the restifrom body (Fig. 104). The arcuate nuclei are small irregular patches of gray matter situated on the ventromedial aspect of the pyramid and continuous rostrally with the nuclei pontis, with which they Spinal vestibular nucleus Dorsal external arcuate fibers Tractus solitaries and nucleus Nucleus of hypoglossal nerve Internal arcuate fibers Dorsal spinocere- bellar tract. Medial longitudinal fasciculus Ventral spinocere- bellar tract Tectospinal trad Medial lemniscus Inferior olivary nucleus Hilus of olivary nucleus Pyramid, cortico- spinal tract Arcuate nucleus Dorsal motor nucleus of vagus Nucleus cuncatus Restiform body Central canal Spinal trad and nucleus N. V Nucleus ambiguus Reticular substance Lateral reticular nucleus Medial accessory olivary nucleus Inferior olivary nucleus Hypoglossal nerve Ventral external arcuate fibers Fig. 101.— Section through the medulla oblongata of a child at the level of the olive. Pal-Weigert method. (X 6.) seem to be homologous (Figs. 101, 103). They probably receive fibers from the cerebral cortex by way of the pyramidal tracts; and, if so, the external arcuate fibers which arise from them are homologous with the transverse fibers of the pons. Although the facts stated above are pretty well established, only a small part of the ventral external arcuate fibers are thus accounted for. The origin and course of the majority of these fibers is still obscure. According to Cajal (1909) they arise from the nuclei of the posterior funiculus, curve ventrally and medially over the surface of the medulla oblongata, penetrate the pyramids or the anterior median fissure, cross in the median raphe, and join the medial lemniscus of the opposite side. On the other hand, Edinger (1911) gives to THE STRUC'IM'I "i mi. mi. mi. I. \ OBLONGATA 141 them the name "tractus cerebello-tegmentalis Imllii," and believes that thej from the cerebellum by way of the restiform body, then arch vent rally over the surface of the medulla, penetrate tin- pyramid or the anterior median fissure, and end in the reticular formal ion of the opposite side I Fig. 153). A< cording to Van Gehui hten (1904 of the ventral external arcuate fibers arise from cells in the reti< ular formation of the same and the opposite side, and run through the restiform body to the < erebellum. Olivary Nuclei. — The oval prominence in the literal area of the medulla, known as the olive, is produced by the presence just beneath the surface of a large gray mass, the inferior olivary nucleus, with which there are associated > - :- . Fig. 102. — Diagram to illustrate the structure of the inferior olivary nucleus. (Cajal, Edinger.) two accessory olivary nuclei. The inferior olivary nucleus is very conspicuous in the sections of this part of the medulla (Fig. 101). It appears as a broad, irregularly folded band of gray matter, curved in such a way as to enclose a white core, which extends into the nucleus from the medial side through an opening, known as the hilus. Considered as a whole this nucleus resembles a crumpled leather purse, with an opening, the hilus, directed medially. Sec- tions at either end of the nucleus do not include this Opening, and at these points the central core of white matter is completely surrounded by the gray lamina. The fibers which stream in and out of the hilus constitute the olivary 142 THE NERVOUS SYSTEM peduncle. The two accessory olives are plates of gray substance, which in trans- verse section appear as rods. The medial necessary olivary nucleus is placed be- tween the hilus of the inferior olive and the medial lemniscus, while the dorsal accessory olivary nucleus is located close to the dorsal aspect of the chief nuclear mass. Structure and Connections.— The gray lamina of the inferior olivary nucleus consists of neuroglia and many rounded nerve-cells beset with numerous short, frequently branching dendrites, the axons of which run through the white core of the nucleus and out at the hilus as olivocerebellar fibers (Fig. 102). About these cells there ramify the end branches of several varieties of afferent fibers, the origin of which is not well understood. Some come from a tract, designated Fourth ventricle Principal vestibular nucleus Spinal vestibular nucleus Nucleus intcrcalatus—^g; Rest if or in body-h Spina! tract and /fellfe'/ nucleus N. V [IIP Pontobulbar body -\^m Glossopharyngeal nerve *¥ Nucleus ambiguus Ventral spinocerebellar tract Dorsal accessory olivary nut Hilus of olivary nucleus Inferior olivary nucleus Tania of fourth ventricle Nucleus of liypoglossal nerve Dorsal motor nucleus of vagus Tractus solitarius and nucleus L. Medial longitudinal % fasciculus a" Reticular substance m ' ■■ Olivocerebellar fibers r~-rZ'<-f ' ~v— ]'a''us nerve Lateral reticular nucleus ^m^ ^^y^ :? ^\C Thalamo-olivary tract ||lps3s§§y^ " , --^~ n Inferior olivary nucleus 0>3 ^P^-w Medial lemniscus r ~— ^-— Hypoglossal nerve it - — Pyramid, corticospinal tract Medial accessory olivary nucleus ' «:^. / s -'-':^-,- Ventral external arcuate fibers ^^^^ ^~**^—-~ Arcuate nucleus Fig. 103. — Section through the medulla oblongata of a child at the level of the restiform body. Pal-Weigert method. (X4.) as the thalamo-olivary fasciculus; but it is not certain that they have their origin in the thalamus; quite possibly they come from some other gray mass in that neighborhood. Another group of fibers, consisting chiefly of collaterals, comes from the ventral funiculus of the spinal cord and may be regarded as ascending sensory fibers (Cajal, 1909). These belong to the so-called spino- olivary fasciculus. Olivocerebellar Fibers. — The axons from the cells of the inferior olivary nucleus stream out of the hilus, cross the median plane, and either pass through or around the opposite nucleus. Here they are joined by some uncrossed fibers from the olivary nucleus of the same side. Thence they curve dorsally toward the restiform body, passing through the spinal tract of the trigeminal nerve Mil STRUCT! RE I HP Mil. Ml l»i II \ I IBLONG \ I \ J 43 whirh becomes split up into several bundle- (Fig. 103). They form an Im- portant group of internal annate fibers, which run through the restiform bod) to the cerebellum and constitute the olivocerebellar tra>;•." X • : ^'"'.'^ ,. , ... in,, "i-'vY' s .''■•'>•''•■••'• '•^/ Dorsal spinocerebellar tract Medial longitudinal fasciculus --mk~^\ ^^M--- Spinal tract N. V Tectospinal tract — 1$*\ tti^mr — Vagus nerve Dorsal accessory olivary nucleus— -p$r V* f *'ty$'<§$ml'.~' Rubrospinal tract Medial lemniscus ■ \ <>%'> ..,, " - : •. '""-Ventral spinoierebellar tract Medial accessory olivary nucleus ■ ''^V^^R- "^^ Spinothalamic tract k-ViAySs \ "'--••■'-:'.;:' ,•;••\'-'.• .■' I halamo-olivary tract Corticospinal tract — Inferior olivary nucleus II ypoglossal nerve Fig. 105.— Diagram showing the location of the nuclei and fiber tracts of the medulla oblongata at the level of the olive. Rolandi, with which it is directly continuous, and designated as the nucleus of the spinal tract of the trigeminal nerve (Figs. 92, 98, 99, 101, 103). The tract lies along the lateral side of the nucleus and is superficial except in so far as it is covered by the external arcuate fibers, the dorsal spinocerebellar tract, and the restiform body. It forms an elongated elevation, the tuberculum cinereum on the surface of the medulla oblongata (Fig. 88). The formatio reticularis fills the interspaces among the larger fiber tracts and nuclei. It is composed of small islands of gray matter, separated by fine bundles of nerve-fibers which run in every direction, but which are for the most part either longitudinal or transverse. It is subdivided into two parts. The formatio reticularis alba is located dorsal to the pyramid and medial to the root filaments of the hypoglossal nerve and is composed in large part of longi- Tin: s l kl ( it ki iif mi mi im i i.\ OBLONGATA [45 tudinal nerve-fibers belonging to the medial lemniscus, tectospinal tract, and the medial longitudinal fasciculus (Fig. 105). The latter is closely associated with the vestibular nerve and ran best be described with the central connectioi that nerve. The formaiio reticularis grisea \> found dorsal t<> theoliveand lateral to the hypoglossal nerve. Tn it the nerve-cells predominate and the trans- versely coursing internal anuatc fibers form a conspicuous feature It- longi tudinal fibers, though less prominent, arc of great Importance. The descend- ing fibers include those of the rubrospinal tract, which tan be followed into the lateral funiculus o\ the spinal cord, and the thalamo-olivary fasciculus, which ends in the olive. Among the ascending libers are those of the ventral and dorsal spinocerebellar, the spinothalamic, and spinotectal tracts. The nerve-cells of the reticular formation are scattered through the mesh of interlacing fibers. In certain localities they are more closely grouped and form fairly- well-defined nuclei. Among these we may select two for special atten- tion. The lateral reticular nucleus or nucleus of the lateral funiculus i> a long column of cells found along the deep surface of the ventral spinocerebellar tract. from which it is said by Andre Thomas to receive afferent fibers. At any rati'. it receives fibers from the lateral funiculus of the spinal cord (Cajal, 1909) and sends its axons to the cerebellum by way of the restiform body (Van Gehuchten, 1904; Yagita. 1906). It seems, therefore, to be a way station on a sensory path from the spinal cord to the cerebellum. Some large cells in the gray part of the reticular formation may be grouped together and called the motor nucleus of the tegmentum (nucleus magnocellularis of Cajal). Their axons become as- cending or descending fibers or may bifurcate into ascending and descending branches within the reticular formation. Kohnstamm has traced such fibers by means of the degeneration method, and has shown that they run for the most part in a caudal direction and that some of them reach the cervical por- tion of the spinal cord (tractus reticulospinalis — Fig. 115). The nuclei of the cranial nerves can best be considered in a separate chapter. At this point it will only be necessary to enumerate and locate the nuclei of those nerves which take origin from the medulla oblongata. The nucleus of the hypoglossal nerve contains the cells of origin of the motor fibers which compose that nerve. It forms a long column of nerve-cells on either side of the median plane in the ventral part of the gray matter sur- rounding the central canal and in the floor of the fourth ventricle (Figs. 99, 101, 103). In the latter region it lies immediately beneath that part of the floor which was described in the preceding chapter under the name of the trigonum 146 THE NERVOUS SYSTEM hypoglossi (Fig. 89). In reality, it corresponds only to the medial part of this eminence, for on its lateral side there is found another group of cells known as the nucleus intercalatus (Fig. 103). From their cells of origin the fibers of the hypoglossal nerve stream forward through the reticular formation to emerge at the lateral border of the pyramid. The nucleus ambiguus is a long column of nerve-cells which give origin to the motor fibers that run through the glossopharyngeal, vagus, and accessory nerves to supply the striated musculature of the pharynx and larynx. It is located in the reticular formation of both the open and the closed portions of the medulla, ventromedial to the nucleus of the spinal tract of the trigeminal nerve (Figs. 101, 103). The dorsal motor nucleus of the vagus lies along the lateral side of the nucleus of the hypoglossal. It occupies the ala cinerea of the rhomboid fossa and extends into the closed part of the medulla oblongata along the lateral side of the central canal (Figs. 89, 99, 101, 103). From the cells of this nucleus arise the efferent fibers of the vagus nerve which innervate smooth muscle and glandular tissue. The afferent fibers of the vagus and glossopharyngeal nerves bend caudally and run within the tractus solitarius. The nucleus of the tractus solitarius is the nucleus of reception of the affer- ent fibers of the facial, glossopharyngeal, and vagus nerves, i. e., it contains the cells about which these afferent fibers terminate. The tractus solitarius can be traced throughout almost the entire length of the medulla. It decreases in size as the descending fibers terminate in the gray matter which surrounds it (Figs. 92, 101, 103). CHAPTER X INTERNAL STRUCTURE OF THE PONS The pons consists of two portions which differ greatly in structure and sig- nificance. The dorsal or tegmental part resembles the medulla oblongata, of which it is the direct continuation. The ventral or basilar portion contains the longitudinal fibers which go to form the pyramids; but except for these it is composed of structures which are peculiar to this level. It is a recent phyletic development and forms a prominent feature of the brain only in those mam- mals which have relatively large cerebral and cerebellar hemispheres, as might be expected from the fact that it forms part of a conduction path uniting these structures. THE BASILAR PART OF THE PONS The basilar portion of the pons is the larger of the two divisions. It is made up of fascicles of longitudinal and transverse fibers and of irregular masses of gray substance, which occupy the spaces left among the bundles of nerve- fibers and which are known as the nuclei pontis. The longitudinal fasciculi of the pons consist of two kinds of fibers: (1) those of the corticospinal tract, which are continued through the pons into the pyra- mids of the medulla oblongata ; and (2) those which end in the nuclei of the pons and are known as corticopontine filers (Fig. 106). As they pass through the pons the corticospinal fibers give off collaterals which also end in these nuclei. The longitudinal fibers enter the pons at its rostral border from the basis pedunculi. At first they form on either side a single compact bundle; but this soon becomes broken up into many smaller fascicles, which are separated from each other by the transverse fibers and nuclei of the pons (Fig. 108). At the caudal border these bundles again become assembled into a compact strand, which is con- tinued as the pyramid of the medulla oblongata (Fig. 107). It is evident, how- ever, that the volume of the bundles is much greater at the rostral than at the caudal border. This is to be explained by the fact that the corticopontine fibers have left these bundles during their passage through the pons and have come to an end by arborization within the nuclei pontis. The transverse fibers are designated as fibre pontis and are divisable into a superficial and a deep group (librae pontis superficiales and libra? pontis pro- 147 148 THE NERVOUS SYSTEM fundae). Those of the superficial group lie ventral to the longitudinal fasciculi; while the deep transverse bundles interlace with the longitudinal ones or lie dorsal to them. The majority of the nbra; pontis cross the median plane. These are joined by some uncrossed fibers and gathered together on either side of the pons to form a compact and massive strand, known as the brat Mum pontis or middle cerebellar peduncle, which curves dorsally to enter the white center of the cerebellum (Figs. 88, 108). ~" " Cerebral cortex - Corticobulbar tract — J- Corticospinal tract Temporopontine tract — Frontopontine tract — Pons — Cerebellum """* Nuclei pontis Brachium pontis Lateral corticospinal tract Ventral corticospinal tract Fig. 106. — Diagram of the corticopontocerebellar pathway and the corticospinal and cortico- bulbar tracts. Along the rostral border of the pons and brachium pontis one or two fiber bundles are sometimes found which run an isolated course to the cerebellum. These are known as the fila lateralia pontis or tcenia pontis (Fig. 88). According to Horsley (1906) the constituent fibers arise from a ganglion situated caudal to the interpeduncular ganglion, decussate at once, and end in the cerebellum in the neighborhood of the dentate nucleus. Perhaps they rep- resent slightly displaced fibrae pontis. Some of the transverse fibers on reaching the median plane bend at right angles and run as fibrae recta? toward the pars dorsalis pontis (Fig. 108). According to Edinger (1911) these belong in part at least to the tractus cerebellotegmentalis pontis, which arises in the nuclei of the cerebellum and runs through the brachium pontis to end in the reticular formation of the opposite side (Fig. 153). Cajal (1909) is doubtful about the existence of such efferent fibers from the cerebellum in the brachium pontis. The nuclei pontis, which are continuous with the arcuate nuclei of the medulla oblongata, contain stellate nerve-cells of varying size, the axons of INTERNAL STRUCTURE OF THE PONS I40 which arc continuous with the fibrae pontis. There- art- also some small nerve- cells of dole's Type II. the short axons of which end in the adjacenl gray mat- ter. Within these nuclei terminate the fibers of the corticopontine tracts and Some collateral- from the corticospinal fibers. Collaterals from the medial lemniscus are also found arborizing in those nuclei of the pons which lie im- mediately ventral to that bundle. This gray matter, therefore, represent- an important association apparatus within which there terminate fibers from several different sources. From what has been said it will be apparent that the pons serves to estab- lish an important and for the most part crossed connection between the cere- bral hemispheres and the cerebellum, a cortico- ponto-ccrcbcllar path. The cor- ticopontine fibers take origin from pyramidal cells in the frontal and temporal lobes and end in the nuclei pontis. Arising from the cells in these nuclei, most of the transverse fibers cross the median plane and reach the opposite cerebellar hemisphere through the brachium pontis (Fig. 106). THE DORSAL OR TEGMENTAL PART OF THE PONS The dorsal or tegmental part of the pons (pars dorsalis pontis) resembles in structure the medulla oblongata (Fig. 108). On its dorsal surface there is a thick layer of gray matter which lines the rhomboid fossa. Between this layer and the basilar portion of the pons is the reticular formation divided by the median raphe into two symmetric halves. This has essentially the same struc- ture here as in the medulla oblongata, and contains the continuation of many longitudinal tracts with which we are already familiar. The restiform body at first occupies a position similar to that which it has in the medulla, along the lateral border of the rhomboid fossa; but it soon bends dorsally into the cerebellum. The Cochlear Nuclei. — At the point of transition between the medulla and pons the restiform body is partly encircled on its lateral aspect by a mass of gray matter formed by the terminal nuclei of the cochlear division of the acoustic nerve (Fig. 107). There may be distinguished a dorsal and a ventral cochlear nucleus at the dorsal and ventral borders of the restiform body. Within these nuclei the fibers of the cochlear nerve end; while those of the vestibular nerve plunge into the substance of the pons ventromedially to the restiform body to reach the floor of the fourth ventricle (Fig. 134). Fibers from the dorsal cochlear nucleus run medially upon the floor of the fourth ventricle in the stria? medullares (Fig. 89) , and sinking into the tegmentum join the fibers from the ventral coch- lear nucleus in the trapezoid body. IS© THE NERVOUS SYSTEM The trapezoid body (corpus trapezoideum), which in most mammals appears on the surface of the medulla near the border of the pons (Fig. 83), is covered in man by the enlarged pars basalis pontis. In sections through the more caudal portions of the pons the trapezoid body forms a conspicuous bundle of trans- verse fibers in the ventral portion of the reticular formation (Fig. 108). The fibers are associated with the terminal nuclei of the cochlear nerve, especially the ventral one, and with the superior olivary nucleus, around the ventral border of which they swing in such a way as to form a bay for its reception. Farther medialward they pass through the medial lemniscus at right angles to its con- Dorsal cochlear nucleus Fourth ventricle Strice medullares Vent, spinocerebellar tract Vent, external arcuate fibers Medial lemniscus Nucleus of emineniia teres Principal vestibular nucleus Lateral vestibular nucleus Nucleus of tractus solitarius Glossopharyngeal *C nerve , }X Dorsal cochlear [ || nucleus 'stiform body entral cochlear nucleus Spinal trad and nucleus N. V Trapezoid body Pontobulbar body Medial longitudinal fasciculus Thalamo-olivary tract Inferior olivary nucleus Pyramid, corticospinal tract Arcuate nucleus Foramen caecum Pons Fig. 107.— Section through caudal border of the pons and the cochlear nuclei of a child. Pal- Weigert method. (X 4.) stituent fibers and decussate in the median raphe. The trapezoid body de- scribes a curve with convexity directed rostrally as well as ventrally, and as a result its lateral portions are seen best in sections through the lower border of the pons (Fig. 107), while the rest of it is in evidence in sections at a higher level (Fig. 108). Arising from the ventral nucleus of the cochlear nerve (Fig. 107) these fibers pass, with or without interruption in the superior olivary nucleus, across the median plane (Fig. 108); and, on reaching the lateral border of the opposite superior olivary nucleus, they turn rostrally to form a longi- tudinal band of fibers known as the lateral lemniscus (Fig. 110). This is a INTERNAL STKIVTI ' k K OF Mil. I'OXS 151 part of the central auditory pathway, the connections of which are represented diagrammatically in Fig. 134. The superior olivary nucleus is a small mass of gray matter located in the ventrolateral portion of the reticular formation of the pons in close relation to the trapezoid body and not far from the rostral pole of the inferior olivary nucleus (Figs. 108, 110). It consists of two or three separate but closely associated nuclear masses composed of small fusiform nerve-cells, among which there ramify collaterals from the fibers of the trapezoid body. From the dorsal aspect Superior vestibular nucleus I v -f •v.M'v. A bducens nerve Genu of facial N . / Medial longitudinal fasciculus Fourth ventricle Restiform body Brachium pontis Nucleus of abducens N. Facial nerve Spinal tract and nu- cleus N. V Nucleus of facial N. Thalamo-olivary tract Superior olivary huch u s Trapezoid body and medial lemniscus Deep stratum of pons Corticospinal and cortico- pontine tracts Nuclei pontis Superficial stratum of pons Fig. 108. — Section through the pons of a child at the level of the facial colliculus. Pal-Weigert method. (X 4.) of this nucleus a bundle of fibers, known as the peduncle of the superior olive, makes its way toward the nucleus of the abducens nerve, and it may be that some of these fibers enter the medial longitudinal bundle (Fig. 124). The nuclei of the vestibular nerve lie in the floor of the fourth ventricle, where they occupy a field with which we are already familiar, namely, the area acustica (Fig. 89). The vestibular fibers on approaching the rhomboid fossa divide into ascending and descending branches, and terminate in four nuclear masses: (1) the medial (dorsal or principal) vestibular nucleus (Figs. 103, 107), (2) the lateral vestibular nucleus of Deiters (Fig. 107), (3) the superior vestibular i=;2 THE NERVOUS SYSTEM nucleus of Bechterew (Fig. 108), (4) the spinal or descending vestibular nucleus (Fig. 103). These are represented diagrammatically in Fig. 136. The medial longitudinal fasciculus is an important bundle which extends from near the floor of the third ventricle to the spinal cord, and is especially concerned with the reflex control of the movements of the head and eyes. A large proportion of its fibers are derived from the lateral vestibular nucleus. .1/. rectus medialis M . rectus lateralis Nucleus of med. long. fasc. Nucleus of oculomotor nerve Nucleus of trochlear nerve Nucleus of abducens nerve Medial longitudinal fasciculus Lateral vestibular nucleus Vestibular nerve Fig. 109. — Diagram showing the connections of the medial longitudinal fasciculus. (Modified from Yilliger.) From this origin the fibers pass horizontally through the reticular formation to the median longitudinal fasciculus of the same or the opposite side, and there divide into ascending and descending branches (Fig. 109). The former terminate in the nuclei of the oculomotor, trochlear, and abducens nerve, the latter in the nucleus of the spinal accessory nerve and in the columna anterior of the cervical portion of the spinal cord. In this way there is established a path for IN I I.KN AX STRUCTCTRE OF THE PONS 153 the reflex control of the movement of the head, neck, and eyes in response to Stimulation of the nerve endings in the semicircular canals of the ears. Another important group of fibers within this fasciculus takes origin from a collection of cells situated in the hypothalamus ju>t rostral to the red nucleus, which Cajal (1911) has called the interstitial nucleus} but which might properly be designated as the nucleus of the medial longitudinal fasciculus. According to Cajal the fascicle also contains ascending fibers from the ventral fasciculus proprius of the spinal cord. Still other fibers serve to connect the nuclei of the oculomotor and abducens nerves. The medial longitudinal fasciculus is continued into the ventral fasciculus proprius of the spinal cord. These fibers are displaced dorsolaterally by the decussation of the pyramids (Fig. 98) and then still farther dorsally by the decussation of the lemniscus (Fig. 99) until they come to lie in the most dorsal part of the substantia reticularis alba (Fig. 101), which position they occupy throughout the remainder of their course. The fasciculus is found ventral to the nucleus of the hypoglossal nerve (Fig. 103) and in close apposition to the nuclei of the three motor nerves of the eye (Figs. 108, 114, 116). The medial lemniscus can also be traced within the reticular formation from the medulla into and through the pons. But this broad band of longitudinal fibers, which was spread out along the median raphe in the medulla, shifts ventrally in the pons, assuming first a somewhat triangular outline and a ven- tromedian position (Fig. 107); then by shifting farther lateralward it takes again the form of a flat band (Figs. 108, 110). But now it is compressed ven- trodorsally and occupies the ventral part of the reticular formation, its fibers crossing those of the trapezoid body at right angles. It must not be forgotten that the medial lemniscus is composed of longitudinal fibers, and it is by the gradual shifting of these that the bundle as a whole changes shape and posi- tion. As it is displaced ventrally it separates from the medial longitudinal bundle, which retains its dorsal position. The motor nucleus of the facial nerve occupies a position in the reticular formation dorsal to the superior olive (Fig. 108). It is an oval mass of gray matter, which extends from the lower border of the pons to the level of the facial colliculus, and contains the cells of origin of the fibers which innervate 1 The interstitial nucleus of Cajal must not be confused with the nucleus of the posterior commissure of Darkschewitsch which lies in the mesencephalon just rostral to the oculomotor nucleus and which, according to Cajal, may or may not send fibers into the medial longitudinal bundle. 154 THE NERVOUS SYSTEM the platysma and muscles of the face. These fibers emerge from the dorsal surface of the nucleus and run dorsomedially toward the floor of the fourth ventricle. Somewhat widely separated at first, they become united on the medial side of the abducens nerve into a compact strand, which as the genu of I lie facial nerve partly encircles this nucleus, and which then runs ventrolaterally between the spinal tract of the trigeminal nerve and its own nucleus toward its exit from the brain (Figs. 108, 124). Anterior medullary velum Medial longitudinal fasciculus^ Ventral spinocerebellar tract Trapezoid b Superior ol Lateral lemniscu Brachium pontis Fourth ventricle Brachium conjunctivum Mesencephalic root of trigem- inal nerve Motor nucleus of trigeminal nerve Sensory nucleus of trigem- inal nerve Medial lemniscus Superficial stratum of pon Trigeminal nerve Corticospinal and cortico- pontine tracts pontis Fig. 110. — Section through the pons of a child at the level of the motor nucleus of the trigeminal nerve. Pal-Weigert method. (X 4.) The nucleus of the abducens nerve along with the genu of the facial pro- duces a rounded elevation in the rhomboid fossa, known as the facial colliculus (Figs. 89, 108). It is a spheric mass of gray matter containing the cells of origin of the fibers which innervate the lateral rectus. These emerge from the dorsal and medial surfaces of the nucleus and run ventrally more or less parallel to the median raphe toward their exit at the lower border of the pons. The Nuclei of the Trigeminal Nerve. — In transverse section through approxi- IN l I K\ \1. STRU< 11 RE I >] I 111. PONS j-- mately the middle of the pons we encounter the fibers of the trigeminal nerve ami two associated masses of gray matter, the motor and main sensory nuclei of that nerve (Fig. 110). These are located close together in tin dorsolateral part of the reticular formation near the groove between the middle an. I supe- rior cerebellar peduncles. ( If the two, the sensory nucleus is the more superfi< ial. It is. in reality, not a new structure, but rather the enlarged rostral extremity of the column of gray matter which we have followed upward from the sub- stantia gelatinosa Rolandi of the spinal cord and have designated as the nucleus of the spinal tract of the trigeminal nerve (Figs. 98, 101). On its medial side is found the motor nucleus, a large oval mass of gray matter from the cells of which arise the motor fibers for the muscles of mastication. Some of the fibers of the trigeminal nerve, passing between these two nuclei, are continued as the mesen- cephalic root of the trigeminal nerve (Figs. 110, 111). Reaching the gray matter in the lateral wall of the rostral part of the fourth ventricle, this bundle of fibers turns rostrally along the medial side of the brachium conjunctivum (Fig. 112). It extends into the mesencephalon in the lateral part of the gray matter which surrounds the cerebral aqueduct (Fig. 114). The fibers of this root take origin from unipolar cells scattered along its course and known as the mesencephalic nucleus of the trigeminal nerve. It will be apparent from this description that there are four nuclear masses associated with the trigeminal nerve, namely, the nucleus of the spinal tract, and the main sensory, motor, and mesencephalic nuclei. The relations which each of these groups of cells bear to the fibers of the trigeminal nerve are illus- trated in Fig. 111. Note that those fibers which arise from cells in the semi- lunar ganglion divide into short ascending and long descending branches. The former end in the main sensory nucleus; while the latter run in the spinal tract of the trigeminal nerve and end in the nucleus which accompanies it. The brachium conjunctivum or superior cerebellar peduncle (Fig. 89) is seen in sections through the rostral half of the pons, where it enters into the lateral boundary of the fourth ventricle. It is a large strand of fibers which runs from the dentate nucleus of the cerebellum to the red nucleus of the mesencephalon (Fig. 115). As it emerges from the white center of the cerebellum this brachium is superficially placed, with its ventral border resting on the tegmental portion of the pons (Fig. 110). To its dorsal border is attached a thin plate of white matter, the anterior medullary velum, which roofs in the rostral part of the fourth ventricle. As the brachium ascends toward the mesencephalon it -inks deeper and deeper into the dorsal part of the pons until it is entirely submerged i<6 THE NERVOUS SYSTEM (Fig. 112). Near the rostral border of the pons it assumes a crescentic outline and lies in the lateral part of the reticular formation. From its ventral border Fig. 111. — Diagram of the nucle^and central connections of the trigeminal nerve: A, Semi- lunar ganglion; B, mesencephalic nucleus, N. V.; C, motor nucleus, N. V.; D, motor nucleus, N. VII; E, motor nucleus, X. XII ; F, nuchus of the spinal tract of X. V; G, sensory fibers of the sec- ond order of the trigeminal path; a, ascending and b, descending branches of the sensory fibers, X. V; c, ophthalmic nerve; d, maxillary nerve; e, mandibular nerve. (Cajal.) fibers stream across the median plane, decussating with similar fibers from the opposite side. This is the most caudal portion of the decussation of the brachinm I\ I KRXAL SUM ill RE OF THE PONS 157 conjunctivum, which increases in volume as it i> followed rostrally, reachin maximum in the mesencephalon at the level <>!' the interior colliculi. In this decussation the fibers of the brachium undergo a complete crossing. The ventral spinocerebellar tract, which has made it> way through the retic- ular formation of the pons, turns dorsolaterally near the rostral >i ofbrachi conjunctiv Interpeduncular fos Substantia, nigra issurc of inft rior colliculi crior quadrigeminal brat hium uclcus of inferior colliculus Lateral lemniscus Trochlear m Thalamo-olivary trad —Nucleus of trochlear nerve ^ Medial lemniscus Basis pedunculi Posterior perforated substance Pons Fig. 114.— Section through the mesencephalon of a child at the level of the inferior colliculus. Pal-Weigert method. (X 4.) deeply placed in the tegmentum; and here they cross the median plane in the decussation of the brackium conjunctivum (Fig. 114). After crossing, each brach- ium turns rostrally and forms a rounded bundle of ascending fibers, which al- most at once comes into relation with the red nucleus (Fig. 116). Many of the fibers enter this nucleus directly, while others are prolonged over its surface to form a capsule that is best developed on its medial surface. While the majority of these fibers ultimately end in the red nucleus, some reach and end within the ventral part of the thalamus (Fig. 115). By way of summary we may repeat that the fibers of the brachium conjunctivum. or at least the greater part of them, i6o THE NERVOUS SYSTEM arise in the dentate nucleus of the cerebellum; they cross the median plane in the tegmentum at the level of the inferior colliculi and end either in the red nucleus or in the thalamus. According to Cajal (1911) the fibers of the brachium conjunctivum give off two sets of descending branches, which he has seen in Golgi preparations of the mouse, rabbit, and cat. The first group are collaterals given off as the brachium enters the dorsal part of the pons and before its decussation (Fig. 115). They descend into the pons and medulla oblongata and constitute a direct descending tract from the dentate nucleus of the cerebellum to the reticular formation of the pons and medulla oblongata. The second group of descending From frontal lobe and corpus striatum " Thalamus Rubrospinal trad s Rubroreticular tract Red nucleus Brachium conjunctivum Dentate nucleus \ Pons Rubrospinal tract Medulla oblongata -Reticulospinal tract Spinal cord m Fig. 115. — Diagram showing the connections of the red nucleus: A, Ventral tegmental decussation; B, decussation of the brachium conjunctivum; Cand D, descending fibers from bra- chium conjunctivum, before and after its decussation respectively. branches is formed by the bifurcation of the fibers of the brachium conjunctivum just beyond the decussation, and constitute a crossed descending tract from the dentate nucleus, which can be followed by degeneration methods through the reticular formation of the brain stem and probably into the anterior and lateral funiculi of the spinal cord (Fig. 115). The red nucleus (nucleus ruber) is a very large oval mass of gray matter, which in the fresh brain has a pink color. It is located on the path of the brach- ium conjunctivum in the rostral part of the tegmentum (Fig. 116). In trans- verse sections it presents a circular outline and can be followed from the level of the inferior border of the superior colliculus into the hypothalamus. In its caudal portion it contains great numbers of libers derived from the brachium THE DJTERNAL STRUCTURE 01 CHE MESENCEPHALON 1O1 conjunctivum, and stains deeply in Weigerl preparations, hut farther rostrally these fibers are less numerous and the nucleus take- on more and more theap pearance of gray substance. Afferent fibers reach the red nucleus chiefly through the brachium con junctivum, but it also receives fibers from the cerebral cortex of the frontal lobe and other- from the corpus striatum (Fig. 115). These descending fibers help to form the capsule of the nucleus and are most abundant along its medial surface. Efferent Fibers. From the cells of the red nucleus arise the fiber- of the rubrospinal tracts which after crossing the median plane descend into the spinal cord. Other cells give origin to liber-, which decussate along with those of the rubrospinal tract and terminate in the nuclei of the reticular formation and in the nucleus of the lateral lemniscus. These form the tractus rubroreticular (Fig. 115). Other fibers from the red nucleus reach the thalamus. The nerve-cells which are found in the red nucleus vary greatly in size. The smaller ones have the character of the cells of the reticular formation and send their axons into the tegmentum of the same and the opposite side. Another group of very large cells furnishes the axons that constitute the rubrospinal tract. This collection of large cells is phylogenetic- ally the older and forms the chief part of the red nucleus in the lower mammals. But in man, where the two parts are rather sharply differentiated, the chief mass is composed of the smaller cells. The red nucleus may be regarded as an especially highly developed portion of the motor nuclei of the tegmentum. In the lower mammals it serves as a center through which the cerebellum can influence the motor functions of the spinal cord and medulla oblongata. In man it has the same function, but is also more closely linked with the reticular formation of the pons by way of the rubroreticular tract. It is a significant fact that in man where the rubrospinal tract is relatively small the rubroreticular tract is especially well developed. This suggests the possibility that impulses from the red nucleus may be relayed through the reticular nuclei of the pons to the spinal cord (Fig. 115). The Tegmental Decussations. — At the level of the superior colliculus and between the two red nuclei the median raphe presents an unusual number of crossing fibers (Fig. 116). Among these are included the dorsal tegmental de- cussation (fountain decussation of Meynert) and the ventral tegmental decussa- tion (fountain decussation of Forel). The latter is composed of fibers from the red nucleus, which, after crossing the median plane, descend through the brain stem into the lateral funiculus of the spinal cord as the rubrospinal tract I Fig. 115). The dorsal tegmental decussation is composed of fibers which arise in the superior colliculi of the corpora quadrigemina, sweep in broad curves around the central gray stratum, and after crossing the median plane in the dorsal part of the raphe, go to form the tectobulbar and tectospinal tracts. 162 THE NERVOUS SYSTEM The median longitudinal fasciculus is more conspicuous in the mesencephalon than in other parts of the brain stem, but it occupies the same relative position, that is, near the median plane close to the central gray matter. At the level of the superior colliculus it forms a rather broad obliquely placed lamina, extending dorsolaterally from the median raphe, which together with the corresponding lamina of the opposite side produces in transverse sections a V-shaped figure (Fig. 116). The apex of this V is directed ventrally; and included between its two limbs are the oculomotor nuclei. At the level of the inferior colliculi the Stratum zonule Stratum griscum-y£ 4g Stratum optic urn • d Stratum lemnisci Stratum profundum— Aqueduct of cere- ^p brum . '• . Medial lemnis CUS ffi Superior colliculus Nucli us of oculomotor nerve Medial longitudinal fasciculus Thalamo-olivary tract Inf. quadrigemina! brack. Med. gen. body Basis pedunculi Dorsal tegmental decussation Ventral tegmental decussation Red nucleus Oculomotor nerve Substantia nigra Fig. 116. — Section through the mesencephalon of a child at the level of the superior colliculus. Pal-W'eigert method. (X 4.) medial longitudinal fasciculus lies immediately ventral to the nucleus of the trochlear nerve (Fig. 114). In the pons the nucleus of the abducens nerve is placed on its dorsolateral border. The close relation of this fascicle to the nuclei for the motor nerves of the eye is of considerable significance, since according to the law of neurobiotaxis (p. 179) it is an expression of the fact that the majority of the afferent fibers to these nuclei come from this fascicle. This bundle of fibers, the composition of which is discussed on pages 152 and 329, is a chief factor in the reflex control of the movements of the eyes, and especially in the coordination of these movements with those of the head and neck. nil. IMI l:\.\I. STRUCTURE OS CHE Ml 51 W( I I'll The Lemnisci. — In sections through the rostral border of the pons the two temmsci form a broad curved band in the ventral and lateral portions of the tegmentum. The fibers of the lateral lemniscus are cut obliquely, indicating that they have- begun to turn dorsally toward the inferior colliculus (Fig. 112). ()n entering the midbrain this lateral portion of the fillet separates from the medial lemniscus and run- toward the corpora quadrigemina, where it forms a capsule for the nucleus of the inferior colliculus (Fig. 114). Some of these fibers are prolonged beyond the nucleus and decussate with similar fibers from the opposite side. A large proportion of the fibers of the lateral lemniscus end in the inferior colliculus. but other- form the inferior quadrigeminal brachium Fig. 114). through which they reach the medial geniculate body I igs. 116, 134 In the mesencephalon the lateral lemniscus, which, it will be remembered, is the central auditory tract from the cochear nuclei, is joined by the fiber- of the spinotectal tract; and these run with it to the corpora quadrigemina. The medial lemniscus, or bulbothalamic tract from the gracile and cuneate nuclei of the opposite side, is continued through the tegmentum of the mesen- cephalon to end in the lateral nucleus of the thalamus (Fig. 235). Incorporated with it in this upper part of its course are the fibers of the spinothalamic tract and a portion of the central sensory tract of the trigeminal nerve (Figs. 132. 234). In the caudal part of the mesencephalon this broad band of longitudinal fiber.-, occupies the ventrolateral portion of the tegmentum (Fig. 1 14) ; but at the level of the superior colliculus it has been displaced dorsolaterally by the red nucleus. Here it lies not far from the medial geniculate body and inferior quadrigeminal brachium (Fig. 116). The Central Gray Stratum. — The cerebral aqueduct is lined by ependymal epithelium and surrounded by a thick layer of gray matter, the central gray stratum, which, because of its paucity in myelinated fibers, is nearly colorless in Weigert preparations. This layer is continuous with the gray matter surround- ing the third ventricle, on the one hand, and with that covering the rhomboid fossa on the other. Numerous nerve-cells of various size and -hape are tered through this central gray substance; and. in addition, there are three compact groups of cells, which are the nuclei of the oculomotor and trochlear nerves and of the mesencephalic root of the trigeminus. The nucleus of the trochlear nerve contains the cells of origin of the motor fibers for the superior oblique muscle of the eye. It is a small oval mass -ituated in the ventral part of the central gray Stratum at the level of the inferior collic- ulus (Fig. 114). The fibers of the trochlear nerve emerge from the dorsolateral 164 THE NERVOUS SYSTEM aspect of this nucleus, curve dorsally around the central gray matter, and decus- sate in the anterior medullary velum (Fig. 112). The nucleus of the oculomotor nerve is composed of the cells of origin of the motor fibers for all of the ocular muscles except the superior oblique and lateral rectus. It lies in the ventral part of the central gray substance beneath the superior colliculus (Fig. 116). This nucleus, a part of which occupies a median position and supplies fibers to the nerves of both sides, is 6 or 7 mm. long and extends from a little beyond the rostral limit of the mesencephalon to the nucleus of the trochlear nerve, from which it is not sharply separated. From the nucleus the fibers of the oculomotor nerve stream forward through the tegmentum and red nucleus. They emerge through the oculomotor sulcus along the ventromedial surface of the basis pedunculi. The interpeduncular ganglion is a median collection of nerve-cells in the posterior perforated substance situated between the two cerebral peduncles near the border of the pons (Fig. 114). It receives fibers from the habenular nucleus of the epithalamus by way of the fasciculus retroflexus of Meynert; and from it spring fibers that run to the dorsal nucleus of the tegmentum (Tig. 211). The substantia nigra is a broad thick plate of pigmented gray matter, which separates the basis pedunculi from the tegmentum and extends from the border of the pons throughout the length of the mesencephalon into the hypothalamus. In transverse section it presents a semilunar outline. Its medial border is super- ficial in the oculomotor sulcus and is thicker than the lateral border, which reaches the lateral sulcus of the mesencephalon. Its constituent nerve-cells, irregular in shape and deeply pigmented, send their axons into the tegmentum. But we are still ignorant as to the destination these may have; and the func- tion of the substantia nigra is equally obscure. It receives collaterals from the corticifugal fibers of the basis pedunculi. Furthermore, there terminates within it a bundle, consisting of both direct and crossed fibers from the corpus striatum, the strionigral tract (Fig. 117). The basis pedunculi is a broad compact strand, crescentic in transverse sec- tion, which consists of longitudinal fibers of cortical origin. These are con- tinued from the internal capsule into the longitudinal bundles of the pons through the basis pedunculi. It consists of four tracts. The medial and lat- eral fifths are occupied by fibers which terminate in the nuclei pontis. Those of the medial one-fifth arise from the cortex of the frontal lobe of the cerebral hemisphere and constitute the frontopontine tract. Other fibers, arising from the temporal lobe, form the temporopontine tract and occupy the lateral one- THE l.MKKNAI. STRUCTUR] "I Mil MESENCEPHALON [65 fifth of the basis pedunculi. The intermediate portion, approximately three- fifths, is formed by the corticospinal tract, the fibers of which after giving off collaterals to the nuclei pontis are continued into the pyramids of the medulla oblongata and thence into the spinal cord. Many of the fibers of the cortico- bulbar trad are intermingled with the more medially placed corticospinal fibers; but even at this level two large Fascicles destined for the nuclei of the cranial nerves have separated from the main strand of motor fibers (Dejerine, 1914). These have been called the medial and lateral corticobulbar tracts (Figs. 1.06, 117). The Corpora Quadrigemina. — The rostral portion of the midbrain roof or latum mesencephali is in all vertebrates an end-station for the optic tracts. In the lower vertebrates there are but two elevations in the roof, the optic lobes or corpora bigemina, and these, which correspond in a general way to the superior Temporopontine tract Tr. corticobulbaris tat. Strionigral tract Corticospinal tract Frontopontine tract Tr. corticobulbaris med. Fig. 117. — Diagram of the basis pedunculi. colliculi, are visual centers (Fig. 13). In mammals the development of a spir- ally wound cochlea is associated with the appearance of two additional eleva- tions, the inferior colliculi, within which many of the fibers of the central audi- tory path terminate. The entire tectum receives fibers from the spinal cord and medulla oblongata and sends other fibers back to them; it also receives fibers from the cerebral cortex. It contains important reflex centers, those in the superior colliculus being dominated by visual, those in the inferior colliculus by auditory, impulses. The inferior colliculi or inferior quadrigeminal bodies each contain, in addi- tion to the laminated gray matter of the tectum, a large gray mass, oval in transverse section, and known as the nucleus of the inferior colliculus (Fig. 114). 1 66 THE NERVOUS SYSTEM The lateral lemniscus has been traced to this nucleus, and while some of the libers plunge directly into it, others sweep around it to form a capsule, within which it is enclosed. The majority of these fibers ultimately end in this nu- cleus, but some pass beyond it, reach the median plane, and decussate with sim- ilar fibers from the opposite side (Fig. 118). The ramifications of fibers from the lateral lemniscus form an intricate interlacement within the nucleus, and throughout this network are scattered many nerve-cells of various shapes and Fig. 118. — Semidiagrammatic section through the inferior colliculus of the mouse: A, Nucleus of inferior colliculus; B, gray matter of the lamina quadrigemina; C, inferior quadrigeminal bra- chium; D, central gray substance; K, decussation of the brachium conjunctivum; a, b, c, d, fibers of the lateral lemnisus. Golgi method. (Cajal.) sizes. On the medial side of this circumscribed nuclear mass we find some of the laminated gray matter of the tectum, within which are embedded large mul- tipolar cells with axons directed ventrally in the stratum profundum. These partially encircle the central gray matter and after undergoing a partial decus- sation enter the tectobulbar and tectospinal tracts. The inferior quadrigeminal brachium begins on the lateral side of the nucleus of the inferior colliculus and consists of fibers from the lateral lemniscus which THE 1XTKKNAL STRIX'TI'KK OF Till; MESENCEPHALON [67 run to and terminate within the medial geniculate body (Figs. 114. 116 The fibers of the lateral Lemniscus carry auditory impulses from the terminal uuclei of the cochlear nerve. Some of these terminate in the inferior colliculus and are concerned with reflexes in response to sound. Other fibers, some of which are branches of those to the inferior colliculus, run to the medial genii ulate body, from which the impulses that they carry are relayed to the cerebral cor tex. The inferior quadrigeminal brachium also contains fibers of i ortii al origin, chiefly from the temporal lobe, which end within the inferior colliculus (Beevor and Horsley. 1902). The superior colliculi, or superior quadrigeminal bodies, are composed of laminated gray matter. Each consists of four superimposed, dorsally convex layers (Fig. 116). The most superficial of these is a thin lamina with many transversely coursing nerve-fibers, the stratum zonale. The second layer is much thicker, contains few myelinated fibers, and is know r n as the stratum grtseum. The third and fourth layers, stratum opticam and stratum lemnisci, are rich in myelinated fibers. The majority of the afferent fibers of the superior colliculus come from the optic tract by way of the superior quadrigeminal brachium and enter the stratum opticum. Many of these end in the superimposed stratum griseum. The superior colliculus also receives fibers from the. cerebral cortex and from the spinotectal tract. It has been generally supposed that the fibers of the stratum zonale come from the optic tract, but according to Cajal (1911) this cannot be the case, since they remain intact in animals which have been operated on in such a way as to produce degeneration of the optic fibers. According to him it is also probable that the fibers from the cerebral cortex, which reach the colliculus by way of the superior quadrigeminal brachium, end in the stratum lemnisci. The fibers of the spinotectal tract run with the lateral lemniscus in the upper part of its course and enter the superior colliculus by way of the stratum profundum. The tectobulbar and tectospinal tracts have their origin within the tectum of the mesencephalon, more of the fibers coming from the superior than from the inferior colliculi. These fibers, arising from cells in more superficial layers, are assembled in the stratum profundum and sweep ventrally in broad curves around the central gray substance (Figs. 116, 118). The majority of the fibers, after crossing the median plane in the dorsal tegmental decussation, run in a caudal direction just ventral to the medial longitudinal bundle in the tectospinal tract. They give off collaterals to the reticular formation and the red nucleus. But some of them, instead of taking part in this decussation, leave the mesencephalon by way of the lateral lemniscus of the same side, constituting the lateral tecto- bulbar and tectospinal tracts (Cajal. 1911; Edinger, 1911). CHAPTER XII THE CRANIAL NERVES AND THEIR NUCLEI The cranial nerves contain, in addition to the general somatic and visceral components, which were encountered in the study of the spinal nerves, also other functional groups of fibers of more restricted distribution and specialized function. These special somatic and visceral components supply the organs of special sense and the visceral musculature, derived from the branchial arches, which differs from other visceral musculature in that it is striated. The fibers which supply this special musculature are designated as special visceral efferent fibers. The eye and ear, being special somatic sense organs, are supplied by special somatic afferent fibers. The olfactory mucous membrane and the taste buds are special visceral sense organs and are supplied by special visceral af- ferent fibers. From what has been said it will be evident that there are seven distinct functional components in the cranial nerves, namely: somatic efferent, general somatic afferent, special somatic afferent, general visceral efferent, special vis- ceral efferent, general visceral afferent, and special visceral afferent components (Figs. 119, 120). No single nerve contains all seven types of fibers and the individual cranial nerves vary greatly in their functional composition. On entering the brain a nerve breaks up into its several components, which separate from each other and pass to their respective nuclei, enumerated below. These nuclei may be widely separated in the brain stem. Fibers having the same func- tion tend to be associated together within the brain irrespective of the nerves to which they belong. For example, all the visceral afferent fibers of the facial, glossopharyngeal, and vagus nerves are grouped in the tractus solitarius (Fig. 120, yellow). The nerve-cells, with which the fibers of the several functional varieties are associated within the brain stem, are arranged in longitudinal nuclear columns. The analysis of the cranial nerves into their functional com- ponents has involved a great amount of labor which has been carried through for the most part by American investigators. Among those who have made important contributions to this subject may be mentioned the following: Gas- kell (1886), Strong (1895), Herrick (1899), Johnston (1901), Coghill (1902), Norris (1908), and Willard (1915). 1 68 Special somatic afferent,, nucleus Gent ral somatii afferent m .[. \ r*^ nucleus Alar lamina" Visceral afferent nucleus' Genera! visceral efferent _ nucleus Special visceral efferent- nucleus Basal lamina' Somatic efferent nucleus' Somatic muscle Sympathetic ganglion Visceral mucous membrane Smooth muscle * N Sensory ganglion Branchial muscle Fig. 119. Sensory nucleus N. I Nucleus of abducens nerve\ \ Facial nerve* \ ^ Vestibular nuclei \ \ \ \ \ Vestibular ganglion and nerve < Bulbar rootlet of accessory nerve Spinal root of accessory nerve-' Nucleus ambiguus-' Traclus solitarius Nucleus of Edinger-Westphal Nucleus of oculomotor nerve Nucleus of trochlear nerve Mesencephalic nucleus N. V Trigeminal nerve and semilunar ganglion . Spinal trad and nucleus X. V .- Cm hlear nuclei ■ Spiral ganglion and cochlear nerve .Glossopharyn- geal nerve ■ Vagus nerve Nuc. salivatorius superior Xuc. salivatorius inferior Dorsal motor nucleus N. X Cervical spinal nerve Fig. 120. Figs. 119 and 120.— Diagrams showing the origin, course, and termination of the functional components of the cranial nerves. Somatic afferent and efferent, red; visceral afferent, yellow; general visceral efferent, black; special visceral efferent, blue. Fig. 1 19 shows the locations of the several functional cell columns in a section through the medulla oblongata of a human embryo and the peripheral terminations of the several varieties of fibers. Fig. 120, dorsal view of the human brain stem, showing the location of the nuclei and the intramedullary course of the fibers of the cranial nerves. 170 THE NERVOUS SYSTEM Longitudinal Nuclear Columns. — In a previous chapter we learned that at an early stage in its development the lateral wall of the neural tube consists of a dorsal or alar and a ventral or basal plate, separated by a groove, the sulcus li nutans (Fig. 119). The sensory nuclei of the cranial nerves develop within the alar plate and the motor nuclei within the basal plate. In the rhombencephalon both plates come to He in the floor of the fourth ventricle, the alar occupying the more lateral position. And, in spite of the changes of position which occur during development, the sensory nuclei retain, on the whole, a lateral, and the motor nuclei a more medial, location. From the basal plate there differentiate a somatic and a visceral column of efferent nuclei, and from the alar plate a visceral and a somatic column of afferent nuclei. The somatic efferent column includes the nuclei of those motor nerves which supply the striated musculature derived from the myotomes, i.e., the extrinsic muscles of the eye and the musculature of the tongue (Figs. 119-121). The visceral efferent column undergoes subdivision into: (1) a ventrolateral column of nuclei, from which arise the special visceral efferent fibers to the striated visceral or branchial musculature, and which includes the nucleus ambiguus and the motor nuclei of the fifth and seventh nerves; and (2) a more dorsally placed group for the innervation of involuntary musculature and glandular tissue, of which the dorsal motor nucleus of the vagus is the chief example. The former may be called the special visceral efferent and the latter the general visceral ef- ferent column. The visceral afferent column is represented by the nucleus of the tractus solitarius, within which end the afferent fibers from the visceral mucous membrane and the taste buds, i. e., both the general and special visceral afferent fibers. The somatic afferent column splits into two: a general somatic afferent column. within which terminate the sensory fibers from the skin; and a special somatic group of nuclei for the reception of the fibers of the acoustic nerve and, in aquatic vertebrates, of the lateral line nerves also. THE SOMATIC EFFERENT COLUMN As can be seen by reference to Figs. 101. 108, 114. and 116 the nuclei of the hypoglossal, abducens, trochlear, and oculomotor nerves are arranged in linear order in the central gray matter near the median plane. They represent the continuation into the medulla oblongata of the large cells of the anterior column of the spinal cord. The cells of these nuclei are large and multipolar with well-developed Xissl bodies (Fig. 126). From them arise large myelinated THE CRANIAL NERVES AND Til KIR NUCLEI I 7 I fibers, which innervate the striated musculature derived from the myotomes. This group of nuclei is indicated in red in Fig. 120 and by small circles in Figs. 121 and 122. The nucleus of the oculomotor nerve is an elongated mass of cells in the ceil tral gray matter ventral to the cerebral aqueduct at the level of the superi< r colliculus (Figs. 121, 122). Even a superficial examination shows that it is divided into a lateral paired and a medial unpaired portion (Tig. 116). The Nuc. Ill E-W. Nuc. Ill [at. Trigonum hypoglossi Nuc. spinalis V Nuc. com. Cajal Fig. 121. — Dorsal view of the human brain stem with the positions of the cranial nerve nuclei projected upon the surface. Sensory nuclei on the right side, motor nuclei on the left. Circles indicate somatic efferent nuclei; small dots, general visceral efferent nuclei; large dots, special visceral efferent nuclei; horizontal lines, general somatic sensory nuclei; cross-hatching, visceral sensory nuclei; stipple, special somatic sensory nuclei. (Herrick.) lateral groups of cells spreads out upon the surface of the medial longitudinal bundle, extends throughout the entire length of the nucleus, and may be divided into ventral and dorsal portions (Fig. 123). The medial group of cells is placed exactly in the median plane and is found only in the rostral half of the nucleus. Dorsolateral from this median group, and restricted to the most rostral part of the nucleus, is a collection of small cells which form the nucleus of Edinger- Westphal. This is a visceromotor nucleus and will be considered elsewhere. 172 THE NERVOUS SYSTEM The fibers from the medial nucleus enter both right and left nerves. Some from the caudal portion of the dorsal division of the lateral nucleus cross the median plane. The others remain uncrossed. After sweeping in broad curves through the tegmentum and red nucleus the fibers emerge through the oculo- motor sulcus. All of the extrinsic muscles of the eye except the lateral rectus and superior oblique are supplied by the medial and lateral groups of cells just described. t , Nucleus of Edinger-W est phal ,. Nucleus of oculomotor nerve ) . — -Corpora quadrigcmina Cerebral aqueduct Nucleus of trochlear nerve Trochlear nerve Anterior medullary velum ..-■' Motor nucleus N. V _, - 'Nucleus of facial nerve Fourth ventricle Nucleus of abducens nerve Nuc. salivatorius superior Nuc. salivatorius inferior - Nucleus of hypoglossal nerve — Dorsal motor nucleus N. X ^--Central canal — Nucleus ambiguus Mesencephalon Oculomotor nerve Pons- • Portio minor N. V- Facial nerve ■ Abducens nerve - Medulla oblongata Fig. 122. — Motor nuclei of the cranial nerves projected on a median sagittal section of the human brain stem. Circles indicate somatic efferent nuclei; small dots, general visceral efferent nuclei; large dots, special visceral efferent nuclei. As one might expect from the fact that the oculomotor nerve supplies several distinct muscles, its nucleus seems to be made up of a number of more or less distinct groups of cells; but the efforts to locate subordinate nuclei have given rise to contradictory results. The most significant work in this field has been done by Bernheimer (1904), who extirpated in- dividual eye muscles in monkeys and studied the resultant changes in the cells of the oculo- motor nuclei. According to him, the various muscles are supplied by the lateral nucleus in the following order, beginning at the rostral end: levator palpebral superioris, rectus supe- rior, rectus medialis, obliquus inferior, and rectus inferior. Bernheimer says that the fibers for the rectus inferior are entirely crossed, those for the obliquus inferior are in greater part crossed, those for the rectus medialis for the most part uncrossed, those for the rectus superior and levator palpebral superioris entirely uncrossed. IHE (RAMAI. NERVES AND I III Ik M I l.u 173 The nucleus of the trochlear nerve has already been located in the central pray matter ventral to the cerebral aqueduct at the level of the inferior collic- uluSj close to the caudal extremity of the oculomotor nucleus Figs. 111. 121, 122). The fibers of the trochlear nerve emerge from the dorsal and lateral aspects of this nucleus, and. encircling the central gray matter along an angular course which carries them also caudally, enter the anterior medullary velum, decussate within it. and make their exit from its dorsal surface 1 Fig. 112). The) supply the superior oblique muscle. The nucleus of the abducens nerve was encountered in the dorsal portion of the pons as a spheric gray mass, which with the genu of the facial nerve forms the facial colliculus of the rhomboid fossa (Figs. 108, 121, 122). The fibers of the abducens nerve leave the nucleus chiefly on its dorsal and medial surfaces and become assembled into several root bundles, which are directed ventrally toward their exit from the lower border of the pons near the pyramid of the medulla oblongata. It supplies the lateral rectus muscle. The axons, which ramify within the three nuclei for the motor nerves of the eye, are derived from many sources. The most important of these sources are the corticobulbar tract, the medial lon- gitudinal bundle, and the tectospinal tract. The nucleus of the abducens receives fibers also from the central auditory apparatus through the pe- duncle of the superior olive. These various fibers provide for voluntary movements of the eyes, and for reflex ocular movements in response to vestibular, visual, and auditory impulses. The nuclei probably- also receive branches from the central sensory path of the fifth nerve. The nucleus of the hypoglossal nerve is a slender cylindric mass of gray matter nearly 2 cm. in length, extending from the level of the fovea inferior to that of the decussation of the pyramids. We have already identified it in both the open and the closed portions of the medulla oblongata (Figs. 99, 103). In the floor of the fourth ventricle it lies beneath the trigonum hypoglossi. while more caudally it lies ventral to the central canal (Figs. 121, 122). The root fibers Fig. 123. — Diagram of the nuclei of the oculomotor nerve: M, Median nucleus; £.11*., nu- cleus of Edinger-\Y estphal; V.L., D.L., ventral and dorsal portions of the lateral nucleus. (Ober- steiner.) 174 THE NERVOUS SYSTEM are assembled into bundles which run ventrally toward their exit along the lateral border of the pyramid. A conspicuous plexus of myelinated fibers gives the hypoglossal nucleus a characteristic appearance in Weigert preparations. Fibers from many sources reach the nucleus and ramify within it. These include some from the cortico- bulbar tract and others from the sensory nuclei of the fifth nerve and from the nucleus of the tractus solitarius. The part which such fibers may play in reflex movements of the tongue is illustrated in Fig. 92. THE SPECIAL VISCERAL EFFERENT COLUMN The special visceral efferent column of nuclei contains the cells of origin of the motor fibers for the striated musculature derived from the branchial arches, as distinguished from the general skeletal musculature that develops from the myotomes. The branchial musculature includes the following groups of muscles: the muscles of mastication, derived from the mesoderm of the first branchial arch and innervated by the trigeminal nerve; the muscles of expression, derived from the second or hyoid arch and innervated by the facial nerve; the musculature of the pharnyx and larynx, derived from the third and fourth arches and innervated by the glossopharyngeal, vagus, and accessory nerves; and prob- ably also the sternocleidomastoid and trapezius muscles, innervated through the spinal root of the accessory nerve. Some authors prefer to call this column, which includes the motor nuclei of the fifth and seventh nerves and the nucleus ambiguus, the lateral somatic column, because the cells in these nuclei and the fibers which arise from them possess the characteristics of somatic motor cells and fibers (Malone, 1913). The nuclei are composed of large multipolar cells with well-developed Nissl bodies. These cells give origin to large myelinated fibers which run through the corresponding nerve and terminate in neuromus- cular endings in one or another of the muscles indicated above. The motor nuclei of the fifth and seventh nerves and the nucleus ambiguus of the ninth, tenth, and eleventh nerves form a broken column of gray matter, located in the ventrolateral part of the reticular formation of the pons and medulla oblongata some distance beneath the floor of the fourth ventricle (Figs. 121, 122). The cells of this column and the special visceral efferent fibers which arise from them have been colored blue in Figs. 119 and 120. The motor nucleus of the trigeminal nerve lies on the medial side of the main sensory nucleus of that nerve, and is located at the level of the middle of the pons in the lateral part of the reticular formation some distance from the THK CRANIAL NKRVKS AND T1IKIK M I II I '75 ventricular floor (Figs. 110, 121, 122). The fibers, which take their origin here, arc collected in the motor root or portio minor of the fifth nerve and run with ii mandibular division to the muscles of mastication. Within the nucleus then- terminate fibers from the corticobulbar tract and many fibers, chiefly collaterals from the central sensory tract of the trigeminal nerve. It also receives collat- erals from the mesencephalic root of the trigeminal and from other sources (Fig. 131). The motor nucleus of the facial nerve is located in the ventrolateral part of the reticular formation of the pons near its caudal border (Figs. 108, 121, 122). Its constituent cells are arranged so as to form a varying number of sub- groups which may possibly be concerned with the innervation of individual facial muscles. Root of facial nerve, first part Ibducens nucleus Root of facial nerve, genu Root of facial nerve, second part Facial nucleus Nucleus of ahducens nerve Root filaments of ahducens nerve Stalk of superior olive Root of facial nerve, first part Spinal root and nuch u AT A ucleus of facial m Root official )i.. Superior olive [part A bducens nerve Fig. 124. — Diagram of the root of the facial nerve, shown as if exposed by dissection in a thick section of the pons. From the dorsal aspect of this nucleus there emerge a large number of fine bundles of libers, directed dorsomedially through the reticular formation. These rather widely separated bundles constitute the first part of the root of the facial nerve (Fig. 124). Beneath the floor of the fourth ventricle the fibers turn sharply rostrad and are assembled into a compact strand of longitudinal fibers, often called the ascending part of the facial nerve. This ascends along the medial side of the abducens nucleus dorsal to the medial longitudinal bundle for a consid- erable distance (5 mm.). The nerve then turns sharply lateralward over the dorsal surface of the nucleus of the abducens nerve, and helps to form the eleva- tion in the rhomboid fossa, known as the facial colliculus. This bend around the abducens nucleus, including the ascending part of the facial nerve, is known 176 TI1K NERVOUS SYSTEM as the genu. The second part of the root of the facial nerve is directed ventro- laterally and at the same time somewhat caudally, passing close to the lateral side of its own nucleus, to make its exit from the lateral part of the caudal border of the pons (Fig. 108). Fibers from many sources terminate in the facial nucleus in synaptic rela- tion with its constituent cells. Those from the corticobulbar tract place the facial muscles under voluntary control. Others are collaterals from the sec- ondary sensory paths in the reticular formation and are concerned with bulbar reflexes. Some of these collaterals are given off by fibers arising in the trapezoid body and carry auditory impulses. Others are collaterals of fibers arising in the nucleus of the spinal tract of the fifth nerve; and still others are given off by ascending sensory fibers from the spinal cord (Cajal, 1909). - Vagus nerve ^" Jugular foramen Internal ramus I , c. . 1 - accessory nerve hxtcrnal ramus] J Ibar rootlets of accessory nerve ^.-zs ; Foramen magnum -\ Spinal root of accessory nerve V / / / / / / / A ( 1 Fig. 125. — Diagram of the roots of the vagus and accessory nerves. The nucleus ambiguus is a long slender column of nerve-cells, extending through the greater part of the length of the medulla oblongata in the ventro- lateral part of the reticular formation (Figs. 103, 121, 122). Its constituent cells give rise to the special visceral efferent fibers that run through the glosso- pharyngeal, vagus, and accessory nerves to supply the musculature of the pharynx and larynx. It reaches from the border of the pons to the motor de- cussation, but is most evident in transverse sections through the caudal part of the rhomboid fossa. Here it can be found in the reticular formation ventral to the nucleus of the spinal root of the trigeminal nerve. The fibers arising from its cells are at first directed dorsally; then curving laterally and ventrally they join the root bundles of the ninth, tenth, and eleventh nerves with which they Nil . R \\l \l. \l i:\ I S AND nil II' \i | 1.1 | i;; emerge from the brain (Fig. 105). A lew of tin- fibei the median plane ami join the corresponding root bundles of tin- opposite side. The accessory nerve consists of a bulbar am! a spinal portion. The fibers "i the spinal root take origin from a linear group of cells in the lateral pan of the anterior gray column in the upper cervical segments of the spinal cord. This rool ascends along ili< side of the spinal cord, pasM-s through the foramen magnum, and is joined by the bulbar rootlets of the sorj (Fig. I-'.m. The nerve then divides into an internal and an external branch. In the latter run all the fibers of spinal origin and these are distributed to the trapeziu sternocleidomastoid muscles. If. as seems probable, these muscles are derived from the branchial arches (Lewis, 1910), the fibers which supply them may he regarded as special viscera] efferent tihers; and the spinal nucleus of the accessory nerve may he considered as homologous to the nucleus ambiguus. Tin- bulbar rootlets of the accessory nerve, which con- tain both general and special visceral efferent fibers, form a well-denned fascicle, readily distinguished from the spinal portion of the nerve, which, as the internal ramus, joins the vagus nerve and is distributed through its branches (Fig. 120 — Chase and Ranson, Y>\ I;. The sensory collaterals which arborize among the cells of the nucleus am- biguus are derived from the central tracts of the trigeminal, glossopharyngeal, and vagus nerves, from ascending sensory fibers of spinal origin, and from other longitudinal fibers in the reticular formation. Other fibers reach this nucleus from the corticobulbar tract. THE GENERAL VISCERAL EFFERENT COLUMN The general visceral efferent column of nuclei is composed of the cells fri im which arise the efferent libers innervating cardiac and smooth muscle and glan- A ■ A B Fig. 126. — Two types of motor nerve-cells from medulla oblongata of lemur: .1, Cells of the somatic motor type from the hypoglossal nucleus; B, cells of the visceral efferent type Irom the rostral part of the dorsal motor nucleus of the vagus. Toluidin blue stain. I Mai dular tissue. The cells of these nuclei are of small or medium size and their Xissl bodies are not well developed (Fig. 126). They give rise to the general IJo THE NERVOUS SYSTEM visceral efferent fibers of the cranial nerves. These are small myelinated fibers, which end in sympathetic ganglia, where they arborize about sympathetic cells, the axons of which terminate in smooth or cardiac muscle or in glandular tissue. The neurons of this series are. therefore, characterized by the fact that the impulses which they transmit must be relayed by neurons of a second order before reaching the innervated tissue (Fig. 119). This group of nuclei is indi- cated by black in Fig. 120 and by fine stipple in Figs. 121 and 122. The dorsal motor nucleus of the vagus nucleus vagi dorsalis medialis) has been noted in the transverse sections through the medulla oblongata (Figs. 99, 103). It lies along the dorsolateral side of the hypoglossal nucleus, subjacent to the ala cinerea of the rhomboid fossa, and along the side of the central canal in the closed part of the medulla oblongata. The general visceral efferent fibers, which arise from the cells in this nucleus, leave the medulla oblongata through the roots of the vagus and accessory nerves; but those entering the accessory nerve leave that nerve by its internal ramus and join the vagus (Fig. 120). Hence all of the fibers from this nucleus are distributed through the branches of the vagus to the vagal sympathetic plexuses of the thorax and abdomen for the innervation of the involuntary musculature of the heart, respiratory passages, esophagus, stomach, and small intestines (Van Gehuchten and Molhant. 1912), and for the innervation of the pancreas, liver, and other glands. There are relatively few sensory collaterals reaching the dorsal motor nucleus, and these come in large part from sensory fibers of the second order, arising in the receptive nuclei of the trigeminal, glossopharyngeal, and vagus nerves. The nucleus salivatorius is located in the reticular formation, some distance from the floor of the fourth ventricle at the junction of the pons and medulla oblongata near the caudal end of the facial nucleus and the rostral end of the nucleus ambiguus (Figs. 121, 122). The more caudal portion, or nucleus sal- ivatorius inferior, sends general visceral efferent fibers by way of the glosso- pharyngeal nerve to the otic ganglion for the innervation of the parotid gland. The rostral part, or nucleus salivatorius superior, lies dorsal to the large motor nucleus of the facial nerve, to which nerve it sends general visceral efferent fibers. These run from the facial nerve through the chorda tympani to the sub- maxillary ganglion for the innervation of the submaxillary and sublingual sal- ivary glands 'Kohnraamm. 1902. 1903. 1907: Yagita, 1909; Feiling, 1913). The Edinger-Westphal nucleus i- a group of small nerve-cells located in the rostral part of the nucleus of the oculomotor nerve. Here it is placed dorsolateral to the median unpaired portion of that nucleus (Figs. 121-123). Mil: CRANIAL NERVES AND mill' NUCLEI 179 This group of small cells gives origin to the general visceral efferent fibers of the oculomotor nerve which run to the ciliary ganglion for thi innervation of the intrinsic muscle of the eye. Neurobiotaxis. The position of the motor nuclei of the brain stem varii - greatly in different orders of vertebrates, and is determined by the source of tin- principal al impulses which reach them. The perikarya of the neurons migrate under the influent c of an attraction, which has been called neurobiotaxis, in the direction of the chief fiber tra14. 1 ( )17; Black, 1917). "When from different places stimuli proceed to a cell, its chief dendrite grows out and its < el) body shifts in the direction whence the majority of the stimuli proceed," while the axon grows in the op- posite direction (Fig. 127). The nature of the attractive force is not altogether clear. Kap- Axiscyli ndcf B Fig. 127. — Diagram to illustrate the principle of neurobiotaxis. The axis-cylinder grows in the direction of the nervous current, indicated by the arrow, while the dendritic outgrowth and the final shifting of the cell body occur against the nervous current: A, Dendrites grown out to- ward the center of stimulation; B, the cell body has shifted toward the center of stimulation; the axis-cylinder is consequently elongated. (Kappers.) pers believes that it is a galvanotropic phenomenon, on the basis of the fact that the stimu- lation center is electrically negative, i. e., a cathode with reference to the surrounding tissue. Numerous instances might be cited of the action of this taxis, but two will suffice. It has already been noted that the eye-muscle nuclei receive most of their collaterals from the optic and vestibular reflex tracts; and these appear to be the most important factors in the determination of the positions occupied by those nuclei. The changes in position of the nuclei in the vertebrate series appear to run parallel to the changes in these tracts. The reader will now appreciate the significance of the close relation of these nuclei to the medial longi- tudinal and tectospinal fasciculi which convey to them impulses from the vestibular and optic centers. The position of the nucleus of the facial nerve and the curved course of its fibers within the pons may be explained in the same way. In a 10 mm. human embryo the nucleus ot the facial nerve lies rostral to that of the abducens and the motor fibers pass directly lateralward ISO THE NERVOUS SYSTEM to their exit from the brain (Fig. 128). This nucleus, which supplies the muscles that sur- round the mouth, receives axons from the primary taste center in the medulla oblongata (the nucleus of the tractus solitarius) which is located at a more caudal level. Accordingly, the facial nucleus migrates caudally toward that center. It also receives fibers from the nucleus of the spinal tract of the trigeminal nerve and migrates ventrolaterally toward it. Thus is explained the adult position of the nucleus of the facial nerve, not far from the spinal tract of the trigeminal nerve and near the rostral end of the nucleus of the tractus solitarius. In the same way the curved course of the facial nerve within the pons may be explained. These examples are perhaps sufficient to illustrate the general principle of neuro- biotaxis. Nuclei of Origin and Terminal Nuclei. — The efferent nuclei, which we have examined, all have this in common, that the axons, which take origin from their constituent cells, leave the brain through the efferent roots of the cranial nerves. Hence they may all be included under the term nuclei of origin. On the other hand, the afferent fibers of the cerebrospinal nerves have their cells of origin located © Sulcus Sulcus Genu internum n. facialis Sulcus Fig. 128. — Diagram illustrating three stages in the development of the genu of the facial nerve, the youngest, A, corresponding to the 10 mm. embryo, and the oldest, C, the newborn child. The relative position of the nucleus of the n. abducens is represented in outline. Sulcus, Sulcus medianus fossae rhomboideae. (Streeter, in Keibel and Mall's Embryology.) outside the central nervous system and, with the exception of the first two cranial nerves, in the cerebrospinal ganglia. These fibers enter the central nervous system and end by entering into synaptic relations with sensory neurons of the second order located in terminal nuclei. These are classified according to the function of the fibers which end in them as visceral afferent and somatic afferent nuclei. THE VISCERAL AFFERENT COLUMN All of the visceral afferent fibers of the cranial nerves, except those of the first pair, are contained in the facial, glossopharyngeal, and vagus nerves. These include: (1) the fibers from the taste buds, which since they mediate the special sense of taste, may be called special visceral afferent fibers; as well as (2) others from the posterior part of the tongue, and from the pharynx, larynx, trachea, esophagus, and thoracic and abdominal viscera, which are known as general THE CRAN1 \l. NERVES WD mill: \i CL1 I l8l visceral afferent fibers. The majority of the taste fibers run through the seventh (via the chorda tympani and lingual) and ninth nerves (Cushing, 1903), but a few reach the epiglottis by way of the tenth (Wilson, 1905 Fig. 129). All of these general and special visceral afferent libers, whether contained in the seventh, ninth, or tenth nerves, enter the tractus solitarius, within which they descend for varying distances (Fig. 120. yellow). They terminate in a column of nerve-cells, which in part surround the tract and in part are scattered among its fibers. This is known as the nucleus of the tractus solitarius (Figs. 121, 1 -Id . It is a long slender nucleus, which extends throughout the entire length of the medulla oblongata and is best developed at the level of origin of the vagus nerve, t ^W^"}' ana%t \\-r ^3^— ~N ■ max. cr& n,ffi Gotic. Fig. 129. — Diagram of the trigeminal, facial, and glossopharyngeal nerves showing the course of the taste fibers in solid black lines. The broken and dotted lines indicate the course which ac- cording to certain investigators some of the taste fibers are supposed to take: G. G., Gasserian ganglion; G. g., geniculate ganglion; G. sp., sphenopalatine ganglion; g.s.p., great superficial petro- sal nerve; N. Jac, the tympanic nerve of Jacobson; N, vid., vidian nerve; s.s.p., small superficial petrosal nerve. (Cushing.) where it lies ventrolateral to the dorsal motor nucleus of that nerve and some little distance below the floor of the fourth ventricle (Fig. 103). The fibers from the seventh and ninth nerves terminate in the rostral- portion of the nucleus, which is therefore the part especially concerned with the sense of taste, while those from the vagus end in the caudal part. Some of these vagus fibers after undergoing a partial decussation terminate in a cell mass, the commissural nucleus, which lies dorsal to the central canal in the closed part of the medulla and unites the nucleus of the tractus solitarius on one side with the correspond- ing nucleus on the other side (Fig. 121). The secondary afferent paths from the nucleus of the tractus solitarius are not well defined. Since gustatory impulses arouse sensations of taste and the l82 THE NERVOUS SYSTEM afferent impulses from the viscera may be vaguely represented in conscious- ness, there must be a visceral afferent path to the thalamus; but concerning the character and location of this path we are entirely ignorant. 1 The fibers arising from the nucleus of the tractus solitarius enter the reticular formation, and it is probable that a majority of them are distributed to the visceral motor nuclei of the medulla oblongata, including the nucleus ambiguus and the dorsal motor nucleus of the vagus. In this way arcs are established for a large and important group of visceral reflexes. Some of these fibers descend to the spinal cord and may play an important part in the reflex control of respiration and in initiating reflex coughing and vomiting (Figs. 245, 246). THE GENERAL SOMATIC AFFERENT NUCLEI The general somatic afferent nuclei receive fibers from the skin and ecto- dermal mucous membrane of the head by way of the trigeminal nerve. These have their cells of origin in the semilunar ganglion, and within the pons they divide into short ascending and long descending branches (Fig. 131). The as- cending branches terminate in the main sensory nucleus; the descending branches run through the spinal tract and terminate in the nucleus of the spinal tract of the trigeminal nerve. Since these nuclei receive sensory fibers from the skin and ectodermal mucous membrane of the head, they are exteroceptive in function. The spinal tract and its nucleus also receives a few cutaneous afferent fibers through the glossopharyngeal and vagus nerves from the skin of the external ear (Fig. 120). The main sensory nucleus of the trigeminal nerve is located at the level of the middle of the pons in the lateral part of the reticular formation some dis- tance from the floor of the fourth ventricle (Figs. 110, 121, 130). The spinal nucleus, with which it is continuous, at first lies deeply under cover of the resti- form body; but when it is traced caudally it approaches the surface and, covered by the spinal tract, forms the tuberculum cinereum (Figs. 99, 103). It finally becomes continuous with the substantia gelatinosa Rolandi of the spinal cord. Thus we have a continuous column of gray matter extending from the sacral por- tion of the spinal cord into the brain stem and ending abruptly in an enlarge- ment, the main sensory nucleus of the trigeminal nerve. This entire column receives afferent fibers from the skin and belongs to the exteroceptive portion of the somatic ajferent division of the nervous system. 1 Kohnstamm and Hindelang (1910) and von Monakow (1°13) have described a secondary visceral afferent path which arises from the gray matter in and around the tractus solitarius and terminates in the thalamus. THE CRANIAL NERVES AND Till Ik \i I u i l8 3 Secondary Afferent Paths.— From the cells of the mail] sensor) and spinal nuclei of the trigeminal nerve arise fibers which enter the reticular formation and are there grouped into Longitudinal bundles from which collaterals are given off to the motor nuclei of the brain stem ( Fig. 131 ). There are at least two su< li longitudinal bundles in each lateral half of the brain. The ventral secondary afferent path of the trigeminal nerve consists for the most part of crossed liber- and is located in the ventral part of the reticular formation, close to the spino thalamic tract in the medulla, and dorsal to the medial lemniscus in the pons Mesencephalon Pons- Ventral cochlear nucleus Medulla oblongata -**j Cerebral aqueduct - - Inferior eollirulus "Mesencephalic nucleus of N. V Sensory nucleus of N. V -/^"Fourth ventricle ■-■Vestibular nucleus Dorsal cochlear nucleus Nucleus of tractus solitarius Nucleus of spinal trad X. V Central canal Fig. 130. — Sensory nuclei projected upon a median sagittal section of the human brain stem. Horizontal lines, general somatic sensory nuclei; cross-hatching, visceral sensory nucleus; stipple, special somatic sensory nuclei. and mesencephalon (Fig. 132). It is composed in large part of long fibers which reach the thalamus. The dorsal secondary afferent path of the trigeminal nerve consists chiefly of uncrossed fibers and lies not far from the floor of the fourth ventricle and the central gray matter of the cerebral aqueduct. It consist- in considerable part of short fibers (Cajal, 1911; Wallenberg, 1905; Economo, 1911; Dejerine, 1914). The proprioceptive nuclei of the cranial nerves are not well known. They have to do with afferent impulses arising in the muscles of mastication and in 1 84 THE NERVOUS SYSTEM the extrinsic muscles of the eye. Johnston (1909) has shown that the large unipolar cells of the mesencephalic nucleus of the fifth nerve which give rise Fig. 131. — Diagram of the nuclei and central connections of the trigeminal nerve: A, Semi- lunar ganglion; B, mesencephalic nucleus, N. Y.; C, motor nucleus, N. Y. ; D, motor nucleus, N. VII; E, motor nucleus, N. XII; F, nucleus of the spinal tract of N. V.; G, sensory fibers of the sec- ond order of the trigeminal path: a, ascending and b, descending branches of the sensory fibers, N. V.; c, ophthalmic nerve; d, maxillary nerve; e, mandibular nerve. (Cajal.) to the fibers of the mesencephalic root of that nerve, are probably sensory in function. Willems (1911) and Allen (1919) believe that these are sensory fibers THE CRANIAL NERVES AND iiii.ik NUCLEI 185 to the muscles of mastication. If this interpretation is correct we are pre- sented with an exception to the rule that the afferent fibers of the cerebrospinal nerves take origin from cells located outside the cerebrospinal axis. This nu< leus lies in the lateral wall of the rostral portion of the fourth ventricle and in the lateral part of the gray matter surrounding the cerebral aqueduct (Figs. 114, 121, 130). The origin and termination of the afferent fibers for the extrin>ic Fig. 132.— Diagram to show the location of the secondary sensory tracts of the trigeminal nerve (solid black) in the tegmental portion of the rostral part of the pons: B.C., Brachium con- junct ivum; D. T.T.N. , dorsal secondary sensory tract of the trigeminal nerve; L.L., lateral lemnis- cus; M. L., medial lemniscus; M.L.F., medial longitudinal fasciculus; V.T.T.N., ventral secondary sensory tract of trigeminal nerve. muscles of the eye are unknown, although we know that such afferent fibers are present in the oculomotor, trochlear, and abducens nerves. SPECIAL SOMATIC AFFERENT NUCLEI The special somatic afferent nuclei are associated with the acoustic nerve, which is composed of two divisions. One part, the cochlear nerve, conveys im- pulses aroused by sound waves reaching the cochlea through the outer ear and tympanic cavity. Since it responds to stimuli from without, the cochlear apparatus subserves exteroceptive functions. The vestibular nerve, on the other hand, conveys impulses from the semicircular canals of the ear. These are im- portant proprioceptive sense organs and give information concerning the move- ments and posture of the head. The cochlear nuclei are the terminal nuclei of the cochlear nerve, the fibers of which take origin in the spiral ganglion of the cochlea. This is composed of bipolar cells, each having a short peripheral and a longer central process (Fig. 133). The peripheral process terminates in the spiral organ of Corti. The central process is directed toward the brain in the cochlear nerve. These central fibers terminate in two masses of gray matter, located on the restiform body near the point where the latter turns dorsally into the cerebellum (Figs. 107, 121, i86 THE NERVOUS SYSTEM 130). One of these masses, the dorsal cochlear nucleus, is placed on the dorso- lateral aspect of the restiform body and produces a prominent elevation on the surface of the brain (Fig. 91). The other, known as the ventral cochlear nucleus, is in contact with the ventrolateral aspect of the restiform body. Secondary Auditory Path. — From the cells of the ventral cochlear nucleus arise fibers which stream medialward in the ventral part of the pars dorsalis pontis and form the trapezoid body (Figs. 108, 134). The fibers cross the median plane and on reaching the lateral border of the opposite superior olivary nucleus turn rostrally as a compact bundle known as the lateral lemniscus (Figs. 110, Fig. 133. — Section of the spiral ganglion and organ of Corti of the mouse: A, Bipolar cells of the spiral ganglion; B, outer hair cells; C, sustentacular cells; D, terminal arborization of the peripheral branch of a bipolar cell about an inner hair cell; T, tectorial membrane. Golgi method. (Cajal.) 112, 114). Some of the fibers of the trapezoid body end in the superior olivary nuclei and in the nuclei of the trapezoid body, while others give off collaterals to these nuclear masses. Some of the fibers arising in these nuclei, especially in the nuclei of the trapezoid body, join in the formation of the lateral lemniscus; but according to Cajal (1909) a majority of the fibers from the superior olivary nucleus belong to short reflex pathways in the reticular formation connecting the cochlear nerve with the nuclei of the motor nerves of the head and neck. Fibers arising in the dorsal cochlear nucleus, and possibly also some from the ventral cochlear nucleus, sweep over the dorsal surface of the restiform body and the floor of the fourth ventricle as the strice medullares acusticce. These may THi: CRANIAL XKRVKS AM) TIIKIK \l ( l.l.l l8 7 lie just beneath the ependyma or may be buried in the gray matter of the rhom- boid fossa. On reaching the median plane the>e fibers decussate, ^ink into the reticular formation, and join the trapezoid body or lateral lemniscus of the opposite side. Some probably fail to cross, since clinical experience and evi- dence based on animal experiments tend to show that a part of the fibers in the lateral lemniscus represent an uncrossed path from the cochlear nuclei of the same side (Kreidl, 1914). Transverse temporal gyms Auditory radiation Medial geniculate body Inferior colliculus Lateral lemnisci Collaterals to nucleus of lateral lemniscus Rostral portion of the pons /Stria: medullares , Dorsal cochlear nucleus "Ventral coclilear nucleus Cochlear nerve > Vestibular nerve Caudal portion of the pons--\- Superior olive ■ Trapezoid body ' Nucleus of the trapezoid body Fig. 134. — Diagram of the auditory pathway. (Based on the researches of Cajal and Kreidl.) As the lateral lemniscus ascends in the reticular formation of the pons, there are scattered among its fibers many nerve-cells which together constitute the nucleus of the lateral lemniscus. To these cells it gives off collaterals and pos- sibly also terminal branches, and from them it is said to receive additional fibers. But according to Cajal the axons arising here do not ascend in the lateral lem- niscus, but are directed medially into the reticular formation. On reaching the mesencephalon the lateral lemniscus terminates in part in the inferior colliculus, but also sends branches and direct fibers by way of the inferior quadrigeminal brachium to the medial geniculate body. While the me- 1 88 THE NERVOUS SYSTEM dial geniculate body is a way-station on the auditory path to the cerebral cor- tex, the inferior colliculus serves as a center for reflexes in response to sound. The Vestibular Nuclei. — The fibers of the vestibular nerve take origin from the bipolar cells of the vestibular ganglion located in the internal auditory meatus (Fig. 135). The cochlear and vestibular divisions of the acoustic nerve sepa- rate at the ventral border of the restiform body. Here the vestibular nerve Fig. 135. — The vestibular ganglion and the termination of the peripheral branches of its bi- polar cells in a macula acustica: A, Hair cells and B, sustentacular cells of the macula; D, terminal arborization of the peripheral branches of the bipolar cells of the vestibular ganglion (G) about the hair cells of the macula; F, facial nerve; R, central branches of the bipolar cells directed toward the medulla oblongata T. Mouse. Golgi method. (Cajal.) penetrates into the brain, passing between the restiform body and the spinal tract of the trigeminal nerve toward the area acustica of the rhomboid fossa. Under cover of the area acustica the fibers divide into short ascending and longer descending branches (Figs. 134, 136). There may be enumerated five cellular masses within which these fibers terminate, namely: (1) the principal or medial nucleus, (2) the descending or spinal nucleus, (3) the superior nucleus THE CRANIAL NERVES AND IIIKIK NUCLEI of Bechterew, (4) the lateral amicus of Deiters, and (5) the cerebellum I 130, 136). The principal, medial, or dorsal vestibular nucleus is very large. 1 1 lies sub- jacent to the major portion of the area acustica and belongs, therefore, to both the pons and the medulla oblongata (Figs. 89, 103, 107). The -ray matter. associated with the descending branches from the vestibular nerve and lying on the medial side of the restiform body, constitutes the spinal or descending vestibular nucleus. Along with the descending fibers it can be followed in serial Nuc. of oculomotor nerve Nuc. of trochlear nerve — -^~- Brachium pontis^^ Xuc. of abducens nerve. Rhomboid fossa- Medulla oblongata. Superior colliculus Inferior colliculus Med. longitudinal fast ii ulus Superior vestibular I 'eslibuloccrcbcllar tract f— Lateral vestibular nuc. Vestibular nerve ' Spinal vestibular nuc. •Principal vestibular nucleus Fig. 136. — Diagram of the nuclei and central connections of the vestibular nerve. (Based on figures by Herrick and Weed.) sections as far as the rostral extremity of the nucleus gracilis. The lateral vestib- ular nucleus of Deiters is situated close to the restiform body at the point where the fibers of the vestibular nerve begin to diverge (Fig. 107). It is composed of large multipolar cells like those found in motor nuclei. Directly continuous with the medial and lateral nuclei is a mass of medium-sized cells, the superior vestib- ular nucleus of Bechterew, located in the floor and lateral wall of the fourth ventricle lateral to the abducens nucleus, and the emergent fibers of the facial nerve (Fig. 108). It extends as far rostrad as the caudal border of the main sensory nucleus of the trigeminal nerve (Weed, 1914). 190 THE NERVOUS SYSTEM Many of the ascending branches of the vestibular nerve, after giving off collaterals to the nuclei of Deiters and Bechterew, are prolonged in the tractus vestibulocerebellaris, to end in the cortex of the cerebellum (Cajal, 1909). These are joined by fibers arising in the superior and lateral vestibular nuclei which also run to the cerebellum (Fig. 136). From the standpoint of its embryologic development the cerebellum may properly be regarded as a highly specialized vestibular nucleus (p. 196). Secondary Vestibular Paths. — In addition to the fibers to the cerebellum mentioned in the preceding paragraph two important tracts of fibers take origin in the superior and lateral vestibular nuclei. One of these was encountered in the study of the medial longitudinal bundle. Cells in the superior and lateral vestibular nuclei give rise to fibers which run to the medial longitudinal fascicle of the same and of the opposite side, and through it reach the motor nuclei of the ocular muscles (Fig. 136). In this way there is established an arc, which makes possible the reflex response of the eye muscles to afferent impulses arising in the vestibule and semicircular canals of the ear. The other bundle was considered in connection with the spinal cord as the vestibulospinal tract, the fibers of which take origin from the cells of the lateral nucleus and descend into the anterior funiculus of the same side of the cord. These fibers serve to place the primary motor neurons of the spinal cord under the reflex control of the vestib- ular apparatus. From the medial border of the principal vestibular nucleus many scattered fibers cross the raphe and enter the reticular formation of the opposite side, where they become longitudinal fibers. No tract to the thalamus is known, a fact which is in keeping with this other, that ordinarily the activities of the vestib- ular apparatus are not clearly represented in consciousness. SUMMARY OF THE ORIGIN, COMPOSITION, AND CONNECTIONS OF THE CRANIAL NERVES The olfactory and optic nerves and the nervus terminalis, which have not yet been considered in detail, have been included in this summary for the sake of completeness. The nervus terminalis is a recently discovered nerve which arises from the cerebral hemisphere in the region of the medial olfactory tract or stria. It is closely associated with the olfactory nerve and its fibers run to the nasal septum. The origin, termination, and function of its component fibers are not yet under- stood (McKibben, 1911; Huber and Guild, 1913; McCotter, 1913; Johnston, THE CRANIAL NER\ ES \\n Mil 1 R \i . 1.1 [ 191 1914; Brookover, 1914, 1 ( >17; Larsell, 1918, 1919). Since it was unknown at the time the cranial uerves were first enumerated, it bears no numerical d nation. I. Olfactory Nerve. Superficial origin from the olfactory hull) in the form of a number of line fila which separately pass through the openings in the cribri- form plate. It is composed of special visceral afferent fibers with cells of origin in tin- olfactory mucous membrane. The fibers terminate in the glomeruli of the olfactory bulb. II. Optic Nerve.- Not a true nerve; but both from the standpoint of its structure and development a fiber tract of the brain. Superficial origin, from the optic chiasma. or after partial decussation, from the lateral geniculate body, pulvinar of the thalamus, and superior colliculus. Component fibers^, special somatic afferent — exteroceptive; origin, ganglion cells of the retina; terminations in the lateral geniculate body, pulvinar of the thalamus and superior colliculus. The fibers from the nasal half of each retina cross in the optic chiasma. 1 III. Oculomotor Nerve. — Superficial origin, from the oculomotor sulcus on the medial aspect of the cerebral peduncle. Composition: 1. Somatic Efferent Fibers. — Cells of origin, in the oculomotor nucleus of the same and to a less extent of the opposite side (Fig. 120). Termination, in the extrinsic muscles of the eye except the superior oblique and the lateral rectus. 2. General Visceral Efferent Fibers. — Cells of origin in the Edinger-Westphal nucleus. Termination in the ciliary ganglion, from the cells of which post- ganglionic fibers run to the intrinsic muscles of the eye. 2 IV. Trochlear Nerve. — Superficial origin, from the anterior medullary ve- lum. Composed of somatic efferent fibers; cells of origin in the trochlear nucleus; decussation in the anterior medullary velum; termination in the superior oblique muscle of the eye (Fig. 120). V. Trigeminal Nerve .—Superficial origin, from the lateral aspect of the middle of the pons by two roots: the portio major or sensory root and the portio minor or motor root. Composition (Fig. 120): 1. General Somatic Afferent Fibers. — A, Exteroceptive — Cells of origin in the semilunar ganglion (Gasserii), chiefly unipolar with T-shaped axons, peripheral 1 It has been demonstrated by Arey that there are also efferent fibers in the optic nerves of fishes which control the movement of the retinal elements in response to light, Jour. Com p. Xeur., vol. 26, p. 213. 2 It is probable that the oculomotor, trochlear, and abducens nerves contain proprioceptive fibers for the extrinsic muscles of the eye, but the cells of origin and the central connections of these sensory components are unknown. 192 THE NERVOUS SYSTEM branches to skin and mucous membrane of the head, central branches by way of the portio major to the brain. Termination in the main sensory nucleus and nucleus of the spinal tract of the trigeminal nerve. 2. General Somatic Afferent Fibers. — B, Proprioceptive — Cells of origin prob- ably located in the mesencephalic nucleus of the fifth nerve. Fibers by way of the portio major, distributed as sensory fibers to the muscles of mastication. 3. Special Visceral Efferent Fibers. — Cells of origin in the motor nucleus of the fifth nerve. Fibers by way of the portio minor and the mandibular nerve to the muscles of mastication. VI. Abducens Nerve. — Superficial origin, from the lower border of the pons just rostral to the pyramid. Composed of somatic efferent fibers; cells of origin in the abducens nucleus; termination in the lateral rectus muscle of the eye. VII. Facial Nerve and Nervus Intermedius. — Superficial origin from the lateral part of the lower border of the pons separated from the flocculus by the x eighth nerve. Composition (Fig. 120) : 1. Special Visceral Afferent Fibers. — Cells of origin in the ganglion geniculi, chiefly unipolar, with T-shaped axons. The peripheral branches run by way of the chorda tympani and lingual nerves to the taste buds of the anterior two- thirds of the tongue. The central branches run by way of the nervus intermedius to the tractus solitarius and end in the nucleus of that tract. It is probable that the taste fibers terminate in the rostral part of this nucleus. 1 2. General Visceral Efferent Fibers. — Cells of origin in the nucleus salivatorius superior. These fibers run by way of the- nervus intermedius, facial nerve, chorda tympani, and lingual nerve to the submaxillary ganglion for the in- nervation of the submaxillary and sublingual salivary glands. 3. Special Visceral Efferent Fibers. — Cells of origin in the motor nucleus of the facial nerve. These fibers run by way of the facial nerve to end in the super- ficial musculature of the face and scalp, and in the platysma, posterior belly of the digastric, and stylohyoid muscles. VIII. Acoustic Nerve. — Superficial origin from the lateral part of the lower border of the pons near the flocculus. Consists of two separate parts known as the vestibular and cochlear nerves. 1 Herrick (1918) describes general visceral afferent fibers in the facial nerve which he says mediate deep visceral sensibility and are probably found in all the branches of the facial. And Rhinehart (1918) has described a cutaneous branch of the facial in the mouse. This branch con- tains general somatic afferent fibers, which arise in the geniculate ganglion and terminate in the skin. I 111 CB \M M. M l^\ ES \\l» I III.IK \l I III The Vestibular Nerve. The componenl Gibers belong to the special som>^ Culmen r ■ , tar lobule { PosL p:irlillll 5SS=|^ fl^v Declhe > m °" l ' C " l " S Cerebellar hem i- w - - Primary fissure sphere superior j surface MMR \ \ Superior semi- lunar lobule Cerebellar folia^^^B^ Inferior semilunar lobule ' -Post clival sulcus nzontal cerebellar sulcus Folium of vermis Posterior cerebellar notch Fig. 138. — Dorsal view of the human cerebellum. (Modified from Sobotta-McMurrich.) incisures cerebelli. The anterior cerebellar notch (semilunar notch) is broad and deep; and as seen from above it is occupied by the brachia conjunctiva and the inferior colliculi of the corpora quadrigemina. The posterior cerebellar notch (marsupial notch) is smaller, and within it is lodged a fold of the dura mater, the falx cerebelli. The superior vermis is divided by transverse fissures into the following lobules (Fig. 138): 1. Lingula, closely applied to the anterior medullary velum between the two brachia conjunctiva. 2. Central lobule, associated with the small alae lobuli centralis of the hemi- sphere. 198 THE NERVOUS SYSTEM 3. Monticulus. which is further subdivided into the c id men and declive. The former goes over laterally without line of demarcation into the anterior portion of the quadrangular lobule, and the latter into the posterior portion of the same lobule in the hemisphere. 4. Folium vermis at the posterior extremity of the superior vermis. The rostral or dorsal surface of the hemisphere is subdivided by curved transverse fissures, which are continued across the vermis, into the following parts : 1. The anterior part of the quadrangular lobule, continuous with the oilmen monticuli of the vermis. 2. The posterior part of the quadrangular lobule, continuous with the declive monticuli. Nodule of vermis Flocculus Inferior n rmis Cerebellar hemisphere inferior surface** Tonsil - Bivenlral lobule Inferior semi- lunar lobule Horizontal cere- bellar sulcus Superior semilunar lobule Ui da of vermis ' / p oster ; or ' N Tuber of vermis Pyramid of vermis' cerebellar Folium of vermis notch Fig. 139.— Ventral view of the human cerebellum. (Sobotta-McMurrich.) 3. The superior semilunar lobule, occupying a large crescentic area along the dorsolateral border of the rostral surface. The inferior vermis (Fig. 139) is divided by transverse sulci into the follow- ing lobules : 1. The tuber vermis, next to the folium. 2. The pyramis. 3. The inula. 4. The nodidus. The caudal surface of the hemisphere presents the following subdivisions : 1. The inferior semilunar lobule, occupying a large part of this surface along its dorsolateral border. THE CEREBELLUM 199 2. The bivcnlral lobule, occupying the ventrolateral part of the inferior surface. 3. The tonsil, a small rounded lobule near the inferior vermis. 4. The flocculus is the smallest of the lobules; and from it there runs toward the median plane a thin white band, the posterior medullary velum, and the peduncle of the flocculus. Structure of the Cerebellum. — The cerebellum is composed of a thin super- ficial lamina of gray matter, spread over an irregular white center that con- tains several compact nuclear masses. This white medullary body forms a compact mass in the interior and is continuous from hemisphere to hemisphere through the vermis, within which, however, it is smaller than in the hemi- spheres (Figs. 140, 141). As is most readily seen in sagittal sections through the cerebellum, the medullary body gives off numerous thick laminae, which pro- Dentaie nucleus Central lobule Lingula Culmen Fissura prima / ^ — Declive Tuber Pyramis Nodule Uvula Fig. 140. Fig. 141. Figs. 140 and 141. — Sagittal sections of the human cerebellum: Fig. 140 passes through the hemisphere and dentate nucleus; Fig. 141, through the vermis in the median plane. ject into the lobules of the cerebellum; and from these there are given off sec- ondary and tertiary laminae at various angles. Thus a very irregular white mass is formed, over the surface of which the much folded cortex is spread in a thin but even layer. Supported by the white laminae, the cortex forms long narrow folds, known as folia, which are separated by sulci and which are aggre- gated into lobules that, in turn, are separated by more or less deep fissures. Sections through the cerebellum at right angles to the long axis of the folia thus present an arborescent appearance to which the name arbor -cita has been ap- plied. This is particularly evident in sections through the vermis (Fig. 141). MORPHOLOGY OF THE CEREBELLUM According to Elliott Smith (1903) and Bolk (1906). who have carried out extensive investigations on the morphology of the mammalian cerebellum, the fissura prima is an 200 THE NERVOUS SYSTEM important and constant fissure. It extends in a continuous curved line across the rostral : of the vermis and both hemispheres. It has been found by Ingvar (1918) in reptiles and birds. All investigators who have given attention to this subject in recent years agree in designating the portion of the cerebellum which lies rostral to the fissura prima as the anterior lobe. The portion behind this fissure is composed of several individual lobules, each of which, though subject to considerable variation in form in the different genera, can be identified in every mammalian cerebellum. These lobules have been variously grouped into lobes by different invest igators. Here we will follow the grouping employed by Ingvar, which is based on a comparison of the mammalian cerebellum with that of birds and reptiles (Fig. 142). lb- recognizes three major divisions of the cerebellum, which he designates as the anterior, middle, and posterior lobes. The middle lobe contains those parts of the cerebellum which have been the last to appear during phyletic development, and it is here that the greatesl variations are found in the different orders of mammals. 1. Fig. 142. — Schematic drawing of the cerebellum of 1, lizard; 2, crocodile; 3, bird, and 4, mammal. Vertical lines, anterior lobe; stipple, middle lobe; horizontal lines, posterior lobe; white, lobus ansoparamedianus. (Ingvar.) The anterior lobe includes all that part of the cerebellum that lies on the rostral side of the fissura prima (Figs. 143, 144, 146). In this lobe the folia have a transverse direction and extend without interruption across the vermis into both hemispheres. In the sheep the an- terior lobe is bounded laterally by the parafloccular fissure. It includes the three most rostral lobules of the superior vermis, which are designated in order from before backward, the lingula, lobulus centralis, and admen monlieuli. In man it also includes a large wing-shaped portion of each hemisphere (the pars anterior lobuli quadrangularis) ; and the entire lobe has the shape of a butterfly (Fig. 146). Morphologically, it is a median unpaired structure. The middle lobe is subdivided into four parts (Fig. 142). The most rostral of these is the lobulus simplex. It is separated from the anterior lobe by the fissura prima, and like that lobe it consists of transverse folia which extend across the superior vermis into both I HE CEREB1 l.l.i M 20I hemispheres ( Figs. 143, 144). In man the lobulus simplex forms a broad crescentii band across the rostral surface of the cerebellum, including what is ordinarily designated as the posterior part of the quadrangular lobule and the declive monticuli (Fig. 146 . Like the anterior lobe, ii is a median unpaired structure. The remainder of the middle lobe is sub- divided into median and lateral portions. The median part, known as the tuber :*r»iis Fissura prima Lobulus ansiformis Lobulus paramedianus *— LobuS anleriiir f t Lobulus simplex .- Parafloi < ulus Fissura Parafloct ularis —^Tuber vermis Fig. 143. — Cerebellum of the sheep, dorsorostral view. (lobulus medius medianus of Ingvar and lobulus C 2 of Bolk), forms a conspicuous S-shaped lobule in I he vermis of the sheep (Fig. 145) and may be readily identified at the occipital extremity of the inferior vermis in man (Figs. 139, 141). The paired lateral portions of the middle lobe each consist of two parts, called the lobulus ansiformis and lobulus paramedianus. The lobulus ansiformis, relatively small in most mammals (Fig. 144), is very large in man, Fissura prima i , Lobus anterior ,Lobulus simplex Flocculus-" ~\lfS Parajlocculus *" Lobulus paramedianus '' Lobulus ansiformis -~ a " Tuber vermis Lobulus medianus posterior Fig. 144.— Cerebellum of the sheep, lateral view. and forms approximately the dorsolateral half of the hemisphere, occupying considerable parts of both the rostral and caudal surfaces. It corresponds to what has been known as the superior and inferior semilunar lobules and the biventral lobule (Figs. 146, 147). The lobulus paramedianus, or tonsilla of the B. N. A., is located on the lateral surface of the sheep's cerebellum, but is displaced on to the caudal surface in man by the great expansion of the lobulus ansiformis. 202 THE NERVOUS SYSTEM The posterior lobe, as outlined by Ingvar, is composed of median and lateral portions. The median part, known as the posterior median lobule, comprises all of the inferior vermis except the tuber, from which it is separated by the prepyramidal sulcus. It is subdivided into three sublobules, known as the nodule, uvula, and pyramid (Figs. 139, 141, 145). The lateral part of the posterior lobe is formed on either side by two lobules, known as the flocculus and paraflocculus. These form the most lateral portion of the hemisphere in most mammals (Figs. 142, 144). In man the paraflocculus is rudimentary and the flocculus lies upon the caudal surface of the hemispheres (Fig. 147). It is connected with the nodule by a thin sheet of white matter, the posterior medullary velum. Functional Localization in the Cerebellum. — We have described the cerebellum in terms of the subdivisions of Bolk and Ingvar, because these have morphologic and physio- logic significance, which is not true of the parts into which the cerebellum had previously been divided. By comparison of the size of these subdivisions with the degree of develop- ment and functional importance of the various groups of muscles in different animals Bolk endeavored to show that each of these parts was related to a particular group of muscles. On the basis of these comparative studies he concluded that the median unpaired portions of the cerebellum serve as coordination centers for the muscles which function in bilateral Tuber vermis Prepyramidal sulcus v ^--j^k y^^ ^c^^ ,tui •/ tJ ^jC- r~*J?ix ■- " x§c\ -"' Lobulus ansijormis Paraflocculus-- --^a^ ^^%^C^^^-~^^rY0r^^-^t — Lobulus paramedianus ~" Lob id us medianus posterior / Fig. 145. — Cerebellum of the sheep, caudal view. synergy. The muscles of expression and mastication, those of the eyes, pharynx, larynx and neck, and many of the trunk muscles are called into action simultaneously on both sides of the body, and should, according to this theory, have a median unpaired representation in the cerebellum. Bolk located the coordination center for the musculature of the head in the anterior lobe, that for the muscles of the neck in the lobulus simplex (Figs. 146, 147). A median center for those movements of the extremities which are strictly bilateral is found in the most dorsal sublobule of the vermis inferior, known as lobulus C" 2 or tuber vermis. The remainder of the inferior vermis forms, according to this theory, a center for the bilateral movements of the trunk. In addition to a median center in the tuber vermis, the limbs are represented in the cerebellum by lateral centers for the coordination of unilateral move- ments. The lateral center for the arm is located in the rostral part or crus primum of the lobulus ansiformis (superior and inferior semilunar lobules) and that for the legs in the caudal part or crus secundum (biventral lobule), and perhaps also in the lobulus paramedianus (tonsil). The conclusions concerning the localization of function in the cerebellum, reached by Bolk on the basis of morphologic studies, have been confirmed in so far as the centers for the neck and extremities are concerned by animal experimentation (Van Rynberk, 1908, 1912; Till. < I REBELLUM 203 Andre Thomas and Durupt, 1914) and by clinical observations (B&rany, 1'>1_'). There are, however, good reasons for skepticism regarding his localization ol centers for the head and trunk, [ngvar (1918) presents evidence which indicates thai the anterior and posterior lobes arc probably concerned with the maintenance of the equilibrium of the bod) as a whole. I he middle lobe, on t he ol her hand, contains a number of separate < enters, wbi< h correspond to those out lined by Hoik, for t he eont rol of i he rnus< ulat ure of the neck and extremit It has long been known thai the degree of development of the cerebellar hemispheres in the different classes of vertebrates i-* closely < orrelated with that of the pons and c erebral 1 ortex. Tin's is particularly true of the lobulus ansiformis and lobulus paramedianus, win. h, like the aeopallium, are recent phyletic developments. These belong to what Edinger (1911) calls /: N. A. Ala liilmli centralis Lobulus centralis Culmen monticuli J'ars anterior lobuli quadrangularis Pars posterior lobuli quadrangularis Declive montu uli Lobulus semilunaris superior Fig. 146. Lobulus centralis A 'a lobuli centra' is Brae liiuni pontis Flocculus Brae hi urn conjunct ivum Nodulus I 'vula 'J' on si' I a Lobulus biventer Pyramis Tuber Lob. semilun. inf. Sulcus horizontalis Lobulus semilunaris superior Figs. 146 and 147. — Outline drawings of the human function according to the theory of Bolk. On the right to Bolk's terminology, on the left according to the B. X ventral view. (Herrick.) BOLR Lobui anterior Sulcus pri mar ius Lobulus simplex S. postt livalis Lobulus ansiformis f.obus anterior Cerebellar peduncles (cut) Flocculus Sulcus uvulo-nodularis Lobulus paramedianus Fissura sccunda Lobulus ansiformis cerebellum showing the localization of side the parts are designated according A. Fig. 146, dorsal view. Fig. 147, the neocerebellum, receive the majority of the fibers from the brachium pontis, and may properly be regarded as cortical dependencies. They take an important part in the co- ordination of the voluntary movements of the extremities. THE NUCLEI OF THE CEREBELLUM The dentate nucleus is a crumpled, purse-like lamina of gray matter within the massive medullary body of each cerebellar hemisphere (Fig. 148). Like the inferior olivary nucleus, which it closely resembles, it has a white center and a medially placed hilus. In close relation to this hilus lies a plate of gray matter, the cmboliform nucleus, and medial to this is the small globose nucleus. 204 THE NERVOUS SYSTEM Close to the median plane in the medullary body of the vermis, where this forms the tent-like covering of the fourth ventricle, is the nucleus of the roof or nucleus fastigii. The dentate nucleus is well developed only in those animals which possess large cerebellar hemispheres. It receives fibers from the cortex of the cere- bellar hemisphere, while the nuclei fastigii and globosi receive fibers chiefly from the vermis (Clark and Horsley, 1905; Edinger, 1911). It is probable that Rhomboid fossa Decussation of brachia conjunctiva --,.-• Medial longitudinal fasciculus" / A ^rior medullary velum Brachium conjunctivum s Molecular Granular layer Lingula of cerebellum Fastigial nucleus llilus of dentate nucleus Dentate nucleus Medullary la miner' \fr^ <\/t\K£ Cerebellar folia^'-*^ .. Medullary substance of -•' hemisphere Emboliform nucleu Globose nucleus Vermis Capsule of dentate nucleus Posterior cerebellar notch Fig. 148. — Horizontal section through the cerebellum showing the location of the central nuclei. (Sobotta-McMurrich.) a functional localization similar to that in the cerebellar cortex will be found to exist in the central nuclei. In histologic structure the central nuclei closely resemble the inferior olive. THE CEREBELLAR PEDUNCLES The white core of the cerebellum is formed in large part of fibers which enter and leave the cerebellum through its three peduncles. The brachium pontis, or middle cerebellar peduncle, is formed by the trans- verse fibers of the pons and carries impulses which come from the cerebral cortex of the opposite side. It enters the cerebellum on the lateral side of the other two, and is distributed in two great bundles: one from the rostral part of the pons radiates to the caudal part of the cerebellar hemisphere; the other, from the caudal part of the pons, spreads out to the rostral portion of the hemisphere. In man, as might be expected from the large size of the pons and cerebellar T1IK CKRKHKLLUM 205 hemispheres, the brachium pontis is the largesl of the three peduncles (Fig. 89). lUit this is not true in most mammals, where, as in the sheep, the cere- bellum receives the majority of its afferent fibers from the spinal cord and medulla oblongata by way of the relatively large restiform bodies (Fig. 91). The restiform body ascends along the lateral border of the fourth ventricle; and at a point jusl rostral to the lateral recess it makes a sharp turn dorsally to enter the cerebellum between the other two peduncles (Figs. 87, 88). It consists of ascending fibers from the spinal cord and medulla oblongata and prob- ably also of descending fibers from the cerebellum to the reticular formation of the medulla ( fastigiobulbar tract, p. 211). Among the ascending fibers are those of the following bundles: (1) dorsal spinocerebellar tract, which arises Res Dorsal spinoco Ventral spinocc Tcctoccrebellar trad ' -/ Corpora quadrigemina ■ Brachium con junctivum Pons Fig. 149. — Diagram of the spinocerebellar and tectocerebellar tracts. from the cells of the nucleus dorsalis of the same side of the spinal cord and ends in the cortex of the vermis; (2) the olivocerebellar tract, which consists of fibers from the opposite inferior olivary nucleus and to a less extent from that of the same side and which ends in the cortex of the vermis and of the hemi- sphere and in the central nuclei; (3) the dorsal external arcuate fibers, from the nuclei of the posterior funiculi of the same side; (4) ventral external arcuate fibers from the arcuate and lateral reticular nuclei (Fig. 104). The so-called medial part of the restiform body consists of bundles of fibers belonging to the tractus nucleocercbcllaris, which course along the medial side of that peduncle as it turns dorsally into the cerebellum (Fig. 110). These come from the sensory nuclei of the cranial nerves. Most of them arise from the superior and lateral vestibular nuclei or represent the ascending branches of the 206 THE NERVOUS SYSTEM fibers of the vestibular nerve and constitute the tractus vestibulocerebellaris. According to Cajal (1911) the fibers of this tract are distributed to the cortex of the cerebellum, the majority of them going to the vermis, a smaller proportion to hemisphere. In view of the newer ideas concerning the morphology of the cerebellum, the statements concerning the termination of all these cerebellar afferent fibers require re-examination. The brachium conjunctivum (Fig. 88) consists of efferent fibers from the dentate nucleus to the red nucleus and the thalamus of the opposite side. It is the smallest and most medial of the three peduncles. The ventral spinocere- bellar tract enters the cerebellum in company with the brachium conjunctivum. It ascends through the medulla oblongata and pons, curves over the brachium conjunctivum (Fig. 110), and enters the anterior medullary velum, within which it runs to the cerebellum (Fig. 149). Its fibers terminate in the rostral part of the vermis and in the nucleus fastigii (Fforrax, 1915). According to Edinger, a bundle of fibers, the tcctocerebellar tract, arises in the tectum of the mesencepha- lon and descends alongside of the brachium conjunctivum to the cerebellum, probably conveying impulses from visual centers. According to MacNalty and Horsley (1909) and Ingvar (1918) the fibers of the ventral spinocerebellar tract end in the lobulus centralis, culmen, and most rostral part of the declive. The fibers of the dorsal spinocerebellar tract have the same termination and, in addition, many of them go to the pyramis, and smaller numbers to the uvula and nodule. Practically all of the fibers which end in the cortex, therefore, go to the anterior and posterior lobes (Ingvar). The fact that the anterior lobe receives the majority of these fibers, which convey proprioceptive impulses from the trunk and extremities, is a strong argument against Bolk's conception of the anterior lobe as a co-ordination center for the musculature of the head. HISTOLOGY OF THE CEREBELLAR CORTEX The cerebellar cortex differs from that of the cerebral hemispheres in pos- sessing essentially the same structure in all the lobules. This would indicate that it functions in essentially the same way throughout, though as a result of different fiber connections the various lobules act on different muscle groups. A section through the cerebellum, taken at right angles to the long axis of the folia, shows each folium to be composed of a central white lamina, covered by a layer of gray cortex. Within the white lamina the nerve-fibers are arranged in parallel bundles extending from the medullary center of the cerebellum into the lobules and folia. A few at a time these bundles turn off obliquely into the gray matter, and there is no sharp demarcation between the cortex and the sub- jacent white lamina. The cortex presents for examination three well-defined THE CEREBELLUM 207 zones: a superficial molecular layer, a layer of Purkinje cells, and a subjacent granular layer. The cells of Purkinje have large flask-shaped bodies and are arranged in an almost continuous sheet, consisting of a single layer of cells and separating the other two cortical zones (Fig. 150). They are more numerous at the summit than at the base of the folium. Each has a pyriform cell body. The part directed toward the surface of the cortex resembles the neck of a flask and from Fig. 150. — Semidiagrammatic transverse section through a folium of the cerebellum. (Golgi method): A, Molecular layer; B, granular layer; C, white matter; a, Purkinje cell; b, basket cells; d, pericellular baskets, surrounding the Purkinje cells and formed by the arborizatiens of the axons of the basket cells; e, superficial stellate cells;/, cell of Golgi Type II; g, granules, whose axons enter the molecular layer and bifurcate at i\ h, mossy fibers;.; and m, neuroglia; n, climb- ing fibers. (Cajal.) it spring one or two stout dendrites. These run into the molecular layer and extend throughout its entire thickness, branching repeatedly. This branching occurs in a plane at right angles to the long axis of the folium; and it is only in sections, taken in this plane, that the full extent of the branching can be ob- served. In a plane corresponding to the long axis of the folium the dendrites occupy a more restricted area (Fig. 151). In this respect the dendritic ramifica- tions resemble the branches of a vine on a trellis. From the larger end of the cell, directed away from the surface of the cortex, there arises an axon which 208 THE NERVOUS SYSTEM almost at once becomes myelinated and runs through the granular layer into the white substance of the cerebellum. According to Clarke and Horsley (1905) and Cajal (1911) these axons end in the central cerebellar nuclei. Near their origin they give off collaterals, which run backward through the molecular layer to end in connection with neighboring Purkinje cells — an arrangement designed to bring about the simultaneous discharge of a whole group of such neurons. The granular layer, situated immediately subjacent to that which we have just described, is characterized by the presence of great numbers of small neurons, the granule cells. Each of these contains a relatively large nucleus, surrounded by a small amount of cytoplasm; and from each there are given off from three to five short dendritic branches with claw-like endings. These are synaptically related with the terminal branches of the moss fibers, soon to be described, and Purkinje cell" Basket cell" Granule cell" "Purkinje cell "~ Granule cell Fig. 151. — Diagrammatic representation of the structure of the cerebellar cortex as seen in a section along the axis of the folium (on the right), and in a section at right angles to the axis of the folium (on the left). form with them small glomeruli comparable to those of the olfactory bulb (Fig. 208). Each granule cell gives origin to an unmyelinated axon, which extends toward the surface of the folium and enters the molecular layer. Here it divides in the manner of a T into two branches. These run parallel to the long axis of the folium through layer after layer of the dendritic expansions of the Purkinje cells, with which they doubtless establish synaptic relations (Fig. 151). Besides the granules just described, this layer contains some large cells of Golgi's Type II (Fig. 150, /). Most of these are placed near the line of Purkinje cells and send their dendrites into the molecular layer, while their short axons resolve themselves into plexuses of fine branches in the granular zone. The molecular layer contains few nerve-cells and has in transverse sections a finely punctate appearance. It is composed in large part of the dendritic ramifications of the Purkinje cells and the branches of axons from the granule THE CEREBELLUM 209 cells (Fig. 150). It contains a relatively small number of stellate neurons, the more superficial of which possess short axons and belong to Golgi's Type II. Those more deeply situated have a highly specialized form and arc known as basket cells. From each of these there arises, in addition to several stout brandl- ing dendrites, a single characteristic axon, which runs through the molecular layer in a plane at right angles to the long axis of the folium (Fig. 151). These axons are at first very fine, but soon become coarse and irregular, giving off numerous collaterals which are directed away from the surface of the cortex. These collaterals and the terminal branches of the axons run toward the Purkinje cells, about which their terminal arborizations form basket-like networks (Fig. 29). Purkinje < Dentate nucleus Brachium conjunc- tivum Brachium pontis Restiform body Climbing fibers'' Mossy fibers - Basket cell " Granule cell Fig. 152. — Diagram to illustrate the probable lines of conduction through the cerebellum. Nerve-fibers. — The axons of the Purkinje cells form a considerable volume of fibers directed away from the cortex. There are also two kinds of afferent fibers which enter the cortex from the white center, and are known as climbing and mossy fibers respectively. The latter are very coarse and give off numerous branches ending within the granular layer. The terminal branches are provided with characteristic moss-like appendages. These mossy tufts are intimately related to the claw-like dendritic ramifications of the granule cells (Fig. 152). The climbing fibers, somewhat finer than those of the preceding group, pass through the molecular layer and become associated with the dendrites of the Purkinje cells in the manner of a climbing vine. Branching repeatedly, they 2IO THE NERVOUS SYSTEM follow closely the dendritic ramifications of these neurons and terminate in free varicose endings. It would seem reasonable to suppose that the two kinds of afferent fibers, just described, have a separate origin and functional significance. According to Cajal (1911) it is probable that those entering the cerebellum through the brachium pontis are distributed as climbing fibers, and those from the restiform body as mossy fibers. The accompanying diagram represents the probable course of impulses through the cerebellum (Fig. 152). The mossy fibers, prob- ably derived from the restiform body, transfer their impulses to the granule cells; and these, in turn, relay them, either directly or through the basket neu- rons, to the Purkinje cells. The climbing fibers, which probably come from the brachium pontis, transfer their impulses directly to the dendrites of the Purk- inje cells. We do not known to which class the fibers of the vestibulocerebellar tract should be assigned. The efferent path may be said to begin with the Purkinje cells, whose axons terminate in the central cerebellar nuclei. From these nuclei, especially the dentate, arise the fibers of the brachium conjunc- tivum, the great efferent tract from the cerebellum. By means of the axons of the granule cells, basket cells, and neurons of Golgi's Type II, as well as by the collaterals from the axons of the Purkinje cells, an incoming impulse may be diffused through the cortex. The cerebellum probably receives fibers from all the somatic sensory centers, but especially from those o* the pioprioceptive group, through which afferent impulses are err eyed to it from the muscles, joints and tendons, and from the semicircular canals of the ear. Its connection with the vestibular appa- ratus is especially intimate. In fact, as already stated, it may be regarded from the standpoint of development as a very highly specialized portion of the ves- tibular nucleus. It is the great proprioceptive correlation center. Further- more, it sends efferent impulses to the various somatic motor centers and plays an important part in the coordination of muscular contraction and in the main- tenance of muscular tone. It is the chief center for equilibration, which depends upon the proper adjustment of the muscles in response, very largely, to the impulses from the semicircular canals. In man and mammals it also receives impulses from the cerebral cortex by way of the pons, which probably set the coordinating cerebellar mechanism into activity to bring about the proper adjustment of voluntary movements. For additional details concerning the functions of the cerebellum the reader should consult the recent paper by Holmes (1917). THE CKKKHKLLl.'M 211 THE EFFERENT CEREBELLAR TRACTS The efferent cerebellar tracts arise in the central nuclei. It is probable that no fibers of cortical origin leave the cerebellum except, perhaps, some to Deiter's nucleus (Clarke and Horsley, 1905). The brachium conjunctivum, or tractus cerebellotegmentalis mesencephali, arises for the most part at least in the dentate nucleus and terminates in the red nucleus and thalamus of the opposite side (Fig. 153). It constitutes the chief tract leading from the cerebellum and has been more fully described on page 159. It undergoes a complete decussation beneath the inferior colliculus in the tegmentum of the mesencephalon. Both before and after this crossing its Brachium conj Fastigiobulbar tract Thalamus Red nucleus y Nucleus fastigii - Nucleus dentatus --Tractus cerebellotegmentalis pontis "' 'Lateral vestibular nucleus a ..'.' ■'obulbar tract Fig. 153. — Efferent tracts which arise in the central nuclei of the cerebellum. (Modified from Edinger.) fibers give off branches, which descend in the reticular formation of the pons and medulla. Some of the impulses reach the thalamus, but the others are relayed in the red nucleus along the rubrospinal and rubroreticular tracts to motor neurons in the brain stem and spinal cord (Fig. 115). Other efferent tracts arise in the nucleus fastigii of the same and opposite side, and run, probably by way of all three cerebellar peduncles, to the retic- ular formation of the pons and medulla oblongata. One bundle of these fibers winds around the brachium conjunctivum before descending through the pons and medulla (Fig. 153). It is probable that other fibers descend by way of the restiform body, and are distributed in the reticular formation of the medulla 212 THE NERVOUS SYSTEM oblongata on the same side, or are continued as ventral external arcuate fibers to end on the opposite side. The bundles which run from the nucleus fastigii to the medulla oblongata may be designated as the fastigiobulbar tracts (tractus cerebellotegmentales bulbi). These include fibers which terminate in the lateral vestibular nucleus. It is said that some fibers belonging to this system leave the cerebellum by way of the brachium pontis (tractus cerebellotegmentalis pontis) . Since the dentate nucleus receives fibers from the cortex of the correspond- ing cerebellar hemisphere, and the nucleus fastigii receives similar fibers from the vermis, it may be inferred that the brachium conjunctivum is the chief efferent tract for the hemisphere and that the fastigiobulbar tracts serve the same purpose for the vermis (Strong, 1915). CHAPTER XIV THE DIENCEPHALON AND THE OPTIC NERVE Development. — In an earlier chapter we traced briefly the development of the prosencephalon and showed that the cerebral hemispheres were developed through the evagination of the lateral walls of the telencephalon (Fig. 16). It is, however, only the alar lamina which is involved in this evagination. The basal lamina of the telencephalon retains its primitive position and forms the pars optica hypothalami. This part of the hypothalamus, along with the lamina tcrminalis and the most rostral part of the third ventricle, constitutes the telencephalon medium (Johnston, 1912). Through the excessive growth of the hemisphere the diencephalon becomes covered from view (Fig. 17), and appears to occupy a central position in the adult human brain. It is separated from the hemisphere by the transverse cerebral fissure, which is formed by the folding back of the hemisphere over the diencephalon. The differentiation of the alar lamina of the diencephalon into the thalamus, epithalamus, and meta- t ha lam us, and of its basal lamina into the hypothalamus was briefly traced on page 34. The sulcus limitans, which separates these two plates in the embryo, corresponds to the more caudal portion of the hypothalamic sulcus of the adult; but, since the latter can be followed to the interventricular foramen, while the former ends near the optic chiasma, the rostral ends of these two sulci are not related. The roof plate of the prosencephalon remains thin and constitutes the epithelial roof of the third ventricle, which along the median plane becomes invaginated into the ventricle as the covering of a vascular network to form the chorioid plexus. THE THALAMUS The thalamus is a large ovoid mass, consisting chiefly of gray matter, placed obliquely across the rostral end of the cerebral peduncle (Figs. 154, 155). Be- tween the two thalami a deep median cleft is formed by the third ventricle. The rostral end is small and lies close to the median plane. It projects slightly above the rest of the dorsal surface, forming the anterior tubercle of the thalamus, and helps to bound the interventricular foramen (Fig. 154). The caudal ex- tremity is larger and is separated from its fellow by a wide interval, in which the 214 THE NERVOUS SYSTEM corpora quadrigemina appear. It forms a marked projection, the pulvinar, which overhangs the medial geniculate body and the brachia of the corpora quadrigemina (Figs. 88, 154). For purposes of description it is convenient to recognize four thalamic surfaces, namely, dorsal, ventral, medial, and lateral. The dorsal surface of the thalamus is free (Figs. 91, 154). It forms the floor of the transverse fissure of the cerebrum and is separated by this fissure from the parts of the cerebral hemisphere which overlie it, that is, from the Free portions of columns of fornix. Head of caudate nucleus^ Medullary stria, s '"• Third ventricle > Habenular trigone v \ Pineal body~j Superior colliculus \] Tail of caudate nucleus- Super, quadrigeminal brack. Infer, quadrigeminal brack. Cerebral peduncle Corpora quadrigemina Lateral filaments of pons Anterior medullary velum Lingula of cerebellum Tela chorioidea of fourth ventricle Corpus callosum Lamina of septum pdlucidum / Columns of fornix y A ntcrior commissure ' y Optic recess of ventricle III ■' •'« - A ntcrior tubercle of thalamus '' y Terminal stria s ^,'Tcenia chorioidea ■ Habenular commissure Lamina affixa Superior quadrigeminal brachium .-'Pulvinar of thalamus ^Lateral geniculate body Medial geniculate body Inferior colliculus Trochlear nerve .Brachium conjuctivum Lateral recess of fourth ventricle ' Brachium pontis .Peduncle of flocculus Flocculus of cerebellum Lateral aperture of ventricle IV i Chorioid plexus of ventricle IV (Rhomboid fossa (intermediate portion) Medial aperture of ventricle IV Funiculus gracilis Medulla oblongata Fig. 154.— Dorsal view of the human brain stem. (Sobotta-McMurrich.) fornix and corpus callosum. Laterally it is bounded by a groove, which separates it from the caudate nucleus and contains a strand of longitudinal fibers, the stria terminalis and a vein, the vena terminalis (Figs. 154, 155). The dorsal surface is separated from the medial by a sharp ridge, the tcenia thalami, which represents the torn edge of the ependymal roof of the third ventricle. The taeniae of the two sides meet in the stalk of the pineal body. The prominence of this torn edge of the roof is increased by a longitudinal bundle of fibers, THE DIENCEPHALON AND Till! (tl'TIC NERVE 2I 5 the stria medullaris thalami. This fascicle, together with the closely related habenular trigone and the pineal body, belong to the epithalamus and will be described later. 'The dorsal surface of the thalamus is slightly convex and is divided by a faint groove into two parts: a lateral area, covered by the lamina affixa and forming a part of the floor of the lateral ventricle; and a larger medial area, which forms the floor of the transverse fissure of the cerebrum. The oblique groove separat- ing these two areas corresponds to the lateral border of the fornix (Figs. 154, 155). The lamina affixa is part of the ependymal lining of the lateral ventricle superim- Fornix t , Transverse fissure of the cerebrum Stratum zonale Chorioid plexus of lateral ventricle Lamina affixa Internal medullary lamina Chorioid plexus of third ventricle Third ventricle Lenticular nucleus Stria medullaris ■, Corpus callosum Lateral ventricle Internal capsule Hypothalamic nucleus Caudate nucleus Stria tcrminalis and vena ter- minalis _ External medull- ary lamina --^Anterior nucleus of thalamus «§»-, ^Lateral nucleus of thalamus - Medial nucleus of thalamus Red nucleus' ■ Substantia nigra •' Optic tract Basis pedunculi Fig. 155. — Diagrammatic frontal section through the human thalamus and the structures which immediately surround it. posed upon this part of the thalamus. It is not present in the sheep, where the fornix is larger and the entire dorsal surface of the thalamus belongs to the floor of the transverse fissure. These features are well illustrated in Figs. 179 and 180, as is also the position of the transverse fissure. This fissure intervenes be- tween the thalamus and the cerebral hemisphere, and contains a fold of pia mater, known as the tela chorioidea, of the third ventricle. The medial surface of the thalamus forms the lateral wall of the third ven- tricle (Figs. 158, 159). It is covered by the ependymal lining of that cavity. The medial surfaces of the two thalami are closely approximated, being separated 2i6 I HE NERVOUS SYSTEM from each other by the cleft-like space of the third ventricle, and are united across the median plane by a short bar of gray substance, the massa intermedia. The lateral surface is hidden from view. It lies against the broad band of fibers, known as the internal capsule, which connects the cerebral hemispheres with the lower levels of the central nervous system. This surface is best examined in sections through the entire cerebrum (Figs. 155-157). Many fibers stream out of the thalamus through its lateral surface and enter the internal capsule, through which they reach the cerebral cortex. To this important stream of fibers the name thalamic radiation is applied. The ventral surface of the thalamus is also covered from view and lies on the hypothalamus, by which it is separated from the tegmentum of the mesencepha- lon ( lugs. 155, 157). Many fibers, representing such ascending tegmental paths as the medial lemniscus, spinothalamic tract, and brachium conjunctivum, enter the thalamus through this surface. Structure of the Thalamus. — The thalamus consists chiefly of gray matter, within which there may be recognized a number of nuclear masses. Its dorsal surface is covered by a thin layer of white matter, called the stratum zonale, which in the region of the pulvinar consists in large part of fibers derived from the optic tract. On the lateral surface of the thalamus next the internal cap- sule there are many myelinated fibers, which constitute the external medullary lamina (Figs. 155, 156). The medial surface is covered by a layer of central gray matter, continuous with that which lines the cerebral aqueduct and forms the floor of the third ventricle. This central gray matter consists of neuroglia and of scattered nerve-fibers and cells (the nucleus paramedianus of Malone, 1910). Some of these fibers are continued through the gray matter that lines the aqueduct and the floor of the fourth ventricle, as the dorsal longitudinal bundle of Schutz (Fig. 112). It is probable that this portion of the thalamus forms a center for vasomotor and visceral reflexes, since lesions in this region are often accompanied by disturbances in the nervous control of the blood- vessels and viscera (Edinger, 1911; Rogers, 1916). If this be true, it is probable that the dorsal longitudinal bundle of Schutz serves to bring this thalamic mechanism for visceral adjustments into connection with the visceral efferent nuclei of the brain. From the stratum zonale, which clothes its dorsal surface, there penetrates into the thalamus a vertical plate of white matter, called the internal medullary lamina. This subdivides the thalamus into three parts: the anterior, medial, and lateral nuclei. At the rostral extremity of its dorsal border the internal THE DIENCEPHALON AND nil: OPTIC NERVE 217 medullary lamina bifurcates and includes between its two Limbs the anterior nucleus. The anterior nucleus (or dorsal nucleus) of the thalamus is located in the dorsal pari of the rostral extremity of the thalamus and penetrates like a wedge between the medial and lateral nuclei. It protrudes somewhat above the genera] level of the dorsal surface, forming the anterior tubercle of the thalamus. It receives a large bundle of fibers from the mammillary body, the mamillotha- lamic tract or bundle of Vicq d'Azyr (Figs. 156, 204, 205), and sends fibers to the caudate nucleus of the corpus striatum (Fig. 196). Taenia ttcta Stria* La Corpui callosum Ventrtiulus literati's Strat. tubffenjymalt Fasc. frontcoccipitaUt Nucleus (nittt \xtus Nucleus anterior t/ialami Nucleus tat.-ratis Ihalnmi Mass.i fmtrrmedi Corpus subthalatu. Substantia nigra Fig. 156. — Frontal section through the mammillary body, thalamus, and adjacent structures. Weigert method. (Villiger-Piersol.) The medial nucleus of the thalamus is situated between the central gray- matter of the third ventricle and the internal medullary lamina, which separates it from the lateral nucleus except in the caudal part, where the line of separation between the two is not distinct. It is said to receive fibers from the olfactory centers and to send fibers to the caudate nucleus and the subthalamus. The lateral nucleus of the thalamus is by far the largest of the three. It extends farther caudad than the medial nucleus and includes all of the pulvinar. Through the external medullary lamina and the internal capsule it sends fibers to the cerebral cortex in the thalamic radiation and receives corticothalamic 2l8 THE NERVOUS SYSTEM fibers in return. Especially in its ventral subdivision it receives all of the as- cending sensory tracts from the tegmentum of the mesencephalon, as well as libers from the brachium conjunctivum and red nucleus. It is much more richly supplied throughout with myelinated fibers than are the other nuclei of the thala- mus. The lateral nucleus is subdivided into a dorsal portion, the lateral nucleus proper, and a ventral part, better known as the ventral nucleus of the thalamus. Within the latter are two well-defined nuclear masses. The more medial of "• VtuMculm lateralis Taenia semicircula. Nucleus caudatus Nucleus anterior thalami 'JgW '^k Nucleus lateralis thalami %SS?T S*" Nucleus medialis thalami Cor/us gemcultltunt latere, Gyrus dentatus Centrum mediant Luys Nucleus semilunaris ( I'lechsig) Zcna lateralis - Wernicke Nucleus ruber Cornu Ammont* Fig. 157.-Frontal section through the human pons, basis pedunculi, thalamus and adjacent structures. Weigert method. (Villiger-Piersol.) the two is known as the nucleus centralis (nucleus globosus or centrum media- num) and is surrounded by a well-defined capsule of myelinated fibers (Fig. 157). Ventrolateral to this is the well-defined nucleus arcuatus, which because of its shape is also called the nucleus semilunaris. The pulvinar is a very large mass which forms the most caudal part of the thalamus and is usually considered as a part of the lateral nucleus. Function.— The medial and anterior thalamic nuclei are closely associated in function and from a phylogenetic point of view represent the older part of the thalamus. They serve as centers for the more primitive thalamic correlations THE DIEXCEPHALON AXD THE OPTIC NERVE 2IO, such as occur in lower vertebrates that lack the cerebral cortex (Herrick, 1917) Both receive fibers from the olfactory centers and both send fibers to the corpus striatum, but none to the cerebral cortex (Sachs, 1909). There is some evidence of a clinical nature to show that the activity of these centers may be accompanied by a crude form of consciousness (Head and Holmes, 1911; Head. 1918). Pa- tients in whom the paths from the thalamus to the cortex have been interrupted are aware of many sensations, but cannot discriminate among them. The thalamus seems to be the chief center for the perception of pain and the affec- tive qualities of other sensations, and in this respect it plays an important role in consciousness independently of the cerebral cortex. The more lateral group of centers, which includes the lateral nucleus of the thalamus, the pulvinar, and the geniculate bodies, is of more recent origin and has been called the neothalamus. They serve as relay stations on the somatic sensory paths to the cerebral cortex. The medial lemniscus and spinothalamic tracts terminate in the ventral subdivision of the lateral nucleus. In the pul- vinar and lateral geniculate body terminate fibers from the optic tracts, while the lateral lemniscus ends in the medial geniculate body. From these nuclei sensory fibers of the third order run to the cerebral cortex. The lateral nucleus, exclusive of the pulvinar, is therefore a relay station on the paths of cutaneous and deep sensibility, and it is connected with the parietal and frontal cortex through the thalamic radiation. The pulvinar and lateral geniculate body are stations on the optic pathway, and the medial geniculate body on that for hearing. The thalamic radiation can best be considered in detail after we have ac- quired some familiarity with the structure of the cerebral hemisphere (p. 263). The fiber tract connections, established by the various nuclear masses composing the thalamus, among themselves and with other parts of the brain, are not as yet well known. This is particularly true of the descending tracts. It is known that from the region of the thalamus a large bundle, the thalamo-olivary tract, descends to the inferior olivary nucleus. Some authors also describe a thalamospinal tract which arises in the thalamus and is closely associated with the rubrospinal tract. It is fairly well established that each of the ascending sensory tracts of the tegmentum has its own particular field of distribution within the ventral nucleus of the thalamus; and it is, therefore, probable that there are corresponding functional differences in the various subdivisions of this nucleus. Beginning at the lateral side and passing mediahvard. the terminals of these various tracts are as follows: The spinothalamic tract ends in the most lateral part of the ventral nucleus. Next comes the field, within which terminate the fibers of the central tract of the trigeminal nerve, and which includes the nucleus arcuatus and nucleus centralis. The medial lemniscus ends in the most medial part of the inferior nucleus, including the nucleus centralis. This corresponds to the relative position which these tracts occupy in the tegmentum of the mesencephalon, where the spinothalamic tract is the most lateral of the three. 22Q THE NERVOUS SYSTEM THE METATHALAMUS The mctathalamus is composed of two small protuberances, the geniculate bodies, which, having been displaced by the excessive development of the thalamus, are situated upon the dorsolateral surface of the rostral end of the mesencephalon (Figs. 87-89, 154, 161). The lateral geniculate body is an oval swelling in the course of the optic tract. Its connections will be more fully considered in connection with the discussion of the course of the visual impulses. The medial geniculate body is overhung by the pulvinar, from which it is separated by a deep sulcus. It receives fibers by way of the inferior quadrigeminal bra- chium from the lateral lemniscus, which we have learned to know as the central auditory path from the cochlear nuclei. From it fibers run to the auditory ana of the cerebral cortex (the thalamotemporal or acoustic radiation). THE EPITHALAMUS The epithalamus includes the pineal body, stria medullaris, and habemilar trigone. The latter is a small triangular depressed area located on the dorso- medial aspect of the thalamus rostral to the pineal body (Fig. 158) . In the sheep, as in most other mammals, it is much larger than in man and bulges both dor- sally and medially beyond the surface of the thalamus (Figs. 91, 159). It marks the position of a nuclear mass, called the habenular ganglion, which receives fibers from the stria medullaris, a fascicle which runs along the border between the dorsal and medial surfaces of the thalamus subjacent to the taenia thalami (Figs. 154, 155). The stria medullaris takes origin from the anterior perforated substance and other olfactory centers on the basal surface of the cerebral hemi- sphere and, partially encircling the thalamus, reaches the habenular ganglion, in which it ends. (See p. 281.) Not all of the fibers terminate on the same side; some cross to the ganglion of the opposite side, forming a transverse bundle of myelinated fibers which joins the caudal end of the two ganglia together and is known as the habenular commissure. From the cells in this ganglion arises a bundle of fibers, known as the fasciculus retroflexus of Meynert or the tractus habenulopeduncularis. This bundle of fibers is directed ventralward and at the same time caudally along the medial side of the red nucleus toward the base of the brain, where it crosses to the opposite side and ends in the inter- peduncular ganglion (Fig. 189). The stria medullaris, habenular ganglion, and fasciculus retroflexus are all parts of an arc for olfactory reflexes, as indi- cated in Fig. 211. According to Edinger (1911) the cells, from which the stria medullaris arises, are intimately related to a bundle of ascending fibers from rill: D1EXCEPHALON AND THE OPTIC NERVE 221 the sensory nuclei of the trigeminal nerve. I!" this he true, the mechanism in question may receive afferent impulses from the nose, mouth, and tongue and he concerned with feeding reflexes. The pineal body is a small mass, shaped like a fir cone, which re>ts upon the mesencephalon in the interval between the two thalami. Its base is attached by a short stalk to the habenular and posterior commissures, and into the stalk there extends the small pineal recess of the third ventricle. The pineal body i- a rudimentary structure and is not composed of nervous elements. In some Posterior com m issun Pirn ill body Splcnium of corpus callosum - Lamina quadrigemina x \ Ccnhrai aqueduct s \ Anterior medullary velum s \ Four Ui ventricle ^ Sup. verm, of cerebellum „ Fissura prima . £ L >: Inferior vermis ,,- M of cerebellum Hypothermic sulcus , Body of fornix II abeu ula ^ \ , Chorioid 'plexus of third ventricle Habenular commissure l i i / Massa intermedia Suprapineal recess \ • ' ' / / Epithelial roof of third ventricle I i Lamina commissures hippocampi ( Corpus col I os urn Epithelial roof and chori- oid plexus of fourth-'' ventricle Genu of corpus callosum / -^„ Septum pclluci- dum ^-~ Ros.ofcor. callosum Lamina rostral is „ "' Columna fornicis - "• Interventricular foramen «^ v Anterior commissure N,/ Lamina terminalis * -COptic recess '^^ Optic chiasma ^^Infundibulum \ \ \ \ Hypophysis » \ \ Mammillary body * * Oculomotor nerve \ \ y Si Hippocampal com. Roofs of third ventricle or tela chorioidca \ \ Stria med. /Haben. com. \ \ \ Habcnular / /Splcnium Suprapineal recess , \ \ \ ■ Trigone ///Pineal /', Superior colliculus J / / /Primary fissure /White center of vermis Olfactory bulb . Medial olfactory gyrus' / / Anterior perf. substance'/, Lamina terminalis / Diagonal band W//, Tnfundib. • / ,' Third vent. ! / Massa intermedia ' Optic c hi as ma Preoptic recess \ 1 'Pons ^ \ -Aqueduct \Lamina quad. \ Posterior com. >. v Hypophysis Mam miliary body Central canal ' Medulla v 1 Medial aperture of \ \ fourth ventricle \ \Tcla chorioidca \ ' Fourth ventricle x Anterior medullary velum Fig. 159. — Medial sagittal section of the sheep's brain. the same time an account of the parts of the telencephalon which help to form its walls. These include the lamina terminalis, anterior commissure, and the optic chiasma (Figs. 158, 159). The latter, formed by the decussation of the fibers of the optic nerve, projects as a transverse ridge in the floor of the ven- tricle. The lamina terminalis is a thin plate joining the two hemispheres, which stretches from the optic chiasma in a dorsal direction to the anterior commis- sure. Here it becomes continuous with the thin edge of the rostrum of the corpus callosum, known as the rostral lamina. As indicated on page 26, the 224 THE NERVOUS SYSTEM lamina terminalis is to be regarded as forming the rostral end of the brain; and the part of the third ventricle, which lies behind it and dorsal to the optic chiasma, belongs to the telencephalon. The anterior commissure is a bundle of fibers which crosses the median plane in the lamina terminalis and serves to connect certain parts of the two cerebral hemispheres, which are associated with the olfactory nerves. The anterior commissure and the lamina terminalis form the rostral boundary of the third ventricle, and between the latter and the optic chiasma is a diverticulum, known as the optic recess. The third ventricle is a narrow vertical cleft, the lateral walls of which are formed for the greater part by the medial surfaces of the two thalami. Ventral to the massa intermedia is seen a groove known as the hypothalamic sulcus, which if followed rostrally leads to the interventricular foramen, while in the other direction it can be traced to the cerebral aqueduct. Below this groove the lateral wall and floor of the ventricle are formed by the hypothalamus. In the floor of the ventricle there may be enumerated the following structures, beginning at the rostral end: the optic chiasma, infundibulum, tuber cinereum, mammillary bodies, and the subthalamus. The roof of the third ventricle is formed by the thin layer of ependyma, which is stretched between the striae medullares thalami of the two sides, and whose torn edge, in the dissected specimen, is represented by the taenia thalami (Figs. 85, 155, 159). Upon the outer surface of this ependymal roof is a fold of pia mater in the transverse fissure. This is known as the tela chorioidea; and from it delicate vascular folds are invaginated into the ventricle, carrying a layer of ependyma before them by which they are, in reality, excluded from the cavity. These folds are the chorioid plexuses. There are two of them extending side by side from the interventricular foramina to the caudal extremity of the roof. Here they extend into an evagination of the roof above the pineal body, known as the suprapineal recess. There are three openings into the third ventricle. The aqueduct of the cere- brum opens into it at the caudal end; while at the opposite extremity it com- municates with the lateral ventricles through the two interventricular foramina. THE VISUAL APPARATUS Development of the Retina and Optic Nerve.— There is but one pair of nerves associated with the diencephalon, and these, the optic nerves, are not true nerves, but fiber tracts joining the retinae with the brain. It will be re- membered that the retina develops as an evagination of the lateral wall of the prosencephalon in the form of a vesicle whose cavity is continuous with that of THE nil \< l I'll \l.35 frontal Lobe. Within it one may identify three chief sulci, which are, however, subject to considerable variation. The precentral sulcus is more or lessparallel with the centra] sulcus and is often subdivided into two part-, the superior and inferior precentral sulci (Fig. 168). The superior frontal sulcus usually begins in the superior precentral sulcus and runs rostrally, following in a general way the curvature of the dorsal border of the hemisphere which it gradually ap- proaches. The inferior frontal sulcus usually begins in the inferior precentral sulcus and extends rostrally. arching at the same time toward the base of the hemisphere. Between the precentral and central sulci lies the anterior central gyrus in which is found the motor area of the cerebral cortex. The remainder of this Anterior central gyms Superior precentral sh/chs ; Superior frontal gyrus -- Superior frontal sulcus -- Middle frontal gyrus— MiddU frontal side Inferior frontal sulcus luf rior precentral sulcus . Inf. Parsopcrcularis front.- Pars triang. ■ gyrus Pars orbitalis- Lateral Ant. hor. ram. ' / cerebraHAnt. ascend, ram./ y fissure Post, ram.y' , Superior temporal sulcus,'' Superior temporal gyrus , Central sulcus , Posterior central gyrus Postcentral sulcus , Supra marg. gyrusl ';.,' ^Angular gyrus ^^ ---' Superior parietal lobule — - Interparietal sulcus / \\ r' Trails, occipital sulcus Sulcus lunatus \\ Inferior temporal gyrus S? Middle temporal sulcus Middle temporal gyrus Fig. 168.— Sulci and gyri on the lateral aspect of the human cerebral hemisphere. surface of the frontal lobe is composed of three convolutions, the superior, middle, and inferior frontal gyri. separated from each other by the superior and inferior frontal sulci. The inferior frontal gyrus, which in the left hemisphere is also known as Broca's convolution, is subdivided by the two anterior rami of the lateral sulcus into three parts, known as the orbital, triangular, and oper- cular portions. The orbital part of the inferior frontal gyrus lies rostral to the anterior horizontal ramus of the lateral sulcus; the triangular part is a wedge- shaped convolution between the two anterior rami of that fissure; while the opercular portion lies in the frontal operculum between the precentral sulcus and the anterior ascending ramus of the lateral fissure. The Temporal Lobe.— Ventral to the lateral fissure is the long tongue-shaped 236 THE NERV01 - SYS1 EM temporal lobe which terminates rostrally in the temporal pole. The superior temporal sulcus is a very constant fissure, which begins near the temporal pole and runs nearly parallel with lateral cerebral fissure. Its terminal part turns dorsally into the parietal lobe. The middle temporal sulcus, ventral to the pre- ceding and in general parallel with it. is usually composed of two or more dis- connected parts. The interior temporal sulcus is located for the most part on the basal surface of the temporal lobe. Dorsal to each of these fissures is a gyrus which bears a similar name: the superior temporal gyrus, between the lateral fissure and the superior temporal sulcus; the middle temporal gyrus, be- tween the superior and middle temporal sulci; and the inferior temporal gyrus, between the middle and inferior temporal sulci. The lateral fissure is very deep; and the surface of the superior temporal gyrus that bounds it is broad and marked near its posterior extremity by horizontal convolutions, known as the transverse temporal gyri. One of these, more marked than the others, has been called the anterior transverse temporal gyrus or Heschl's convolution and represents the cortical center for hearing (Fig. 174). The Parietal Lobe. — The postcentral sulcus runs nearly parallel with the central sulcus and consists of two parts, the superior and inferior postcentral sulci, which may unite with each other or with the interparietal sulcus. Often all three are continuous, forming a complicated fissure, as shown in Fig. 168. The interparietal sulcus extends in an arched course toward the occiput and may end in the transverse occipital sulcus. These four sulci are often included under the term ' "interparietal sulcus." The interparietal sulcus proper is then designated as the horizontal ramus. The posterior central gyrus lies between the central and postcentral sulci. The interparietal sulcus separates the superior parietal lobule from the inferior parietal lobule. Within the latter we should take note of two convolutions: the supramarginal gyrus, which curves around the upturned end of the lateral fissure; and the angular gyrus, similarly related to the terminal ascending por- tion of the superior temporal fissure. The Occipital Lobe. — Only a small part of the dorsolateral surface of the hemisphere is formed by the occipital lobe. This is a triangular area at the occipital extremity, bounded rostrally by a line joining the parieto-occipital fissure and the preoccipital notch (Fig. 167). The transverse occipital fissure may help to bound this area or may lie within it. Other inconstant sulci help to divide it into irregular convolutions. Sometimes the visual area which lies on the mesial aspect of this lobe is prolonged over the occipital pole to the lateral THE EXTERNAL CONFIGURATION OE THE CEREBRAL BEMISPHERES 237 aspect. In this case a small semilunar furrow develops around it on the lateral surface and is known as the sulcus lunatus (Fig. 168). This sulcus, tailed b) Rudinger the "Affenspalte," forms a conspicuous feature of the lateral surface of the cerebral hemisphere in the lower ( >ld World apes ( [ngalls, \ { >\ 1 The Insula. The part of the cortex which overlies the corpus striatum la<^> behind in its development and becomes overlapped by the surrounding pallium. The cortex, which thus becomes hidden from view at the bottom of the lateral fissure, forms in the adult a somewhat conical mass called the insula or island of Keil (Fig. 169). Its base is surrounded by a limiting furrow, the circular sulcus, which is, however, more triangular than circular, and in which we may recognize three portions: superior, inferior, and anterior. The apex of this conical lobe 1'ariclal lobe { 'entral sulcus of insula m^-: Circular sulcus «# V Frontal lobe Occipital lobe / , ^^■■•^ (Pr Short gyri of insula Temporal lobe £ ong gyrus f i nsu la Fig. 169. — Lateral view of the human cerebral hemisphere with the insula exposed by removal of the opercula. (Sobotta-McMurrich.) is known as the limen insula; and the remainder is subdivided by an oblique groove (sulcus centralis insula?) into the long gyrus of the insula and a more rostral portion, which is again subdivided into short gyri. The Operculum. — As the adjacent portions of the pallium close over the insula (Fig. 164) they form by the approximation of their margins the three rami of the lateral fissure. These folds constitute the opercula of the insula. Each of the three surrounding lobes takes part in this process; and we may accordingly recognize a frontal, a temporal, and a parietal operculum (Fig. 166). At this point it will be instructive to examine the lateral surface of the cerebral hemisphere of the sheep. It will be seen that the region which corresponds to the insula is on a level with the general surface of the hemisphere; no opercula have developed, and the lateral sulcus is only a shallow groove (Fig. 173). 2 3 8 THE NERVOUS SYS I EM THE MEDIAN AND BASAL SURFACES The occipital lobe comes more nearly being a structural and functional entity than any of the other lobes. It corresponds in a general way to the "regio occipitalis" as outlined by Brodman (Figs. 216, 217). and it is probably all concerned directly or indirectly with visual processes. We have seen that it forms a small convex area on the lateral surface near the occipital pole; and we now note that it is continued on to the medial surface of the hemi- sphere, where it forms a somewhat larger triangular field between the parieto- occipital and anterior portion of the calcarine fissure dorsorostrally and the Sulcus cinguli Sulcus of corpus callosum . Body of corpus callosum Paracentral lobule Central sulcus Sup. frontal gyrus Frontal por. of sulcus cinguli Frontal pole Genu of corp. cat. Septum pellucidum Rosl. of corpus callosum A nlerior parolfactory sulcus Parolfactory area ' ,' Temporal pole Uncus Anterior commissure Fimbria Hippocampal gyrus ^Marginal portion of sulcus cinguli Precuneus Column of fornix y Subparielal sulcus Crus of fornix .- Paricto-occip. fis. 'Splcn.ofcorp.cal. I "lli. of gyrus fornicatus Cuneus Calcarine fissure -Occipital poh Lingual gyrus Inferior temporal gyrus Inferior temporal sulcus Fusiform gyrus Collateral fissure Fasciola cinerca Fig. 170. — Human cerebral hemisphere seen from the medial side. The brain has been divided in the median plane and part of the thalamus has been removed along with the mesen- cephalon and rhombencephalon. (Sobotta-McMurrich.) collateral fissure ventrally. On this aspect of the brain it includes two constant and well-defined convolutions: the cuneus and the Ungual gyrus (Figs. 170, 171). The calcarine fissure begins ventrally to the splenium of the corpus callosum and extends toward the occipital pole, arching at the same time somewhat dorsally. It consists of two portions. The rostral part, the calcarine fissure proper, is deeper, more constant in form and position, and phylogenetically much older than the rest, and produces the elevation on the wall of the lateral ventricle known as the calcar avis (Fig. 181). This part terminates at the point nil EXTERNAL CONFIGURATION 01 nil. CEREBRAL HEMISPHERES 239 when- the calcarine is joined by the parieto-o< < ipital fissure. The other portion. sometimes called the "posterior calcarine sulcus," arches downward and back- ward from this junction toward the occipital pole, and occasionally cuts across the border of the hemisphere to its dorsolateral surface. The parieto-occipital fissure, which i> really a deep fossa with much buried cortex at it.- depth, appears to be the direct continuation of the rostral part of the calcarine fissure. It cuts through the dorsal border of the hemisphere somewhat nearer to the occipital pole than to the central sulcus. These fissures form a Y-shaped figure whose stem is the calcarine figure and whose two limbs are the parieto-occipital fissure and the "posterior calcarine sulcus." If the fissures are opened up the stem is seen to be marked off from the two limbs by buried annectant gyri. The chucks is a triangular convolution with apex directed rostrally. which lie- between the diverging parieto-occipital and calcarine fissures. The rest of the medial surface of the occipital lobe belongs to the lingual gyrus, which lies between the calcarine and collateral fissures. The remaining sulci and gyri on the median and basal surfaces may now be briefly described. The sulcus of the corpus callosum (sulcus corporis callosi) begins ventrally to the rostrum of the corpus callosum. encircles that great commissure on its con- vex aspect, and finally bends around the splenium to become continuous with the hi ppocam pal fissure (Fig. 171). The latter is a shallow groove, which runs from the region of the splenium of the corpus callosum toward the temporal pole near the dorsomedial border of the temporal lobe. It terminates in the bend between the hippocampal gyrus and the uncus. The sulcus cinguli (callosomarginal fissure) begins some distance ventral to the rostrum of the corpus callosum and follows the arched course of the sulcus of the corpus callosum. from which it is separated by the gyrus cinguli. It terminates by dividing into two branches. One of these, the subparietal sulcus, continues in the direction of the sulcus cinguli and ends a short distance behind the splenium. The other, known as the marginal ramus, turns off at a right angie and is directed toward the dorsal margin of the hemisphere. A side branch, directed dorsally. is usually given off from the main sulcus some dis- tance rostral to its bifurcation, and is known as the paracentral sulcus. The collateral fissure begins near the occipital pole and runs rostrally. sepa- rated from the calcarine and hippocampal fissures by the lingual and hippo- campal gyri. It is sometimes continuous with the rliinal fissure. The latter separates the terminal part of the hippocampal gyrus, which belongs to the archi- 2-J.O IHI. NERVOUS SYSTEM pallium, from the rest of the temporal lobe, and is a very conspicuous fissure in most mammalian brains (Fig. 83). Convolutions.— Dorsal to the corpus callosum is the gyrus cinguli between the sulcus of the corpus callosum and the sulcus cinguli. The superior frontal gyrus is continued over the dorsal border of the hemisphere from the dorso- lateral surface and reaches the sulcus cinguli. Surrounding the end of the central sulcus is a quadrilateral convolution, known as the paracentral lobule. It is bounded by the sulcus cinguli, its marginal ramus and the paracentral sulcus. Another quadrilateral area, known as the precuneus, is bounded by the parieto-occipital fissure, the subparietal sulcus, and the marginal ramus of the sulcus cinguli. The hippocampal gyrus lies between the hippocampal fissure Superior frontal gyrus Sulcus of corpus callosum . v Gyrus cinguli :, Sulcus cinguli --. Corpus callosum Gyrus fornicatus Frontal lobe Post, parolfactory sulcus-^- Parolfactory area- S. centralis Paracentral sulcus Ant. parolfactory sulcus- Paracentral lobule „ , • Parietal lobe _..- Marginal ramus __.— Precuneus 'Subparietal sulcus -- Parieto-occipital fissure — Cuneus -- Cakarine fissure - Lingual gyrus . Isthmus of gyrus Temporal lobe ' Rhinal fissure' Uncm ! Hippocampal gyrus \ Inf. temporal gyrus fornicatus ~- Hippocampal fissure Collateral fissure v > Fusiform gyrus Inferior temporal sulcus Fig. 171. — Diagram of the lobes, sulci, and gyri on the medial aspect of the human cerebral hemisphere. dorsally and the collateral and rhinal fissures ventrally. Its rostral extremity bends around the hippocampal fissure to form the uncus. It is connected with the gyrus cinguli by a narrow convolution, the isthmus of the gyrus fornicatus. Under the name gyrus fornicatus it has been customary to include the gyru- cinguli. isthmus, hippocampal gyrus, and uncus. Between the collateral fissure and the inferior temporal sulcus is the fusiform gyrus which lies on the basal surface of the temporal lobe in contact with the tentorium of the cerebellum (Figs. 170. 172 ). It has been customary to apportion parts of the medial and basal surfaces of the cerebral hemisphere to the frontal, parietal, occipital, and temporal lobes, as indicated in Fig. 171. According to this scheme the gyrus fornicatus 1111 EXTERNAL CONFIGURATION OP I III CEREBRAL II I. MI -I'll I 241 stand- by it-di' ami i- sometimes designated a- tin- limbic lobe. This plan of subdivision, which was based <>n the erroneous belief that all portions of the gyrus fornicatus belonged t<> the rhinencephalon, should In- abandoned. A simpler ami more logical arrangement assigns the hippocampal gyrus and uncus to the temporal lobe and divides the gyrus cinguli between the frontal and parietal lobes. Optic chiasma Orbital gyri Anterior perforated substance .. / . mporal pole Lateral cerebral (Sylvian) fissure Longitudinal fi \urt of cerebrum frontal pole Gyrus rectus ^^0//./( lory sulcus "M Vj. Orbital sulci &/ V^k Olfn lory trigone Mammillary body .- 1 'ncus Middle temporal sulcus- Tuber cinereum Hippocampal fissure- T Collateral fissure-'' Inferior temporal sulcus Cerebral aqueduct •' Collateral fissure Cuneus Middle temporal sulcus Base of cerebral peduncle Substantia nigra 'Inferior temporal gyrus Fusiform gyrus Hippocampal gyrus Corpus quadrigeminum Isthmus of gyrus fornicatus Lingual gyrus Gyrus cinguli Splenium of corpus callosum Parieto-occipital fissure Occipital pole (Sobotta-McMurrich.) Fig. 172. — Basal aspect of the human cerebral hemisphere The basal surface of the hemisphere (Fig. 172) consists of two parts: (1) the ventral surface of the temporal lobe, whose sulci and gyri have been de- scribed in a preceding paragraph, and which rests upon the tentorium cerebelli and the floor of the middle cranial fossa; and (2) the orbital surface of the frontal lobe resting upon the floor of the anterior cranial fossa. The latter surface presents near its medial border the olfactory sulcus, a straight, deep furrow, directed rostrally and somewhat medially, that lodges the olfactory tract and bulb. To its medial side is found the gyrus rectus. The remainder of the orbital surface of the frontal lobe is subdivided by irregular orbital sulci into equally irregular orbital gyri. 16 242 THE NERVOUS SYSTEM From the foregoing account it will be apparent that almost the entire sur- face of the human cerebral hemisphere is formed by neopallium. Of the parts already described only the uncus and adjacent part of the hippocampal gyrus belong to the archi pallium. Other superficial portions of the rhinencephalon, such as the olfactory bulb, tract and trigone, and the anterior perforated sub- stance, will be described in connection with the hidden parts of the rhinen- cephalon in Chapter XVII. Suprasylvian fissure Cerebral hemisphere , Cerebellum Poslmedian lobulet*-^ Ansiform lobn Paraiiocculus\~ Paramedian lobule* Flocculus 1 Chorioid plexus of fourth ventricle Lateral fissure , Insula Vin Olive VII Trapezoid body V TV I VI Pons fissure fissure Mammillary body Hippocampal gyrus Cerebral peiuncle » Olfactory bulb Lateral olfactory gyrus Fig. 173. — Lateral view of the sheep's brain. The surface form of the cerebral hemisphere of the sheep is illustrated in Figs. 83. 84. and 173. On these figures are indicated the names of the chief sulci and gyri. It will be of interest to note the position of the motor cortex in the sheep as given in Fig. 82. Since this corresponds to the precentral gyrus in man, it will be seen that there is little in the sheep's brain to correspond to the ro.-tral part of the frontal lobe in man. CHAPTER XVI THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES Whin a horizontal section is made through the cerebral hemisphere at the level of the dorsal border of the corpus callosum the central white substance will be displayed in its maximum extent and will appear as a solid, semioval mass, known as the centrum semiovaU (Figs. 174, 175). It will also Ik- apparent that lamella' extend from this central white substance to form the medullary centers of the various convolutions, and that over this entire mass the cortex is spread in an uneven layer, thicker over the summit of a convolution than at the bottom of a sulcus. This medullary substance is composed of three kinds of fillers: (1) fibers from the corpus callosum and other commissures joining the cortex of one hemisphere with that of the other; (2) fibers from the internal cap- sule, uniting the cortex with the thalamus and lower lying centers; and (3) fibers running from one part of the cortex to another within the same hemi- sphere (p. 296). The Corpus Callosum. — At the bottom of the longitudinal fissure of the cerebrum is a broad white band of commissural fibers, known as the corpus callosum, which connects the neopallium of the two hemispheres. While the medial portion of this commissure is exposed in the floor of the longitudinal fissure, its greater part is concealed in the white center of the hemisphere where its fibers radiate to all parts of the neopallium, forming the radiation of the corpus callosum. When examined in a median sagittal section of the brain the corpus callosum is seen to be arched dorsally and to be related on its ventral surface to the fornix and septum pellucidum (Figs. 84, 158, 170). The latter consists of two thin membranous plates, stretched between the corpus callosum and the fornix and separated by a narrow cleft-like space, the cavum septi pellucidi (Fig. 177). If the septum has been torn away it will be possible to look into the lateral ventricle and see that the corpus callosum forms the roof of a large part of that cavity. At its rostral extremity it curves abruptly toward the base of the brain, forming the genu, and then tapers rapidly to form the rostrum. The latter is triangular in cross-section, with its edge directed toward the anterior commissure to which it is connected by the rostral lamina. The 243 244 THE NERVOUS SYSTEM body of the corpus callosum (truncus corporis callosi), arching somewhat dor- sally, extends toward the occiput and terminates in the splenium, a thickened rounded border situated dorsal to the pineal body and corpora quadrigemina. Related to the concave or ventral side of the corpus callosum are the fornix, septum pellucidum, lateral ventricles, tela chorioidea of the third ventricle, and the pineal body (Fig. 170). Genu of corpus callosum Cingulum (cut) Centrum semi-. Medial I ■ tudinal stria \ Cingulum (cut Splenium of corp. callosum Frontal part of ,- radiation of corp. callosum Intersection of fibers from cor- _. pus callosum and corona radiata __ Superior longi- tudinal fas- ciculus ~ ' Temporal lobe ~:--Insula Radiation of " corp. callosum ^JTransverse tem- poral gyri •Optic radiation '--^-Tapctum Occipital part of " radiation of corp. callosum Fig. 174. -Dissection of the human telencephalo Dorsal view. show the radiation of the corpus callosum. Turning again to the dorsal aspect of the corpus callosum. a careful inspec- tion will show that at the bottom of the great longitudinal fissure it is covered by a very thin coating of gray matter, continuous with the cerebral cortex in the depths of the sulcus of the corpus callosum (Figs. 174. 175). This is a rudi- mentary portion of the hippocampus and is known as the supracallosal gyrus or indusium griseum. In this gray band there are embedded delicate longitudinal mi IMI RNAI « ONI K.i RATIO*! OP mi. I I ii BRAL BEMISPH1 I 245 strands of nerve fibers. Two of these, placed close together on either side of the median plain', are known as the medial longitudinal stria. Further lateral- ward on cither side, hidden within the sulcus of the corpus callosum, Is a less well developed band, the lateral longitudinal stria. The corpus callosum is transversely striated and is composed of fibers thai pass from one hemisphere to the other. By dissection these may be foil* into the centrum semiovale, where they constitute the radiation of the corpus Genu of corpus callosum ; Medial longitudinal stria Hippocampal rudiment \ -Body of corpus callosum Radiation of corpus callosum - Corona radiata Intersection of corona ra- diata and radiation of corpus callosum Lateral longitudinal stria Splenium of corpus callosum Fig. 175. — Dissection of the telencephalon of the sheep to show the radiation of the corpus cal- losum. Dorsal view. callosum and intersect those from the internal capsule in the corona radiata (Figs. 174, 175). The fibers of the genu sweep forward into the frontal lobe, constituting the frontal part of the radiation. Fibers from the splenium bend backward toward the occipital pole, forming the occipital part of the radiation or forceps major. In the human brain fibers from the body and splenium of the corpus callosum sweep outward over the lateral ventricle, forming the roof and lateral wall of its posterior horn and the lateral wall of its inferior cornu. Here they constitute a very definite stratum called the tapctum. 246 THE NERVOUS SYSTEM THE LATERAL VENTRICLE When the corpus callosum and its radiation arc cut away a cavity, known as the lateral ventricle, is uncovered. It is lined by ependyma, continuous with the ependyma] lining of the third ventricle by way of the interventricular for- amen. This cavity, which contains cerebrospinal fluid, varies in size in differ- ent parts, and in some places is reduced to a mere cleft between closely apposed walls. The shape of the ventricle is highly irregular (Fig. 176). As constit- uent parts we recognize a central portion, anterior and inferior horns, and in man also a posterior horn. The latter part develops rather late in the human fetus as a diverticulum from the main cavity. Third ventricle -Ant. horn Centra', part \ Latcral vn " n Inf. horn' Q viral pari Ant. horn Fourth ventricle Fourth ventricle v Interventricular for. \f ' Optic recess UJ\\ ' Infundibulum \ \ \ ' Third ventricle \ \ ^ Inf. horn K \* Suprapineal recess ^ Cerebral aqueduct lateral recess B Fig. 176. — Two views of the brain ventricles of man: A, Dorsal view; B, lateral view. The anterior horn, or cornu anterius, is the part which lies rostral to the interventricular foramen. Its roof and rostral boundary are formed by the corpus callosum. Its medial wall is vertical and is formed by the septum pellu- cidum, which is stretched between the corpus callosum and the fornix (Figs. 177, 178). The sloping floor is at the same time the lateral wall, and is formed by the head of the caudate nucleus, which bulges into the ventricle from the ventrolateral side. In frontal section the cavity has a triangular outline; and in such a section its walls and the relation which the)' bear to the rest of the brain can be studied to advantage (Fig. 186). The central part or body of the lateral ventricle extends from the inter- ventricular foramen to the splenium of the corpus callosum, where in man the cavity bifurcates into posterior and inferior horns. The roof of the central nil IMU'WI CONFIGURATION O] mi CEREBRAL HEMISPHE] j.47 part is formed 1>\ the corpus callosum, and the medial wall by the septum pellu- ddum. The floor, which slants to meet the roof al the lateral angle, is com posed l'n>m within outward of the following structures: the fornix, chorioid plexus, lateral part of the dorsal surface of the thalamus in man. but not in the sheep), the stria terminalis, vena terminalis, and the caudate nucleus (Figs. 177 180, 188). The caudate nucleus tapers rapidly as it Is folli from tin- anterior horn into the body of the ventricle (Fig. 177 . The cavity Longitudinal fissure of cerebrum ■"" "/ cor P u ina of septum peUucidum Corpus callosum umn of fornix v^^HM^ i ^Wl^h-^ / Cavity of u plum pel!,,, i Column of fornix Caudate nucleus Interventricular foramen Thalamus Body of fornix ■ Chorioid plexus Transverse fissure of cerebrum • septum peUucidum Anterior horn of lateral ventricle audate nucleus Chorioid plexus of lateral ventricle Terminal stria C> ntral portion of lateral ventricle Chcrioid glomus J^-Crus of fornix ^Inferior horn of lateral ventricle S pi 'en 1 urn of corpus callosum /' :>rior horn of lateral ventricle Calcarine fissure Cerebellum Fig. 177 — Dissection of the human telencephalon. The corpus callosum has been partly removed, and the lateral ventricles have been exposed. Dorsal view. (Sobotta-Mc Murrich.) is lined throughout by an ependymal epithelium, indicated in red in Fig. 155. Between the caudate nucleus and the fornix this layer of ependyma constitutes the entire thickness of the wall of the hemisphere. In man. where the fornix and caudate nucleus are more widely separated than in the sheep, this epithelial membrane rests upon the thalamus and becomes adherent to it as the lamina affixa (Figs. 154. 155). At the margin of the fornix a vascular network from the tela chorioidca, i. c, from the pia mater in the transverse cerebral hs>ure. i- 248 THE NERVOUS SYSTEM invaginated into the ventricle, pushing this epithelial layer before it and con- stituting the chorioid plexus. The posterior horn, or cornu posterius, extends into the occipital lobe of the human brain, tapering to a point, and describing a gentle curve with con- cavity directed medially (Figs. 177, 181). The tapetum of the corpus callosum forms a thin but distinct layer in the roof and lateral wall of the posterior horn, and is covered in turn by a thicker layer of fibers belonging to optic radiation or radiatio occipitothalamica (Fig. 190). In the medial wall two longitudinal elevations may be seen. Of these, Corpus callosum -. Head of fa m! a If initials " Body of fornix -. Fimbria of hippo- campus ' Hippocampus - Splcnium of corpus callosum -- -Genu of corpus callosum . — A nterior horn of lateral ventricle ■ - Thick portion of septum pell tie id um ~~ Lateral fissure Interventricular foramen Lateral ventricle Fig. 178. — Dissection of the telencephalon of the sheep to show the lateral ventricle and the structures which form its floor. Dorsal view. the more dorsal one is known as the bull of the posterior horn (bulbus cornu), and is formed by the occipital portion of the radiation of the corpus callosum or forceps major. The other elevation, known as the ealcar avis, is larger and is produced by the rostral part of the calcarine fissure, which here causes a fold- ing of the entire thickness of the pallium (p. 238). The inferior horn, or cornu inferius, curves ventrally and then rostrally into the temporal lobe (Fig. 181). The angle between the diverging inferior and posterior horns is known as the collateral trigone. This horn lies in the medial part of the temporal lobe and does not quite reach the temporal pole. The roof Ill I : IMKkNAI. CONFIGURATION OF THE CEREBRAL BEMISPHERES is formed by the while substance of the hemisphere, and along ii 3 medial bord< r are the stria terminalis and tail of the caudate nucleus. At the end of the latter {'iDiii of < or pus caUosurn" Septum pell ik 'ilium - . Thick portion of sip I ) turn pellut idiim'"^" --^--- / Hippocampus -• — y s^T~" // /' Inferior horn of. lateral ventricle ^^^^^^H_ Transverse fissure of cerebrum ' Thalamus Fig. 179. Lateral ventricle*-*. Head of caudate nth h us --/nli rn utricular foramen Churioiil fissure -Fimbria of hippocampus \ Hippocampal commis uri Pineal body Septum pcllucidum Thick portion of septum pcllucidum Column of fornix Genu of corpus call os urn - Head of caudate nucleus - Interventricular foramen V Fimbria of hippo- campus -Inferior horn of lateral ventricle Thalamus i Thai a m us ** Hippoca mpus _, . , . . , / > Tcenia of thalamus Third ventricle ! ! , , " . Pineal bodv Habcuular trigone Fig. 180. Figs. 179 and 180.— Dissections of the rostral part of the sheep's brain to show the relation of the lateral ventricles, fornix, fimbria, and hippocampus to the transverse fissure, thalamus, and third ventricle. Dorsal views. In Fig. 180 a triangular piece, including portions of the fornix, fimbria, and hippocampus, has been removed. the amygdaloid nucleus bulges into the terminal part of the inferior horn (Fig. 185). The floor and medial wall of the inferior horn are formed in large part 2 5° THE NERVOUS SYSTEM by the following structures, named in their order from within outward: the fimbria, hippocampus, and (in man) the collateral eminence (Figs. 181, 182, 189). Upon the fimbria and hippocampus there is superimposed the chorioid plexus (Fig. 183). The hippocampus is a long, prominent, curved elevation, with whose medial border there is associated a band of fibers, representing a continuation of the fornix and known as the fimbria. These parts will be de- Lamina of septum pellucidum Columns of fornix Anterior tubercle of thai a- /fp Uncus , Ilippocampal (limitations Hippocarnpa gyrus 1 Collateral eminence'-^ Fimbria of hippo- campus . Collateral trigone Posterior commissure Hippocampus Calcar avi Longitudinal fissure of cerebrum Corpus callosum Cavity of septum pellucidum Interventricular foramen Anterior horn of lateral ventricle Head of caudate nucleus Jlassa intermedia Third ventricle ■ Habenular commissure .- Habenular trigone A Inferior horn of lateral ventricle ,■■ Posterior horn of lat- eral ventricle Pineal body Posterior horn of lateral ventricle "^-I___ i Corpora quadrigemina Vermis of cerebellum Fig. 181. — Dissection of the human brain to show the posterior and inferior horns of the lateral ventricle. The body and splenium of the corpus callosum have been removed, as have also the body of the fornix and the tela chorioidea of the third ventricle. A sound has been passed through the interventricular foramina. Dorsal view. (Sobotta-McMurrich.) scribed in connection with the rhinencephalon. The collateral eminence is an elevation in the lateral part of the floor produced by the collateral fissure. The thin epithelial membrane, described above as joining the edge of the fornix with the caudate nucleus (Fig. 155), continues to unite these structures as they both curve downward, the former in the floor, the latter in the roof, of the inferior horn. A vascular plexus from the pia mater is invaginated into the \ THE [NTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES lateral ventricle along this curved line, carrying before it an epithalial covering from this thin membrane. In this way there is formed the chorioid plexus of the lateral ventricle (Figs. 183, 184). The line along which this invagination oc< unci is the chorioid fissure; and when the plexus is torn away, the position of the Lateral ventricle \ Interventrii ular foramen Hippocampus / Fimbria of hippocampus' Body of fornix Optic tract 1 Internal capsule / f | Olfactory bulb I ! I Rhinoccsle > Genu of corpus callosum I Body of corpus callosum Septum pcllucidum Fig. 182. — Dissection of the cerebral hemisphere of the sheep to show the lateral ventricle. Lateral view. fissure is indicated by an artificial cleft extending into the ventricle, which be- gins at the interventricular foramen and follows the fornix and fimbria in an arched course into the temporal lobe (Fig. 205) . Hippocampus Chorioid plexus of lateral ventricle Fig. 183.— Outline drawing from Fig. 182, to show the location of the chorioid plexus of the lateral ventricle. The chorioid plexus of the lateral ventricle (Figs. 183, 184. 188) is continuous with that of the third ventricle at the interventricular foramen, from which point it can be followed backward through the central part into the inferior horn. It is coextensive with the chorioid fissure and is not found in the anterior or posterior horns. It consists of a vascular network derived from the pia 2«C2 THE NERVOUS SYS1 EM mater, and especially from that part of it enclosed in the transverse fissure and known as the tela chorioidea of the third ventricle. It is covered throughout Body of corpus callosum .Lamina of septum pdlucidum Longitudinal fissun of cerebrum Anterior horn of lateral ventricle Corpus striatum Interventricular for. Columns of fornix C( ntral portion of lateral ventricle Internal cerebral veins Clwrioid vein Chorioid arlcrv Inferior horn of lateral ventricle Collateral trigone ,• Posterior horn "■ Calcar a\ Great cerebral vein ., -Cavity of sept, pdlucidum Lamina of septum S pellucid urn ]'< in of septum pellucid um - Terminal vein ' Thalamus -■Corpus striatum Lateral chorioid ■ plexus ■ Tela chorioidea of third ventricle Chorioid glomus Hippocampal Body of corpus Body of fornix Crura of fornix com missure callosum Fig. 184. — Dissection of the human brain to show the tela chorioidea of the third ventricle and the hippocampal commissure. The body of the corpus callosum and the fornix have been divided and reflected. Dorsal view, except that the ventral surfaces of the reflected corpus callosum and hippocampal commissure are seen. (Sobotta-McMurrich.) by a layer of epithelium of ependymal origin, which is adapted to every uneven- ness of its surface (Fig. 155). THE BASAL GANGLIA OF THE TELENCEPHALON There are four deeply placed masses of gray matter within the hemisphere, known as the caudate, lentijorm and amygdaloid nuclei, and the claustrum. The I III [NTERNAL CONFIGURATION 01 nil CEREBRAL HEMISPHERES 253 two former, together with the white fascicles of the internal capsule which separate them, constitute the corpus striatum (Fig. 185 I he caudate nucleus (nucleus caudatus) is an elongated mass of gray matter bent "ii itself like a horseshoe, and is throughout it- entire extent closely re Caudate nucleus Thalamus Lenticular nucleus Amygdaloid nucleus Caudate nucleus Thalamus Tail of caudate nucleus Internal capsule Lenticular nucleus Caudate nucleus Thalamus f\ Tail of caudate nucleus Internal capsule Fig. 185. — Diagrams of lateral view and sections of the nuclei of the corpus striatum with the internal capsule omitted. A and B below represent horizontal sections along the lines A and B in the figure above. The figure also shows the relative position of the thalamus and the amygda- loid nucleus. (Jackson-Morris.) lated to the lateral ventricle (Figs. 91, 177, 178, 186. 187, 188, 191). Its swol- len rostral extremity or head is pear shaped and bulges into the anterior horn of the lateral ventricle. The remainder of the nucleus is drawn out into a long, slender, highly arched tail. In the floor of the central part of the ventricle the head gradually tapers off into the tail, which finally curves around into the roof 2 54 THE NERVOUS SYSTEM of the inferior horn and extends rostrally as far as the amygdaloid nucleus. Because of its arched form it will be cut twice in any horizontal section which passes through the main mass of the corpus striatum, and in any frontal section through that body behind the amygdaloid nucleus (Figs. 185, 189, 191). The head of the caudate nucleus is directly continuous with the anterior perforated substance; and ventral to the anterior limb of the internal capsule it is fused with the lentiform nucleus (Fig. 186). The lentiform or lenticular nucleus (nucleus lentiformis) is deeply placed in the white center of the hemisphere and intervenes between the insula, on the Stria longitu diu3lis I lateralis Nucleus lentifor mis (Putamen Polus temporalis Fissura longitiuli nalis cerebri ..Gyrus cinguli Sulcus corporis callosi Cornu anterius ventriculi lateralis .Vena, septi pe'.tuciili _ Septum pellucidum -.Fissura cerebri lateralis(Sylvii) vlnsula Rostrum cor- poris callosi . Gyrus sub- callosus Area parolfac- toria (Brocae) ^Fissura cerebri lateralis (Sylvii) Fig. 186. — Frontal section of the human brain through the rostral end of the corpus striatum and the rostrum of the corpus callosum. (Toldt.) one hand, and the caudate nucleus and thalamus on the other (Figs. 185, 191, 194). In shape it bears some resemblance to a biconvex lens. Its lateral, moderately convex surface is nearly coextensive with the insula from which it is separated by the claustrum. Its ventral surface rests upon the anterior per- forated substance and the white matter forming the roof of the inferior horn of the lateral ventricle (Figs. 187-189). Its sloping medial surface is closely applied to the internal capsule. The lentiform nucleus is not a homogeneous mass, but is divided into three zones by internal and external medullary lamina. The most lateral zone is the largest and is known as the putamen. The two medial zones together form the globus pallidus. THE INIIkWI. CONFIGURATION OF Mil. CEREBRAL HEMISPHERES 255 (facleu 1 caudatus 1 lapul Capsula intei na (Pars i: tormina Foramen inters' vcntriculare (Monrol Substantia per- , forata anterior Uncus idinalia cet- tllotum mterius ricull laterlais 'triculi Intel 1 ptum pellu- cidum una fornicis Fissura cerebri lateral *-" Gyri insulae _ Recessus opti- cus vcntriculi terlii Tractus opticus Chiasma opti- ■ cum (po part; N Commissure inferior Fig. 187. — Frontal section of the human brain through the anterior commissure. (Toldt.) Ventriculus lateralis (Pars centralis Plexus chorioideus^ ,. vcntriculi lateralis ^ Nucleus ,\ caudatus~"\- Massa inter-. -. media Capsula interna--, iPutamen-Jiii Globus =; pallidus Capsula externa.. Claustrum.^- Ansa peduncu- laris Tractus opticus- — - Pedunculus. tha -„--' lami inferior Cornu inferius ve: -x" triculi lateralis Digitationes;-" :\ hippocampi N N. oculomotorius'' Corpus callosum Ventriculus tertius Thalamus Fasciculus -;' \ ^.J-- thalamo- mamillaris — Ansa lenticularis .Nucleus hypo- thalamics (Corpus Luysi) Substantia - nigra - — Basis pedunculi Corpus '•■- mamillare I Fossa inter- peduncularis -Pons (Varoli) Fig. 188. — Frontal section of the human brain through the mammillary bodies. (Toldt.) The putamen is larger than the globus pallidus and is encountered alone in frontal sections through either the rostral or caudal extremities of the corpus striatum (Fig. 189), and also in horizontal sections above the level of the globus 2^6 THE NERVOUS SYSTEM pallidus (Fig. 191). It is fused rostrally with the caudate nucleus, which it resembles in color and structure. The globus pallidus is lighter in color and is subdivided into two parts, of which the medial is the smaller. Both parts are traversed by many fine white fascicles from the medullary laminae. Especially in the anterior part of the internal capsule bands of gray sub- stance stretch across from the lentiform to the caudate nucleus, producing a striated appearance (Fig. 187). This appearance, which is accentuated by the medullarv laminae and the liner fiber bundles in the lentiform nucleus, makes Tela chorioidea^ ventriculi tertii N^tf ' Cauda nuclei caudati Thalamus » Capsula interna-^ Putamen, _.•• - Claustrum-..':. Nucleus habenulae Cauda nuclei caudati ■ Tractus opticus- Fimbria hippo- campi Fascia dentata-' hippocampi Pedunculus cerebri V cerebri interna Plexus chorio- ideus ventriculi tertii Commissura • -' habenularum " Commissura V posterior Aditus ad aquae- srf* ductum cerebri Fasciculus retro- "" flcxus(Mcynerti) _ Cornu inferius ventriculi lateralis --...Nucleus ruber --- Nucleus bypo- thalamicus (Corpus Luysi) x - Substantia nigra "^Pons (Varoli) Recessus posterior fossae interpeduncularis'' Fig. 189. — Frontal section of the human brain through the rostral part of the pons. (Toldt.) the term corpus striatum an appropriate name to apply to the two nuclei and the internal capsule, which separates them. The claustrum is a thin plate of gray substance, which, along with the white matter in which it is embedded, separates the putamen from the cortex of the insula. Its lateral surface is somewhat irregular, being adapted to the convolu- tions of the insula, with which it is coextensive (Figs. 188, 191). Its concave medial surface is separated from the putamen by a thin lamina of white matter, known as the external capsule. By some authorities the claustrum is thought to be a detached portion of the lentiform nucleus, while others believe that it ha- been split off from the insular cortex. It is probable that neither of these views is strictly correct. However, according to the recent work of Elliot [HE INTERNAL CONFIGURATION OP nil CEREBRA1 II MISPHERES 257 Smith (1919), the claustrum, putamen, amygdaloid nucleus, and the greater part of the caudate nucleus are pallia! derivatives and arc closelj related mor phologicaUy to the neopallium; while the globus pallidus is the representative in the mammalian brain i)\ the Corpus Striatum of lower forms, a- seen in the shark (Fig. 9). The Amygdaloid Nucleus. In the roof of the terminal part of the inferior ventricular horn, at the point where the tail of the caudate nucleus ends, there is located a small mass of gray matter, known as the amygdaloid nucleus (Fig. Radiatio corporis callosi Hippocampus Corpora quadrigemina ■ Nucleus colliculi.^ inferioris Aquaeductus___'_ cerebri Nucleus n trochlearis Fasciculus longitudinalis medialis Cerebellum. - Brachium pontis -- Flocculus- — Pyramis medullae oblongatae Splenium cor- poris callosi Tela chorio- idea ven- triculi tcrtii Corpus -'' pineale < ornu poste- .-' riiis-ventri- culi lateralis Glomus . - ' \ chorioideum - Tapetum Radiatio occi- pitothalamica -- Eminentia collatcralis Fissura collateralis Lemniscus lateralis -Brachium con- junctivum -Stratum griseum centrale --Lemniscus medialis - N. vagus Fig. 190. — Frontal section of the human brain through the splenium of the corpus callosum. View into the posterior horn of the lateral ventricle. (Toldt.) 185"). It is continuous with the cerebral cortex of the temporal lobe lateral to the anterior perforated substance (Fig. 198; Landau. 1919). The external capsule is a thin lamina of white matter separating the claus- trum from the putamen. Along with the internal capsule it encloses the lenti- form nucleus with a coating of white substance. THE INTERNAL CAPSULE The internal capsule is a broad band of white substance separating the lentiform nucleus on the lateral side from the caudate nucleus and thalamus on the medial side (Figs. 191, 192). In a horizontal section through the middle K -^ THE NERVOUS SYSTEM of the corpus striatum it has the shape of a wide open V. The angle, situated in the interval between the caudate nucleus and the thalamus, is known as the Truncus corporis callosi Septum pellucidum Corpus fornicis Comu inferius ventriculi lateralis Glomus chorio- ideum Radiatio occi- pitothalamica (Gratioleti) , Genu corporis callosi ,Cornu anterius ventriculi lateralis uclei caudati urr.na fornicis Capsula interna Insula , Capsula externa t Claustrum ^Putameni Nucleus _. . } lend- ^Globus I fonnis pallidus I Massa inter- media Ventriculus tertius Stria medullaris thai ami - Nucleus habenulae iK--Habenula S"""" Cauda nuclei ;^3 ' caudati jj"' Fimbria hippo- campi t^-Corpus pineale Hippocampus Splenium corporis callo v Eminentia colla'.eralis Calcar avis Cornu posterius ventriculi lateralis Fissura calcarina Fig. 191. — Horizontal sections of the human brain through the internal capsule and corpus striatum. The section on the right side was made 1.5 cm. farther ventralward than that on the left. (Toldt.) genu. From this bend the frontal part or anterior limb of the internal capsule extends laterally and rostrally between the thalamus and the head of the caudate nucleus; while the occipital part or posterior limb of the internal capsule extends Till IMKRNAL CONFIGURATION OF Till > I II BRAL III M I - 1 • 1 1 1 II 259 lateralis and toward the occiput between the lentiform nucleu and the thala mus. The anterior limb of the internal capsule, intervening between the caudate and lentiform nuclei, is broken up by hand- of gray matter connecting these two nuclei. It consists of corticipetal and corticifuga] fibers. The former belong to the frontal stalk of the thalamus or anterior thalamic radiation from the lateral nucleus of the thalamus to the cortex of the frontal lobe. The- cortici Septum pettucidum % Fornix^ Chorioid fissur Third ventricle-* Thalamus-. Habenular trigone . , Habcnular commis- „ '_ sure I Transverse fissure -*p Pineal hotly -- f- Inferior horn of lateral ventricle Superior colliculus -<„ Genu of corpus callosum ..'Anterior horn of lateral ventricle ^..Anterior limb r -" sale Anterior commissure Optic tract '' Temporal lobe '' \ Basis peduneuli Fig. 195. — Dissection of the human cerebral hemisphere, showing the internal capsule exposed from the medial side. The caudate nucleus and thalamus have been removed. under the name basis peduneuli. By removing the optic tract, temporal lobe, insula, and lentiform nucleus this strand can easily be traced into the internal capsule where it is joined by many fibers radiating from the thalamus and spreads out in a fan-shaped manner (Figs. 87, 88), forming a curved plate which partially encloses the lentiform nucleus. As seen from the lateral side, the line along which the libers of the internal capsule emerge from behind the lentiform nucleus forms three-fourths of an ellipse (Fig. 194). Beyond the lentiform nu- cleus the diverging strands from the internal capsule, known as the corona radiate, join the central white substance of the hemisphere and intersect with those from the corpus callosum (Figs. 174, 238). An instructive view of the internal capsule may also be obtained by remov- 262 THE NERVOUS SYSTEM ing the thalamus and caudate nucleus from its medial surface. It is then seen to bear the imprint of both of these nuclei, and especially of the thalamus; and between the two impressions it presents a prominent curved ridge (Fig. 195). This ridge is responsible for the sharp bend known as the genu, which is evi- dent in horizontal sections at appropriate levels through the capsule. Many broken bundles of fibers, representing the thalamic radiation, are seen enter- ing the capsule upon its medial surface. THE CONNECTIONS OF THE CORPUS STRIATUM AND THALAMUS What is the function of the corpus striatum, and what connection does it have with other parts of the nervous system? These questions, to which no Caudate nucleus Thalamus Parietal stalk of thalamus Corticospinal tract Insula Claustrum Putamen Globus pallidus Ansa pcditncularis Red nucleus \lnsa lenticularis 'Substantia nigra Hypothalamic nucleus Fig. 196. — Diagram of the connections of the caudate and lenticular nuclei. final answer can as yet be given, have recently become of great importance, because of the frequency with which degeneration of the lentiform nucleus has been found at autopsy in patients who have shown serious disturbances of the motor mechanism (Wilson, 1912-1914). It seems probable that the corpus striatum exerts a steadying influence upon muscular activity, the abolition of which results in tremor during voluntary movement. The probable connec- tions of the corpus striatum are indicated in Fig. 196. Strio petal fibers reach the caudate nucleus from the anterior and medial nuclei of the thalamus (Sachs, 1909). According to Cajal, the corpus striatum also receives collaterals from the corticospinal tract. Intemuncial fibers join together various parts of the corpus striatum. The majority of these seem to run from the caudate nucleus THE [NTERNAL CONFIGURATION OP THE CEREBRAL BEMISPHERES 263 to the putamen, on the one hand, and from the putamen to the globus pallidus on the other. The striofugal fibers arise, for the most pari at Least, in the globus pallidus. They arc collected into a bundle of transvefselj dire* ted fibers, known as the ansa lenticularis (Fig. 188), which is distributed to the thalamus, red nucleus, hypothalamic nucleus, and substantia nigra, other fibers belonging to the same general system break through the ventral third of the internal capsule to reach the thalamus (Wilson, 1^14). The importance of the connec- tion with the red nucleus is obvious, since by way of the rubrospinal and rubro- reticular tracts the corpus striatum is able to exert its influence upon the pri- mary motor neurons of the brain stem and spinal cord. The fibers to the sub- stantia nigra have already been mentioned under the name strio nigral tract (p. 164). The impulses which travel along them are, in all probability, re- layed through the substantia nigra to krwer lying motor centers, although the functions and connections of this large nuclear mass are still obscure. The Thalamic Radiation. — We are now in position to understand the course and distribution of the fascicles, which unite the thalamus with the cerebral cortex and which consist of both thalamocortical and corticothalamic fibers. This thalamic radiation may be divided into four parts: the frontal, parietal, occip- ital, and ventral stalks of the thalamus, which will now be traced as fasciculi, without reference to the direction of conduction in the individual fibers. The ventral stalk, or inferior peduncle of the thalamus, streams out of the rostral portion of the ventral thalamic surface and is directed lateralward under cover of the lentiform nucleus. Some of these fibers belong to the ansa lentic- ularis and run from the lentiform nucleus to the thalamus. The others, form- ing a bundle known as the ansa pcduncularis, runs lateralward ventral to the lentiform nucleus and are distributed to the cortex of the temporal lobe and insula (Fig. 196). The frontal stalk, or peduncle of the thalamus, consists of fibers which run through the anterior limb of the internal capsule from the lateral thalamic nucleus to the cortex of the frontal lobe (Fig. 193), and in small part to the cau- date nucleus also. The parietal stalk, or peduncle, emerges from the lateral surface of the thalamus, and runs through the posterior limb of the internal capsule in close association with the great motor tracts (Figs. 193, 196). It connects the lateral nucleus of the thalamus with the cortex of the parietal and posterior part of the frontal lobe. Many of these fibers, especially those terminating in the posterior central 264 THE NERVOUS SYSTEM L r yrus. are afferent fibers of the third order mediating sensations of touch, heat, cold, and perhaps also pain as well as sensations from the muscles, joints, and tendons (Head. 1918). These sensory fibers are located behind the corticospinal tract in the posterior limb of the internal capsule. According to Wilson (1914) the medullary lamina' of the lentiform nucleus do not contain any thalamocor- tical fibers. The occipital stalk, or peduncle, is also known as the optic radiation and as the radiatio occipitothalamica. Its fibers stream out of the pulvinar and lateral geniculate body, pass through the retrolenticular part of the internal capsule, and run in a curved course toward the occiput, around the lateral side of the rior horn of the lateral ventricle to the cortex of the occipital lobe, and es- pecially to the region of the calcarine fissure (Figs. 190. 191). It also contains some fibers arising in the occipital cortex and ending in the superior quadrigeminal body. We have learned that it forms an important part of the visual path Fig. 162). Closely associated with the optic radiation in the retrolenticular part of the internal capsule is the acoustic radiation (radiatio thalamotemporalis). This connects the medial geniculate body with the anterior transverse temporal gyrus and the adjacent part of the superior temporal gyrus, and mediates auditory sensations. It should be included as a part of the thalamic radiation. CHArTER XVII THE RHINENCEPHALON The olfactory portions of the cerebral hemisphere may all be grouped to- gether under the name rkinencephalon. Phylogenetically very old. this part of the brain varies greatly in relative importance in the different classes of verte- brates. The central connections of the olfactory nerves form all or almost all of the cerebral hemispheres in the selachian brain (Fig. 13); while in the mammal the non-olfactory cortex or neopallium has become the dominant part. Even among the mammals there is great variation in the importance and relative size of the olfactory apparatus. The rodents, for example, depend to a great extent on the sense of smell in their search for food, and possess a highly developed rhinencephalon. Such mammals are classed as macrosmatic. Man, on the other hand, belongs in this respect with the microsmatic mammals, because in his activities the sense of smell has ceased to play a very important part, and his olfactory centers have undergone retrogressive changes. The carnivora and ruminants are in an intermediate group. The sheep's brain furnishes a good illustration of this intermediate type, and displays much more clearly than the human brain the various parts of the rhinencephalon and their relation to each other. Parts Seen on the Basal Surface of the Brain. — A comparison of the basal surface of the sheep's brain with that of the human fetus of the fifth month shows a remarkable similarity in the parts under consideration (Figs. 197. 198). The olfactory bulb, which is the olfactory center of the first order, is oval in shape and attached to the hemisphere rostral to the anterior perforated substance. It lies between the orbital surface of the cerebral hemisphere and the cribriform plate of the ethmoid bone. Through the openings in this plate numerous fine filaments, the olfactory nerves, reach the bulb from the olfactory mucous mem- brane. It contains a cavity, the rhinoccele. continuous with the lateral ventricle (Fig. 182). In the adult human brain the cavity is obliterated and the connec- tion between bulb and hemisphere is drawn out into the long olfactory tract. This is lodged in the olfactory sulcus on the orbital surface of the frontal lobe and in transverse section presents a triangular outline (Fig. 172). It contains 265 266 THE NERVOUS SYSTEM olfactory fibers of the second order connecting the bulb with the secondary ol- factory centers in the hemisphere. At its point of insertion into the hemisphere the olfactory tract forms a triangular enlargement, the olfactory trigone. From the point of insertion of the olfactory bulb or tract a band of gray matter, the medial olfactory gyrus, can be seen extending toward the medial surface of the hemisphere (Figs. 159, 197, 198). A similar gray band, the lateral olfactory gyrus, runs caudalward on the basal surface of the sheep's brain. Along Longitudinal fissure of ccrcbriu Optic nerve Optic chiasma^ Rhinal fissure J Insula— Lateral fissure. Optic tract . Infundibulum - Mammillary body - Cerebral pedunch Interpeduncular fossa and nucleus Trigeminal nerve Abducens nerve--- Acoustic{ Veslibular "-- nerve ] ~ , , [toe lit car n. Glossopharyngeal nerve Vagus nerve'' Hypoglossal nerve' Anterior median fissure Olfactory bulb ' Medial olfactory gyrus Anterior perforated substance {■'Lateral olfactory stria Lateral olfactory gyrus --—Diagonal band Amygdaloid nucleus - Pyriform area - Ilippocampal gyrus ■ Trochlear nerve T,-'-Pons ..-' Abducens nerve .-- Facial nerve ■tl'i^r; Trapezoid body — Cerebellum Olive ^'Chorioid plexus * Accessory nerve ^Tractus lateralis minor Fig. 197. — Ventral view of the sheep's brain. its lateral border it is separated from the neopallium by the rhinal fissure; while its medial border contains a band of fibers, the stria olfactoria lateralis (Fig. 197). The same gyrus is seen in the brain of the human fetus, but here it is directed outward toward the insula (Fig. 198). In the adult human brain these olfactory convolutions are very inconspicuous, and with the fibers from the olfactory tract which accompany them are usually designated as the medial and lateral olfactory stria. Till: RHINKNCKI'HALON 267 The medial olfactory gyrus and stria require further investigation. It has been gen- erally supposed that the stria is formed by olfactory fibers of the second and third order running to the olfactory centers in tin- rostral part of the medial surface of the hemisphere. These are certainly few in number in the higher mammals, and Cajal (1 ( >11), who worked Chiefly with rodents, has been unable to identify any SUCh fibers in these animals. 'The sig- nificance of the medial olfactory gyrus is also obscure. According to Elliol Smith (1915), "the rudiment of the hippocampal formation that develops on the medial surface begins in front alongside the place where the stalk of the olfactory peduncle (which becomes the trigonum olfactorium) is inserted; it passes upward to the superior end of the Lamina tcrmi- nalis, from the rest of which it is separated by a triangular mass of gray matter called the corpus paraterminale" I Fig. 2(H)). This description, as well as the figure which accompanies il, suggests a close relation between the rostral end of the hippocampal rudiment and what is ordinarily known as the medial olfactory gyrus. The subdivision of the olfactory lobe into anterior and posterior portions by the morphologically unimportant sulcus parol factorius posterior, although adopted in the B. N. A., is without justification and leads only to confusion (Elliot Smith, 1907). Olfactory bulb Lateral olfactory gyrus (stria) Posterior parolfactory sulcus A mygdaloid nucleus Medial olfactory gyrus (stria) Olfactory tract Limen insula Anterior perforated substance Hippocampal gyrus Fig. 198. — Brain of a human fetus of 22.5 cm. Ventral view. (Retzius, Jackson-Morris.) Between the olfactory trigone and the medial olfactory gyrus, on the one hand, and the optic tract on the other, is a depressed area of gray matter known as the anterior perforated substance, through the openings in which numerous small arteries reach the basal ganglia (Figs. 172, 197). The part immediately rostral to the optic tract forms a band of lighter color, known as the diagonal gyrus of the rhinencephalon or the diagonal band of Broca (Fig. 197). This can be followed on to the medial surface of the hemisphere, where it is continued as the paraterminal body or subcallosal gyrus (Fig. 200). Rostral to this gyrus the hippocampal rudiment, which corresponds in part to the parolfactory area of Broca, extends as a narrow band from the rostrum of the corpus callosum toward the medial olfactory gyrus. In those mammals which possess an espe- cially rich innervation of the nose and mouth, the region of the anterior per- forated space is marked by a swelling, sometimes of considerable size, called 2 68 THE NERVOUS SYSTEM the tubcrculum olfactorium. According to Retzius, a small oval mass is present in the anterior perforated substance of man immediately adjacent to the ol- factory trigone, which represents this tubercle. Olfactory bulb Anterior commissure interior perforated substance ■A mygdaloid nucleus Pyriform area Fig. 199. — Ventral view of a sheep's brain, pyriform area shaded and anterior commissure exposed. The Pyriform Area. — The lateral olfactory gyrus is continuous at its caudal extremity with the hippocampal gyrus (Figs. 197. 198). and the two together form the pyriform area or lobe (Tig. 199). In the adult human brain it is more difficult to demonstrate the continuity of these parts. As the temporal lobe is Hippocampal rudiment - v Corpus callosum Septum pellucid um - Fornix Anterior commissure . Parat-rminal body ^ Hippocampal rudiment Olfactory trigone Olfaxtory tract Olfactory bulb K Intermediate olfactory stria' Lateral olfactory gyrus and stria' Anterior perforated substance Limen insula • .Hippocampus (gyri Andrea; Rctzii) ) — Fascia dentata - Fimbria of hippocampus Hippocampus (proper) '"-- Hippocampus ~ Hippocampal gyrus '*■ Cauda fascia; dentata Uncus Diagonal band Fig. 200. — Diagram of the rhinencephalon. thrust rostrally and the insula becomes depressed, the pyriform area is bent on itself like a V (Fig. 198). The knee-like bend forms the limen insula, and with the rest of the insula becomes buried at the bottom of the lateral fissure. The continuity of the pyriform area is not interrupted in the adult, though part THE RHINEN< I I'll \l,u\ of it is hidden from view. It includes the lateral olfactory stria and the cortex subjacent to it (or lateral olfactory gyrus), the limen insula-, the uncus, and at least a part of the kippocampal gyrus (Figs. K> ( ). 172. 200). It i- not easy to determine just how much of the human hippocampa] gyrus should be included. Cajal (1911) apparently includes the entire gyrus, while Elliol Smith (1915) limits it to the part of the gyrus dorsal to the rhinal fissure. In Fig. 200 we have followed the outlines of the hippocampa] region as given by Brodmann (1909). The Hippocampus.- An olfactory center of still higher order is represented by the hippocampus, which was seen in connection with the study of the lateral Inferior horn of lateral ventr'n le Hippocampus Collateral eminence. Tapelitm C 'ollateral trigone Posterior horn of lateral ventricle Hippocampal digitations ■ I'm us Dentate fascia of hippocampus Hippocampal gyrus Hippocampa! fissure 'imbria of hippocampus Bulb of posterior horn Calcarine fissure Calcar avis Fig. 201. — Part of temporal lobe of human brain showing inferior horn of lateral ventricle and the hippocampus. Dorsal view. (Sobotta-McMurrich.) ventricle. If we turn again to the floor of the inferior horn of the lateral ven- tricle we shall see a long curved elevation projecting into the cavity (Figs. 181, 201). This is the hippocampus and is formed by highly specialized cortex which has been rolled into the ventricle along the line of the hippocampal fissure (Figs. 204, 209). It is covered on its ventricular surface by a thin coating of white matter, called the alveus, which is continuous along its medial edge with a band of fibers known as the fimbria of the hippocampus. This, in turn, is continuous with the fornix (Fig. 201). In Figs. 201 and 204 there may be seen, along the border of the fimbria, a narrow serrated band of gray matter, the fascia dentata, which lies upon the medial side of the hippocampus. It is sepa- rated from the hippocampal gyrus by a shallow groove, called the hippocampal 270 THE NERVOUS SYSTEM fissure, that marks the line along which the hippocampus has been rolled into the ventricle. The hippocampus and fascia dentata belong to the archipallium. In the marsupials and monotremes this extends dorsally on the medial surface of the hemisphere in a curve, which suggests that of the corpus callosum (Fig. 202). In the higher mammals the presence of a massive corpus callosum seems to inhibit the development of the adjacent part of the hippocampal formation, which remains as the vestigial indusium griseum, or supracallosal gyrus. This hippocampal rudiment is a thin layer of gray matter on the dorsal surface of the corpus callosum, within which are found delicate strands of longitudinal fibers. Two of these strands, placed close together on either side of the median plane, Cerebral cortex ; — ^^ .Hippocampal fissure yr • x. Hippocampus and fascia / / /( dentata I jite side, it forms the fornix. The fornix, which is represented diagrammatically in Fig. 203. is an arched filter tract, consisting of two symmetric lateral halves, whieh are separate at either extremity, but joined together beneath the corpus callosum. This medially placed portion is known as the body of the fornix. From its caudal extremity the fimbria diverge, and one of them runs along the medial aspect of each hippocampus. In man the hippocampus does not reach the under surface Column of fornix Body of fornix Hippocampal commissure — Cms of fornix hippocampus Fig. 203. — Diagram of the fornix. of the corpus callosum, and the part of the fimbria which joins the body of the fornix, being unaccompanied by hippocampus, is known as the cms fornicis. Rostrally the fornix is continued as two arched pillars, the columna fornicis, to the mammillary bodies. The body of the fornix is triangular, with its apex directed rostrally. It con- sists in large part of two longitudinal bundles of fibers, representing the con- tinuation of the fimbria?, widely separated at the base of the triangle, but closely approximated at the apex, whence they are continued as the columns fornicis. At the point where these longitudinal bundles diverge toward the base of the triangle they are united by transverse fibers which join together the two hippo- campi by way of the fimbria?. These fibers constitute the hippocampal com- missure. This part of the fornix, because of its resemblance to a harp, was formerly known as the psalterium (Fig. 184). The hippocampal commissure 272 THE NERVOUS SYSTEM is not very evident in the human brain, but can be easily dissected out in the sheep (Fig. 204). The coin nunc J or n iris are round fascicles which can be traced ventrally in an arched course to the mammillary bodies (Figs. 203-205). They are placed on either side of the median plane. Each consists of an initial free portion, which forms the rostral boundary of the interventricular foramen, and a cov- ered part, which runs through the gray matter in the lateral wall of the third ventricle to reach the mammillary body (Figs. 204, 205). The relations of the fornix are well shown in Figs. 155, 200, and 205. The body of the fornix intervenes between the corpus callosum, septum pellucidum, Body of corpus callosum Lateral ventricle \ Genu of corpus callosum Body of fornix ' Hippocampal commissure ,' Thalamus Splenium of corpus callosum _ Lateral ventricle r « Chorioid fissure ~ Hippocampus - Fimbria of hippo- campus -/--- Hippocampal fissure --■- — Hippocampal gyrus Dentate fascia Mammittothalamic tract Mammillary body Infundibulum Fig. 204. — Dissection of the cerebral hemisphere of the sheep to show the fornix and hippocampus. Anterior commissure Lamina lerminaHs I Optic chiasma Column of forni.\\ Median view. and cavity of the lateral ventricle on the one hand, and the transverse fissure of the cerebrum and the thalamus on the other. The fimbria and body of the for- nix form one boundary of the chorioid fissure. This fissure, which is shown but not labeled in Fig. 205, represents the line along which the chorioid plexus is invaginated into the lateral ventricle. When this plexus has been torn out, the fissure communicates with the interventricular foramen. The septum pellucidum is the thin wall which separates the two lateral ven- tricles and fills in the triangular interval between the fornix and the corpus callosum (Fig. 205). It consists of two thin vertical lamina? separated by a cleft-like interval, the cavity of the septum pellucidum (Fig. 177). Each lamina TIIK KlIINK.\(i:i'HAI.().\ 273 forms part of tin- medial wall of the corresponding hemisphere i MEDULLARY. CENTEB "I un CEREBRAL EEMISPHER] Myelination. The fibers in the various parts of the cortex acquire their myelin sheaths at different times. On this ba>i> Flech [g (1896 Identified thirty-six areas, which arc numbered in Fig. 21 ( ) in the order of myelination. I [e recognizes three main groups: primary (Nos. 1 to I2) s intermediate (Nos. 13 I ig 218. — Diagram showing the differences in thickness and in the arrangemenl (if t In- lighter ami darker bands in the human cerebral cortex in different regions as seen with the naked eye: .1, Motor cortex from anterior central gyrus; B, sensory cortex from the posterior central gyrus; (', \isiial cortex from the region of the calcarine fissure; D, auditory cortex from the anterior transverse temporal gyrus. (Redrawn after Elliot Smith.) to 28), and late (Nos. 28 to 36). According to Flechsig, the primary areas, which are myelinated at birth, are projection centers and receive the sensory radiation from the thalamus; while the other parts of the cortex, not being pro- vided with projection fibers, serve only as association centers. He believed that Fig. 219. — Lateral view of the human cerebral hemisphere, showing the cortical areas as outlined by Flechsig on the basis of differences in the time of myelination of their nerve-fibers. The primary areas (first to become well myelinated) are cross-hatched; the intermediate are indicated by vertical lines; the late areas are unshaded. (Lewandowsky.) myelination of nerve-fibers takes place in the order of conduction, that is, the sheaths are developed first on the afferent fibers, reaching the cortex from the thalamus, and later on the association fibers, linking the various areas together. According to this conception fibers of like function tend to become myelinated 19 290 THE NERVOUS SYSTEM at the same time. Much of Flechsig's work has failed to stand the test of rigid examination. It is now known that practically all regions of the cortex, in- cluding those designated by him as association centers, are connected with the thalamus or lower lying centers by afferent or efferent projection fibers. It has been shown that the more mature areas fade off gradually into those whose differentiation is less advanced, and that sharply outlined zones such as are indicated in his figures do not exist. Nevertheless, it is true that the regions designated by him as primary areas, though not sharply outlined by this method from the surrounding cortex, do mature first, and the myelination spreading from these areas reaches its completion last in those areas included in his late group (Brodmann, 1910). The primary areas include the region surrounding the central fissure, the region around the calcarine fissure, a portion of the superior temporal gyrus, and a part of the hippocampal gyrus. These areas are associated with especially important projection tracts and may properly be spoken of as projection centers. CORTICAL OR CEREBRAL LOCALIZATION In opposition to the crude conceptions of the localization of cerebral functions introduced by Gall (1825), which formed the basis for phrenology, the French physiologist Florens maintained the doctrine that all parts of the cerebrum are functionally equivalent. In 1861 Broca demonstrated that destruction of the left third frontal convolution may result in a loss of ability to speak; and nine years later Fritsch and Hitzig (1870) discovered that electric excitation of the cortex in the region of the central sulcus will elicit movements from muscles of the opposite side of the body. These observations, confirmed and extended by many observers, definitely proved that certain cortical areas possess spe- cialized functions. Physiologic and pathologic researches have served to out- line a number of these with considerable precision, and it is possible to identify them with regions of characteristic cell and fiber lamination. In this way evi- dence derived from histologic studies reinforces that drawn from physiology and pathology. The motor projection center is located in the anterior wall of the central sulcus, in the adjacent part of the anterior central gyrus, and in that part of the para- central lobule which lies rostral to the continuation of the central sulcus on the medial surface of the hemisphere (Figs. 220, 221). It coincides fairly closely with Area 4 of Brodmann's charts (Figs. 216, 217). This is the center from which the impulses initiating voluntary movements on the opposite side of the body THE CORTEX AND MEDULLARY CENTER OF Mil. CEREBRAL EEMISPHERE 2gi descend to the motor nuclei of the cerebrospinal nerves, it is subdivided into areas, eaeli of which controls the muscles moving a given part of the opposite hall" of the body; an MEDULLAR? CENTEB 01 Mil CEREBRAL HEMISPHERE 293 as the motor cortex, and the outer line of Baillarger is greatly in< reased in thn k ness and known as the line of German (Fig. 218, C). Because of the prominence of this line the region i> known as the area striata. It is surrounded \>\ cortex of quite different structure; and uowhere « an the diflerem es in adja< < al < orti< al areas he better illustrated than at its border, where the prominent line of ( lennari i- -een to terminate abruptly (Fig. 222). The fibers of the opti< radiation from tin- pulvinar and lateral geniculate body terminate in the visual projection center. These fibers carry impulses from the temporal side of the corresponding retina and the nasal side of the opposite one. The visual cortex of one hemisphere, therefore, receives impressions from the objects on the opposite side of the line of vision (Figs. 162, 163). The auditory receptive center is located in the anterior transverse temporal gyrus, which lies buried in the floor of the lateral sulcus. The area comes to the surface near the middle of the dorsal border of the superior temporal gyrus (Fig. 220). It receives the auditory radiation from the medial geniculate body. The olfactory receptive center is located in the uncus and adjacent portions of the hippocampal gyrus (principal olfactory area of Cajal). Within it ter- minate the fibers of the lateral olfactory stria. They form a rather thick layer of tangential fibers on its surface, which increases the thickness of the plexiform layer. Association Centers. — It will be seen that the sensory and motor projection centers occupy only a small part of the entire area of the cortex. The remaining parts are connected with these centers by association fibers and are known as association centers. Each area of sensory projection is surrounded by a zone closely linked up with it by such fibers, and therefore probably under the dom- inating influence of the particular sensory impulses reaching that projection center. Their positions are indicated by lighter shading in Figs. 220 and 221. Campbell (1905) has applied to them the designations "audito-psychic" and "visuo-psychic fields" (Figs. 223, 224). The same author has designated the portion of the frontal cortex immediately rostral to the motor projection center the "intermediate precentral area," and is of the opinion it is especially concerned with the "execution of complex movements of an associated kind, of skilled movements, and of movements in which consciousness or volition takes an active part." There still remains more than half of the cortical area, in- dicated in white in Figs. 220 and 221, which is probably less intimately related to any particular projection center. The fact that the increased size of the human cerebral hemisphere over that of the higher apes is due to the much 294 THE NERVOUS SYSTEM greater development of the association centers in man, suggests that these are of especial significance for the higher intellectual functions. Vi.iuo-seiisojy J- Fig. 224. Figs. 223 and 224.— Areas of the human cerebral cortex each of which structure. (Campbell.) possesses a distinctive In the present state of our knowledge of cortical activity and its relation to consciousness it is the part of wisdom to be very conservative in locating any mental faculty or fraction of our conscious experience in any particular part of Nil: CORTEX AM) MEDULLARY CENTEB OF Mil. CEREBRAL BEMISPHERE 295 the cerebral cortex. We know upon which areas the auditory, visual, and olfa< tory impulses impinge, and less accurately that in which the thalamic radiation. mediating general bodily sensibility, terminates. Destruction of these areas causes impairment or loss of the corresponding sensations with reference to the opposite side of the body or the opposite half of the field of vision. Total loss of cutaneous sensibility even within circumscribed areas never results from cor- tical lesions; and it seems probable that the thalamic centers are in themselves sufficient for a certain low grade, non-discriminative consciousness or awareness of cutaneous stimulation. This is particularly true of painful sensations, which seem to be for the most part of thalamic origin (Head, 1918). Furthermore, the various parts of the cerebral cortex are so intimately linked together by as- sociation fibers that when afferent impulses reach a given projection center they must not only activate this center, but be propagated to other parts of the cortex Motor speech ccnttr Auditory speech center Visual s P eech ccnlcr Fig. 225. — The cortical areas especially concerned with language. as well. In view of these facts it is best to express the known facts of cortical localization in terms of the relation of particular areas to the known projection fiber systems. Aphasia. — Some idea of the significance of the so-called association centers may be obtained from a study of the group of speech defects included under the term "aphasia." In right-handed individuals these result from lesions in the left hemisphere. Destruction of the triangular and opercular portions of the inferior frontal gyrus usually causes loss of ability to carry out the coordinated movements required in speaking, but does not impair the ability to move the tongue or lips (Fig. 225). This defect is known as motor aphasia. Broca's center, as this particular part of the cortex is sometimes called, is located in Campbell's intermediate precentral area; and motor aphasia serves as a good illustration of the importance of the entire intermediate precentral area for the THE NERV01 S -',-11 M ution of -killed volitional movements of an associated kind. In the same way. after a lesion in the posterior part of the left superior temporal gyrus, the patient may hear the spoken word, but no longer comprehend its meaning. This is sensory aphasia or word deafness. Word blindness, the inability to under- stand the printed or written language, although there is no impairment of vision, may result from lesions in the angular gyrus. These three areas are often spoken <>!' ;i- speech centers and are closely united together by association fibers. In fact, it is not altogether clear to what extent such defects as those mentioned above arc dependent upon the destruction of these association tracts which lie subjacent to the speech centers. THE MEDULLARY CENTER OF THE CEREBRAL HEMISPHERE The medullary center of the cerebral hemi>phere underlies the cortex and -epa rates it from the lateral ventricle and corpus striatum. It varies greatly in thickness, from that of the thin lamina separating the insula and the claus- trum (Fig. 191 ) to that of the massive centrum semiovale (Fig. 174). The myelinated nerve-fibers of which it is composed are of three kinds, namely, as- sociation libers, projection fibers, and commissural fibers. Commissural Fibers. — As was stated in Chapter XV, there are three com- missures joining together the cerebral hemispheres. Of these, the corpus callo- sum is by far the largest and its radiation contributes largely to the bulk of the centrum semiovale (Fig. 174). The fibers which compose it arise in the various parts of the neopallium of each hemisphere; they are assembled into a broad compact plate as they cross the median plane, and then spread out again to terminate in the neopallium of the opposite side. As they spread through the centrum semiovale they form the radiation of the corpus callosum. Some cor- tical areas are better supplied with these fibers than others, few. if any. being associated with the visual cortex about the calcarine fissure (Van Valkenburg, 1913). The majority of the callosal fibers do not connect together symmetric portion- of the cortex; but. after crossing the median plane, the fibers from a given point in one hemisphere spread out to many parts of the opposite side. The anterior and hippocampal commissures connect portions of the rhinencephalon in one hemisphere, with similar parts on the opposite side. The anterior com- missure connects together by its rostral part the two olfactory bulbs and by its caudal part the two pyriform areas (Figs. 187, 194. 195). The hippocampal commissure is composed of fibers which join together the two hippocampi by way of the fimbria.* and the psalterium. Mil CORTEX Wl> MEDULLAR? CENTER OF llll CEREBRAL HEMISPHER] '97 Projection Fibers. Many of the fibers of the medullary white < enter < oiu the cerebral cortex with the thalamus and lower lying portions of the nervous system. These are known as projection fibers, and may be divided into two groups according as they convey impulses to or from the cerebral cortex. The corticipetal or afferent projection fibers include the following: (1) the optu radio Hon, which arises in the pulvinar of the thalamus and the lateral geniculate \nn\y and ends in the visual cortex about the calcarine fissure (Fig. 221); (2) the auditory radiation, which arises in the medial geniculate body and terminates in the auditory cortex of tin- anterior transverse temporal gyrus; (3) the thalamic radiation which unites the lateral nucleus of the thalamus with various parts of the cerebral cortex, and which forms the ventral, frontal, and parietal -talks of the thalamus (Fig. 105V The fibers of the parietal stalk include the sensory fibers to the somesthetic cortex in the posterior central gyrus. The lateral ol- factory stria, which conveys impulses from the olfactory bulb to the pyriform area, is not a projection system in the strict sense of the word, since it begins and ends within the telencephalon. Efferent projection fibers convey impulses from the cerebral cortex to the thalamus, brain stem, and spinal cord. They represent the axons of pyramidal cells. The most important groups are those of the corticospinal and corticobulbar tracts, which together form the great motor or pyramidal system. These fibers begin in the motor cortex of the anterior central gyrus as axons of the giant cells of Betz. Entering the white medullary center of the hemisphere, they are as- sembled in the corona radiata (Fig. 194) and enter the internal capsule (Fig. 88). Their course beyond this point has been traced in the preceding chapters. They convey impulses to the primary motor neurons of the opposite side of the brain stem and spinal cord. Another important group of corticifugal fibers is contained in the corticopontine tracts. Of these there are two main strands. The frontopontine tract consists of fibers which begin as axons of cells in the cortex of the frontal lobe, traverse the centrum semiovale. corona radiata, frontal part of the internal capsule and medial one-fifth of the basis pedunculi. and finally terminate in the nuclei pontis. The temporopontine tract has a similar origin from the cortical cells of the temporal lobe and possibly of the occipital lobe also, passes through the sublenticular part of the internal capsule and lateral one- fifth of the basis pedunculi, and finally terminates in the nuclei pontis (Figs. 88, 106). The ascending thalamic radiation is paralleled by descending corticothalamic fibers, which should be included among the efferent projection systems, although their physiologic significance is not fully understood. Similar 298 THE NERVOUS SYSTEM efferent fibers are contained in the optic radiation. They arise in the cortex about the calcarine fissure and terminate in the pulvinar. lateral geniculate body, and superior colliculus of the corpora quadrigemina (Fig. 162). A corti- coruhral tract descend?- from the frontal lobe through the posterior limb of the internal capsule to end in the red nucleus of the mesencephalon. There do not appear to be any strictly corticostriate fibers, but. according to Cajal (1911), collateral- from the corticospinal fibers are given off to the corpus striatum. The efferent projection tracts which we have considered all have their origin in the neopallium. There are several projection tracts from the rhincncephaJon, and of these the most important is the fornix. The fibers of this fascicle take origin in the hip- Fig. 226. — Some of the important association bundles projected upon the medial aspect of the cerebral hemisphere. (Sobotta-McMurrich.) pocampus, follow an arched course already described, and. entering the dien- cephalon. terminate in part in the mammillary body and in part in the teg- mentum of the brain stem (Fig. 205). The frontal olfactory projection tract arises from the gray matter of the ol- factory peduncle and the lateral olfactory gyrus. It enters the brain stem and terminates in the pons and the medulla oblongata (Fig. 211). Association Fibers. — The various parts of the cortex within each hemisphere are bound together by association fibers of varying length. The short associa- tion fibers are of two kinds: (1) those which run in the deeper part of the cortex and are designated as intracortical, and (2) those just beneath the cortex, which are known as the subcortical fibers. The greater number of these subcortical association fibers unite adjacent gyri. curving in U-shaped loops beneath the THE CORTEX AND MEDULLAR? CENTER OF Mil CEREBRAL BEMISPHER] intervening sulci, and are accordingly often designated as annate fibei i 226). Others unite somewhat more widely separated gyri. The Un lotion fibers form bundles of considerable size, deeply situated in the medullary center of the hemisphere, and unite widely separated cortical area-. 'I lure are five of these which may be readily displayed by dissection of the human cerebral hemisphere, namely, the uncinate, inferior occipitofrontal, inferior longitudinal, and superior longitudinal fasciculi, and the cingulum. Another, known as the fasciculus occipitofrontal^ superior, is less easily displayed. The cingulum is an arched bundle which partly encircles the corpus callosum not far from the median plane (Figs. 174, 226). It begins ventral to the n»trum of the corpus callosum, curves around the genu and over the dorsal surface of Optic radiation External capsule ami leniiform nucleus Corona radiata ; Frontal lobe --Fas. occipitofrontalis inferior 'Fas. uncinatus -Temporal lobe Fig. 227. — Lateral view of a dissection of a human cerebral hemisphere. The dorsal part of the hemisphere has been cut away. On the lateral side the insula, opercula, and adjacent parts have been removed. that commissure to the splenium, and then bends ventrally to terminate near the temporal pole. It is closely related to the gyrus cinguli and the hippocampal gyrus and is composed for the most part of short fibers, which connect the various parts of these convolutions. The uncinate fasciculus connects the orbital gyri of the frontal lobe with the rostral part of the temporal lobe. It is sharply bent on itself as it passes over the stem of the lateral fissure of the cerebrum (Figs. 227, 228). The inferior longitudinal fasciculus is a large bundle which runs through the entire length of the temporal and occipital lobes (Fig. 226). It connects the occipital pole, the cuneus, and other parts of the occipital lobe with the temporal cortex, ex- tending as far forward as the temporal pole. According to Curran (1909) the 3°° THE NERVOUS SYSTEM uncinate and inferior longitudinal fascicles are formed by the shorter and more superficial libers of a larger and longer tract, the inferior occipitofrontal fasciculus, Superior longitudinal fasciculus Uncinate ft Inferior occipitofrontal fasciculus Fig. 228. — Some of the long association bundles projected upon the lateral aspect of the cerebral hemisphere. which unites the cortex of the frontal and occipital lobes (Figs. 227, 228). Along with the uncinate fasciculus it may easily be exposed by dissection, as it courses along the ventrolateral border of the lentiform nucleus. Cingulum Fas. occipilofrontalis sup. Corpus call os um — Fas. longitudinalis sup. Caudate nucleus — sUfJ Internal capsule Lentiform nucleus-^ Insula" Fas. occipitofrontalis inf.-'' Fas. uncinatus Amygdaloid nucleus Fig. 229. — Frontal section of the cerebral hemisphere through the anterior commissure showing the location of the long association bundles. The superior longitudinal fasciculus (fasciculus arcuatus) is a bundle of as- sociation fibers which serves to connect many parts of the cortex on the lateral Till CORTEX AND MEDULLARY CENTEB OF THE CEREBRAL HEMISPHERE 301 surface of the hemisphere (Tig. 228). It sweeps over the insula, occupying the base of the frontal and parietal opercula, and then bends downward into the temporal lobe (Fig. 174). Tt is composed for the most part of bundles of rather short fibers which radiate from it to the frontal, parietal, occipital, and temporal cortex. The superior occipitofrontal fasciculus runs in an arched course close to the dorsal border of the caudate nucleus and just beneath the corpu> callosum. It is separated from the superior longitudinal fasciculus by the corona radiata (Fig. 229). The weight of the brain varies with the sex, age, and size of the individual. The average weight of the brain in young adult men of medium stature is 1360 grams. It is less in women and in persons of small size or advanced age. It is doubtful if there is any close correlation between the brain weight and intelligence or between the latter and the size and arrangement of the cerebral convolutions (Donaldson, 1898). CHAPTER XIX THE GREAT AFFERENT SYSTEMS EXTEROCEPTIVE PATHWAYS TO THE CEREBRAL CORTEX As has been intimated elsewhere, it is chiefly those nervous impulses, which are aroused by stimuli acting upon the body from without, that rise above the subconscious level and produce clear-cut sensations. The importance of these sensations in our conscious experience is no doubt correlated with the fact that it is through the reactions, called forth by such external stimuli; that the organism is enabled to respond appropriately to the various situations in its constantly changing environment. To meet these complex and variable situations cor- rectly requires the nicest correlation of sensory impulses from the various sources as well as their integration with vestiges of past experience, and it is in connec- tion with these higher correlations and adjustments that consciousness appears. The responses initiated by interoceptive and proprioceptive afferent impulses are more stereotyped and invariable in character; and these reactions are for the most part carried out without the individual being aware either of the stimulus or the response. It is known that the cerebral cortex is the organ within which occur at least the majority of those complex and highly variable correlations and integrations which have consciousness as their counterpart. A single object may appeal to many sense organs, and our perception of that object involves a synthesis of a corresponding number of sensations and their comparison with past experience. For example, when I meet a friend and grasp his hand in greeting, my perception of him includes not only the image of his face but also the sound of his voice and the warm contact of his hand. Thus thermal, tactile, auditory, and visual sensations may be fused in the perception of a single object, and this involves an integration of the corresponding afferent impulses within the cerebral cortex. Accordingly, it becomes of special interest to trace the course of these afferent impulses from the various exteroceptive sense organs to their cortical receptive centers. As we shall see, the outer world has for the most part a crossed representation in the cerebral cortex. Cutaneous stimuli, received from objects touching the 302 nil- i.kl.Al .\l l l ki.m SYSTEMS ^ . Hght side of the body, and optic stimuli produced by light waves coming from the right half of the field of vision, are propagated to the cortex of the left hemi- sphere. The crossed representation in the case of hearing is less i omplete, partly because every sound wave reaches both ears, but also because the crossing of the central auditory pathway seems to be incomplete. The grouping of the afferent fibers in the peripheral nerves differs from that in the spinal cord. In each of the spinal nerves several varieties of sensory liber- are freely mingled. In the cutaneous branches are found conductors of thermal, tactile, and painful sensibility; while the deeper nerves contain liber- for pain and sensations of pre>sure-touch as well as for muscle, joint, and tendon sensi- bility. Because of the mtermingling of the variou> kind- of fibers a lesion of a spinal nerve results in a loss of all modalities of sensation in the area supplied exclusively by that nerve. But in the spinal cord a regrouping of the afferent impulse occurs, such that all of a given modality travel in a path by themselves. All those of touch and pressure, whether originally conveyed by the superficial or deep nerves, find their way into a common path in the cord. In the same way all painful impulses, whether arising in the skin or deeper parts, follow a special course through the cord. Another intramedullary path conveys impulses from the muscles, joints, and tendons. These various lines of conduction within the cord are so distinct from each other that a localized spinal lesion may interrupt one without affecting the others. A striking illustration of this is the loss of sensibility to pain and temperature over part of the body surface without any impairment of tactile sensibility as a result of a disease of the spinal cord, known as syringomyelia. While we shall here confine our attention to the afferent channels leading directly toward the cerebral cortex, it should not be forgotten that these are in communication with the reflex apparatus of all levels of the spinal cord and brain stem. The Spinal Path for Sensations of Touch and Pressure. — Tactile impulses which reach the central nervous system by way of the spinal nerves are relayed to the cerebral cortex by a series of at least three units. Neuron I.— The first neuron of this conduction system has its cell body, which typically is unipolar, located in the spinal ganglion; and its axon divides in the manner of a T or Y into a central and a peripheral branch. The per- ipheral branch runs through the corresponding spinal nerve to the skin, or in the case of those fibers subserving the tactile functions of deep sensibility, to the underlying tissues. The central branch from the stem process of the spinal 3°4 Tin: \Kkvors systkm ganglion cell enters the spinal cord by way of the dorsal roots. The touch fibers arc probably myelinated and enter the cuneate fasciculus through the medial division of the dorsal root; and, like all other dorsal root libers, they divide into ascending and descending branches. The ascending branches run for varying distances in the posterior funiculus, giving off collaterals before they terminate Internal capsule Thalamus Spinothalamic tract Medial lemniscus -■ Ascending branches of dorsal root fibers Ventral spinothalamic tract — Mesencephalon Medulla oblongata Spinal cord Dorsal root and spinal ganglion Fig. 230. — Diagram of the tactile path. m the gray matter of the spinal cord, some few at least even reaching the nucleus gracilis and cuneatus in the medulla oblongata. At varying levels they enter the gray substance of the columna posterior and form synapses with tht of the second order (Fig. 230). le neurons Tin: GREAT \i i i ki \ i SYST] 305 Neuron II, with its cell body located in the posterior gra\ column, sends ii axon across tin- median plane into the ventral spinothalamic tra< t in the oppo ite anterior funiculus. In this the liber ascends through the spinal cord and brain stem to the thalamus. This trait gives oil fibers, either collateral or terminal, to the reticular formation of the brain stem. Other neurons of the second order in the tactile path are located in the gracile and euneate nu< lei of the medulla oblongata, and their axons after crossing the median plane ascend in the median lemniscus of the opposite side to end in the thalamus. All of these secondary tactile libers end within the ventral part of the lateral thalamic nucleus. The course of the ventral spinothalamic tract through the medulla oblongata anil pons is not accurately known. It has generally been figured as joining the lateral spinothalamic tract dorsolateral to the olive (Fig. 230. See also Herrick, Fig. 81). But, since lesions in the lateral area of the medulla oblongata may cause a loss of pain and temperature sensation over the opposite half of the body without affecting tactile sensibility, it is not improhahle that DejVrinc (1<)14) is correct in supposing that it follows a median course, its fibers inter- mingled with those of the tectospinal tract which run, however, in the opposite direction (Fig. 234; Economo, 1911; Spiller, 1915). There is reason to believe that the ventral as well as the lateral spinothalamic tract consists in part of short relays with synaptic interruptions in the gray matter of the spinal cord and brain stem, and the two tracts are sometimes designated as the spino-reticulo-thala- mic path. In the spinal cord there appear to be two tracts which convey tactile im- pulses toward the brain, an uncrossed one in the posterior funiculus and another that crosses into the opposite anterior funiculus. Since these overlap each other for many segments, this arrangement would account for the fact that con- tact sensibility is usually unaffected by a purely unilateral lesion (Head and Thompson, 1906; Rothmann, 1906; Petren, 1902). Among the fibers of contact sensibility, which ascend in the posterior funiculus to the euneate and gracile nuclei of the same side, are those that subserve the function of tactile discrim- ination, or, in other words, the ability to recognize the duality of two closely juxtaposed points of contact, as when the two points of the compasses or dividers are applied simultaneously to the skin. Furthermore, those elements of tactile sensibility, which underlie the appreciation of the form of objects or stereognosis, ascend uncrossed in the posterior funiculus to the gracile and euneate nuclei. Neuron III. — The neurons located in the ventral portion of the lateral nucleus of the thalamus, with which the tactile fibers of the second order enter into syn- aptic relations, send their axons by way of the thalamic radiation through the posterior limb of the internal capsule and the corona radiata to the somesthetic area of the cerebral cortex in the posterior central gyrus (Fig. 220). 3°6 THE NERVOUS SYSTEM THE SPINAL PATH FOR PAIN AND TEMPERATURE SENSATIONS Tain and temperature sensations are mediated by closely associated though not identical paths, and it is convenient to consider them at the same time. Neuron I. — The first neuron of this system has its cell of origin located in the spinal ganglion. Its axon divides into a peripheral branch, directed through Internal capsule Thalamus Mesencephalon Medulla oblongata Lateral spinothalamic tract Spinal cord Dorsal root and spinal ganglion Fig. 231. — Diagram of the path for pain and temperature sensations. the peripheral nerve to the skin, or in the case of the pain fibers also to the deeper tissues, and a central branch, which enters the spinal cord through the dorsal root and almost at once terminates in the gray matter of the posterior gray column (Fig. 231). As was shown in Chapter VII, there is reason to believe that the [HE GREAT AITKKINl SYSTEMS 307 libers of painful sensibility, and possibly those of temperature sensation well, are unmyelinated and enter the cud through the lateral division of the dorsal root to end in the substantia gelatinosa Rolandi. Neuron II.- From these dorsal root fibers the Impulses arc transmitted (perhaps through the intermediation of one or more intercalated neurons) to the leurons of the second order. These have their cell bodies Located in the pos- terior gray column, and their axons cross the median plane and ascend in the lateral spinothalamic tract to end in the ventral part of the lateral nucleus of the thalamus. In addition to this long uninterrupted path, there probably also exists a chain of short neurons with frequent interruptions in the gray matter of the spinal cord, which serves as an accessory path to the same end station. In the medulla oblongata the spinothalamic tract lies dorsolateral to the inferior olivary nucleus. In the pons it joins the medial lemniscus and runs in the lateral part of this fillet through the pons and mesencephalon to the thalamus (Figs. 231, 234). Neuron III.— Fibers, arising from nerve-cells located in the lateral thalamic nucleus, convey thermal and possibly also painful impulses to the somesthetic area of the cerebral cortex in the posterior central gyrus by way of the thalamic radiation, and the posterior limb of the interal capsule. It is important to note that it is not necessary for painful afferent impulses to reach the cerebral cortex before they make themselves felt in consciousness, the thalamus being in itself sufficient for the perception of pain (Head and Holmes, 1911 ; Head, 1918). The Exteroceptive Paths Associated with the Trigeminal Nerve.— The tri- geminal nerve mediates tactile, thermal, and painful sensations from a large part of the cutaneous and mucous surfaces of the head. While there is reason to be- lieve that the tactile impulses mediated by this nerve follow a central course distinct from that of thermal and painful sensibility, we cannot as yet assign definite paths to either group, and shall consider the exteroceptive connections of this nerve as a unit. Neuron I.— The axon of a unipolar cell in the semilunar ganglion divides into a peripheral branch, distributed to the skin or mucous membrane of the head, and a central branch, which runs through the sensory root (pars major) of the trigeminal nerve into the pons. Here it divides into a short ascending and a long descending branch. The former terminates in the main sensory nucleus, and the latter in the spinal nucleus of that nerve (Fig. 232). Neuron II.— The fibers of the second order in the sensory paths of the tri- geminal nerve arise from cells located in the main sensory and the spinal nucleus 3 o8 THE NERVOUS SYSTEM of that nerve; and alter crossing the raphe they run in two tracts to the ventral part of the lateral nucleus of the thalamus. The ventral secondary afferent path is located in the ventral part of the reticular formation, close to the spino- thalamic tract in the medulla oblongata and dorsal to the medial lemniscus in the pons and mesencephalon (Figs. 132, 234). The dorsal tract lies not far from the floor of the fourth ventricle and the central gray matter of the cerebral ' N Thalamus 'Medial lemniscus Mesencephalon I I -1- Medial lemniscus -Pons '" Dorsal secondary tract N. V j Ventral secondary tract N. V Main sensory nucleus N. V — Pons N. V Spinal tract N. V I Spinal nucleus N. V -*■ Medulla oblongata Fig. 232. — Diagram of the exteroceptive pathways associated with the trigeminal nerve. aqueduct. It consists in considerable part of uncrossed fibers and of fibers hav- ing a short course (Wallenberg, 1905; Economo, 1911; Dejerine, 1914). Neuron III. — The afferent impulses are relayed from the thalamus to the cortex of the posterior central gyrus by fibers of the third order, which run through the posterior limb of the internal capsule. Their cells of origin are located in the lateral nucleus of the thalamus. THE GREA1 \l I I K l \ l SYSTEMS .W) The Neural Mechanism for Hearing. The spiral organ of Corti within the cochlea is connected with the auditory center in the cerebral cortex l>\ a chain of three or more units. Neuron I. The bipolar cells of the spiral ganglion within the cochlea send each a peripheral process to end in the spiral organ of Corti. Ku< h -end- a < entral branch to ramify in the cochlear nuclei, where it forms synaptic connections with the auditor)' neurons of the second order (Fig. 233). Transverse temporal ^yrus 1 Auditory radiation Medial geniculate body Inferior colliculus - Lateral lemnisci Collaterals to nucleus of lateral lemniscus Roslral portion of the pons ,Stri AM) K 1.1 1.1 \ mm S travel along the descending fibers, which arise in that nucleus, to the primary motor neurons of the spinal cord that give rise to the fibers innervating the dia- phragm an: Nerve to abdominal muscles Sympathetic ganglion Postganglionic Fig. 246. — Reflex mechanism of coughing and vomiting. (Herrick, Cajal.) disturbance is propagated along the afferent fibers of the vagus, through the nucleus of the tractus solitarius and the descending fibers arising in it to the spinal primary motor neurons, which innervate the diaphragm and the inter- costal and abdominal muscles. The corpora quadrigemina are important reflex centers. The path for re- flexes in response to sound begins in the spiral organ of Corti and follows the coch- lear nerve and its central connections, including the lateral lemniscus, to the inferior colliculus of the opposite side, and to a less extent of the same side also $3* THE NERVOl S SYS'l KM see p. 309). Thence the path follows the tectospinal and tectobulbar tracts to the primary motor neurons of the cerebrospinal nerves (see p. 167). The visual reflex arc begins in the retina, follows the optic nerve and optic tract with partial decussation in the chiasma, to the superior colliculus of the corpora quadrigemina (p. 226) ; thence it is continued by way of the tectospinal and tecto- bulbar paths to the primary motor neurons of the cerebrospinal nerves (Fig. 162). Pupillary Reactions. — The iris is innervated by two sets of sympathetic nerve-libers derived from the ciliary and the superior cervical sympathetic ganglia respectively. Impulses reaching the iris through the latter ganglion induce dilatation of the pupil; those through the ciliary ganglion cause constric + io~ . The latter reaction always accompanies accommodation. When vision is fo- Sup. colliculus Sensory nuc. .V. V Pons -- Upper thoracic segments of < spinal cord N. V \ Carotid plexus Sup. cervical sympathetic ganglion Cervical sympathetic trunk Fig. 247. — Pupillary reflex arcs. cused on a near object, contraction of the ciliary muscle results in accommoda- tion; and at the same time contraction of the two internal rectus muscles brings about a convergence of the visual axes. These two movements are always associated with a third, the contraction of the sphincter pupillae. In addition to this constriction of the pupil, which accompanies accommodation, two other pupillary reactions require attention (Fig. 247). The Pupillary Reflex (Light Reflex).— When, light impinges on the retinae there results a contraction of the sphincter pupillae and a corresponding constric- tion of the pupil. The reflex circuit, which is traversed by the impulses bringing about this reaction, begins in the retina and includes the following elements: the fibers of the optic nerve and tract, with a partial decussation in the optic EFFERENT PATHS AND REFLEX ARCS 333 chiasma; synapses in the superior colliculus of the corpora quadrigemina; fibers of the tectobulbar tract ending in the nucleus of Edinger-Westphal (visceral efferent portion of the oculomotor nucleus); the visceral efferenl fibers of the oculomotor nerve, ending in the ciliary ganglion; and the postganglioni< fibers extending from the ciliary ganglion to iris. The pupillary-shin reflex is a dilatation of the pupil following scratching of the skin of the cheek or chin. This is hut one example of the fact that dilatation of the pupil can he induced by the stimulation of many sensory nerves and i on stantly occurs in severe pain. The path includes the following parts: the fibers of these sensory nerves and their central connections in the brain stem and spinal cord; preganglionic visceral efferent fibers, which arise from the cells of the inter- mediolateral column of the spinal cord and run through the upper white rami and the sympathetic trunk to the superior cervical sympathetic ganglion; and postganglionic fibers, which arise in that ganglion and run through the plexus on the internal carotid artery to end in the iris (Fig. 247). We have in the case of the pupillary reactions an illustration of the double and antagonistic innervation, which, as we shall see in the next chapter, is a rather characteristic feature of the autonomic nervous system. CHAPTER XXI THE SYMPATHETIC NERVOUS SYSTEM The sympathetic nervous system is an aggregation of ganglia, nerves, and plexuses, through which the viscera, glands, heart, and blood-vessels, as well as Ciliary ganglion Maxillary nerve Sphenopalatine ganglion v Superior cervical ganglion of sympathetic \ \ Cervical plexus Brachial plexus Greater splanchnic nerve Lesser splanchnic nerve - Lumbar plexus Sacral plexus ■Pharyngeal plexus Middle cervical ganglion of sympathetic Inferior cervical gang, of sympathetic Recurrent nerve Bronchial plexus Cardiac plexus Esophageal plexus Coronary plexus Left vagus nerve Gastric plexus Celiac plex~us Superior mesenteric plexus —j- Aortic plexus ^s— Inferior mesenteric plexus Hypogastric plexus Pelvic plexus Bladder Vesical plexus Fig. 248. — The sympathetic nervous system. (Schwalbe, Herrick.) smooth muscle in other situations, receive their innervation. As illustrated in Fig. 248 it is widely distributed over the body, especially in the head and neck 334 Mil. SYMPA l III l H \l l:\ OUS SYS1 I.M 335 and in the thoracic and abdominal cavities. It musl tiol be too sharply de- limitated from the cerebrospinal nervous system, sin< e it contains greal numbers of fibers which run to and from the brain and spinal cord. For example, the vagus nerve containsmany fibers which are distributed through the thoracic and abominal sympathetic plexuses for the innervation of the viscera. In the same way the spinal nerves are connected by communicating brani hes or rami communicates with the sympathetic trunks. The sympathetic trunks are two nerve cords which extend vertically through the neck, thorax, and abdomen, one on each side of the vertebral column Fig. 248). Each trunk is composed of a series of ganglia arranged in linear order and bound together by short nerve strands. Every spinal nerve is connected with the sympathetic trunk of its own side by one or more gray rami commu- nicantes through which it receives libers from the sympathetic trunk. Fibers reach this trunk from the thoracic and upper lumbar nerves by way of the white rami communicantes (Fig. 257). The sympathetic trunk also gives off branches which enter into the formation of the nerve plexuses which are associated with the larger arteries. The largest of these plexuses is the celiac, which is associ- ated with the upper portion of the abdominal aorta and its branches. In this plexus and located in close relation to the abdominal aorta are the celiac, mesenteric, and aorticorenal ganglia, all of which are in man grouped in a pair of large irregular masses designated as the celiac ganglia and placed one on either side of the celiac artery (Fig. 257). The sympathetic ganglia may be grouped into three series as follows: (1) the ganglia of the sympathetic trunk, arranged in linear order along each side of the vertebral column and joined together by short nerve strands to form the two sympathetic trunks; (2) col- lateral ganglia, arranged about the aorta and including the celiac and mesenteric ganglia; and (3) terminal ganglia, located close to or within the structures which they innervate. As examples of the latter group there may be men- tioned the ciliary and cardiac ganglia and the small groups of nerve-cells in the myenteric and submucous plexuses (Fig. 257). FUNDAMENTAL FACTS CONCERNING VISCERAL INNERVATION General visceral afferent fibers are found in the ninth and tenth cranial nerves and in many of the spinal nerves, especially in those associated with the white rami (thoracic and upper lumbar nerves) and in the second, third, and fourth sacral nerves. These afferent fibers take origin from cells in the cerebro- spinal ganglia (Fig. 249). From these ganglia the fibers run through the corres- 336 THE NERVOUS SYSTEM ponding cerebrospinal nerves to the sympathetic nervous system, through which they pass without interruption in any of its ganglia to end in the viscera. These fibers are of all >izes. including large and small myelinated fibers and many which are unmyelinated (Chase and Ranson, 1914; Ranson and Billingsley, 1918). The afferent impulses mediated by these fibers serve to initiate visceral re- flexes, and for the most part remain at a subconscious level. Such general vis- ceral sensations as we do experience are vague and poorly localized. Tactile sensibility is entirely lacking in the viscera and thermal sensibility almost so, although sensations of heat and cold may be experienced when very warm or cold substances enter the stomach or colon (Carlson and Braafladt, 1915). Pain cannot be produced by pinching or cutting the thoracic or abdominal viscera. Acute visceral pain may, however, be caused by disease, as in the pas- sage of a stone along the ureter. From the cerebrospinal ganglia the visceral afferent impulses are carried to the brain and spinal cord by the sensory nerve roots. The relations within the cerebrospinal ganglia are not entirely clear; but it seems probable that the visceral afferent impulses are conducted through the ganglion by way of the two branches of the typical unipolar sensory neuron (Fig. 249). Many authors believe that there are also sensory fibers which arise from cells in the sympathetic ganglia and terminate in the spinal ganglia in the form of pericellular plexuses (Fig. 40, C). Through these plexuses visceral sensory impulses are supposed to be transmitted to somatic sensory neurons and to be relayed by them to the spinal cord. Since it has not been clearly demonstrated that any sensory fibers arise from cells in the sym- pathetic ganglia, this interpretation of the pericellular plexuses of the spinal ganglia must be regarded as purely hypothetic. Langley (1903) has presented strong evidence that few if any sensory fibers arise in the sympathetic ganglia. Physiologic experiments show that the visceral afferent fibers run in the white rami, yet all or practically all of the fibers of a white ramus degenerate if the cor- responding spinal nerve is severed distal to the spinal ganglion. Huber (1913) states that "it has not been determined that the fine medullated fibers or the unmedullated fibers which appear to enter the spinal ganglia from without and end in pericellular plexuses are. in fact, the neuraxes of sympathetic neurones." The hypothesis that these pericellular plexuses represent the termination of visceral afferent fibers is, therefore, not well supported. This subject is treated in more detail in a series of papers on the sympathetic nervous system by Ranson and Billingsley (1918). Visceral Efferent Neurons.— The general visceral efferent fibers of the cerebrospinal nerves take origin from cells located within the cerebrospinal axis. They do not run without interruption to the structures which they innervate; instead, they always terminate in sympathetic ganglia, whence the impulses, which they carry, are relayed to their destination by neurons of a second order (Fig. 249). This important information we owe to Langley (1900 and 1903), who showed that the injection of proper doses of nicotin into rabbits prevents Mil. SYMPATH1 m NERV01 - SYSTEM the passage of impulses through the sympathetic ganglia, although an undi minished reaction may be obtained by stimulation of the more peripheral -\m pathetic nerves By a long series of experiments Langle) bas shown that there are always two and probably never more than two neurons concerned in the conduction of an impulse from the central nervous system to smooth muscle or glandular tissue. The neurons of the first order in this series arc designated as preganglionic, those of the second order as postganglionic, with reference to tin- relation which they bear to the ganglion containing their synap Preganglionic neurons have their cell bodies located in the visceral efferent column of the cerebrospinal axis. The cells of this series are smaller than those Somatic afferent fiber ,, . , . Visceral afferent fiber Dorsa ^ oot \ ^ Spinal ganglion Dorsal ramus f Ventral ramus Ramus cotntnunicans Sympathetic ganglion - < . Visceral efferent fiber ,- , , , ■ c ,- J- . A \ cntral root Somatic efferent fiber Postganglionic fiber . _ Viscus Fig. 249. — Diagrammatic section through a spinal nerve and the spinal cord in the thoracic region to illustrate the chief functional types of peripheral nerve-fibers. of the somatic motor column and contain less massive Nissl granules. From these cells arise the fine myelinated visceral efferent fibers which run through the cerebrospinal nerves to the sympathetic nervous system and terminate in the sympathetic ganglia (Fig. 249). Postganglionic neurons have their cell bodies located in the sympathetic ganglia. In fact, these cells with their dendritic ramifications and the terminal branches of the preganglionic fibers synaptically related to them are the es- sential elements in the sympathetic ganglia. Their axons for the most part remain unmyelinated and run as Remak fibers through the sympathetic nerves 33* THE NERVOUS SYSTEM o > ir. -= Si •• 3 £ M O I; >, -2 biO I III SVMI'Allll I I. NERVOUS SYST] \1 and plexuses, to end in relation with involuntary muscle or glandular ti A very few postganglionic fillers acquire delicate myelin sheaths. Three streams of preganglionic fibers leave the cerebrospinal axis I The cranial stream includes the general visceral efferenl fibers of the 1 1 ulomotor, facial, glossopharyngeal, vagus, and accessory nerves. These fibers end in the terminal ganglia, already mentioned, which are located close to or within the organ which they innervate. In the cervical nerves there are no visceral ef- ferent fibers, the cranial stream being separated from the next by a rather wide gap. The thoracicolumbar stream includes the fibers which arise from the cells of the intermediolateral column of the spinal cord and make their exit through the thoracic and first four lumbar nerves (Langley, 1892; Miiller, 1000). After leaving the spinal nerves by way of the white rami they enter the sympathetic nervous system and terminate in the ganglia of the sympathetic trunk or in the celiac and associated collateral ganglia (Fig. 250). The sacfal stream includes the visceral efferent fibers of the second, third, and fourth sacral nerves. These arise from cells in the lateral column of gray matter in the sacral portion of the spinal cord and run through the visceral branch of the third sacral and a similar branch from either the second or fourth sacral nerves. These fibers end in the ganglia of the pelvic sympathetic plexuses. The Autonomic Nervous System. — For many reasons it is convenient to have a name which vill designate the sum total of all general visceral efferent neurons, both preganglionic and postganglionic, whether associated with the cerebral or spinal nerves. For this purpose the term "auto nomic ner vou s sy stem" is in general use. It designates that functional division of the nervous system which supplies the glands, heart, and smooth musculature with their efferent in- nervation (Fig. 250). It is important to bear in mind that this is a functional and not an anatomic division of the nervous system, that it includes o nly efferent^ elements, and that t he p reganglionic neurons lie in part within the cerebrospinal nervous system. The terminal portions of the preganglionic fibers and the postganglionic neurons are located in the sympathetic system. According to the origin of the preganglionic fibers, we may recognize the following three subdivisions of the autonomic system: (1) the cranial autonomic system, whose preganglionic fibers make their exit by way of the third, seventh, ninth, tenth. and eleventh cranial nerves; (2) the thoracicolumbar autonomic system, whose pre- ganglionic fibers make their exit by way of the thoracic and upper lumbar spinal nerves; and (3) the sacral autonomic system, whose preganglionic fibers run in the visceral rami of the second, third, and fourth sacral nerves (Fig. 250). 34Q THE NERVOUS SYSTEM The fibers of the thoracicoliimbar stream run by way of the white rami to the sympathetic trunk, while the libers of the cranial and sacral streams make no connection with that trunk, but run directly to the sympathetic plexuses. And while the thoracicolumbar preganglionic fibers terminate in the ganglia of the trunk, those of cranial and sacral origin end in the terminal ganglia. In these two respects the cranial and sacral streams agree with each other and differ from the thoracicolumbar outflow. Also in their response to certain drugs, like atropin and adrenalin, the two former agree with each other and differ from the latter. It is, therefore, desirable to group the cranial and sacral systems together as the craniosacral autonomic system. This has been called by many physiologists the parasympathetic system. It stands in contrast to the thoracico- lumbar autonomic system to which many physiologists have unfortunately applied the name "sympathetic system." The importance of recognizing these two principal subdivisions is further emphasized by the fact that most of the struc- tures innervated by the autonomic system receive a double nerve supply and are supplied with fibers from both subdivisions. The thoracicolumbar fibers are accompanied in most peripheral plexuses by craniosacral fibers of opposite func- tion so that the analysis of these plexuses is greatly facilitated by subdividing the autonomic system in this way. Visceral Reflexes. — In the gastro-intestinal tract and perhaps within other viscera there may be a mechanism for purely local reactions as indicated in the following paragraph. With this exception the evidence strongly indicates that all visceral reflex arcs pass through the cerebrospinal axis. In such an arc there are at least three neurons, namely, (1) visceral afferent, (2) pregang- lionic visceral efferent, and (3) postganglionic visceral efferent neurons (Fig. 249) . The purely local reactions which occur in the gut wall after section of all of the nerves leading to the intestine are known as myenteric reflexes and must de- pend upon a mechanism different from that of other visceral reflexes (Langley and Magnus, 1905; Cannon, 1912). Practically nothing is known of this mech- anism beyond the fact that it must be located in the enteric plexuses. Some authors have assumed that within these plexuses there is a diffuse nerve net similar to that found in the ccelenterates (Parker, 1919). While the evidence is far from satisfactory, it may be that such a net does exist in this situation and that it is responsible for these local reactions. Mil: SYMiw illi;iK NERVOUS SYSTEM STRUCTURE OF THE SYMPATHETIC GANGLIA I I), nerve-cells of the sympathetic ganglia arc almosl all multipolar, hut there arc also a feu that art- unipolar or bipolar. Each < ill i surrounded l>\ a leati 'I membranous capsule. Some of the dendrites ramify beneath tlii- capsule and are designated as intracapsular. Others pierce the capsule, run Ion- distances through the ganglia, and are known as extracapsular dendrites. Fig. 251. — Neurons from the human superior cervical sympathetic ganglion (pyridin-silver method): A, Three nerve cells and the intercellular plexus: a, unicellular glomerulus; b, neuron with extracapsular dendrites. B, Tricellular glomerulus. C, Neuron surrounded by subcapsular dendrites. Intracapsular dendrites are numerous in the sympathetic ganglia of man. but rare in those of mammals (Marinesco, 1906; Cajal, 1911; Michailow, 1911; Ranson and Billingsley, 1918). Beneath the capsule these dendrites may form an open network more or less uniformly distributed around the cell (Fig. 251 . O. or they may be grouped on one side of the cell, causing a localized bulging in the capsule (Fig. 251, A, a). Such a localized mass of subcapsular dendrites with interlacing branches is known as a glomerulus. Following Cajal's classifi- cation we may distinguish four types of glomeruli according to the number of 342 THE NERVOUS SYSTEM neurons whose dendrites enter into their formation, namely, unicellular (Fig. 251, A , a), bicellular, tricellular (Fig. 251, B), and multicellular glomeruli. Short intracapsular dendrites with swollen ends are sometimes present in the sym- pathetic ganglia of mammals (Fig. 252, A). Fig. 252. — Sympathetic ganglion cells showing various types of dendrites. Redrawn from Michailow. Methylcne-blue stain. A, From superior mesenteric ganglion, horse; B, from celiac ganglion, horse; C, from stellate ganglion, horse; D, from superior cervical ganglion, dog; E, celiac ganglion, horse; F, superior cervical ganglion, dog. Extracapsular dentrites pierce the capsule, run for longer or shorter dis- tances among the cells, and help to form an intercellular plexus of dendritic and axonic ramifications (Fig. 251, A). These dendrites may end in a variety of ways. Some of these types of endings may be enumerated as follows: (1) brush-like endings (Fig. 252, A); (2) plate-like or bulbous terminals applied I 111 S\ Ml'\ I II I in M l:\ I ; |\[ 343 against the outer surf ace of the capsule of another cell I ' /; i \ inter lacing branches, which form a plexus upon the outer surface of the capsule of an adjacent cell I Fig. 252. D Dogiel (1896) thought that the cells possessing the longest dendriti ry, but Cajal (,1'MI) could find no evidence for this, and was unable to trace anj of them from the ganglia and associated nerves to the viscera. Carpenter and Cone! (1914), usinu the size and arrangement of the Nissl granules as a criterion, were able t<> find only one cell type in the sympathetic ganglia, and concluded that these ganglia do not contain sensory nerve Fig. 253. — Neurons and intercellular plexus from the superior cervical sympathetic ganglion of a dog (pyridin-silver method). The axons of sympathetic ganglion cells are usually unmyelinated, but a few of them acquire thin myelin sheaths. They are the postganglionic libers which relay the visceral efferent impulses to the innervated tissue. According to Cajal (1911). who states that his anatomic studies are in accord with the physio- logic experiments of Langley, the axons of the cells in the ganglia of the sympa- thetic trunk dispose themselves in one of the three following ways: 1 1 Usually they run transversely to the long axis of the ganglion to enter a gray ramus. 344 THE NERVOUS SYSTEM (2) The axons may run through a connecting nerve trunk into another ganglion. He is not able to say whether these axons only run through the second ganglion or whether they make connections with its cells. In the chick embryo he at one time described collaterals coming from those longitudinal fibers of the ganglia, which take origin in neighboring ganglia. Now, however, he is inclined to doubt this observation, and thinks it likely that these collaterals all come from fibers that have entered the sympathetic trunk through white rami at other levels. Fig. 254. Fig. 255. Figs. 254 and 255. — Preganglionic fibers and pericellular plexuses of the frog. Fig. 254, Pre- ganglionic fibers, the branches of which form pericellular plexuses; Fig. 255, a unipolar sympathetic ganglion cell in connection with which a preganglionic fiber is terminating. Methylene-blue. (Huber.) (3) In some cases the axons, arising from cells in the ganglia of the sympathetic trunk, run toward the neighboring arteries in the visceral nerves. There is no anatomic evidence worth mentioning in favor of the existence of association neurons, uniting one sympathetic ganglion with another or one group of cells with another within such a ganglion. But there is strong physiologic evidence against the existence of such association neurons (Langley, 1900 and 1904); and Johnson (1918) has shown that none are present in the sympathetic trunk of the frog. Termination of the Preganglionic Fibers. — The spaces among the cells of a sympathetic ganglion are occupied by a rich intercellular plexus of dendritic THE SVMI'\ in [C NERVOT s SYSTEM branches and fine axons (Figs. 251, A; 253). The fine axons represenl therami fications of preganglionic fibers and they degenerate when the connection between the ganglion and the central nervous system is severed (Ranson and Billingsley, L918). Similar fibers pierce the capsules surrounding the cells and intertwine with the intracapsular dendrites. No doubt synaptic relations are established between the axonic and dendritic ramifications in these plexuses. Another and very characteristic type of synapse is established in the peri- cellular plexuses, formed by the terminal ramifications of preganglionic fibers upon the surface of the cell bodies of postganglionic neurons. Huber (1899) showed that fibers from the white rami branch repeatedly in the sympathetic ganglia and that the branches terminate in subcapsular pericellular plexuses (Figs. 254, 255). In the sympathetic ganglia of the frog the pericellular plexus seems to be the only type of synapse and there is no intercellular plexus. In the mammalian sympathetic ganglion these pericellular plexuses are harder to demonstrate and are probably less numerous, while the intercellular plexus is much in evidence. It is well established that one preganglionic fiber may be synaptically related to several postganglionic neurons, probably in some in- stances to as many as thirty or more (Ranson and Billingsley, 1918). COMPOSITION OF SYMPATHETIC NERVES AND PLEXUSES Some of the sympathetic nerves are as well myelinated as the cerebrospinal nerves and present a white glistening appearance. This is true, for example, of the cervical portion of the sympathetic trunk, the white rami, and the splanch- nic nerves. Such white sympathetic nerves are composed at least in large part of fibers running to and from the central nervous system. Other nerves like the gray rami and branches to the blood-vessels are gray, because they are com- posed chiefly of unmyelinated postganglionic fibers. In preceding paragraphs we have shown that there are probably no association or sensory neurons in the sympathetic ganglia; and, if this be true, there are no axons, arising from such cells, in the sympathetic nerve trunks and plexuses. These nerves and plexuses are composed of the following three kinds of fibers (Fig. 256) : (1) Preganglionic visceral efferent fibers, which are of small size and myelinated, have their cells of origin in the cerebrospinal axis, and terminate in the sympathetic ganglia. (2) Postganglionic fibers, which are for the most part unmyelinated, have their cells of origin in the sympathetic ganglia and terminate in involuntary muscle or glandular tissue. (3) Visceral afferent fibers, which include myelinated fibers of all sizes as well as many that are unmyelinated, have their cells of origin in 346 THE NERVOUS SYSTEM the cerebrospinal ganglia and terminate in the viscera. The statements con- tained in this paragraph should not be applied without qualification to the ter- S pi iinl ganglion Dorsal root Collateral ganglion Gland-, Blood-vessel-, Pacinian corpuscle n smooth muscle Sensory ending Motor ending on smooth muscle'' Ventral root Splanchnic nerve // ,. Ganglion of sympathetic trunk '• fl^~-- Gray ramus (JnM 1 ^-' " White ramus Sympathetic trunk Dorsal ramus ■Ventral ramus jg$ Gland ~g? Blood-vessel - White ramus v Gray ramus Ganglion of sympathetic trunk Sympathetic trunk Fig. 256. — Diagram showing the composition of sympathetic nerves. Black lines, visceral afferent fibers; unbroken red lines, preganglionic visceral efferent fibers; dotted red lines, post- ganglionic visceral efferent fibers. minal ganglia and plexuses, since it is probable that these contain additional elements either in the nature of sensory neurons or of a nerve net. ARCHITECTURE OF THE SYMPATHETIC NERVOUS SYSTEM The sympathetic trunks are two ganglionated cords, each of which consists of a series of more or less segmentally arranged ganglia, bound together by as- cending and descending nerve-fibers and extending from the level of the second cervical vertebra to the coccyx (Figs. 248, 257) . The two trunks are symmetrically placed along the anterolateral aspects of the bodies of the vertebrae. There are 21 or 22 ganglia in each chain; and of these, 3 are associated with the cervical spinal nerves, 10 or 11 with the thoracic, 4 with the lumbar, and 4 with the sacral spinal nerves. The sympathetic trunks are connected with each of the spinal nerves by one or more delicate nerve strands, called rami communicantes (Figs. TILE SYMPATHETIC NERVOUS SYSTEM 248, 257). T<> each spinal nerve there runs a %ray ramus from the sympathetic trunk. The white rami, on the other hand, arc more limited in distribution and unite the thoracic and upper four lumbar nerves with the corresponding portion of tlu- sympathetic trunk. The white rami consist of visceral afferent and preganglioni* visceral efferent fibers directed from the central into the sympathetic nervous system. The\ contribute the great majority of the ascending and descending fibers of the sympathetic trunk (Fig. 257). While some of the fibers may terminate in the ganglion with which the white ramus is associated, and others run directlj through the trunk into the splanchnic nerves, the majority of the fibers turn either upward or downward in the trunk and run for considerable distances within it (Fig. 250). The fibers from the upper white rami run upward, those from the lower white rami downward, while those from the intermediate rami may run either upward or downward. The cervical portion of the sympathetic trunk consists almost or quite exclusively of ascending fibers, the lumbar and sacral portions of the trunk largely of descending fibers from the white rami. The afferent fibers of the white rami merely pass through the trunk and its branches to the viscera. The preganglionic fibers, with the exception of those which run out through the splanchnic nerves, end in the ganglia of the trunk. Here they enter into synaptic relations with the postganglionic neurons. The majority of the postganglionic neurons, located in the ganglia of the sympathetic trunk, send their axons into the gray rami (Figs. 250, 256). The gray rami are composed of postganglionic fibers directed from the sym- pathetic trunk into the spinal nerves. These unmyelinated fibers, after joining the spinal nerves, are distributed with them as vasomotor, secretory, and pilo- motor fibers to the blood-vessels, the sweat glands, and the smooth muscle of the hair-follicles. Especially in the cervical region there are other important branches from the sympathetic trunk, which resemble the gray rami in structure and which convey postganglionic fibers to certain of the cranial nerves and to the heart, pharynx, the internal and external carotid and thyroid arteries, and through the plexuses on these arteries to the thyroid gland, salivary glands, eye, and other structures (Figs. 248, 250, 257). The cranial portion of the sympathetic trunk consists of three ganglia bound together by ascending preganglionic fibers from the white rami. In the cat it has been shown to contain few if any sensory or postganglionic fibers. The superior cervical ganglion is the largest of the three ganglia and from it there are given ofl 348 mi: nervous system numerous gray nerve strands. These are all composed of postganglionic fibers which arise in this ganglion. They run to the neighboring cranial and spinal nerves, to which they carry vasomotor, pilomotor, and secretory fibers, and to the heart, pharynx, and the internal and external carotid arteries (Figs. 248, 250. 257). The most important of these branches of the superior cervical ganglion are the three following: (1) The superior cervical cardiac nerve, which run.-, from the superior cervical ganglion to the cardiac plexus, carries accelerator fibers to the heart. (2) The internal carotid nerve runs vertically from the ganglion to the internal carotid artery, about which its fibers form a plexu.-. known as the internal carotid plexus (Fig. 257). It is by way of this nerve and plexus that the pupillary dilator fibers reach the eye (Fig. 247). (3) The branch of the superior cervical ganglion to the external carotid artery breaks up into a plexus on that artery. A continuation of this plexus extends along the external maxillary artery, and carries secretory fibers to the submaxillary salivary gland. The middle and inferior cervical sympathetic ganglia are smaller. Among the branches from these ganglia we may mention the gray rami to the adjacent spinal nerves and the middle and inferior cardiac nerves to the cardiac plexus (Figs. 248, 257; . The thoracic portion of the sympathetic trunk i> connected with the thoracic nerves by the gray and white rami. In addition to the rami communicantes and some small branches to the aortic and pulmonary plexuses, there are three important branches of the thoracic portion of the sympathetic trunk known as the splanchnic nerves. These run through the diaphragm for the innervation of abdominal viscera (Figs. 248, 257). The greater splanchnic nerve is usually formed by branches from the fifth to the ninth thoracic sympathetic ganglia and after piercing the diaphragm joins the celiac ganglion. The smaller splanch- nic nerve is u.-ually formed by branches from the ninth and tenth thoracic sympathetic ganglia and terminates in the celiac plexus. The loiccrmost splanch- nic nerve arises from the last thoracic sympathetic ganglion and terminates in the renal plexus. These splanchnic nerves, although they appear to be branches of the thoracic sympathetic trunk, are at least in major part composed of fibers from the white rami, which merely pass through the trunk on their way to the ganglia of the celiac plexus (Figs. 250. 257: Langley. 1900; Ranson and Billings- ley. 1918 . THE SYMPATHETIC PLEXUSES The Sympathetic Plexuses of the Thorax. — In close association with the vagus nerve in the thorax are three important sympathetic plexuses. The Mil S"i MI'MIII I [( \l l'\ .,1 3YST] M J4Q cardiac plexus lies in close relation to the arch of the aorta, and from it sub- ordinate plexuses arc continued along the coronary arteries, h receives the three cardiac sympathetic nerves from the cervical portion of each sympathetic trunk, as well as branches from both vagus nen i I \. 248, 257). Thepregan glionic fibers of the vagus terminate in synaptic relation with the cells of the cardiac ganglia. They convey inhibitory impulses which arc relayed through these ganglia to the cardiac musculature (Fig. 250). The cardia* sympathetic nerves contain postganglionic fibers which take origin in the cervical sympa thetic ganglia; and they relay accelerator impulses, coming from the spinal cord by way of the upper white rami and sympathetic trunk to the hearl I Fig. 250 The pulmonary and esophageal plexuses of the vagus arc also to be regarded as parts of the sympathetic system (Fig. 257). The celiac plexus (solar plexus) is located in the abdomen in close relation to the celiac artery (Figs. 248, 257). It is continuous with the plexus which surrounds the aorta. Subordinate portions of the celiac plexus accompany the branches of the celiac artery and the branches from the upper part of the abdominal aorta. These are designated as the phrenic, suprarenal, renal, spermatic or ovarian, abdominal aortic, superior gastric, inferior gastric, he- patic, splenic, superior mesenteric, and inferior mesenteric plexuses. The celiac plexus contains a number of ganglia which in man are grouped into two large flat masses, placed one on either side of the celiac artery and known as the celiac ganglia. These ganglia are bound together by strands which cross the median plane above and below this artery. Somewhat detached portions of the celiac ganglion, which lie near the origin of the renal and superior mesenteric arteries, are known respectively as the aorticorenal and superior mesenteric ganglia. In addition, there is a small mass of nerve-cells in the inferior mesen- teric plexus close to the beginning of the inferior mesenteric artery. This is known as the inferior mesenteric ganglion. Preganglionic fibers reach the celiac plexus from two sources, namely, from the white rami by way of the sympathetic trunk and splanchnic nerves and from the vagus nerve (Fig. 257). Most if not all of the preganglionic fiber- contained in the splanchnic nerves terminate in the ganglia of the celiac plexus. At the lower end of the esophageal plexus the fibers from the right vagus nerve become assembled into a trunk which passes to the posterior surface of the stomach and the celiac plexus. The fibers of the left vagus pass to the anterior surface of the stomach and to the hepatic plexus (Fig. 257). It i> probable that the pre- ganglionic fibers of the vagus do not terminate in the ganglia of the celiac plexus, 6^ THE NERVOUS SYSTEM N. VII I i Internal carol id plexus. To N. X ToN.IX- To cervu al X . I ■ X. II X. Ill X.IY— -^* N. V ^4 N. VI- --*-» ^ N. VII. N. VIII- To thoracic N. I • N. II- N. III- N. IV- N. V- N. VI- N. VII- N. VIII- N. IX- N. X- N. XI. N. XII- To lumbar X. /_ N. II. N. III- X. IV- N. V--- To sacral X. I \^ N. II , N. Ill Visceral branches of \ rrT sacral nerves j 111 ' I v ^- . N. IV — N. V — To coccygeal nerve <•**' -.V. /// "Ciliary ganglion ~ S plena palatine ganglion -N. IX ■•Otic ganglion 'Superior cervical ganglion "Pharyngeal plexus -N. VII -Submaxillary ganglion 'Middle cervical ganglion -Superior cardiac N. -Middle cardiac N. •Inferior cardiac N. ■ Cardiac brandies of vagus ' Vagus and left pulmonary plexus -Cardiac plexus •Left coronary plexus . Esophageal plexus Splanchnic nerves ^Hepatic plexus Left vagus nerve ^ .Gastric plexus \ Myenteric and sub- /^ mucous plexuses '^^~ Splenic plexus Celiac plexus Superior mesenteric plexus Inferior mesenteric plexus "Abdominal aortic plexus Hypogastric plexus Fig. 257.— Diagram of the sympathetic nervous system. The red lines indicate the branches of the cerebrospinal nerves which join the sympathetic system and those sympathetic nerves which are composed in major part of fibers from the cerebrospinal nerves. (Modified from Jackson- Morns.) but merely pass through that plexus to end in the terminal ganglia, such as the small groups of nerve-cells in the myenteric and submucous plexuses of the in- testine (Fig. 250). IHE SYMPATH] I [I NERV01 S SYS1 l.M The myenteric plexus (of Auerbach) and the submucous plexus (of Mei sner), located within the walls of the stomach and intestini .«• filaments from the gastric and mesenteric divisions of the celiac plexus. The] also receive fibers from the vagus either directly, as in the case of the stoma* h. or indire* tly through the celiac plexus (Fig. 257). Unfortunately, very little is known con- cerning the synaptic relations established in the ganglia of these plexuses. A. cording to Langley, the postganglionic fibers from the celiac ganglia run through these plexuses without interruption and end in the muscular coats and glands of the gastro-intestinal tract. The preganglionic fibers from the vagus probably end in synaptic relation to cells in these small ganglia; and the axons of these cells serve as postganglionic fibers, relaying the impulses from the vagus to the glands and muscular tissue. As was indicated in a preceding paragraph, the enteric plexuses must also contain a mechanism for purely local reactions, sun e peristalsis can be set up by distention in an excised portion of the gut. But as yet we are entirely ignorant as to what that mechanism may be. The hypogastric plexus is formed by strands which run into the pelvis from the lower end of the aortic plexus and are joined by the visceral branches of the second, third, and fourth sacral nerves and by branches from the sympathetic trunk (Figs. 248, 257). As the hypogastric plexus enters the pelvis it splits into two parts, which lie on either side of the rectum and are sometimes called the pelvic plexuses. From these plexuses branches are supplied to the pelvic vis- cera and the external genitalia. The Cephalic Ganglionated Plexus. — In close topographic relation to the branches of the fifth cranial nerve are four sympathetic ganglia, known as the ciliary, sphenopalatine, otic, and submaxillary ganglia. Each of these is con- nected with the superior cervical sympathetic ganglion by filaments derived from the plexuses on the internal and external carotid arteries and their branches (Fig. 257). These filaments are designated in descriptive anatomy as the sym- pathetic roots of the ganglia. Each ganglion receives preganglionic fibers from one of the cranial nerves by way of what is usually designated as its motor root (Fig. 257). Thus the ciliary ganglion receives fibers from the oculomotor nerve; the sphenopalatine ganglion receives fibers from the facial nerve by way of the great superficial petrosal nerve and the nerve of the pterygoid canal; the otic ganglion receives fibers from the glossopharyngeal nerve (Miiller and Dahl, 1910) ; and the submaxillary ganglion receives fibers from the facial nerve by way of the nervus intermedius and the Ungual nerve. Postganglionic fibers arising in these ganglia are distributed to the structures of the head. From the ciliary 352 THE NERVOUS SYSTEM ganglion libers go to the intrinsic musculature of the eye. Some of the fibers arising in the sphenopalatine ganglion go to the blood-vessels in the mucous membrane of the nose. Fibers from the otic ganglion reach the parotid gland. And those arising in the submaxillary ganglion end in the submaxillary and sublingual salivary glands (Fig. 250). IMPORTANT CONDUCTION PATHS BELONGING TO THE AUTONOMIC NERVOUS SYSTEM Thanks to the work of Langley, we know that the conduction pathways in the sympathetic nervous system are at least as sharply defined as those in the brain and spinal cord. A great deal has already been done in the way of tracing these pathways; and some of the more important of these are given in the out- line which follows : 1. Paths for the efferent innervation of the eye (Figs. 247, 250): (a) Ocular craniosacral pathway. Preganglionic neurons: Cells in the Edinger-Westphal nucleus, fibers by way of the third cranial nerve to end in the ciliary ganglion. Postganglionic neurons: Cells in the ciliary ganglion, fibers by way of the short ciliary nerves to the ciliary muscle and the circular fibers of the iris. Function: Accommodation and contraction of the pupil. (b) Ocular thoracicolumbar pathway. Preganglionic neurons: Cells in the intermediolateral column of the spinal cord, fibers by way of the upper white rami and sympathetic trunk to end in the superior cervical ganglion. Postganglionic neurons: Cells in the superior cervical ganglion, fibers by way of the internal carotid plexus to the ophthalmic division of the fifth nerve, the nasociliary and long ciliary nerves of the eyeball; other fibers pass from the internal carotid plexus through the ciliary ganglion, without interruption, into the short ciliary nerves and to the eyeball. Function: Dilatation of the pupil by the radial muscle-fibers of the iris. 2. Paths for the efferent innervation of the submaxillary gland (Fig. 250): (a) Submaxillary craniosacral pathway. Preganglionic neurons: Cells in the nucleus salivatorius superior, fibers by way of the seventh cranial nerve, chorda tympani, and mi SYMPATH] i K NERV01 5 SYST] M lingual nerve to end in the portion of the submaxillary ganglion located on the submaxillary duel . Postganglionic neurons: Cells in a number of groups along the chorda tympani fibers as they follow the submaxillary duct, fibers distributed in branches to the submaxillar) gland. Function : [ncreases se< retion. (b) Submaxillary thoracicolumbar pathway. Preganglionic neurons: Cells in the intermediolateral column of the spinal cord, fibers by way of the upper white rami, and the sym- pathetic trunk to end in the superior cervical ganglion. Postganglionic neurons: Cells in the superior cervical ganglion, fibers by way of the plexuses on the external carotid and external maxillary arteries to the submaxillary gland. Function: Increases secretion. 3. Paths for the efferent innervation of the heart: (a) Cardiac craniosacral pathway. Preganglionic neurons: Cells in the dorsal motor nucleus of the vagus, fibers through the vagus nerve to the intrinsic ganglia of the heart, in which they end. Postganglionic neurons: Cells in the intrinsic cardiac ganglia, fibers to the cardiac muscle. Function: Cardiac inhibition. (b) Cardiac thoracicolumbar pathway. Preganglionic neurons: Cells in the intermediolateral column of the spinal cord, fibers by way of the upper white rami and the sym- pathetic trunk to the superior, middle, and inferior cervical ganglia. Postganglionic neurons: Cells in the cervical ganglia of the sym- pathetic trunk, fibers by way of the corresponding cardiac nerves to the musculature of the heart. Function: Cardiac acceleration. 4. Paths for the efferent innervation of the musculature of the stomach exclusive of the sphincters (Fig. 250) : (a) Gastric craniosacral pathway. Preganglionic neurons: Cells in the dorsal motor nucleus of the vagus, fibers by way of the vagus nerve, to end in the intrinsic ganglia of the stomach. 23 354 THE NERVOUS SYSTEM Postganglionic neurons: Cells in the intrinsic gastric ganglia, fibers to end in the gastric musculature. Function: Excites peristalsis. (b) Gastric thoracicolumbar pathway. Preganglionic neurons: Cells in the intermediolateral column of the spinal cord, fibers by way of the white rami from the fifth or sixth to the twelfth thoracic nerves, through the sympathetic trunk without interruption, and along the splanchnic nerves to the celiac ganglion, where they end. Postganglionic neurons: Cells in the celiac ganglion, libers by way of the celiac plexus and its offshoots to the stomach, to end in the musculature of the stomach. Function: Inhibits peristalsis. 5. Paths for the efferent innervation of the musculature of the urinary bladder. (a) Vesical craniosacral pathway. Preganglionic neurons: Cells in the lateral part of the anterior gray column in the sacral portion of the spinal cord, fibers by way of the second and third sacral nerves and their visceral rami through the pelvic plexus to the plexus upon the wall of the bladder. Postganglionic neurons: Cells in the small ganglia of the vesical plexus, fibers to the vesical musculature. Function: Excites contraction of the vesical musculature exclusive of the internal sphincter (trigonal area), the contraction of which it inhibits and thus produces urination. (b) Vesical thoracicolumbar pathway. Preganglionic neurons: Cells in the caudal part of the intermedio- lateral cell column, fibers by way of the lower white rami to the infe- rior mesenteric ganglion. Postganglionic neurons: Cells in the inferior mesenteric ganglion, fibers through the inferior mesenteric plexus to the musculature of the bladder. Function: Excites contraction of the internal sphincter (trigonal area of the vesical musculature), causing retention of urine. It will be noted that the viscera receive a double autonomic innervation, and that the impulses transmitted along the craniosacral pathways are usually antagonistic to those transmitted along the thoracicolumbar paths. A LABORATORY OUTLINE OF NEURO-ANATOMY The following directions for the study of the gross and microscopi< anatomy of the nervous system are intended to aid the student in making the besl use of bis time and laboratory material. Free use is made of the sheep's brain because in mosl in- stitutions the number of human brains available is limited, and these arc often poorly preserved and entirely unsuited for dissection. Even if an unlimited supply of well- preserved human brains were at hand, there would still be an advantage in the use of the sheep's brain because in it certain structures (such as the olfactory trai 1 - and < enters and the really significant subdivisions of the cerebellum) are more easily seen and more readily understood. The outline has been written in such a way that it can be readily adapted by the instructor to meet his own needs. It is assumed that each instructor will furnish his students with a schedule for the laboratory work, showing the number of laboratory periods available and the topics to be covered each period. This will help the student properly to apportion his time and enable the instructor to arrange the order of the laboratory work to his own liking. The paragraphs have been numbered serially in order that in such a schedule they may be referred to by number. It is not necessary that the topics be taken up in their numeric order. And in a course of one hundred hours some of the topics should be omitted altogether. How much should be omitted will depend largely on the amount of drawing required. It is assumed that the in- structor will indicate on the laboratory schedule the drawings which he wishes to have made. For this reason we have, for the most part, omitted specific directions for draw- ings. Since it will be necessary for the student in using the outline to make frequent references to figures in the text, it will be convenient to keep in the book several strips of thin paper to serve as bookmarks. METHODS OF BRAIN DISSECTION Much information concerning the gray masses and fiber tracts of the brain can be obtained by dissection. This should be carried out, for the most part, with blunt instruments. It is rarely necessary to make a cut with a knife. An orangewood mani- cure stick makes an excellent instrument. It should be rounded to a point at one end for teasing, while the larger end should be adapted for scraping away nuclear masses. A pair of blunt tissue forceps of medium size with smooth even edges and fine transverse interlocking ridges is also an essential instrument. This i< useful in grasping and -trip- ping away small bundles of fibers. In dissecting out a fiber tract it is necessary to have in mind a clear idea of the position and course of the tract, and the dissecting instru- ments should be carried in the direction of the fibers. Where it is uecessary to remove nuclear material in order to display fiber bundles, it will be found very helpful to let a stream of water run over the specimen while the dissection is in progn 355 356 THE NERV01 - SYSTEM DISSECTION OF THE HEAD OF THE DOGFISH 1. The dogfish is the smallest of the sharks. Either the spiny rlr^fish (Squalus acanthias or the smooth dogfish (Mustelus canis) may be used for dissection. 2. The special ■ \\ the cartilage in thin slices in the region between the spiracle and the median plane. The membranous labyrinth can be seen through the translucent cartilage, and care should be exercised to avoid injuring it while the cartilage is being removed. It consists of a spheric sac, the utriculosaccular chamber, to which there are attached three semicircular canals (Fig. 12). The endolymphatic duct is a small canal, which extends from this chamber through the roof of the skull to the small opening in the skin, which has previously been identified. Xote the enlarge- ment at one end of each semicircular canal, known as the ampulla, and observe that each of these canals lies in a plane at right angles to the planes of the other two. 7. The Brain and Cranial Serves. — Remove the remainder of the roof of the skull and expose the brain, eyes, and cranial nerves. 8. Examine the brain as seen from the dorsal surface. Note the continuity of the medulla oblongata with the Spinal cord. Identify the cerebellum, the thalamus, epiphysis, habenula, cerebral hemispheres, and olfactory bulbs (Fig. 8 and pp. 26-31 J. 9. By dissection display on the left side the eye-muscles and the nerves which in- nervate them, as well as the optic nerve (Fig. 12). 10. Find the nervus tmninalis (Fig. 8). Now locate each of the cranial nerves from the second to the tenth inclusive, and trace them from the brain as far as possible toward their peripheral terminations (Figs. 12, 13). Note particularly that Nn. VII and X each have an extra root, indicated in black in Pig 13, which carries fibers from the lateral line organs to the acusticolateral area of the medulla. 11. Attention should now be paid to the functional types of nerve-fibers which compo>e each of the cranial nerves (see pp. 168 170 and Figs. 119, 120). The ac- companying table shows in which of the cranial nerves of the dogfish each of the four principal functional groups of liber- are to be found (Herrick and Crosby, 1918). \ LABORATORY 01 I LIN] l IF \i I RO \\ \ \<>\\\ ( u \\i \i \i u\ i ( OMPONl NTS Ol mi I i i^n - Somal ic sensory. Somatit motoi Viscei ■l..r. II. Optic 1. < llfai torj III. Muscle sense 1\ . Muscle sense III. To eye-muscles I V. To eye-mua les III. For inl r i 1 1 -i» mi ol thi Y. ( ieneral cutaneous VI. Muscle sense VI. To eye-muscles V. To i he jaw must les \'l 1. Lateral line fibers VII. ( ieneral \ is* eral VII. To hyoid must ula- VIII. To the ear .tnd gustatory t lire IX. Lateral line fibers IX, X. < Ieneral \ is< eral IX, X To brani hial and X. Lateral line and general cutaneous fibers and gustatorj general \ isi eral mus- culat lire 12. There are six pairs of cranial nerves associated with the medulla oblongata. The tenth cranial or vagus nerve is one of the largest and arises by two series of roots. One group of rootlets springs from the dorsolateral aspect of the medulla oblongata near its lower end, and contains libers which are distributed through the branchial and gastro- intestinal rami of the vagus, while a large root, carrying fibers for the lateral line sense organs, runs farther cephalad and enters the acusticolateral area. The ninth or glosso- pharyngeal nerve, the nerve of the first branchial arch, arises from the medulla ob- longata just ventral to this root of the vagus. Since the gills, as well as the gastro- intestinal tract, are visceral organs, both the ninth and tenth nerves earn - many visceral fibers. The eighth or acoustic nerve arises from the side of the medulla opposite the caudal part of the cerebellum in company with the fifth and seventh nerves, and ends in the membranous labyrinth of the ear. Like the vagus, the facial or seventh cranial nerve has, in addition to its main root, another, which runs further dorsally into the acusticolateral area. This root carries sensory fibers for the lateral line organs of the head. The sixth or abducens nerve arises more ventrally at the same level as the eighth. The fifth, or trigeminal nerve, which sends many branches to the skin of the head, is represented by a large root emerging from the medulla oblongata in company with the seventh. Some idea of the peripheral distribution of these nerves can be gained from a study of Figs. 12 and 13. 13. The floor of the fourth ventricle should now be exposed by carefully tearing away the membranous roof of that cavity. The floor presents for examination a -cries of longitudinal ridges and furrows which are of importance because they mark the position of longitudianl columns (Figs. 8, 13), to each of which a special functon can be assigned. A ridge on either side of the midline represents the position of the median longitudinal bundle, beneath which lie the nuclei of the third, fourth, and sixth cranial nerves. Since these nerves supply somatic musculature, the longitudinal elevation marks the position of the somatic motor column. Separated from this ridge by a broad furrow 358 THE NERVOUS SYSTEM is a more prominent ridge with tooth-like secondary elevations. Within this second ridge terminate the fibers of visceral sensation and taste from the seventh, ninth, and tenth nerves. It is known as the visceral lobe or visceral sensory column. Beneath the groove which separates these two ridges are located the motor nuclei of the fifth, seventh, ninth, and tenth cranial nerves. These nuclei supply visceral musculature and constitute the visceral motor column. The dorsal part of the lateral wall of the fossa forms another prominent ridge, which just caudal to the cerebellum is redundant and folded on itself to form an ear-shaped projection. This auricular fold, sometimes called the lobus linete lateralis, and the prominent margin just caudal to it belong to the acusticolateral area and contain the centers for the reception of impulses coming from the ear (N. VIII) and from the sense organs of the lateral line (Nn. VII and X). Ad- jacent to the acusticolateral area is a portion of the medulla oblongata which is concerned with the reception of sensory impulses from the skin which reach the medulla oblongata along the fifth and tenth nerves. The nuclei of the acusticolateral and general cutane- ous areas together constitute the somatic afferent column. 14. Locate these functional columns on your specimen. Note the close relation of the olfactory bulb to the nasal sac. By comparison with Fig. 13 locate on your speci- men the olfactory portions of the brain. What part of the brain is especially associated with the eyes? 15. Cut the nerve roots at some distance from the brain. Remove the brain, being careful not to injure the olfactory bulbs. Now study the lateral and ventral surfaces of the brain in order to locate more accurately the points of origin of the various cranial nerves (Fig. 10). 16. Now study the parts of the brain which belong to the rhombencephalon. Which parts are they, and what is their relationship to each other? (Figs. 8, 10 and p. 26.) 17. Study the parts of the brain which belong to the mesencephalon. Which are they, and what relationship do they bear to each other? (Figs. 8, 10 and p. 28.) 18. In the same way study the parts belonging to the diencephalon (Figs.. 8, 10 and pp. 28, 29). Make a list of these parts. Tear aw T ay the membranous roof of the third ventricle and examine that cavity. 19. Note the external form of the telencephalon and the parts which compose it (Figs. 8, 10). Students working at adjacent' tables should cooperate in the work which follows in order that two sharks' brains may be available. With a sharp razor blade divide one in the medial sagittal plane; and with a sharp scalpel open up the ventricles in the other as indicated in Fig. 9. Study the ventricles of the brain as they are displayed in these preparations and in Figs. 9 and 11. 20. Find the velum transversum and the ridge produced by the optic chiasma. All that part of the brain which lies rostral to these structures belongs to the telen- cephalon. Study the telencephalon in detail (Figs. 8-11 and p. 30). Of what parts is it composed, and what are their relations to each other? Pay special attention to the several parts of the telencepha'ic cavity. THE BRAIN OF THE FETAL PIG 21. Using a pig embryo of about 35 mm., slice off the skin and a small amount of the underlying tissue on either side of the head with a sharp razor. Then at one carefu 1 A LABORATORY OUTLINE OF NEURO \\\|n\iv stroke split the specimen lengthwise in the median plane. I his provides two prepara- tions for dissection, which should be used l>v two students. ( 'ert oral aqueduct Lamina quadrigetnina Cerebral pedunch Cerebellu Chorioid plexus of fourth ventru l< Fourth ventricl Medulla oblongata Central canal of spinal cord Pineal I Third \> ntrii le Hypothalamus Thalamus Chorioid plexus of lateral Vi iitr'n h I. ati ral ventru le ■Corpus striatum Lamina tcrminalis Rhinent < plialon Hypophysis Tongue Fig. 258. — Medial sagittal section of the head of a 35 nun. pig embryo. (Redrawn from Prentiss- Arey.) 22. First study the medial section of the brain, noting the five divisions of tin brain, the ventricles, and the relation of the cerebral hemispheres to other parts of the Semilunar ganglion N. V Mi sencepkalon Cerebellum Hypothalamus Geniculate gang. X. I'll Ganglion N. VIII Medulla oblongata Jugular gang. N. X Gang, of Froriep Gang. X. cerv. I Accessory nerve Hypoglossal nerve Ganglion nodosum X. X Gang. X. cerv. V Cerebral hemisphere X. 1'. ophthalmic X. Rhinenccplialon X . opticus X . 1'. maxillary X . X . V . mandibular X. Chorda tympani Facial X . Fig. 259- Dissection of the head of a 35 mm. pig embryo. Lateral view. (Redrawn from Prentiss- Arey.) brain (Fig. 258. See also Figs. 16, 17 and pp. 32-36). Of what three parts is the cerebral hemisphere composed? Locate each of the subdivisions of the dieneephalon. 360 THE NERVOUS SYSTEM To which part does the pineal body belong? The hypophysis? Locate the quadri- geminal lamina, cerebral peduncle, cerebellum, and medulla oblongata. 23. Now turn the specimen over and carefully dissect away what remains of the skin and mesodermal tissues so as to expose the brain and cranial nerves from the lateral side. Identify all the parts labeled in Fig. 259. GENERAL TOPOGRAPHY OF THE BRAIN 24. The adult mammalian brain should now be compared with that of the shark and with that of the fetal pig. If two sheeps' brains are available, one should be divided into lateral halves by a cut made exactly 1 mm. to the left of the median sagittal plane. Use a long, thin brain knife and make the cut with a single sweep. Put away the right half for future study. On the left half and on the intact brain identify all of the chief divisions of the brain, determine their embryologic derivation, and compare them with similar parts in the brains of the shark and fetal pig. (See the table on p. 36, pp. 113-116, and Figs. 82-84.) 25- By a study of the medial aspect of the left half of the brain ascertain what relations the various subdivisions bear to each other. (See Fig. 84 and pp. 116-118.) Note the difference in color between the cortex and the white center of the cerebellum. By tearing away the cerebellum a little at a time make a dissection of the cerebellar peduncles on this half of the brain (Figs. 87, 91). Scrape away the superficial gray matter from the rostral end of the left hemisphere and uncover the white substance beneath. The superficial gray matter is known as the cerebral cortex and this covers the white center of the cerebral hemisphere. NEUROLOGIC STAINS 26. Some knowledge of how various stains act on the nervous tissues is essential for an understanding of the special preparations which are to be studied. The technic involved in preparing such material is described in books devoted to technical methods (Hardesty, 1902; Guyer, 1917). 27. Osmic Acid. — Small nerves may be fixed in osmic acid. This stains the myelin sheaths black. Why? Axons remain unstained. 28. The Weigert or Pal-Weigert Method. — When a portion of the brain or spinal cord has been treated for several weeks with a solution containing potassium bichromate (Miiller's fluid) the myelin sheaths acquire a special affinity for hematoxylin, by virtue of which they become deep blue in color when stained by this method. Axons, nerve-cells, and all other tissue elements remain colorless unless the preparation has been counterstained. The method is adapted for the study of the development and extent of myelination and for tracing myelinated fiber tracts. This method may also be used for a study of degenerated fiber tracts, which remain colorless in preparations in which the normal fiber tracts are well stained. 29. The Marchi method is a differential stain for degenerating fibers. These contain droplets of chemically altered myelin. The tissue is fixed in a solution contain- ing potassium bichromate (Miiller's fluid). This treatment prevents the normal myelinated fibers from staining with osmic acid, but does not prevent the droplets of chemically altered myelin in the degenerated fiber from being stained black by this A LABORATORY OTJ I LINE OF \i I RO \\ \ [OlfY reagent. In a section prepared by this method the normal myelinated fibers are light yellow, while the degenerated fibers are represented by rows of bla< I. dots. 30. The newer silver stains, including the Cajal method and the pyridin-silver technic, depend upon the special affinity for silver nitrate possessed by nerve-cells and their processes. After treatment with silver nitrate tin- tissue is transferred t<» a solution of pyrogallic acid or hydroquinon which reduces the silver in the neurons to a metallic state. Nerve-cells and their processes are stained yellow or brown by these methods. Myelin sheaths remain unstained. The axis-cylinders of the myelinated ill i i light yellow, the unmyelinated axons are dark brown or black. The neurofibri stained somewhat more darkly than other parts of the cytoplasm. 31. The Golgi method furnishes preparations which demonstrate the external form of the neurons, and make it possible to trace individual axons and dendrit< considerable distances. The method also stains neuroglia. It is selective and rather uncertain in its results, since only a small proportion of the nerve-cells are impregnated in any preparation. The stain is due to the impregnation of the nerve-cells and their processes with silver. 32. The best stains for demonstrating the tigroid masses or Nis>l bodies are toluidin blue and Nissl's methylene-blue. Both are basic dyes; and in properly fixed nervous tissue they color the tigroid masses as well as the nuclear chromatin of nerve- cells blue. THE PERIPHERAL NERVOUS SYSTEM 33. The Spinal Ganglia. — Study a longitudinal section through a spinal nerve and its roots, including the spinal ganglion, stained by the pyridin-silver method. How are myelinated and unmyelinated axons stained by this method? What kinds of cells do you find? Study their axons. (See Figs. 39, 40 and pp. 62-66.) Look for the bifurcation of the myelinated and unmyelinated fibers. Note the differences in composition of the ventral and dorsal roots. What becomes of the various kinds of fibers when traced peripherally? When traced toward the spinal cord? What is the origin of the unmyelinated fibers? 34. Study the vagus nerve of the dog in osmic acid and pyridin-silver preparations. How are the various kinds of nerve-fibers stained in each? How does the structure of the vagus differ from that of a spinal nerve? 35. Study the cervical portion of the sympathetic trunk, which in the dog lies in a common sheath with the vagus. Of what kind of fibers is it composed? What is the origin and termination of these fibers? (See pp. 345-347.) 36. Study the pyridin-silver preparation from the superior cervical sympathetic ganglion. What is the source of the fine black fibers, and where do they end? Study the ganglion cells. What becomes of their axons? (See Figs. 251, 253 and pp. 341-544.) THE SPINAL CORD 37. Review the development and gross anatomy of the spinal cord (p. 42 and pp. 73-78). Examine the demonstration preparations of the vertebral column, showing the spinal cord exposed from the dorsal side. In these preparations study the meninges and ligamentum denticulatum, as well as the shape and size of the spinal cord. Note 362 THE NERVOUS SYSTEM the level of the termination of the spinal cord, the level of the origin of the various nerve roots and of their exit from the vertebral canal, and the level of the various seg- ments of the cord with reference to the vertebrae. Note the filum terminale and the Cauda equina. From your text-books of anatomy study the meninges and blood- supply of the cord. 38. The Spina! ( 'ord in Section— Examine the Pal-Weigert sections of the cervical, thoracic, lumbar, and sacral regions, and from them reconstruct a mental picture of the topography of the entire cord. How does it vary in shape and size at the different levels? Identify all the fissures, sulci, septa, funiculi, gray columns, commissures and nerve roots, the reticular formation, the substantia gelatinosa and the caput, cervix, and apex of the posterior gray column. (See pp. 78-84.) 39. The Microscopic Anatomy of the Spinal Cord. — Study all of the histologic preparations of the spinal cord which have been furnished you. (See pp. 85-90.) Study the neuroglia in Golgi preparations. Study the pia mater, septa, blood-vessels, and ependyma in hematoxylin and eosin preparations. Study the nerve-cells in Nissl, Golgi, and silver preparations. Study the myelinated fibers in Weigert preparations and both the myelinated and unmyelinated fibers in the silver preparations. Note the arrangement of each of these histologic elements and be sure that you understand the relations which they bear to each other. 40. Draw in outline, ventral side down, each of four Pal-Weigert sections taken, respectively, through the cervical, thoracic, lumbar, and sacral regions of the human spinal cord. Make the outlines very accurate in shape and size, with an enlargement of 8 times. Put in the outline of the gray columns, the central canal, and the substantia gelatinosa Rolandi. Put each outline on a separate sheet and do not ink the drawings at present. 41. Identify the various cell columns in the gray matter and note how they vary in the different levels of the cord (Nissl or counterstained Weigert preparations). (See pp. 89, 90 and Fig. 65.) Indicate these cell groups in their proper places in the four outline sketches of the spinal cord. What becomes of the axons arising from each group of cells? Why are the anterolateral and posterolateral cell groups seen only in the regions associated with the brachial and lumbosacral plexuses? The intermediolateral column only in the thoracic and highest lumbar segments? Why is the gray matter most abundant in the region of the intumescentiae and the white matter most abundant at the upper end of the spinal cord? 42. What elements are concerned in spinal reflexes? (See pp. 91-94.) 43. What connections do the fibers of the spinal nerves establish in the spinal cord? What is the origin and the peripheral termination of the somatic efferent fibers, of the visceral efferent fibers, of the somatic afferent fibers, and of the visceral afferent fibers of the spinal nerves? (See pp. 60-63 and Fig. 37.) What are the proprioceptive and exteroceptive fibers, and in what peripheral structures do they end? (See pp. 66-72.) 44. In a pyridin-silver preparation of the cervical spinal cord of a cat note that as the dorsal root enters the cord the unmyelinated fibers run through the lateral division of the root into the dorsolateral fasciculus (Fig. 72). The medial division of the root is formed of myelinated fibers which enter the posterior funiculus. Read about the intramedullay course of these fibers (pp. 95-98)., A LABORA ruin 01 I LINE OF \l l R( I \\ \ in\i\ 45. The fiber tracts, of which the white substance is compo ed, cannot be distin- guished in the normal adult cord. They can be recognized from dim n of their rnyelination in fetal cords (p. 112 and Fig. 79) and in preparation degeneration resulting from disease or injury in various parts oi the nervou (p. LOS; Figs. 75, 76). From such preparations as arc available for this purpose and from your reading (pp. 95 112) form a clear conception of the origin, course, and ter- mination of each of the fiber traits. 4(>. Indicate the location of each of these tracts in the outline drawing of the cervical portion of the spinal cord, entering the ascending tracts and the ventral i orti< o- spinal tract on the right side, and all of the descending tra< tsex< epl the ventral cortico- spinal tract on the left side. Why should the ventral and lateral corticospinal tracts be indicated on opposite sides of the cord? Wax crayons should be used to give the several tracts a differential coloring. Use the Eollowing color scheme: Somatic afferent tracts: Proprioceptive — yellow. Exteroceptive — blue. Somatic motor tracts: Corticospinal tracts — red. Rubrospinal tract — brown. All other tracts— black. 47. The fasciculus cuneatus and fasciculus gracilis should be colored yellow and then clotted over with blue to indicate that while the proprioceptive fibers predominate, there are also some exteroceptive libers in these tracts. THE BRAIN STEM 48. Now take the human brain and identify all of its principal divisions. Dissect out the arterial circle of }]'illi.s, and identify the branches of the internal carotid, ver- tebral, and basilar arteries. Read about the blood-supply and meninges of the brain in your text-book of anatomy. Identify all of the cranial nerves (Fig. 86). 49. Examine again the cerebellar peduncles in the three specimens of the sheep- brain (Figs. 87, 91). Now remove the cerebellum from the previously intact sheep- brain. Cut through the peduncles on both sides of the brain as far as possible from the pons and medulla, sacrificing the cerebellum to some extent in order to leave as much of the peduncles as possible attached to the brain stem. Be careful not to damage the anterior medullary velum and the tela chorioidea which lie under cover of the cerebellum (Fig. 84). In the same way remove the cerebellum from the human brain. 50. Study the roof of the fourth ventricle in both the human and the sheep's brain (pp. 128, 129 and Figs. 84, 90, 154). Examine the chorioid plexus of the fourth ven- tricle. Note the line of attachment of the tela chorioidea. Tear this membrane away. The torn edge which remains attached to the medulla is the taenia of the fourth ventricle (Figs. 89, 90). Study the attachments of the anterior medullary velum. The decus- sation of the trochlear nerve within the velum can easily be seen in the sheep. Remove this membrane. The floor of the fourth ventricle is now fully exposed. 51. Remove the pia mater from the brain stem, carefully cutting around the roots of the cranial nerves with a sharp-pointed knife to prevent these nerves being torn away from the brain when this membrane is removed. 364 THE NERVOUS SYSTEM 52. Carefully examine the medulla, pons, floor of the fourth ventricle, and the mesen- cephalon, observing all the details mentioned on pp. 118-131 and illustrated in Figs. 84, 86-89, 91. 53. Take selected transverse sections through the human brain stem and, by com- parison with the gross specimen, determine the level of each section. 54. Draw in outline each of these transverse sections through the brain stem. Put each drawing on a separate page, ventral side down, with the transverse diameter corresponding to the longer dimension of the paper. Study each preparation in detail and identify all of the parts, indicating them lightly in pencil. Do not label the draw- ings at this time. Make sure that all proportions are correct. The sections through the medulla should be enlarged eight diameters, those through the pons and mesen- cephalon four diameters. 55. Section Through the Decussation of the Pyramids. — Keep in mind the tracts which extend into the brain from the spinal cord and note the changes in their form and position. Identify the decussation of the pyramids, the nucleus gracilis and nucleus cuneatus, the spinal root of the trigeminal nerve and its nucleus, the reticular formation. Note the change in the form of the gray substance (pp. 132-137; Figs. 94, 95, 98). 56. Section Through the Decussation of the Lemniscus. — Note the rapid change in the form of the gray matter. Identify the internal and external arcuate fibers, the decussation of the lemniscus and the beginning of the medial lemniscus, as well as the structures continued up from the preceding level (Figs. 96, 99; pp. 137-139). 57. Section Through the Olive and the Hypoglossal Nucleus. — At this level the central canal opens out into the fourth ventricle. The posterior funiculi and their nuclei are disappearing or have disappeared. The dorsal spinocerebellar tract lies lateral to the spinal tract of the trigeminal nerve and is directed obliquely backward toward the restiform body. Identify, in addition to those structures which are continued from the preceding level, the inferior olivary nucleus with the olivocerebellar fibers, the dorsal and medial accessory olivary nuclei, the external arcuate fibers, the nucleus and fibers of the hypoglossal nerve, the dorsal motor nucleus of the vagus, the tractus solitarius and its nucleus, the nucleus ambiguus and the lateral reticular nucleus (Figs. 97, 101; pp. 139-142). 58. Section Through the Restiform Body. — The restiform body and the spinal tract of the fifth nerve are conspicuous in the dorsolateral part of the section. In the floor of the fourth ventricle locate the nucleus of the hypoglossal nerve, the dorsal motor nucleus of the vagus, the medial and the spinal vestibular nuclei. The spinal tract of the fifth nerve and its nucleus are deeply situated ventral to the restiform body and broken up by the olivocerebellar fibers (Fig. 103; pp. 143-146). 59. Section Through the Lower Margin of the Tons. — Identify such portions of the pons, brachium pontis, and cerebellum as are contained in the section. Dorsolateral to the restiform body is the dorsal cochlear nucleus, and ventrolateral to it the ventral cochlear nucleus. Identify the stria? medullares and the beginning of the trapezoid body, also the medial and lateral vestibular nuclei (Fig. 107; pp. 149-152). 60. Section 'Through the Facial Colliculus. — Differentiate between the ventral and the dorsal portions of the pons, and in the ventral portion identify the longitudinal fasciculi, transverse fibers, and the nuclei pontis (pp. 147-149). In the dorsal part identify the nuclei and root fibers of the sixth and seventh nerves including the genu A LABORATORY OUTLINE 01 NEURO \\\n>\i\ , () - of the seventh nerve. Locate the spinal trad of the fifth nerve and it nucleus the trapezoid body, and superior olivary nucleus (Fig. 108; pp. 151 154 61. Section Through the Middle of the Pons Showing the Motor and Main Sen Nuclei of the Fifth Nerve. In addition to these nuclei note the beginning of the mi cephalic root of the fifth nerve. The brachium conjunctivum makes it- appearance in the dorsal part of the section (Fig. 110; pp. 154 157). ()_'. Section Through the Inferior Colliculus. Identify the basis pedum uli, substantia nigra, medial and lateral lemnisci, cerebral aqueduct, central gray matter, mesence- phalic root of the fifth nerve, fasciculus longitudinalis medialis, nucleus of the tnw blear nerve, and the decussation of the brachium conjunctivum (Figs. 1 13, 1 14; pp. 158, 165). 63. Section Through the Superior Colliculus. —Identify, in addition to the 3tru< tun- continued upward from lower levels, the red nucleus, the nucleus of the third nerve, and the root libers of that nerve, the ventral and dorsal tegmental decussations, the inferior quadrigeminal brachium, and the medial geniculate body (Fig. 116; pp. 160, 167). THE CEREBELLUM 64. Compare the human cerebellum with that of the shaik and the sheep. How is its size related to the size of the pons and to the extent of the cerebral cortex? 65. On both the human and sheep's cerebellum identify the vermis, hemispheres, and divided peduncles (Figs. 138, 139, 143-145). In the medial sagittal section of the sheep's brain identify the white medullary body of the cerebellum, the arbor vita?, cerebellar cortex, folia, and sulci (Fig. 84; pp. 196-199). 66. Study the morphology of the cerebellum in the sheep (Figs. 143-145). Lo- cate these same fundamental subdivisions in the human cerebellum (Figs. 146, 147). What functions have recently been assigned to each of these subdivisions? (See pp. 199-203.) 67. Divide the human cerebellum in the median plane. Cut the right half into horizontal sections and the left into sagittal sections and study the medullary center and nuclei of the cerebellum (Figs. 140, 141, 148; pp. 199, 203). 68. Study the histologic sections of the cerebellar cortex and master the details of its structure (Figs. 150, 151; pp. 206-210). FUNCTIONAL ANALYSIS OF THE BRAIN STEM 69. Review the sections of the brain stem as directed in the following paragraphs, paying special attention to the functional significance of the various nuclei and fiber tracts as far as they can be followed in the series of sections. In general, the afferent tracts and nuclei should be entered in color on the right side of the drawings already made, and the efferent tracts and nuclei on the left side. But this order must be re- versed in certain cases to allow for the decussation of the tracts. Label the various tracts and nuclei. Use the following color scheme: Somatic afferent: Exteroceptive — blue. Proprioceptive — yellow. Visceral afferent — orange. Visceral efferent — purple. 366 If]]. NERV01 - SYSTEM Somatic efferent — red. All 1 erebellar connections not strictly proprioceptive — brown. Other tracts — black. PROPRIOCEPTIVE PATHS AND CENTERS pp. 311-315) 70. The- cerebellum is the chief proprioceptive correlation center, and the restiform body consists for the most part of proprioceptive afferent path- (Fiji. 235 . Note its shape, position, and connections in all the gross specimens. In the left lateral half of tin- sheep's brain follow it caudally by dissection, separating it from the other peduncles. Cut and reflect the dorsal cochlear nucleus of the eighth nerve. Trace the restiform body backward and note the accession of external arcuate fibers. At the level of the inferior olive it receives the dorsal spinocerebellar tract. Trace this by dissection from the restiform body obliquely across the upper end of the tuberculum cinereum and then caudally along the ventral border of this elevation to the spinal cord. (See Figs. 87, 88, 104; pp. 143, 205.) 71. Now take the sections of the medulla, locate the dorsal spinocerebellar tract in each, and indicate its position in yellow on the right side of your outlines (p. 144 . Locate the external arcute fibers (p. 139). From where do they come and where do they go? Draw in yellow those belonging to the right peduncle. Locate in your sections the olivocerebellar tract, and with brown indicate in your outline the fibers running into the right peduncle (Fig. 103). 72. From your texts ascertain the course of the ventral spinocerebellar tract and indicate its position in yellow on the right side of the outlines (Fig. 149; p. 157;. 73. Proprioceptive Path to the Cerebral Cortex. — Indicate in yellow the terminal portion of the right dorsal funiculi, and with yellow stipple the right nucleus gracilis and nucleus cuneatus (Figs. 98, 99). Study the internal arcuate fibers and the medial lemniscus, drawing the internal arcuate fibers from right to left and the medial lemniscus on the left side (yellow). Where do the fibers of the medial lemniscus terminate? What is the source and what the destination of the impulses which they carry? (See Figs. 101, 103, 107, 108, 110, 114, 116, 235 and pp. 138, 312.) 74. Locate the vestibular nuclei and indicate them with yellow stipple on the right side of the outlines (Figs. 101, 103, 107, 108). Locate the vestibulocerebellar tract (pp. 151, 188; Fig. 136). EXTEROCEPTIVE PATHS AND CENTERS (pp. 302-310) 75. The Cochlear Nerve and its Connections. — On the sheep's brain note the two divisions of the acoustic nerve as well as the ventral and dorsal cochlear nuclei and the trapezoid body (Fig. K7). Examine the cochlear nuclei and the striae medullares in the human brain (Fig. 89). Locate the lateral lemniscus where it forms a flat band of fibers directed rostrally and dorsally upon the lateral surface of the mesencephalon. It occupies a triangular sp ice dorsal to the basis pedunculi and rostral to the pons and is superficial to the brachium conjunctivum (Fig. 88). 76. Now take the section through the lower border of the pons and study the cochlear nuclei, the s trice medullares, and the beginning of the trapezoid bod\ (Fig. 107). In the section through the facial colliculus study the trapezoid body and the superior \ I \r.< IB \ lnkV 01 I LINE "I Ml Ri I VNAT01TV olivary nuclei (Pig. 108). In the section through the middle of the pons identify the lateral lemniscus. Trace this trad to the inferior colliculu^s I ig. 11 I and through the inferior quadrigeminal brachium to the medial geniculate bod) Figs. 114, 116 Color these centra] connections of the co< blear nerve blue, indicating the co< blear nuclei on the right side and the lateral lemniscus on the lefl ' I ig. 134; pp. 1 19, 1 5 77. Dissection of the spinal tract of the fifth inn,. On the lefl half of the - brain locate the fifth nerve and tear away the transverse fibers of the pons caudal to that nerve until the longitudinal fibers of its spinal tract arc exposed. By carefully scraping away the structures superficial to this tract follow it to the lower end of the medulla. 78. Locate the sensory nuclei of the fifth nerve in your sections and indicate them with colored stipple on the right side of your drawing (pp. 154, 182; Fig. 131): the m cephalic nucleus, yellow (Fig. 114); the main sensory nucleus, blue (Fig. 11<) ; the nucleus of the spinal tract, blue (Figs. 98, 99, 101, 103, 107, 108). At the same time color the spinal tract of the right side blue. What becomes of the fibers which arise from the cells of the main sensory and the spinal nuclei of the trigeminal nerve? pp. 183, 507: Fig. 232.) 79. From the text ascertain the course of the spinothalamic tract and trace it up through the brain stem (Figs. 105, 230, 231, 234). Where do these fibers come from, and where do they end? What kind of sensations do they mediate? Enter it in blue on the right side of your drawings. (See pp. 101, 102, 145, M)^J VISCERAL AFFERENT PATHS AND CENTERS 80. Identify the tr actus solitarius and its nucleus (Figs. 101, 103, 120). What is the origin, termination, and function of the fibers constituting this tract? (See pp. 180, 181.) Indicate the tract with orange and the nucleus with orange stipple on the right side of your drawing. VISCERAL MOTOR CENTERS 81. In the sections of the brain stem identify the dorsal motor nucleus of the vagus (Figs. 101, 103) and the following special visceral motor nuclei: the nucleus ambiguus (Figs. 101, 103), the motor nucleus of the fifth (Fig. 110), and the motor nucleus of the seventh nerve (Fig. 108). Stipple these nuclei purple on the left side. How are visceral afferent and efferent elements connected to form visceral reflex arcs? (See pp. 1 74 178.) SOMATIC MOTOR TRACTS AND CENTERS 82. The Corticospinal and Corticopontine Tracts. — From the cerebral cortex the fibers of the pyramidal tract run through the internal capsule and brain stem to the somatic motor and special visceral motor nuclei of the cranial nerves and to the anterior gray column of the spinal cord. Along with these it will be convenient to study the cortico-ponto-cerebellar pathway. Take the left lateral half of the sheep's brain and, being careful not to injure the optic tract and optic radiation, follow the fibers ot the basis pedunculi by dissection through the internal capsule to the cerebral cortex 260). Now tear away the transverse fibers of the pons a few at a time and follow them by dissection into the brachium pontis. Observe that some of the fibers oi the basis pedunculi end in the pons (corticopontine fibers) and that others (corticospinal i. 368 THE NERVOUS SYSTEM can be traced through the pons into the pyramid of the medulla. Carrying the dis- section caudally, observe the decussation in the lower end of the medulla. 83. Examine again the series of sections through the brain stem and color the corticospinal tract red on the right side of your drawings. Draw the fibers from right to left in the decussation (Fig. 237; pp. 136, 317). N4. With red stipple indicate the somatic motor nuclei on the left side of your draw- ings. Which nuclei are they? (See pp. 170-173.) CEREBELLAR CONNECTIONS 85. The inferior peduncle has already been studied and the cortico-ponto-cerebellar path has been dissected. Review this path in your sections. Color the corticopontine tracts of the left side brown ( Fig. 117). Indicate the nuclei pontis of the left side by brown stipple. Draw the transverse fibers of the pons from the left nuclei pontis to the right brachium pontis (Fig. 106; pp. 147-149). 86. In the left lateral half of the sheep's brain follow the brachium conjunctivum by dissection into the tegmentum of the mesencephalon and note its decussation beneath the inferior colliculus. In your sections trace it rostrally, noting its decus- sation and termination (Figs. 110, 112, 114-116). Indicate it in brown on your drawings, beginning on the right side and tracing it through the decussation to the left red nucleus. Stipple both red nuclei with brown. (See pp. 159, 326.) 87. The Rubrospinal Tract. — Trace the rubrospinal tract from the red nucleus through the ventral tegmental decussation (Fig. 116) and the reticular formation of the brain stem. In the reticular formation it occupies a position ventromedial to the nucleus of the spinal root of the trigeminal nerve (Figs. 115, 234; pp. 161, 326). Color it brown on the left side of your drawings. THE RETICULAR FORMATION 88. Study the reticular formation in the various sections. Of what is it composed? How many kinds of internal arcuate fibers can you find? What is the source of the longitudinal fibers of the reticular formation? Locate the tectospinal tract and in- dicate it in black on the left side of your drawings. (See pp. 144, 145). 89. The Fasciculus Longitudinalis Medialis. — Examine all nine sections, and enter this bundle in black on both sides of your drawings. What is the source of its fibers and what is its function? (See Fig. 109; pp. 152, 162). PROSENCEPHALON 90. With a sharp brain knife divide the human brain exactly in the median sagittal plane, and then cut the left cerebral hemisphere into a series of frontal sections. The planes of the sections should pass through (1) the rostrum of the corpus callosum, (2) the anterior commissure, (3) the mammillary body, (4) the habenular nucleus, (5) the pineal body and the splenium of the corpus callosum (Figs. 186-190). 91. Take the right half of the sheep's brain and make such dissections as may be necessary to secure a good preparation of the structures indicated in Fig. 84. Begin at the rostral angle of the fourth ventricle and follow the cerebral aqueduct, tearing away with tissue forceps any parts of the left lateral wall which have not been cut away. \ I \r.ok\lMRY 01 II. IM "| mi ,,, xx \|,,my Follow the aqueduct into the third ventricle, removing from the hitter the remai it- left lateral wall. Care is required in removing the rostral part of this wall in that the lamina terminalis may he left intact. X<»w remove such portions of the kit cerebral cortex a- are Mill attached t<> the preparation. By this diss* tion a much mere instructive preparation is obtained than when the original section is made exactly in the median plane. { >2. Take the left lateral hall" of the sheep"- brain and tear away what remain- of the septum pellucidum and body of the fornix and Locate the « an. late nucleus, lor the identification of these structures see Figs. 84 and 204. i Cm through the internal capsule, which has previously been exposed from the lateral side in this specimen, along a line extending horizontally toward the occipital pole from the highesl part of the dorsal border of the caudate nucleus. Remove the portion of the cerebral hemisphere that lies dorsal to the plane of this section and thus expose the dorsal surface of the thalamus (Fig. 91). 93. Diencephalon. — Study the thalamus as it appears in all of these preparation- (pp. 213-216). Examine the dorsal surface of the thalamus on the left half of the sheep's brain ( Figs. 89, 91, 180). The lateral surface of the thalamus rests against the internal capsule, as can be readily understood from a study of this dissection. The medial surface forms a part of the wall of the third ventricle (Figs. 158, 159). 94. Study the cpithalamus in both the human and the sheep's brain. Of what parts is it composed? (See Figs. 91, 158, 159; pp. 220, 221.) 95. Locate all the parts which belong to the hypothalamus in both the human and the sheep's brain (Figs. 84, 86, 158, 159; pp. 222, 223). 96. Study the shape and boundaries of the third ventricle (Figs. 158, 159; pp. 223, 224). 97. The Metathalamus. — On the left half of the sheep's brain identify the medial geniculate body (Fig. 87). Immediately rostral to this body is a slight elevation in the optic tract produced by the subjacent lateral geniculate body. Identify both of these bodies on the human brain (Figs. 88, 89, 154). 98. In the frontal sections of the left human cerebral hemisphere identify the various parts of the diencephalon (Figs. 188, 189). From these sections something can be learned concerning the internal st nature of the thalamus, but more information can be obtained on this subject from sections stained by the Weigert method (Figs. 156, 157; p. 216). In these sections trace the basis pedunculi into the internal capsule and the medial lemniscus into the thalamus. 99. Dissection of the Optic Trad.— Take the left lateral half of the sheep's brain and, grasping the optic chiasma with the tissue forceps, pull the optic tract lateralward. separating it from the surface of the peduncle. It separates easily until the position of the lateral geniculate body is reached just rostral to the medial geniculate body. Stronger traction will cause it to tear away from the lateral geniculate body, which is now exposed as a prominent curved ridge of gray matter. This nucleus extends r.-strally and dorsally from the medial geniculate body and is continuous with the pulvinar of the thalamus. Continued traction will cause the optic fibers to strip off from the sur- face of the pulvinar. Here they form a rather thick white lamina, the stratum ztinale. Continue the dissection, raising the fibers of the optic tract as far as the groove rostral to the superior colliculus. Now cut the transverse peduncular tract, which lies in this 37° IIII. NERVOUS SYSTEM groove, by making a superficial incision across the groove along the lateral border of the optic fibers. Scrape away the superficial gray matter (about 1 mm. ) of the superior colliculus and expose the stratum opticum (Fig. 116). Now continue the traction on the optic tract and a striking demonstration will be obtained of the fact that the stratum opticum is composed of fibers from this tract (Figs. 161, 162; pp. 226, 227). 100. Dissection of the Optic Radiation. — In the left half of the sheep's brain scrape away part of the ,L r ray matter of the pulvinar. Follow fibers from the pulvinar into the posterior limb of the internal capsule. These belong to the optic radiation, which may now be followed by dissection to the cortex near the occipital pole of the cerebral hemi- sphere (Fig. 260; pp. 227, 228). Now take the right half of the cerebral hemisphere and identity the visual area of the cerebral cortex (Fig. 221;. Optic radiation ' ' Superior colliculus-' » Inferior colliculus ' ■ Pulvinar ' [ Medial geniculate body Cerebral peduncle Mam miliary body ! Optic tract > Posterior limb of internal capsule • Optic nerve ; ; Intersection of corona radiata and '< ! radiation of corpus callosum \ 'Anterior limb of internal capsule Anterior perforated substance Fig. 260. — Dissection of the cerebrum of a sheep showing the internal capsule and corona radiata. The lentiform nucleus has been removed. 101. Surface Form of the Cerebral Hemispheres. — Compare the basal surface of the human brain with that of the sheep. Note in each the parts belonging to the rhinen- cephalon and locate the rhinal fissure, which separates the neopallium and the archi- pallium. Nearly all of the surface of the human cerebral hemisphere is formed by the neopallium (Figs. 83, 86; pp. 115, 116). 102. Examine the right cerebral hemisphere of the human brain and identify the poles, fissures, sulci, lobes, and gyri (Figs. 166-168, 170, 171; pp. 232-242). Draw the margins of the lateral fissure apart and locate the insula (Fig. 169). Study the insula in the frontal sections through the left cerebral hemisphere (Figs. 186-189; p. 237). 103. Internal Configuration of the Cerebral Hemisphere. — Take the sheep's brain from which the cerebellum has been removed and slice away successive thin layers from the dorsal aspect of both hemispheres. These thin sections should be cut in planes parallel to the dorsal surface of the corpus callosum and the last cut should be } inch dorsal to that commissure. The direction and relative depth of the dorsal surface of \ LABOB \K)KV in I i.i\i. 01 \i i RO \\ \Im\in 71 the corpus callosum can be determied by examination of the medial aspecl of the right half of the sheep's brain. As the sections are removed note the relation of the gray andwhite matter I Fig. 175). Gently pressaparl the two hemisphen and note the< orpus callosum at the bottom of the longitudinal fissure. Now with a blunl instrument dissect away the gray and white matter from the dorsal surfai e of the 1 orpus 1 alio um (Fig. 175). Be careful not to injure a thin layer of gray matter, the indusium griseum, which covers this surface. Study the corpus callosum in this specimen and in the median sagittal sections of the sheep and human brains (Figs. 158, 159, 175; pp. 243 245). Examine the septum peUucidum in the median sagittal sections. 104. The Lateral Ventricles (pp. 246 251). — Cut through the corpus < allosum of the sheep's brain as indicated in Fig. ITS, leaving a median strip in position. Male a careful examination of all the parts thus exposed, including the septum pellucidum. On the right side of the specimen expose the entire extent of the inferior horn of the lateral ventricle by freely cutting away the lateral portion of the hemisphere a- indi< ated in Fig. 182. Remove the caudate nucleus to demonstrate the entire extent of the ante- rior horn, and finally demonstrate the continuity of the lateral ventricle with the cavity of the olfactory bulb (Fig. 182). Now study the lateral ventricle and the structures which form its walls as these are illustrated on the tw r o sides of this specimen. Note the chorioid plexus (Fig. 183) and chorioid fissure. 105. Study the lateral ventricle as seen in the frontal sections of the left hemi- sphere of the human brain (Figs. 186-189). It has an additional part, the posterior horn, not seen in the sheep. Endeavor to reconstruct a mental picture of its shape (Fig. 176). 106. The Corpus Striatum (pp. 253-257). — Examine again the caudate nucleus as it bulges into the lateral ventricle (Fig. 178). Take the right lateral half of the sheep's brain and make a horizontal section through the cerebral hemisphere, passing through the lower border of the genu of the corpus callosum and the lower border of the habenular trigone. Locate the lentiform and caudate nuclei, the claustrum, and the internal and external capsules (Fig. 192). 107. Dissection of the Lentiform Nucleus and the Internal Capsule.— On the left side of the sheep's brain, in which the lateral ventricles have been exposed, remove the cortex and white matter superficial to the lentiform nucleus. Begin by grasping with tissue forceps the olfactory bulb close to its peduncle and tear it away, pulling in a lateral and caudal direction. There should come away with it the superficial part of the anterior perforated substance and part of the lateral olfactory gyrus (Fig. 83). This will expose the ventral part of the lentiform nucleus, and the structures lateral to that nucleus can now be removed. With a blunt dissecting instrument scrape away everything superficial to the lentiform nucleus and continue the dissection until the nucleus and the corona radiata are fully exposed (Fig. 87). Now scrape away the lentiform nucleus and expose the internal capsule (Fig. 260). In removing the nucleus you can obtain a clear idea of its shape and size. 108. Dissection of the Internal Capsule.— In the same specimen remove the optic tract and trace the basis pedunculi into the internal capsule and follow the libers from the internal capsule into the corona radiata. Trace the optic radiation from the poste- rior extremity of the internal capsule to the cortex near the occipital pole (Fig. 260). 109. Dissection of the Caudate Xueleus.— On the left side of the same sheep's 372 THE NERVOUS SYSTEM brain note that the tail of the caudate nucleus extends ventrally into the roof of the inferior horn of the lateral ventricle. With a blunt instrument scrape away the head and first part of the tail of the nucleus, exposing the medial surface of the internal cap- sule (Fig. 91). Note the shape and size of this nucleus as you are removing it. 110. Study a horizontal section stained by the Weigert method through the internal capsule and basal ganglia. From this section and from the dissections endeavor to form a clear mental picture of the internal capsule and its relations (Figs. 191, 193; pp. 257-261). 111. Now take the frontal sections of the left hemisphere of the human brain and identify the various parts of the corpus striatum and internal capsule (Figs. 186- 190). 112. Rhinencephalon. — Study the olfactory portions of the brain to be seen on the ventral surface of the cerebral hemisphere in the human and sheep's brains (Figs. 172, 197, 199; pp. 265-269). Study the hippocampus, alveus, and fimbria as they lie exposed in the inferior horn of the lateral ventricle of the sheep's brain (Figs. 178, 182). Open up the inferior horn of the lateral ventricle on the left side of this specimen so as to expose the hippocampus and fimbria. Raise the hippocampus and fimbria on both sides at the same time, leaving them still attached to the fornix. This should be done without damaging the underlying tela chorioidea of the third ventricle, which occupies the great transverse fissure. Examine the under surface of the hippocampus, fimbria, and for- nix. Note that the two fimbriae unite to form the triangular body of the fornix. The transverse fibers in this triangle constitute the hippocampal commissure (lyra). Note the fascia dentata and hippocampal fissure. Figure 204 will help you to interpret the parts seen in this dissection. 113. The chorioid plexuses of the prosencephalon are now fully exposed, and their relations to each other and the brain ventricles can be readily studied (pp. 224, 251). 114. Remove the tela chorioidea of the third ventricle and again identify the parts of the thalamus and epithalamus which may be seen from above (Figs. 91, 180). 115. Replace the fornix and hippocampus in position and divide the fornix and what remains of the cerebral hemispheres by a sagittal section i millimeter to the right of the median plane. Take the left half of the preparation and, tearing away any por- tions of the right columna fornicis that may still be attached to the preparation, follow the left column of the fornix to the mammillary body. This can be accomplished by scraping away some of the medial surface of the thalamus (Fig. 204). At the same time expose the mamillothalamic tract. Remove the posterior part of the thalamus and the remainder of the brain stem by a cut made just caudal to the mamillothalamic tract, as indicated in Fig. 204. This gives a connected view of the entire fornix system. Find the cut surface of the hippocampal commissure and separate it for a few milli- meters from the rest of the fornix. Identify again the fimbria, fascia dentata, hippo- campal fissure and hippocampal gyrus, and study the fornix as a whole (Figs. 200, 203; pp. 270-272). 116. Study the septum pellueiduni in the right half of the human brain (Fig. 158; p. 272). Also locate the anterior commissure. 117. Dissect the anterior commissure in the right lateral half of the sheep's brain. Locate the commissure on the median surface and by blunt dissection follow it to the olfactory bulb (Fig. 199; p. 273). A LABOK \K>K'V Ml I |.!\i: OF M I I m \\ \|,,\iv 118. In the frontal sections of the left cerebral hemisphere of the human I. rain study the relations of the septum pellucidum, fornix, fimbria, hippocampus, and anterior commissure (Figs. 186 190). 11<). The Cerebral Cortex. On the right hemisphere of the human I. rain identify the motor, somesthetic, auditory, and visual centei I igs. JJO, 221; pp. 290 With a scalpel remove a cube of cortex and subjacent white matter from ea< h of these areas. Each Mock should measure about 1 em. in each dimension. Willi a sharp razor make section through each oi these blocks a1 righl angles to the surfa< e of thee and perpendicular to the long axis of the gyrus from which the block wascut. Not the differences in thickness of the cortex in the various regions. Observe the white striatums in the cortex, and note how these differ in the several specimens (Fig. Study the stained and mounted sections of the cerebral cortex which are furnished you. What details of cell and fiber lamination do these preparation- show, and how- does this lamination differ in the several regions of the cortex? (See Fig. 215; pp. 284-287.) 120. Association Fibers (Figs. 226, 228; pp. 298-301).— If the human brain is reason- ably well preserved the larger bundles of association fibers may be easily exposed by dissection. This can be done on the right hemisphere. But if the material is very soft this half of the brain can more profitably be laid into a series of horizontal sections and these used for a review of the form and relations of the component parts of the cerebral hemisphere. If the material is fairly well preserved, make the following review dissection and at the same time expose and study the various bundles of asso- ciation fibers. 121. Review Dissection of the Human Brain. — Take the right half of the human brain and scrape away the cerebral cortex from a portion of the dorsal surface of the frontal lobe. This will expose the short association or arcuate fibers (Fig. 226). 122. Now make a horizontal section through the hemisphere parallel to the dorsal surface of the corpus callosum and f inch dorsal to it. Note the centrum semiovale. Scrape away the cortex of the gyrus cinguli and the white matter immediately sub- jacent to it. In making this dissection carry the orangewood stick in an anteroposterior direction, removing the white matter a little at a time until a longitudinal bundle of fibers, the cingulum, is exposed (Fig. 174). The indusium griseum and stria' longi- tudinales should now be uncovered. 123. Remove the cingulum, scrape away the indusium griseum, and expose the radiation of the corpus callosum as indicated on the right side of Fig. 174, but do not cut the optic radiation or expose the tapetum at this time. 124. Remove the parietal operculum a little at a time. This can be done with tissue forceps. Grasp small portions and tear them away by upward traction. Note the bundles of transverse fibers which enter this operculum from the corpus callosum and internal capsule. These intersect at right angles with the libers of the superior longitudinal fasciculus which should come into view as the dissection progresses 174). The transverse bundles should be made to break off at the point where they pass through the superior longitudinal fasciculus. Complete the dissection of this fasciculus, carrying the dissecting instrument in the direction of its fibers. Now demonstrate the intersection of the corona radiata with the radiation of the corpus callosum (Fig. 174). 374 THE NERVOUS SYSTEM By this dissection the insula and the dorsal surface of the temporal lobe have been ex- posed. Note in particular the transverse temporal gyri. 125. Now dissect away the dorsal part of the temporal lobe and remove the insula. This will expose the uncinate and inferior occipitofrontal fasciculi as well as the external capsule (Fig. 227). These fiber bundles can best be displayed by carrying the dis- secting instrument in the direction of the fibers. Complete the dissection of the corona radiata and the optic radiation (Fig. 227). 126. Now turn the specimen over and make a dissection of the column of the fornix and the mamillothalamic tract as in Fig. 205, but do not cut away the brain stem as indicated in that figure. 127. Dissection of the Internal Capsule from the Medial Side (Fig. 195). — Tear away the fornix and septum pellucidum, opening up the lateral ventricle. With the brain knife cut away a slice from the medial surface of the hemisphere, varying in thick- ness from t inch at the frontal end to \ inch at the occipital end, cutting through the corpus callosum and into the ventricle, but not into the basal ganglia. With a scalpel and tissue forceps remove what remains of the medial wall of the lateral ventricle, except in the inferior horn. Grasp with tissue forceps the stria terminalis in the rostral end of the sulcus terminalis and tear it away, carrying the forceps toward the occipital pole (p. 214). By blunt dissection remove the thalamus and subthalamus as well as the tegmentum and corpora quadrigemina of the mesencephalon. In scraping away these parts carry the dissecting instrument from the sulcus terminalis in a ventral direction. This will uncover the basis pedunculi and its continuation into the internal capsule. The fibers of the thalamic radiation will be broken off at the point where they enter the internal capsule (Fig. 195). Remove the ependymal lining of the posterior horn of the ventricle and uncover the tapetum. Scrape away the caudate nucleus, carrying the dissecting instrument in the direction of the fibers of the internal capsule (Fig. 195). Trace the anterior commissure to the point where it disappears under the anterior limb of the internal capsule. Study the internal capsule as seen from the medial sur- face, and note particularly the direction of the fibers, the anterior limb, the posterior limb, the optic radiation, and the curved ridge which represents the genu* 128. Now turn again to the lateral side of the specimen (Fig. 227), and grasping with tissue forceps individual strands of the uncinate fasciculus in temporal lobe strip them forward into the frontal lobe. Remove the entire fasciculus in this manner. In the same way strip away the fibers of the inferior occipitofrontal fasciculus, beginning in the frontal lobe and tracing them toward the occiput. Strip off the fibers of the ex- ternal capsule and expose the lentiform nucleus and the corona radiata (Fig. 194). Pay special attention to the fibers of the corona radiata which come from the sublen- ticular part of the internal capsule and enter the temporal lobe. Follow the anterior commissure to the point where it disappears under the lentiform nucleus. 129. Remove what remains of the temporal lobe and examine the hippocampus, fimbria, and inferior horn of the lateral ventricle from the dorsal surface (Fig. 201). 130. Next scrape away the lentiform nucleus and trace the basis pedunculi into the internal capsule (Fig. 88). Study the corona radiata, internal capsule, and basis pedunculi from both sides of this preparation. The thalamus and the caudate and lentiform nuclei produce well-marked impressions on the internal capsule (Figs. 88, 195). BIBLIOGRAPHY Allen. W. I''.. 1 l> 1 ( >: Application of the Marchi Method to the Study of the Radix M cephalica Trigemini in the Guinea pig, Jour. Comp. .Neurol., vol. xxx, p. 169 Andre -Thomas and Durupt, 1914: Localisations cer6belleuses, Paris. \ie>. L. B., 1916: The Function of the Efferenl Fibers of the Opti( Nerve of Fishes, Jour. Comp. Neurol., vol. xxvi, p. 213. Bailey, 1'.. 1916: 'The Morphology and Morphogenesis of the Chorioid Plexuses with Espe- cial Reference to the Development of the Lateral Telencephalic Plexus in Chrysemys Marginata, Jour. Comp. Neurol., vol. xxvi, pp. 507-531. , 1916: Morphology of the Roof Plate of the Forebrain and the Lateral Chorioid Plexuses in the Human Embryo, Jour. Comp. Neurol., vol. xxvi, p. 70. Bar&ny, R., 1912: Lokalisation in der Rinde der Oeinhirnhemispharen des Menschen, Wiener klinisehe W'oehensc hrift . Bd. xxv. p. 2033. Barnes, S., 1901: Degenerations in Hemiplegia with Special Reference to a Ventrolateral Pyramidal Tract, the Accessory Fillet, and Pick's Bundle. Brain, vol. xxiv. p. 4d> Bartelmez. G. \V., 1915: Mauthner's Cell and the Nucleus Motorius Tegmenti, Jour. Comp. Neurol., vol. xxv, p. 87. Batten, F. E., and G. Holmes, 1913: The Endogenous Fibers of the Human Spinal Cord (from the Examination of Acute Poliomyelitis), Brain, vol. xxxv, p. 2r> ( >. Baumgartner, E. A., 1915: The Development of the Hypophysis in Squalus Acanthias, Jour. Morph., vol. xxvi, p. 391. Bayliss, W. B., 1918: Principles of General Physiology, New York. Beevor, C. E., and Victor Horsley, 1902: On the Pallio-tectal or Cortico-mesencephalic System of Fibers, Brain, vol. xxv, p. 436. Bell, C., 1811: Idea of a New Anatomy of the Brain, London. , 1844: The Nervous System of the Human Body, London. Bernheimer, S., 1904: Ueber Ursprung und Yerlauf des Nervus oculomotorius im Mittel- hirn, Monatschrift f. Psych, u. Neurol., Bd. xv, p. 151. Bethe, A., 1903: Allgemeine Anatomie und Physiologie des Nervensystems, G. Thieme, Leipzig. Bing, R., 1906: Experimentelles zur Physiologie der Tractus Spinocerebellares, Arch. f. Anat. u. Physiol., Physiol. Abt., 1906, s. 251. Black, D., 1916: Cerebellar Localization in the Light of Recent Research, Jour. Lab. and Clin. Med., vol. i, p. 467. , 1917: The Motor Nuclei of the Cerebral Nerves in Phylogeny, Jour. Com]). Neurol., vol. xxvii, p. 467, and vol. xxviii, p. 379. Bolk. L., 1906: Das Cerebellum der Saugetiere, Gustav Fischer. Jena, 1906 Bolton. J. S., 1910: A Contribution to the Localization of Cerebral Function, Based on the Clinico-pathologic Study of Mental Disease. Brain, vol. xxxiii. pp. 26-147. Brodmann, K., 1907: Die Kortexgliederung des Menschen, Jour. f. Psychol, u. Neurol.. Bd. x, p. 231. , 1909: Vergleichende Lokalisationslehre der Grosshirnrinde. Barth., Leipzig, 1909. , 1910: Feinere Anatomie des Grosshirns, Lewandowsky's Handbuch der Neurologic Bd. v, pp. 206-307, Berlin, 1910. Brookover, Chas., 1914: The Nervus Terminalis in Adult Man. Jour. Comp. Neurol., vol. xxiv, pp. 131-135. , 1917: The Peripheral Distribution of the Nervus Terminalis in an Infant. Jour. Comp. Neurol., vol. xxviii, pp. 349-360. Bruce, A., and R. Muir, 1896: On a Descending Degeneration in the Posterior Columns in the Lumbo-sacral Region of the Spinal Cord. Brain, vol. xix. p. 333. Bruce, A. N., 1910: The Tract of (lowers, Quart. Jour. Kxper. Phys.. vol. iii. p. 391. , 1914: Arcuate Nucleus in Man, the Anthropoid Apes, and the Microcephalic Idiot, Rev. Neurol, and Psychiat., vol. xii, pp. 51-53. 375 376 BIBLIOGRAPHY Cajal, S. R., 1890: Origcn y terminacion de las fibnis ncrviosas olfatorias, Gac. sanitaria de Barcelona, 1890. — , 1900-06: Studicn iiber die Hirnrinde des Menschcn, Leipzig. — , 1907: Die Structur der sensiblen Ganglien des Menschen und der Tiere, Anat. Hefte, Zweite Abt., Bd. xvi, p. 177. — , 1908: Studien iiber Nervenregeneration, Ubersetzt von J. Bresler, Leipzig, 1908. — , 1909: Histologic du systeme nerveux de l'homme et des vertebres, vol. i, A. Maloine, Paris. 1911: Histologic du systeme nerveux de l'homme et des vertebres, vol. ii, A. Maloine, Paris. Campbell. A. \\\, 1905: Histological Studies on the Localization of Cerebral Function, Cambridge. Cannon, \Y. B., 1912: Peristalsis, Segmentation, and the Myenteric Reflex, Amer. Jour. Physiol., vol. xxx, pp. 114-128. Carlson, A. J., and L. H. Braafladt, 1915: On the Sensibility of the Gastric Mucosa, Amer. Jour. Physiol., vol. xxxvi, p. 153. Carpenter, F. \Y., and J. L. Conel, 1914: A Study of Ganglion Cells in the Sympathetic Nervous System with Special Reference to Intrinsic Sensory Neurones, Jour. Comp. Neurol., vol. xxiv, pp. 269-281. Chase, M. R., and S. \V. Ranson, 1914: The Structure of the Roots, Trunk, and Branches of the Vagus Nerve, Jour. Comp. Neurol., vol. xxiv, p. 31. Clarke, R. EL, and Victor Horsley, 1905: On the Intrinsic Fibers of the Cerebellum, its Nuclei, and its Efferent Tracts, Brain, vol. xxviii, p. 13. Coghill, G. E., 1902: The Cranial Nerves of Amblystoma Trigrinum, Jour. Comp. Neurol., . vol. xii, pp. 205-289. — , 1913: The Primary Ventral Roots and Somatic Motor Column of Amblystoma, Jour. Comp. Neurol., vol. xxiii, pp. 121-143. 1914: Correlated Anatomical and Physiological Studies of the Growth of the Ner- vous System of Amphibia, I, Jour. Comp. Neurol., vol. xxiv, pp. 161-223. Collier, J., and F. Buzzard, 1901: Descending Mesencephalic Tracts in Cat, Monkey, and Man, Brain, vol. xxiv, p. 177. — , 1903: The Degenerations Resulting from Lesions of Posterior Nerve Roots and from Transverse Lesions of the Spinal Cord in Man, Brain, vol. xxvi, p. 559. Cowdry, E. V., 1914: The Comparative Distribution of Mitochondria in Spinal Ganglion Cells of Vertebrates, Amer. Jour. Anat., vol. xvii, p. 1 Curran, E. J., 1909: A New Association Tract in the Cerebrum with Remarks on the Fiber Tract Dissection Method of Studying the Brain, Jour. Comp. Neurol., vol. xix, p. 645. Cushing, H., 1903: The Taste Fibers and Their Independence of the N. Trigeminus, Johns Hopkins Hospital Bulletin, vol. xiv, p. 71. — , 1909: A Note Upon the Faradic Stimulation of the Postcentral Gyrus in Conscious Patients, Brain, vol. xxxii, pp. 44-54. Dejerine, J., 1914: Semiologie des affections du systeme nerveux, Paris, 1914. Dogiel, A. S., 1896: Zwei Arten sympathischer Nervenzellen, Anat. Anz., Bd. xi, pp. 679-687. — , 1908, Der Bau der Spinalganglien des Menschen und der Saugetiere, Gustav Fischer, Jena. Donaldson, H. H., 1898: The Growth of the Brain, Chas. Scribner's Sons, New York. Donaldson, H. H., and D. J. Davis, 1903: A Description of Charts Showing the Areas of the Cross-sections of the Human Spinal Cord at the Level of Each Spinal Nerve, Jour. Comp. Neurol., vol. xiii, p. 19. Economo, C, 1911: Uber dissoziierte Empfindungslahmung bci Ponstumoren und iiber die zentralen Bahnen des sensiblen Trigeminus, Jahrbiicher f. Psychiatrie, vol. xxxii, p. 107. Edinger, L., 1887: On the Importance of the Corpus Striatum and the Basal Forebrain Bundle, Jour. Nerv. and Ment. Diseases, vol. xiv, p. 674. , 1911: Vorlesungen iiber den Bau der Nervosen Zentralorgane des Menschen und der Tiere, F. C. W. Vogel, Leipzig. Edinger, L., and A. Wallenberg, 1903: Bericht iiber die Leistungen auf dem Gebiete der Anatomie des Centralnervensystems, 1901-02, p. 152. Essick, C. R., 1907: The Corpus Ponto-bulbare — A Hitherto Undescribed Nuclear Mass in the Human Hind Brain, Amer. Jour. Anat., vol. vii, p. 119. Feiling, A., 1913: On the Bulbar Nuclei, with Special Reference to the Existence of a Salivary Center in Man, Brain, vol. xxxvi, p. 255. 1 1 1 1 -. I li IGK \\'U\ 77 Feiss, II. 0., 1912: On the Fusion of Nerves, Quart. Jour. Exp. Physiol., vol. v. p. 1. Flechsig, 1'-. 1896: Gehirn und Seele, Leipzig. , 1896: Die Lokalisation der geistigen Vorgange, Leipzig, Fritsch, G., and E. Hitzig, 1870: Qber die elektrische Erregbarkeit des Grossihirns, Vrch. f. Anai., Physiol., u. Wissen. Med., 1870, p. 300. Gall, F. J., 1825: Sur les fonctions du cerveau, Paris. Gaskell, W. H., 1886: < >n the Structure, Distribution, and Function of the Nerv< - which [nnervate the \ isceral and Vascular Systems, Jour. Physiol., vol. vii, p. 1. . 1908: The Origin of Vertebrates, London. Longmans, Goldstein, K.. 1910: Ueber die aufsteigende I legeneral ion und Qers< bnittsunterbrechui . Riickensmarks (Tractus spinocerebellaris posterior, Tractus spino-olivaris, Tractus spino-thalamicus), Neurol. Centralblatt, vol. xxix, p. 897. Grunbaum, A. S. F., and ('. S. Sherrington, 1903: Observations on the Physiology of the Cerebral Cortex of the Anthropoid Apes, Proc. Roy. Soc, vol. Ixxii, p. 152. Guyer, M. I'., I'M 7: Animal Micrology, Chicago. Hardesty, [., 1902: Neurological Technique, Chicago. — , 1904: On the Development and Nature of the Neuroglia, Amer. Jour. Anat., vol. iii, p. 22 the Cerebellum of the Dog by the Method of Degeneration, Jour. Comp. Neurol., vol. ix, p. 307. Horsley, Victor, 1906: Note on the Taenia Pontis, Brain, vol. xxix. ]>. 28. , 1909: The Function of the So-called Motor Area of the Brain, Brit. Med. Jour., 1909, ii, p. 125. Horsley, Victor, and R. H. Clarke, 1908: The Structure and Functions of the Cerebellum Examined by a New Method. Brain, vol. xxxi. pp. 45-124. Huber, G. C, 1899: A Contribution on the Minute Anatomy of the Sympathetic Ganglia of the Different Classes of Vertebrates, [our. Morph., vol. 16, pp. 27-90. ■ , 1913: The Morphology of the Sympathetic Nervous System. NYIIth International Congress of Medicine. London, 1913. Sec. 1, p. 211. Huber, G. C, and S. R. Guild, 1913: Observations on the Peripheral Distribution of the Nervus Terminalis in Mammalia, Anat. Rec, vol. vii, p. 253. 378 BIBLIOGRAPHY Ingalls. X. W., 1914: The Parietal Region in the Primate Brain, Jour. Comp. Neurol., vol. xxiv, pp. 291-341. Ingvar, Sven, 191 8: Zur Phylo- und Ontogenese des Kleinhirns. Folia Neuro-biologica, Bd. xi. p. 205. Johnson, S. E., 1918: On the Question of Commissural Neurones in the Sympathetic Gan- glia, Jour. Comp. Neurol., vol. xxix. p. 385. Johnston, J. B., 1901: The Brain of Acipenser. Zool. Jahrb., Bd. xxv, pp. 1-204. , 1909: The Morphology of the Forebrain Vesicle in Vertebrates, Jour. Comp. Neurol., vol. xix, p. 457. , 1909: The Radix Mesencephalica Trigemini, Jour. Comp. Neurol., vol. xix, pp. 593-644. , 1912: The Telencephalon in Cyclostomes, Jour. Comp. Neurol., vol. xxii. p. 341. , 1913: The Morphology of the Septum. Hippocampus, and Pallial Commissures in Reptiles and Mammals, Jour. Comp. Neurol., vol. xxiii, p. 371. -, 1914: The Nervus Terminalis in Man and Mammals. Anat. Rec. vol. viii, p. 185. Jolly. \Y. A.. 1911: On the Time Relations of the Knee-jerk and Simple Reflexes, Quart. Jour. Exp. Physiol., vol. iv. p. 67. Rappers, C. U. Ariens. 1914: The Phenomena of Neurobiotaxis in the Central Nervous System, XVllth International Cong. Med.. Sec. I, Part II, p. 109. , 1917: Further Contributions on Neurobiotaxis, IX Jour. Comp. Neurol., vol. xxvii, pp. 261-298. Karplus, J. P., and A. Kreidl. 1914: Ein Beitrag zur Kenntnis der Schmerzleitung im Riick- enmark, Pfliiger's Archiv. Bd. clviii. p. 275. Kohnstamm, O., 1902: Der Nucleus salivatorius chordae tympani (nervi intermedii), Anat. Anz.. vol. xxi. pp. 362, 363. , 1903: Der Nucleus Salivatorius inferior und cranio-visceral System. Neurol. Central- blatt, Bd. xxii. p. 699. , 1907: Versuch einer physiologischen Anatomie der Vagusurspriinge und des Kopf- sympathicus, Jour. f. Psych, u. Neurol. VIII. Kohnstamm and Hindelang. 1910: Der nucleus intermedius sensibilis als Ursprung einer gekreuzt aufsteigenden Bahn (Yisceralbahl), Referat in Neurol. Centralbl., Bd. xxix, p. 663. Kolliker. H., 1891: Zur feineren Anatomie des central Nervensystems, Zeit. f. wiss. Zool., Bd. k\p. 1. Kreidl. A., 1914: Zur Frage der sekundaren Horbahnen, Monatschrift f. Ohrenheilkunde und Laryngo-Rhinologie, 1914. H. 1. Kuntz, A., 1910: The Development of the Sympathetic Nervous System in Mammals, Jour. Comp. Neurol., vol. xx. p. 211. Landacre, F. L., 1910: The Origin of the Cranial Ganglia in Ameiurus, Jour. Comp. Neurol., vol. xx, pp. 309-411. , 1910: The Origin of the Sensory Components of the Cranial Ganglia, Anat. Rec, vol. iv, pp. 71-79. Landau, E., 1919: Nucleus Amygdalae, Claustrum and Insular Cortex, Jour, of Anat., vol. liii, p. 351. Langley, J. N., 1892: The Origin from the Spinal Cord of the Cervical and Upper Thoracic Sympathetic Fibers, with Some Observations on White and Gray Rami Communi- cantes, Phil. Trans. Roy. Soc, London, vol. clxxxiii, p. 114. , 1900: The Sympathetic and Other Related Systems of Nerves, Schafer's Text-book of Physiology, vol. ii. , 1900: Remarks on the Results of Degeneration of the Upper Thoracic White Rami Communicantes. Chierly in Relation to Commissural Fibers in the Sympathetic Sys- tem, Jour, of Phys., vol. xxv, p. 468. — , 1903: The Autonomic Nervous System, Brain, vol. xxvi. p. 1. -, 1904: On the Question of Commissural Fibers Between Nerve-cells Having the Same Function, Jour, of Physiol., vol. xxxi. p. 244. Langley and Magnus, 1905: Some Observations on the Movements of the Intestines Before and After Degenerative Section of the Mesenteric Nerves, Jour, of Physiol., vol. xxxiii, p. 34. Larsell, O., 1918: Studies on the Nervus Terminalis: Mammals, Jour. Comp. Neurol., vol. xxx, p. 1. ■ , 1919: Studies on the Nervus Terminalis: Turtle, Jour. Comp. Neurol., vol. xxx, pp. 423-443. BIBLIOGK \l'll\ Lewandowsky, M.. 1907: Die Funktionen des Zentralen nervensj sten fi oa. Lewis, \\ . II.. 1910: The Development of the Mu-. ulai Sj stem, Keibel and Mall's Manual of Hunu.n Embryology, vol. i. p. 454. Linowiecki, A. J., 1 ( >14: The Comparative Anatomy of the Pyramidal Tract, Jour. Comp. Neurol., vol. \\i\ . p. 509. M cNalty, A. S., and Victor Eiorsley, 1909: On the Cervical Spino-bulbar and Spino- cerebellar Trails and on the Question of Topographical Representation in th< hdlum. Brain, vol. \\.\ii. |>. 237. McCotter, R. E., 1913: The Nervus Terminalis In the Adult Dog and Cat, Jour. Comp. Neurol., vol. xxiii, |>. 145 152. .Mi ELibben, 1'. s.. I'M 1 : Tin- Nervus Terminalis in Urodele Amphibia, Jour. Comp. Neurol., vol. wi. p. 261 . Malone, E. I.. 1910: Qberdie KLerne des menschlichen Diencephalon, Neur. Centralbl., 1910. . 1913: Recognition of Members of the Somatic Motor Chain of Nerve-cells by Means of a Fundamental Type of Cell Structure. Anat. Rec, vol. vii. p. 67. 1913: The Nucleus Cardiacus NerviVagi and the Three Distincl Typesof Nerve- cells which Innervate the Three Different Types of Muscle, Amer. Jour. Anat., vol xv. p. 121. Marburg, <>.. 1904: Die I'hv>iologische Funktion der Kleinhirnseitenstrangbahn, Arch. f. Anat. u. Physiol., Physio] Abt., Suppl. 1904, S. 457. Marinesco, M. C. 1906: Quelques recherches sur la morphologie aormale et pathologique des cellules des ganglions spinaux et sympathiques de L'homme, Le Nevraxe, t. viii. p. '). Mauri. K., 1918: On the Finer Structure of the Synapse of the Mauthner Cell, Jour. Comp. Neurol., vol. xxx. p. 127. May. W. P., l n <>6: The Afferent Path. Brain, vol. xxix. p. 742. Michailow, S., 1911: Der Bau der zentralen sympathischen Ganglien, Internat. Monats- schrift f. Anat. u. Physiol., vol. xxviii, pp. 26-115. Minot, C. S., 1901: On the Morphology of the Pineal Region Based Upon its Development in Acanthias. Amer. Jour. Anat.. vol. i. Monakow, C. v.. 1895: Experementelle und pathologisch-anatomische Untersuchungen uber die Haubenregion, den sehhiigel und die Regio subthalamica nebst Beitragen zur Kenntnis friih erworbene Gross und Kleinhirndefecte, Archiv. f. Psych., vol. xxvii. , 1913: Zur Kenntnis der Grosshirnanteile (Yago-glossopharvngeusschleifc I, Neurol. Centr.. 1913. p. 331. Muller. L. R.. 1909: Studien iiber die Anatomic und Histologic des sympathischen Grcnz- stranges, XX\ I Kongr. innerc Med.. Wiesbaden, p. 658. Muller. L. R., and W. Dahl, 1910: Die Beteiligung des sympathischen Nervensvstems an der Koptinnervation, Deutsches Arch. f. klin Med., Bd. xcix. pp. 48-107. Muskens. L. J. J.. 1914: An Anatomico-physiological Study of the Posterior Longitudinal Bundle in its Relation to Forced Movements, Brain, vol. xxxvi. pp. .^^2 42o. Norris. H. YV.. 1908: The Cranial Nerves of Amphiuma Means, Jour. Comp. Neurol.. vol. xviii, pp. 527-568. Parker. G. H., 1919: The Elementary Nervous System, Lippincott, Philadelphia. Petren, K.. 1902: Ein Beitrag zur Frage vom Verlaufe der Bahnen der Hautsinne im Riicken- marke, Skandinav. Archiv f. Physiol., Bd. xiii, s. 9. Ranson, S. \\\, 1911: Non-mcdullated Nerve-fibers in the Spinal Nerves. Amer. Jour. Anat.. vol. xii, p. 67. , 1912: The Structure of the Spinal Ganglia and of the Spinal Nerves. Jour. Comp. Neurol., vol. xxii, p. 159. , 1912: Degeneration and Regeneration of Nerve-fibers, Jour. Comp. Neurol., vol. xxii. p. 487. , 1913: The Fasciculus Cercbrospinalis in the Albino Rat. Amer. Jour. Anat., vol. xiv. p. 411. , 1913: The Course Within the Spinal Cord of the Non-medullated Fibers of the Dorsal Roots. A Study of LissauerV Tract in the Cat, Jour. Comp. Neurol., vol. xxiii, p. 259. , 1914: The Tract of Lissauer and the Substantia (ielatinosa Rolandi, Amer. Jour. Anat.. vol. xvi. p. 97. 1915: Unmyelinated Nerve-fibers as Conductors of Protopathic Sensation. Brain, vol. xxxviii, p. 381. Ranson. S. W., and P. R. Billingsley. 1916: Afferent Spinal Paths and the Ya>omotor Re- flexes. Amer. Jour. Physiol., vol. xlii, p. 16. 380 BIBLIOGRAPHY Ranson, S. \V.. and P. R. Billingsley, 1916: The Conduction of Painful Afferent Impulses in the Spinal Nerves, Amer. Jour. Physiol., vol. \l, p. 571. , 1918: Studies on the Sympathetic Nervous System, Jour. Comp. Neurol., vol. xxix, p. 305. Rasmussen, A. T., 1919: The .Mitochondria in Nerve-cells During Hibernation and Inani- tion in the Woodchuck, Jour. ('oni]>. Neurol., vol. xxxi, pp. 37 49. Rcid, R. \\'., 1889: The Relations Between the Superficial Origins of the Spinal Nerves from I he Spinal Cord and I lie Spinous Processes of the Vertebrae, Jour, of Anal, and Physiol., vol. \\iii, p. M3. Retzius, J., 1880: Untersuchungen iiber die Nervenzellen der cerebrospinalen Ganglien und der iibrigen peripherischen Kopfganglien, Arch. f. Anat. u. Physiol., Anat. Abteil., 1880. Rhinehart, D. A., 1918: The Nervus Fascialis of the Albino Mouse, Jour. Comp. Neurol., vol. xxx, pp. 81-125. Riddoch, George, 1917: The Retlex Functions of the Completely Divided Spinal Cord in Man, Compared with Those Associated with Less Severe Lesions, Brain, vol. xl, p. 264. Rogers, 1*'. T., 1916: The Hunger Mechanism of the Pigeon and its Relation to the Central Nervous System, Amer. Jour. Physiol., vol. xli, pp. 555-570. Rothmann, M., 1903: Zur Anatomic und Physiologic des Yorderstranges, Neurol. Centralb., Bd. xxii, p. 744. — , 1906: tjber die Leitung der Sensibilitat im Ruckenmark, Berlin, Klin. Wochensch., Bd. xliii, pp. 47, 76. — , 1907: Uber die physiologische Wertung der corticospinalen (Pyramiden-) Bahn, Arch. f. (Anat. u.) Physiol., p. 217. Sabin, Florence, 1901: An Atlas of the Medulla and Midbrain, Baltimore. Sachs, E., 1909: On the Structure and Functional Relations of the Optic Thalamus, Brain, vol. xxxii, p. 95. Schaffer, E. A., 1899: Some Results of Partial Transverse Section of the Spinal Cord, Proc. Physiol. Soc, Jour. Physiol., vol. xxiv, p. xxii. — , 1910: Experiments on the Paths Taken by Volitional Impulses Passing from the Cortex to the Cord; the Pyramids and the Ventrolateral Descending Tracts, Quart. Jour. Exp. Physiol., vol. iii, p. 355. Schulte, H. von W., and F. Tilney, 1915: Development of the Neuraxis in the Domestic Cat to the Stage of Twenty-one Somites, Annals of the New York Acad, of Sciences, vol. xxiv, pp. 319-346. Sherrington, C. S., 1894: Experiments in Examination of the Peripheral Distribution of the Fibers of the Posterior Roots of Some Spinal Nerves, Phil. Tr. London (B), vol. clxxxiv, pp. 641-763. , 1894: On the Anatomical Constitution of Nerves of Skeletal Muscles; with Remarks on Recurrent Fibers in the Ventral Spinal Nerve Root, Jour, of Physiol., vol. xvii, p. 211. — , 1906: The Integrative Action of the Nervous System, Yale University Press. New 1 laven. Sherrington, C. S., and Graham Brown, 1913: Note on the Functions of the Cortex Cerebri, Jour. Physiol., vol. xlvi, p. 22. Simpson, S., 1902: Secondary Degeneration Following Unilateral Lesions of the Cerebral Motor Cortex, Internat. Monatsschrift f. Anat. u. Physiol., Bd. xix. Smith, G. Elliot, 1895: Morphology of the True Limbic Lobe, Corpus Callosum, Septum Pellucidum and Fornix, Jour, of Anat. and Physiol., vol. xxx, pp. 157-167 and 185-205. — , 1903: Further Observations on the Natural Mode of Subdivision of the Mammalian Cerebellum, Anat. Anz., Bd. xxiii, p. 368. — , 1907: A New Topographical Survey of the Human Cerebral Cortex, Jour. Anat. and Physiol., vol. xli, p. 237. , 1915: The Central Nervous System, Cunningham's Anatomy, William Wood & Co., New York. — , 1919: The Significance of the Cerebral Cortex, Brit. Med. Jour., 1919, ii, p. 11. 1919: Morphology of the Corpus Striatum and Origin of the Neopallium, Jour, of Anat., vol. liii, p. 271 Solomowicz, von J., 1908: Vom Centrum der Submaxillardriise, Neurol. Centralb., Bd. xxvii, No. 15. Spiller, W. G., 1915: Remarks on the Central Representation of Sensation, Jour. Nerv. and Ment. Diseases, vol. xlii, p. 399. Starling, E. H., 1912: Principles of Human Physiology, Lea & Febiger, New York and Philadelphia. Hllil.hH.k \i'il\ Stewart, P., 1901: Degenerations Following a Traumatii I I the Spinal Cord, Brain vol. \\i\ . p. 222. Streeter, G. I... 1912: The Development of the Nervous System, Keibel and Mall, Human Embryology, vol. ii. Lippincott, Philadelphia. Strong, 0. s.. 1895: The Cranial Nerves of Amphibia, Jour. Morph., vol. s, pp 101 . 1915: A Case of Unilateral Cerebellar Agenesia, Jour. Comp. Neurol., vol. xxv p. 361 . Terry. R. J., 1910: The Morphology of the Pineal Region in I eleosts, lour. Morph., vol Thiele, F. II.. and Victor Horsley, 1901 : A Study of the Degenerations Observed in theCentrai Nervous System in a Case of Fracture dislo< ation of the Spine, Brain, vol. nriv, p. 519. Thompson, T.. 1912: A Case of Subacute Combined Degeneration of the Spinal Cord I >i onstrating the Nature of the Afferent Impulses in the Posterior Columns, Brain, vol. \wiv, p. 510. Tilney, !•'.. 1911: Contribution to the Study of the Hypophysis Cerebri with Especial Ref- erence to its Comparative Histology, Memoirs of the Wistar Institute of Anatomy and Biology, No. 2. , 1913: An Analysis of the Juxtaneural Epithelial Portion of the Hypophysis Cerebri, Internal. Monatsschrifl f. Anal. u. Physiol., Bd. x.xx, p. 258. Van Gehuchten, A., 1901: Recherches sur lcs voies sensitives centrales. La voie centrale du trigemeau, Le Nevraxe, vol. iii, p. 235. — , 1903: Recherches sur la voie acoustique centrale, Le N6vraxe, vol. iv, pp. 253 300. , 1904: Le corps restiforme et lcs connexions bulbo-ccrebcllcuscs, Le Nevraxe. vol. vi, p. 125. Van Gehuchten, A., and M. Molhant, 1912: Contribution a l'etude anatomique du nerf pneu- mogastrique, Le Nevraxe, vol. xiii, p. 55. \ an Rynberk, G., 1908: Die neueren Beitrage zur Anatomic unci Physiologic des Klein- hirns der Sauger Folia Neuro-biologica, vol. i, p 535. , 1908, 1912: Das Lokalizationsproblem im Kleinhirn, Ergebnisse der Physiol., Bd. vii. p. 653, and Bd. xii, p. 533. Van Valkenburg, C. T., 1913: Experimental and Pathologico-anatomical Researches on the Corpus Callosum, Brain, vol. xxxvi, p. 119. Ycs/i, J., 1918: Untersuchungen iiber die Erregunsleitung in Ruckenmark. Ztschr. f. allg. Physiol., vol. xviii, pp. 58-92. Wallenberg, A., 1905: Sekundaren Bahnen aus dem frontalen sensibeln Trigeminuskcrne des Kaninchens., Anat. Anz., Bd. xxvi, p. 145. Walshe, F. M. R., 1919: On the Genesis and Physiological Significance of Spasticity and Other Disorders of Motor Innervation, Brain, vol. xlii, p. 1. Warren, J., 1911: The Development of the Paraphysis and Pineal Region in Reptilia. Amer. Jour. Anat., vol. xi, pp. 313-392. , 1917: The Development of the Paraphysis and Pineal Region in Mammalia. Jour. Comp. Neurol., vol. xxviii, pp. 75-103. Warrington, W. B., and F. Griffith, 1904: On the Cells of the Spinal Ganglia and on the Relationship of their Histological Structure to the Axonal Distribution, Brain, vol. xxvii, p. 297. Weed, L. IP. 1914: A Reconstruction of the Nuclear Masses in the Lower Portion of the Human Brain-stem, Publications of the Carnegie Institution of Washington. 1 ( M4. — , 1914: Observations Upon Decerebrate Rigidity, Jour. Physiol., vol. xlviii. p. 205. Willard, W. A.. 1915: The Cranial Nerves of Anolis Carolinensis. Bui. Museum of Comp. Zool., Harvard, vol. lix, p. 17. Willems. E., 1911: Les noyaux masticateurs et mesencephaliques du trigumeau, be Nevraxe, vol. xii, p. 7. Wilson. J. G., 1905: The Structure and Function of the Taste buds of the Larynx. Brain, vol. xxviii. p. 339. Wilson, J. G., and F. H. Pike. 1915: The Mechanism of Labyrinthine Nystagmus, Arch, ot Int. Med., vol. xv, p. 31. Wilson, S. A. K.. 1912: Progressive Lenticular Degeneration, Brain, vol. x.xxiv. p. 295. , 1914: An Experimental Research into the Anatomy and Physiology of the Corpus Striatum, Brain, vol. xxxvi. p. 427. Yagita. R. von, 1909: Weitere Untersuchungen iiber das Speichelzentrum, Anat. Anzeiger, Bd. xxxv, p. 70. Yagita, K., and S. Hayama, 1909: Uber das Speichelsekretionscentrum. N'eur. C entralb., Bd. xxviii, p. 738. INDEX Note. -In cross references the key words arc italicized. the pages on which the structures arc illustrated. The numbers in Italics refer t<> Accommi idation "t \ ision, 5 $2 Acoustic ana of cortex. See Center, auditory Acousticolateral area, 358 Affenspalte, 237 Ala cinerea, 127 lobuli centralis, 197 Alveus, 27(i, 27s, 279 Ameba, 17 Amnion's horn. Sec Hippocampus. Ampulla of semicircular canal, 358 Amygdala. See Nucleus, amygdaloid. Ansa lent icularis, 2<>S peduncularis, 2(>, : ! Aperture, lateral, of fourth ventricle, 125 medial, of fourth ventricle, 125 Apex col u mnae posterioris, 7 ( » Aphasia, 295 Aqueductus cerebri (aqueduct of Sylvius), 26, 158 Arachnoid, 73 Arbor vitas, 199 Archipallium, 116, 242. 270, 277, 278, 279 Area, acousticolateral, 358 acustica, 127 cortical, 287. (See also Center.) oval, of Flechsig, 107 parolfactoria of Broca, 267 postrema, 129 pyriform, 116, 268, 277 striata, 293 Association bundles of cerebrum, 298 arcuate, 298, 300 cingulum, 299 inferior longitudinal, 299 occipitofrontal, 300 superior longitudinal, 300 occipitofrontal, 301 uncinate, 299 Ataxia, 99 Auditor\ apparatus, 186, 309 Auerbach's plexus, 351 Autonomic system, 339 cranial, 339 craniosacral, 340, 354 sacral, 339 thoracolumbar, 339, 354 Axon (axis-cylinder), 37, 43, 45 hillock. See Cone, implantation. Axonal reaction. See Chromatolysis. BAILLARGER, lines of, 283 Band, diagonal, 267 Basis cerebri, 115, 1 20 pedunculi, 129, 158, 164 Basket-cells, 209 Bell's law, 60 Betz, cells of, 290 Bladder, innervation of, $54 Body of cell, 43 oi fornix, 27 1 geniculate, lateral, 131, 220 medial, 1 $1, 107, 220 mammillary, 111. 280 of Nissl, 48, 51 paraterminal, 267 pineal, 221 pituitary. See Hypophysis. quadrigeminal, 130, 165 restiform, 122, 143, 205 striate. See Corpus striatum. tigroid. See Nissl body. trapezoid, 121, 150, 186 Brachium (or brachia), conjunctivum, 125, 155, 159, 160, 206, 211 of corpora quadrigemina, 131 pontis, 123, 204 quadrigeminum inferius, 131, 163, 166 superius, 131, 167 Brain, 56, 115 development, 25 divisions of, 2F< end-. See Telencephalon. fore-. See Prosencephalon. hind-. See Metencephalon and Rhombenceph- alon. inter-. See Diencephalon. stem. See Medulla oblongata, Pons, Mesen- cephalon, and Ganglia, basal. vesicles, 24, 25 weight, 301 Broca's convolution, 235 Brown-Sequard syndrome, 112 Bulb, olfactory, 265, 274 of posterior horn, 248 Bundle. (See also Fasciculus and Tract.) association, of cerebrum, 298, 299, 500 cornucommissural, 107 ground. See Fasciculus proprius. ol Gudden, tegmental. See Tract, mammillo- tegmental. marginal. See Fasciculus dorsolateralis. Oval. See .1 red. oval. posterior longitudinal. See Fasciculus, medial gil udinal. of Turck. See Tract, ventral corticospinal. ventral longitudinal. See Tract, tectospinal. Burdach, column of. See Fasciculus cuneatus. nucleus of. See Nucleus cuneatus. CAJAL, commissural nucleus of, 330 horizontal cells of, 285 Calamus scriptorius, 127 383 3»4 INDEX Calcar avi- Canal, central fcanalis centralis), 80, 136 lateral line, - - semicircular, 315, 356 spinal, 73 Capsule, external, 2 : 7 internal, 257, 259, 261 nasal of spinal ganglion eel!, 63 Cauda equina, 78 Cavum septi pellucidi, 272 Cell. (See also Neuron.) basket, 209 of Betz, 290 body, 43 ependymal, 37, 85 germinal. $7 granule, of cerebellum, 208 of cerebral cortex. See Neurons, stellate. of olfactory bulb, 276 mitral, 27 neuroglia, 85, 86 of Purkinje, 207 pyramidal, 285 Cell-columns of Clarke. See Nucleus dorsalis. intermediolateral, 89 of spinal cord, 89, go Center, cortical, 290 association, 293 auditory, 293 motor, 290, 317, 318 olfactory, 293 opti. projection, 290 somesthetic, 292 of speech, 295 visual, 292 for pain, 219 projection. 290 respiratory, 330 Central nervous system, 20, 21, 56, 57 Centrum medianum thalami, 218 semiovale, 243 Cerebellum, 195 in birds and reptile- central white matter, 199 cortex, 199, 206, 207, 208, 209 development of, 195 in the dogfish, 27, 28 fiber tracts of, 204. 205, 206, 209, 210, 211 folia, 199 hemispheres of, 197, 198 histogenesis, 196 laminae, 199 lobes or lobules, 197, 198, 200, 201, 202 in mammals, 200 microscopic structure, 206 morphology of, 199 notches, 197 • nucleus dentatus, 203, 211 emboliformis, 2 fastigii or tecti, 204, 211 globosus, 20 } peduncles, 204 inferior, 122, 143, 205 middle, 123, 204 superior, 125, 155, 159, 160, 206, 211 section, median, 199 through hemisphere, 199 Cerebellum in the sheep, 200, 201, 202 vermis of, 196 white matter, 199 Cerebral aqueduct. See Aqueduttus cerebri, cortex, 114, 232, 283 area of, acoustic, 293 association, 293 audito-psychic, 293, 294 audito-sensory, 294 of Broca, 295 motor, 290, 317, 318 striata, 293 visuo-psychic, 293, 294 visuo-sensory, 294 centers of, 290, 292, 2<>? development, 2 50 electric excitability of, 291 frontal olfactory, 277 hippocampal, 278, 279 histogenesis, 230 layers of, 286, 287 localization of function in, 290 myelination of fibers, 289 nerve-cells, 284, 285 nerve-fibers, 283, 284 neuroglia-cells, 284 structure, 283, 284, 285, 286 hemispheres, 113, 229, 232 borders, 232 commissural fibers, 296 convolutions. See Gyri. corticifugal or efferent fibers, 283 corticipetal or afferent fibers, 283 development, 25, 32, 22') in the dogfish, 27, 28, 30 external conformation, 229 fissures. See Fissure. gyri. See Gyrus. lobes. See Lobe. lobules. See Lobule. medullars" center, 243, 296 pallium, 25, 32, 33, 229 poles, 232 sulci. See Sulcus. surfaces, 232 ventricles, lateral, 246 peduncles. See Peduncles. vesicles, 24, 25 Cerebrospinal fluid, 73, 126 system. r : Cerebrum, 117 Cervix, columnar posteriori-. 79 Chiasma, optic, 223, 220 Chorda tympani, 192, 3r2 Chorioid fissure, 229, 251 plexuses. See Plexus. Chromatolysis, 51 Chromophilic bodies. See Nissl bodies. Cingulum, 299 Clarke, column of. See Nucleus dorsalis. Claustrum, 256 Cava, 121, 137 Climbing fibers, 209, 210 Clivus monticuli. See Declke monticuli. Cochlea, 185 nterates, 19 Cold, sensations of, 105, 306 Collateral fiber-. 43, ')7 | Colliculus facialis, 127 INDI \ Colliculus, inferior, 130, 165 superior, 130, 165, U>7 Column, anterior, 80 ol Burdach. See Fasciculus cuneatus. nt Clarke. See Nucleus dorsalis. dorsal (columna < 1< u >-.i 1 i^ gi isea I, 42 of fornix, 272 ol' (.oil. See Fasciculus gracilis. pay, 76 intermediolateral, 89 lateral, 80 nuclear, of brain stem, 168, 170, 171, 174 posterior, 79 somatic afferent, 170, 182, 185 efferent, 17o ventral, 42, 80 vesicular. See Nucleus dorsalis. visceral afferent, 170, 180 efferent, 170, 174, 177 Comma tract of Schultze. See Fasciculus inter- fascicularis. Commissura anterior alba, 80 habenularum, 220 Commissure or commissures, anterior cerebri, 22\ 231, 273, 296 gray, 80 white, 80 great transverse. See Corpus callosum. of Gudden, 227 habenular, 22(i hippocampal, 231, 271, 280, 296 of inferior colliculi, 159 middle. See Afassa intermedia. optic. See Chiasma, optic. posterior, of cerebrum, 221 of spinal cord, 80 superior. See Commissure, habenular. Components of nerves, 61, 168. (See also Nerve-fibers. ) Conduction of nerve impulses, 50 Cone, implantation, 44 of origin. See Cone, implantation. Cones of retina, 226 Consciousness, 2^, 302 Conus medullaris, 74 Convolution. See Gyrus. Coordination, 99, 210, 311 Cornu ammonis. See Hippocampus. Cornucommissural bundle, 107 Corona radiata, 261 Corpus (or corpora) callosum, 243, 296 development, 231 fornicis, 271 geniculatum laterale, 220 mediate, 131, 167, 220 mamillaria, 222, 230 pineale, 221 ponto-bulbare, 123 quadrigemina, 130, 165 restiforme, 122, 143, 205 striatum, 25. 32, 33, 256, 262, 324 subthalamicum (Luysi), 22^-~ trapezoideum, 121, 150, 186 Cortex, cerebellar, 199, 206. 207. 208, 209 localization of function in, 202 neurons of, 207, 208. 209 cerebral. See Cerebral cortex. Corti, ganglion of. See Ganglion, spiral. 25 < oi i i, org in of, ( 'ough, me< nanism of, <^1 ' i rebri. See Peduncle, < erebral. lor nil i-, 27 1 Crusta. S • dunculi. Culmen mom iculi, ' Cuneate tuben le, 121, 1 <7 Cuneus, 239 Cup, opt ic, 32, 3 J, 22r< ( v toplasm ol nerven ells, 42, 1 . I >i i i i\i monticuli, ' nert . See Da ussation, dorsal tegmental. optic. See Chiasma, optic. of pyramids, 11''. 120, 154. 156 tegmental. See Decussations, ventral and dorsal tegmental. ventral tegmental, 161 Degeneration of fiber tracts, 105, 106, 107 of nervc-nlx-r>, 51, 52 Wallerian, 105, 106, 107 Deiters, nucleus of, 151, 189 Dendrites or dendrons, 43 Dermatome, 58 I >i \ elopment of the nervous svstem, 24, 31 Diencephalon, 24, 2'-. 26. 28, 31, 33, 213 Digitationes hippocampi, 269 Dogfish, brain of, 26, 27, 2S Dogiel's Type II cells, 65 Dura mater. 7 J Dynamic polarity, law of, 50 Earthworm, nervous system of, 19 Edinger-Westphal nucleus, 178 Effector, 18, 19, 54, 91 Embrvologv of nervous svstem, 31, 37, 195, 215. 22')' Eminentia cinerea. See Ala cinerea. collaterals, 250 facialis. See Colliculus facialis. hypoglossi. See Trigonum hypoglossi. medialis, 129 teres. See Eminentia medialis. Encephalon. See Brain. End-brain. See Telencephalon. End-plates, motor, 62 Ependyma. 8 ; Epiphysis, 29, 51 Epithalamus, 29, 55, 220 Exteroceptor, exteroceptive, 66, 182, 185 l>. • . development, 22> innervation, 225 retina, 225 1" \-< i \ dentata, 269, 276 Fasciculus, 65. S<-c also Tract and Bundle.) anterior proprius, 1<>7 anterolaterals superficialis, 100 arcuatus, 300 cerelxllospinalis. See Tract, dorsal spinocere- bellar, cerebrospinalis. See Tract, corticospinal. ?86 IM'I X ulus cerebrospinal, anterior. Sec Tract, \ entral corticospinal. lateralis. See Trad, lateral corticospinal, cuneat us, 7o, 8 \ 95, 96, 121, 137 dorsal longitudinal Schutz), 2\<> dorsolati ri -, 104 ilis, 76, 83, 96, 121, 137 interfascicularis, ''7, Ki7 lateralis, minor, 1 21 proprius, 107 longitudinalis inferior, 2'>'> medialis, 145, 152, 162, 190, 328 superior, 300 medial longitudinal, 145, 152, 102, 190, 328 of Meynert, 220 occipitofrontalis, inferior, 300 superior, 301 peduncularis transA posterior longitudinal. See Fasciculus, medial longitudinal. proprius of spinal cord, 107 pyramidal. See Tract, corticospinal. retroflexus, 220 septomarginal, 97, 107 solitarius 152, 181, 330 sulcomargihalis, l l|v superior longitudinal, 300 thalamomamillaris. See Tract, mammillo- thalamic. uncinatus, 299 Fibers, fibrae. (See also Xerce-fibers.) arcuate, of cerebrum, 299 of medulla oblongata, 139 external, 121, 123, 139, 140, 143 internal, 154, 138, 139 association, 92, 298 cerebello-olivarv. See Fibers, olivocerebellar, climbing, 209, 210 commissural, 296 mossy, 209, 210 olivocerebellar, 139, 142, 143, 205 pontis, 147 postganglionic, 537, 343 preganglionic, 55 7, 544 projection, 297 propria?. See Fibers, arcuate, of cerebrum, recta-, 148 Fila lateralia pontis, 148 Fillet. See Lemniscus. Filum durae matris spinalis, 74 terminale, 74 externum, 74 internum, 74 Fimbria hippocampi, 250, 269 Final common path, 94, 311 re or fissura), calcarine, 238, 292 callosal. See Sulcus of corpus callosum. callosomarginal, 240 central, of Rolando, 233 cerebri lateralis, 2 5 1 chorioid, 229, 251 collateral, 239 dentate. See Fissure, hippocampal. development, 230, 231 great longitudinal, 2>2 transverse. See Fissure, transverse cere- bral, hippocampal, 239, 269, 270 lateral cerebral, 233 Fissure, longitudinal cerebral, 114, 232 mediana, anterior, of medulla oblongata, 119 of spinal cord, 76, 82 posterior, of medulla oblongata, 119 parieto-occipital, 239 prima, 196, 199 rhinal, 116, 240 of Rolando. See Sulcus, central. - la, 203 Sylvian. See Fissure, lateral cerebral. transverse cerebral, 215 FleChsig, direct cerebellar tract of. See Tract, dorsal spinocerebellar. Flexure, cephalic, 31, 33 cervical, M, 33 pontine, 31, 33 Flocculus, 199 Fluid, cerebrospinal, 73, 126 Folium vermis, 198 Foramen caecum, 119 interventricular, 26, 118 of Luschka. See Aperture, lateral, of fourth ventricle. of Majendie. See Aperture, medial, of fourth ventricle. of Monro. See Foramen, interventricular. Forceps, major, 245 minor (frontal part of radiation of corpus cal- losum). Fore-brain. See Prosencephalon. Forel, fountain decussation of. See Decussa- tion, ventral tegmental. Formatio reticularis, 80, 136, 144 Fornix, 270, 280 body, 271 columns, 271, 272 commissure, 271, 280 crura, 271 fimbria, 270, 271 longus, 282 Fossa interpedunculans, 115 rhomboid, 126, 127 Fountain decussations of Forel and of Mevnert, 161, 167 Fovea, inferior, 127 superior, 127 Frenulum veli medullaris anterior, 130 Frog, sympathetic ganglia of, 344, 345 Funiculus, 95 anterior, 76, 82 cuneatus, 121, 137 dorsal. See Funiculus, posterior. gracilis, 121, 137 lateralis, 76, 82 posterior, 76, 82 separans, 129 teres. See Eminentia medialis. ventral. See Funiculus, anterior. Cjam.liated cord. See Trunk, sympathetic. Ganglion or ganglia, autonomic. See Ganglia, sympathetic. basal, 252 celiac, 349 cerebrospinal (sensory ganglia on the cerebro- spinal nerves), 38 cervical, inferior, 348 middle, 348 superior, 347 I N 1 > I X hi, i iliarj , {51 ■ it Corti. See Ganglion, Bpiral. enteric, small ganglia be, limbic, lingual, 239, 2<>2 longus insula;, 2 $7 marginalis. . superior frontal. olfactory, lateral, 1 16, 266, J77 medial, 1 16, 266 orbital, 241 postcentral. See Gyrus, posterior central, posterior cenl ral, _' 56, 2 ( >2 precentral 5 ntral. reel u>, 241 subcallosus (pedunculus corporis callosi), 267 supracallosal, 244, 270 supramaximal, 1 $6 temporal, inferior, 236 middle, 2 superior, 2 $6 transverse, 236, 293 uncinatus. See Gyrus, hippocampal. HABENULA. See Xucleus habenulae. Hearing, organs of, 185, 186 187, 309 Heart, innervation of, II eat, sensations of, 105, 306 Hemianopsia, 228 Hemiplegia, 323 Hemispheres, cerebellar, 197, 198 cerebral. See Cerebral hemispheres. Hilus nuclei olivaris, 141 Hind-brain. See Metencephalon and Rhomben- cephalon. Hippocampal gyrus, 116, 240, 277 commissure, 231, 271, 280, 296 Hippocampus, 2^). 269, 277 Histogenesis of cerebellar cortex, 196 of cerebral cortex, 230 of nervous system, 37 of peripheral nervous system, 40 of spinal cord, 38, 39, 42 ganglia, 38, 40 Horizontal cells of Cajal, 285 Horn of lateral ventricle, 246. (See also Column.) Hypophysis, 222 in the dogfish, 29 Hypothalamus, 35, 222 in the dogfish, 2 ( > pars mamillaris, 222 optica, 35 [ncisura. See Notch. Indusium griseum, 244, 270 Infundibulum, 222 Insula, 22'), 237 Inter-brain. See Diencephalon. Interoceptor, interoceptive, 66, 101 Interpeduncular fossa ,115 Interventricular foramen, 2d, 118 Intumescentia cervicalis, T lumbalis, 74, 84 3 88 IXDEX Island of Reil. See Insula. Iter a tertio ad quartum ventriculum. See Aqueductus cerebri. Jelly-fishes, 19 Joints, sensory fibers of, 72 KRAUSE, end-bulb of, 68 Lamina affixa, 215 alar. See Plate, alar. basal. See Plate, basal. medullar is involuta. See Stratum lacunosum. quadrigemina, 130, 158 rostralis, 223, 243 septi pellucidi, 272 terminalis, 25, 33, 223, 231 Laminae medullares of lentiform nucleus, 254 thalami, 216 Lancisi, nerve of. See Stria longitudinalis me- dialis. Lateral line organs, 356 Layers of cerebellar cortex, 208 of cerebral cortex, 286, 287 ependymal, 37 mantle, 37, 42, 196 marginal, 37, 42, 196 of retina, 225 Lemniscus, lateral, 130, 157, 163, 165, 166, 186, 1S7, 309 medial, 135, 138, 145. 153, 163, 219, 313 spinal. See Tract, spinothalamic. trigeminal. See Path, secondary afferent, of trigeminal nerve. Ligamentum denticulatum, 74 Limen insulae, 237, 268 Line (or lines) of Baillarger, 283 of Gennari, 283 Linea splendens, 74 Lingula of cerebellum, 197 Lissauer, tract of. See Fasciculus dorsolater- al. Lobe (lobus or lobes) of cerebellum, 197, 198, 200, 201, 202 of cerebrum, 234 frontal, 234 inferior, 28 insular. See Insula. limbic. See Gyrus fornicatus. linear lateralis, 27 occipital, 236, 238 olfactorv, 267 optic, 27, 28, 165 parietal, 236 pyriform. See Area, pyriform. temporal, 235 visceral, 27 Lobule for lobulus) ansiformis, 201 bi venter, 199 centralis, 197 paracentral, 240, 290 paramedian us, 201 parietal, inferior, 236 superior, 236 postcentral. See Gyrus longus insulae. precentral. See Gyri breves insulae. quadrangularis, 198 quadrate. See Precuneus. semilunaris, inferior, 198 Lobule semilunaris, superior, 198 simplex, 200 Localization of function in cerebellum, 202 in cerebral cortex, 290 in thalamus, 219 Locus caeruleus, 128 Luschka, foramen of. See Aperture, lateral, of fourth ventricle. Luys, nucleus of. See Nucleus hypothalamics. Lyra. See Commissure, hippocampal. Mackosmatic mammals, 265 Magendie, foramen of. See Aperture, medial, of fourth ventricle. Mammillary body, 222, 280 Mantle. See Cerebral cortex. layer. See Layer. Marchi stain for degenerated nerves, 360 Martinotti, cells of, 285 Massa intermedia, 216 Matter, central grav, 136, 158 grav, 42, 79, 87 white, 42, 79, 86 Medulla oblongata, 114, 118 closed portion of, 119 development, 35. (See also Myelencepha- lon.) in the dogfish, 26, 27, 28 fissure, anterior median, 119 posterior median, 119 form, 118, 119, 120, 121, 122 gray matter, 136 internal structure, 132 length, 118 motor nuclei, 170, 174 open portion of, 119 sensorv nuclei, 180, 182 sulci, 119 spinalis. See Spinal cord. Meissner, corpuscles of, 68 plexus of, 351 Meninges, 73, 74 Merkel, corpuscle of, 68 Mesencephalon, 129, 158 development, 24, 31, 35, 36 in the dogfish, 27, 28 form, 129 internal structure, 158 Metamerism, 58. (See also Segmentation.) Metathalamus, 220 Metencephalon, 31, 33, 36 Meynert, fasciculus retroflexus of, 220 fountain decussation of. See Decussation, dorsal tegmental. Microsmatic mammals, 265 Mid-brain. See Mesencephalon. Mitochondria, 49 Molecular layer of cerebellum, 208 of cerebral cortex, 286 Monakcw, bundle of. See Tract, rubrospinal. Monro, foramen of. See Foramen, interventric- ular. Monticulus, 198 Moss-fibers of cerebellum, 209, 210 Motor apparatus, 316 area of cerebral cortex, 290, 317, 318 end-plate, 62 Muscle, branchial, 174 cardiac, innervation of, 353 I\IM \ Muscle ol eyeball, innervation of, 352 ol facial expression, innervation of, 192 ol larj n\, in in i \ .ii ion of, I'M ol 111,1-1 ical ion, innei vati< in of, 1 92 mi \ i endings in, '>-', 72 sense I propnocept ive), 72, ''' , . 100, $11 skeletal. See Muscle, lir.inclii.il and somatic. smooth or unstriated. Sec Muscle, visceral. somal ii\ innei \ al ion of, 62, 1 70 Btriated. See Muscle, branchial and somatic ut tongue, innen al ion of, I'M \ is :eral, innen .ii ion of, '>1 , 1 74, 1 77 Muscle-spindles, 72 Myelencephalon, 31, 32, 33, 36 Myelin, 46 sheal h. See Sheath. Myelination in cerebral cortex, 2S l > in spinal cord, 1 1 2 Myotome, 58, 1 70 \i OPAl i u m, 1 !(•, 2M, 242 Neol halamus, 219 Nerve (Nervus), abducens, 123, 154, 175, 192 accessory, 125, 176, 177, 194 acoustic, 123, 185, 192 auditory. See Nerve, acoustic. cardiac, 348, 349 cerebrospinal, 56 chorda tympani, 1 ( >2, 352 ciliary, 552 cochlear, 149, 185, 193 components, 61. (See also Nerve-fibers.) cranial, 56, 152, 155, 168 facial, 125, 155, 175, 192 glossopharyngeal, 125, L93 hypoglossal, 125, 175, 194 intermedius, 125, 162 of Lancisi. Sec Stria longitudinalis medialis. lingual, 192 oculomotor, 150, 1(>4, 171, 172, I'M olfactory, 191, 2()^< optic, 191, 22'^ phrenic, 59 pneumogastric. See Nerve, vagus. spinal, 5(i, 58, 65 development of, 40 splanchnic, 548 sympal hetic, 545 terminalis, 27, 190 thoracic, ^ s trigeminal, 124, 154, 174, 1S2, 191 trochlear, 125, 165, 175, 191 vagus, 125, 17s, 193 vestibular, 14'), 185, 193, 514 of Wrisberg. See Nervus intermedius. Nerve-cells, 45. (See also Neurons and Cells.) autonomic. See Neurons, sympathetic. motor, lor involuntary muscles, 177 for voluntary muscles, 177 processes, 45 shape, 4 5 struct lire, 47 types of, 45, 44 Nerve-endings, encapsulated, 68 free in epidermis, (>7 in free arborizations, 67 in hair-follicles, 7>7 dc\ elopmenl , 40, 1 1 of dorsal rout , ( >5 efferent, 58 exterocepl ive, 66 gray. See Nerve-fibers, postganglionic, interocepl i\ e, 66 to involuntary muscles, 61 medullated. See Nerve-fibers, myelinated. motor, 59, 62, 94 myelinated, 45, 46, 47, 63, 66, 67, 87 non-medullated. See Nerve-fibers, unmyelin- ated. postganglionic, 557, i I j preganglii inic, 557, 544 primary motor, 62, 90 propriocepl ive, 66, 72 regeneration, r<2 nl Remak. See Nerve-fibers, unmyelinated. soni.it ic afferenl ,61, 66 g< neral, 168, 182, 162, 193 special, 168, I'M, 193 efferent, 61, (.2, 168, I'M, 1<>2, 194 sympathetic. See Nerve-fibers, postgangli- onic. unmyelinated, 47, 63, 66, 67, 87, 98, 104 visceral afferent, (d general, 168, 181, 193, 555 special, U.S. ISO, [92, 193 efferent, (>1 general, 168, 178, 192, 193, 194, 336 special, 168, 17 1, 192, 193, I'M to voluntary muscles. See Nervi matic efferent and special visceral efferent. of white rami, ol ,5 17 substance of brain and cord, 47 Nerve-root. See Root. Nervous system, autonomic, 55') cranial, .^V) craniosacral, 540 sa< ral, ^^^ t horacicolumbar, .^.^'K 540 central, 20, 21. 56, 57 cerebrospinal, development of, 24, >2, 36, J7 diffuse. 18, 1", 540 invertebrate, 1", 20, 21. 22 peripheral, 56 subdivisions ol, sympathetic, 56 vertebrate, 21, Net, nervous, 19, 3 39° INDEX Neural crest, 37 groove, 24, 3 1 tube, 24, 31, 36 Neurilemma, 41, 46, 47 Neurobiotaxis, 179 Neuroblasts, 37, 39 Neurofibrils, 48, (9, 50 Neuroglia, 85, 86 Neuromuscular end-organ, 72 mechanism, 17 Neuron or neurons, 43. (See also Nerve-cells.) basket cell, 50 bipolar, 39, 44, 63 chains, 43, 4", 53, 54 concept, ^2 of cerebellar cortex, 207, 208, 209 of cerebral cortex, 285 development of, 37 form of, 42 horizontal, of Cajal, 285 interrelation of, 49 lower motor, 318 of Martinotti, 285 motor, 22, 44, 46, 177 multipolar, 44 of olfactory bulb, 275 polarization of, 50 postganglionic, 337 preganglionic, 337, 339, 341 of Purkinje, 207 pyramidal, 43, 44, 285 of retina, 225, 226 sensory, 22, 23, 37, 63 stellate, 285 structure of, 47 sympathetic, 341 theory of. See Neuron concept. tvpe I, 44, 87 type II, 44, 45, 87, 88 unipolar, 39, 44, 63 upper motor, 317 Neuropil, 20, 21 Neuropore, 31 Nissl bodies or granules, 48, 51 Nodes of Ranvicr, 47 Nodule of vermis, 198 Non-medullated fibers. See Nerve-fibers, unmye- linated. Notch, anterior cerebellar, 197 posterior cerebellar, 197 preoccipital, 234 Nucleated sheath. See Neurilemma. Nucleus (or nuclei) of abduccns N., 154, 173 accessory cuneate, 138 of accessory N., I'M of acoustic N. See Nuclei, cochlear and vestibular. ambiguus, 146, 176 amygdaloid, 249, 257 anterior thalami, 217, 218 arcuate, 140, 143 arcuatus thalami, 218 of Bechterew, 152, 189 caudatus, 253 centralis, superior, 1 57 of thalamus, 218 of cerebellum, 203, 204 cochlear, 123, 149, 185 commissural, 330 Nucleus of corpus mamillare, 222 cuneatus, 122, 134, 137, 139 of Darkschewitsch, 153 of Deiters, 151, 189 dentatus, 203, 206, 211 dorsalis, 90, 100 of dorsal funiculus. See Nucleus gracilis and Nucleus cuneatus. dorsal motor, of vagus, 146, 178 thalamic. See Nucleus, anterior thalami. of Edinger and Westphal, 178 emboliformis, 203 external round, 138 of facial N., motor, 153, 175, 179 of fasciculus cuneatus. See Nucleus cuneatus. gracilis. See Nucleus gracilis. solitarius. See Nucleus of tractus solitarius. fastigii, 204, 211 of fifth nerve. See Nuclei of trigeminal nerve, of fourth nerve. See Nucleus of trochlear nerve, funiculi cuneati. See Nucleus cuneatus. gracilis. See Nucleus gracilis, globosus of cerebellum, 203 of thalamus, 218 of glossopharyngeal nerve. See Nucleus am- biguus and Nucleus of tractus solitarius. of Goll. See Nucleus gracilis, gracilis, 122, 134, 137 habenula;, 29, 220 of hypoglossal nerve, 145, 173 hypothalamicus (Corpus Luysi), 223 of inferior colliculus, 165 internal round nucleus, 138 interpeduncular, 115, 164 interstitial, 153 of lateral lemniscus, 157, 187 lateral reticular, of medulla oblongata, 143, 145 lateral thalamic, 217, 219 lemnisci lateralis, 157, 187 lenticular, 254 ^ lentiform, 254 of Luys. See Nucleus hypothalamicus. of medial longitudinal fasciculus, 153 medial thalamic, 217, 218 mesencephalic. See Nucleus of trigeminal N. motor, of tegmentum (motorius tegmenti), 145, 161 of nerve-cell, 47 of oculomotor N., 164, 171 olivary, 141, 142 accessory, 142 dorsal, 142 medial, 142 inferior, 141 superior, 151, 186 of origin, 180 pontis, 148, 149 radicis descendentis N. tngemini. See Nu- cleus of tractus spinalis of N. V. red, 159, 160 roof, of cerebellum. See Nucleus fastigii. ruber. See Nucleus, red. salivatory, 178 of Schwalbe. See Nucleus, medial vestibular, semilunar, of thalamus, 218 of sixth nerve, 154, 173 somatic afferent, 182, 185 IMH \ Nucleus, somatit efferenl , 1 7o of spinal tra< I N \ , 136, 144, 155, 182 tecti. Sec Nm Ir its fast igii. tegmental, doi sal, 158 ventral, 158 terminal, 180 thalamic, 217, 218 ol trad us solitarius, 1 l<>, 181, 330 spinalis V trigemini, 1 36, lit, 145, 1 S2 i »l i rapezoid body, 1 86 of i rigeminal \., IS I, 156 main sensory, 1 55, 182 mesencephalic, 155, 184 motor, 1 55, 1 74 spinal, 136, 144, 155, 182 of trochlear V. 163, 173 ot vagus, motor. Sec Nucleus, dorsal motor, of vagus and Nucleus ambiguus. sensory. Sec Nucleus of tract us solitarius. \vnt i.i 1 i halamic, 2 1 8 vestibular, 151, 188 viscera] afferent, 180 efferent, 174, 177 Obex, 129 Olfactory apparatus, 274-282 bulb, 265, 274 cells of nasal mucous membrane, 274 cortex, 277,278,279. (See also ArchipalUum.) glomeruli, 276 gyri, 116, 266, 277 lobe, 267 nerve, 265, 275 roots. See Gyri, olfactory. striae, 266, 277 tract, 265, 277 trigone, 266 tubercle, 268, 282 Olive (oliva, olivary body), 121 accessory, 142 inferior, 141 superior, 151, 186 Opercula, 230, 237 Opt ic apparatus, 225 chiasma, 223, 226 cup, 32, 33, 225 lobes, 27, 1$, 165 nerve, 225, 226 radiation, 227 tectum. See Colliculus, superior. tract, 226 vesicle, 225 Organ of Corti, 185, 186 lateral line, 356 spiral, 185, 186 Pacinian corpuscles, 69 Pain, apparatus of, 68, 103, 105, 306 Palseothalamus. See Thalamus, old. Pallium, 25, 32, 33, 229 Para flocculus, 202 Paralysis, 322, 323 Paraphysis, 31 Parasympathetic system. See Nervous system, craniosacral autonomic. Pars anterior lobuli quadrangularis, 198 basilaris pontis, 124, 147 dorsalis pontis, 124, 149 frontalis capsular internae, 258, 259 Pars intermedia of Wrisb u in- termedius. niaiiiill.ii is hj poi halami, 222 ipitalis i apsulae intei nae, opt ica h\ po1 nalami, 35 postei ior lobuli quadrangulai i Path (or pathwaj I, aff< rent i eretx liar, 11 \, J14 spinal, 91 auditory, 186, 309 • erebello rubra spinal, $26 cortico-ponto i ei ebellai , 1 19, i raniosai ral, 352, $5 J, > ; 1 efferenl ,216 for eye, ^2 for Ileal t ,353 for stomach, 353 for submaxillar) gland, M^2 for urinal \ bladder, $5 I exteroceptive, 66, 101, 102, 502 e\t rapyramidal motor, 524 final n n, 9 I, 311 motor, 109, 216 for cranial nen es, 520 for spinal nerves, 319 for muscular sense. See Path, proprioceptive. olfa y, 280 for pain', 103, 104, 105, 500 proprioceptive, 72, 99, 100, 311 secondary afferent, from tractus solitarius, 181 of trigeminal N., 163, 183, 185, 507 vestibular, 190 for thermal sensibility, 105, 500 thoracicolumbar, 552, 555, 554 for touch, 101, 102, 505 vestibular, 190 visual, 226, 227, 228, 310 Peduncle (or peduncles), cerebellar, 204, 2^5, 206, 211 cerebral, 129, 15S of corpus callosum. See Cyrus subcall of mammillary body, 222 olivary. See Stalk of superior olive. of pineal body. See Stalk of pineal body. Perforated space, anterior. See Substantia per- forata anterior. Perikaryon, 43 Pes pedunculi. See Basis pedunculi. Pia mater, 75 Pine al body. 150 Pituitary body. See Hypophysis. Plate, alar, 34, 42, 194 basal, 54, 42 neural, 24 roof, of prosencephalon, 213 Plexus of Auerbach, 351 brachial, 58 cardiac", 349 celiac, 54 ( > chorioid, lateral, 251 of fourth ventricle, 128 of third ventricle, 22^ ' esophageal, 549 gastric, 349 hypogastric, 351 intercellular, of sympathetic ganglion, 544 lumbosacral. Meissner's, 351 mesenteric, 349 392 INDEX Plexus, myenteric, 351 pelvic, 351 pericellular, of spinal ganglion, 65 of sympathetic ganglion, 345 pulmonary, 540 solar, 349 submucous, 351 sympathetic, 345, 348 vesical, 354 Polarity of the neuron, 50 Poles of cerebral hemisphere, 232 Pons (Varoli), 114, 12.5 basilar or ventral part of, 124, 147 dorsal or tegmental part of, 124, 149 form, 123 internal structure, 147 longitudinal fasciculi, 147 nuclei of, 14S taenia of, 148 transverse fibers of, 147 Ponticulus. See Tcenia of fourth ventricle. Portio major X. trigemini, 125 minor \. trigemini, 125 Postganglionic fibers, 337, 343 Precuneus, 240 Preganglionic fibers, 337, 344 Pressure, apparatus of sensibility to, 66 Presubiculum, 277 Processus reticularis. See Reticular formation of spinal cord. Projection centers, 290 fibers, 297 Proprioceptor, proprioceptive, 72, 99, 100, 183, 185, 311 Prosencephalon, 24, 25. 31, 36, 113 Protoplasm, 17 Psalterium. See Commissure, hippocampal. Pulvinar, 214, 217, 227 Purkinje, cells of, 207 Putamen, 254, 255 Pyramid (or pyramis) of cerebellum, 198 of medulla oblongata, 119, 136 of vermis, 198 Pyriform lobe, 116, 268, 277 Radiation (or radiatio), auditory or acoustic, 261 of corpus callosum, 243, 245 occipitothalamica. See Radiation, optic. optic, 227, 261, 264 sensory, 264 thalamic, 216, 217, 260, 263 thalamotemporal, 264 Radix descendens (mesencephalica) N. tri- gemini. Sec Root, mesencephalic X. Y. X. facialis, 175 Ramus communicans, 335, 346 grav, 337\ 347 white, 335, 347 dorsal, 58 ventral, 58 Ranvier, constrictions or nodes of, 47 Receptor, 10, 53, 91 Recess, lateral, of fourth ventricle, 125 lateralis fossae rhomboidese, 125 optic, 223 pineal, 221 suprapineal, 221 Reflex act, 91 Reflex arc, 20, 53, 91, 92, 93, 327 auditory, 331 of brain stem, 328, M'>, 330, 331, 532 for coughing and vomiting, 330 of medulla oblongata, MX, 329, 330 myenteric, 340 optic, .^M pupillary, 332, 333 respiratory, 330 scratch, 94 of spinal cord, 91, 92, 93, 94, 328 vestibular, 328, 329 visceral, 340 Regeneration of nerve-fibers, 52 Reil, island of. See Insula. Respiratory apparatus, 330 Restiform body, 122, 143, 205 medial part of, 205 Reticular formation (or substance), 80, 136, 144 Retina, 225 Rhinencephalon, 25, 32, 115, 265 Rhombencephalon, 25, 31, 32, 35, 36, 113 Rhombic lip, 195 Rod and cone cells, 226 Rolando, fissure of. See Sulcus centralis, substantia gelatinosa of, 80 tubercle of. See Tuberculum cinereum. Root of abducens nerve, 123 of accessory nerve, 76, 123 of acoustic nerve, 123 anterior spinal. See Root, ventral, dorsal, 58, 76, 95, 96, 07 of facial nerve, 123 field. See Sensory root field, of glossopharyngeal nerve, 123 of hypoglossal nerve, 123 mesencephalic, N. V. 155, 156 of oculomotor nerve, 130 posterior, spinal. See Root, dorsal, spinal, 78 of trigeminal nerve, 124, 125 of trochlear nerve, 191 of vagus nerve, 123 ventral, 58, 76 Rostrum of corpus callosum, 243 Rudiment of hippocampus, 244, 267, 270 Saccule, 193 Saccus vasculosus, 28, 29 Scarpa, ganglion of. See Ganglion, vestibular. Schultze, comma-tract of, 97, 107 Schwalbe, vestibular nucleus of. See Nucleus, medial vestibular. Schwann, sheath of. See Neurilemma. Sea-anemones, 17, 19 Segmentation of spinal cord, 74 Semicircular canals, 193 Septomarginal bundle or fasciculus, 97, 107 Sensation (or sensibility) of cold, 105, 306 of hearing, 185, 186, 187, 309 of heat, 105, 306 muscular, 72, 99, 100, 311 of pain, 68, 103, 105, 306 of pressure, 303 of sight, 225, 228 of smell, 265 of taste, 181 of touch, 66, 77, 101, 303 visceral, 336 IM)I \ 393 Sensory rool field, 59, 60 Sept urn pellucidum, 243, 272 \x istenor intermediate, 83 median, 83 DOBl iriiin, 74 Sliark. Sec Dogfish. Sheath, glial, 86 medullary. See Sheath, myelin, myelin, 1 1 , 16, 47 of Schwann. See Neurilemma. si-iii , organs of, -'25 228 Smell, organs of, 26S 282 Solitary bundle. See Tractus solitarius. Somesl he! ic area, 2 l> 2 Speech, apparal us of, 295, 296 Spider-cells, 86 Spinal cord, 56, 72, 75 cervical enlargement, 73, 79, 84 characters ol different regions, 83 columns of gray matter, 79 of white matter. Sec Funiculus. of cells. See Cell-columns. commissures, 80 coverings, 73 corrua. See Columns. degenerations from brain lesions, 105, 106 from cord lesions, 105, 106 from sect ion of dorsal roots, 106 development, 41 ', 42 in fetus and infant, 77 fissure, anterior median, 76 funiculi, 82 glial sheath, 86 gray matter or substance, 78, 79, 80, 81, 87 cell-columns, 89, 90 columns, 79 horns. See Columns. microscopic structure, 87 nuclei. See Cell-columns. relation to size of nerves, 84 horn. See Column. internal structure, 85 lumbar enlargement, 74, SI, 84 microscopic structure, 85 relation to vertebral canal, 77 reflex mechanism of, 91, 92, 93 sacral region, 74, 81, 84 segmentation, 74 sulcus, anterolateral, 76 posterior, 76 intermediate, 76 posterolateral, 76 thoracic region, 80, 84 tracts, 95-112, 110 white matter (or substance), 81, 86 area in different regions, 82 microscopic structure, 86, 87 ganglion. See Ganglion. nerve. See Nerve. Spiracle, 356 Splanchnic nerves, 348 Splenium corporis callosi, 244 Spongioblasts, 37 Stalk, optic, 32, 225 of pineal body, 221 of superior olive, 151, 175 Stomach, innervation of, 353 Stratum griseum centrale, 163 of superior colliculus, 167 Str.it urn lacunosum, lemni -.i i, Id, lui idum, 27 n opticum, 167 oriens, 279 profundum, 166, W>7 radiatum, 27 u /on. ilc of superior i olliculus, \<>7 ol t halamus, 216 Stria (or striae) acustica. See Stria medullarea acusl ica. of Baillarger, of ( iennari, 283 longil udinalis lateralis, 245, 270 medialis, 2 15, 270 medullaris acusl ica, 1 23, 1 27, 1 *<> thalami, 215, 220, 281 olfactoria lateralis, 266, 277 medialis, 266 semicin ularis. See Stria terminalis. terminalis, 214, 281 Stripe of baillarger, 283 of (iennari, 283 Subarachnoid space, 73 Subiculum, 277, 280 Substantia alba, 42, 79, 86 ferruginea, 128 grisea, 42, 79, 87 centralis, 136, 158, 163 gelatinosa, Rolandi, 80 centralis, 86 externa. See Sheath, glial, nigra, 129, 158, 164 perforata, anterior, 267, 282 posterior, 1 1 5 reticularis. See Reticular formation. alba, 144 grisea, 145 Subthalamic tegmental region. See Subthala- mus. Subl halamus, 222 Sulcus (or sulci), anterior lateral, 76, 119 parol factory, 239 basilar, 124 callosal. See Sulcus of corpus callosum. central, of Rolandi, 233 cerebellar, 199 cerebral, 233, 235, 236, 239 cinguli, 239 circularis insula?, 237 of corpus callosum, 239 cruciate, 1 14 frontal, inferior, 235 middle, 235 superior, 235 horizontals cerebelli, 197 hypothalamicus, ll-~> insulae, 23 7 intermedins, posterior, 76, 127 intraparietal, 236 lateral, of mesencephalon, 130 lateralis, anterior. See Sulcus, anterior lateral. posterior. See Sulcus, posterior lateral. limitans, 34, 42, 129 instihe. See Sulcus circularis insulae. lunatus, 237 medianus posterior of spinal cord, 76 of medulla oblongata, 1 19 occipitalis transversus, 236 394 INDEX Sulcus of oculomotor nerve, 130 olfactory, 241 orbital, 241 paracentral, 239 parolfactorius, anterior, 239 posterior, 267, 239 postcentral, inferior, 236 superior, 236 postclivalis, 1 ( >7 posterior lateral, 76, 1 19 parolfactory, 239, 267 precentral, 235 inferior, 23S superior, 235 prepyramidal, 202 primarius. Sec Fissura prima. rhinalis. See Fissure, rhinal. of spinal cord, 76 subparietal, 239 temporal, inferior, 236 middle, 236 superior, 236 uvulo-nodularis, 203 Sylvius, aqueduct of, 26, 158 fissure of, 233 Sympathetic ganglia. See Ganglion. system, 50, 57, 334 Synapse, 49, 50, 51, 55 Syncytium, 38 System. See Nervous system. Tactile corpuscles, 68 Taenia chorioidea, 214 of fourth ventricle, 126 pontis. See Fila lateralia pontis. teeth See Stria longitudinalis lateralis. thalami, 214, 224 ventriculi quarti, 126 Tapetum, 245 Taste, apparatus of, 181 Tectum mesencephali, 28, 165 Tegmentum, 12°, 1 58 Tela chorioidea of fourth ventricle, 128 of third ventricle, 215, 224 Telencephalon, 36 development, 25, 31, 32, 33 in the dogfish, 27, 28 medium, 212, 229 Temperature, apparatus of, 105, 306 Tendon, nerve endings in, 72 Tentorium cerebelli, 113 Thalamencephalon. See Diencephalon. Thalamus, 213 development, 35, 213 in the dogfish, 29 ending of sensory tracts in, 219 lamina, external medullary, 216 internal medullary, 216 new, 219 nuclei, 217 old, 218 pulvinar, 218 radiation of, 216, 217, 260, 263 stalks, 263 stratum zonalc, 216 thalamocortical fibers, 263 tubercle, anterior, 213 Tigroid bodies. See Nissl bodies. Tonsil (tonsilla cerebelli), 199 Touch, apparatus of, 66, 71, 101, 303 Tract or tracts, 95. (See also Bundle and Fas- I It III US. I bulbospinal, 1 1 1 of Burdach. See Fasciculus cuneatus. central sensory. See Path. cerebellobulbar. See Tract, fastigiobulbar. cerebellotegmental, 211, 212 comma, 97, 107 corticobulbar, 165, 260, 321 corticopontine, 147, 164. (See also Tracts, frontopontine and temporopontine.) corticorubral, 161, 260 corticospinal, 109, 133, 136, 147, 165, 260, 320 lateral, 1() ( ), 134, 136 ventral, 134, 136 corl icothalamic, 263 direct cerebellar. See Tract, dorsal spinocere- bellar. dorsal spinocerebellar, 110, 143, 144, 145, 205 efferent, from cerebellum, 211 from cerebral hemisphere, 297 from mesencephalon. See Tracts, tecto- spinal, tectobulbar, and rubrospinal, fastigiobulbar, 212 of Flechsig. See Tract, dorsal spinocerebellar, frontal olfactory projection, 281 frontopontine, 164, 259 of Goll. See Fasciculus gracilis, of Gowers. See Tract, ventral spinocerebellar, habenulo-peduncular. See Fasciculus retro- rlexus. of Helweg. See Tract, bulbospinal, lateralis minor. See Fasciculus lateralis minor, of Lissauer. See Fasciculus dorsolateral, mamillotegmental, 222, 281 mamillothalamic, 217, 222 mesencephalic, of N. V. See Root, mesen- cephalic, N. V. of Meynerit. See Fasciculus retroflexus. of Monakow. See Tract, rubrospinal, nucleocerebellar, 205 olfactory, 265, 277 olivocerebellar. See Fibers, olivocerebellar, olivospinal. See Tract, bulbospinal, optic, 226 pontocerebellar. See Brachium pontis. pontospinal. See Tract, reticulospinal, predorsal. See Tract, tectospinal, prepyramidal. See Tract, rubrospinal, projection, 297 pyramidal, 109 aberrant, 321 direct, 109 crossed, 109 uncrossed lateral, 320 reticulospinal, 160 rubroreticular, 160, 161 rubrospinal, of Monakow, 110, 145, 161 of Schultze, 107 septomarginal, 97, 107 solitariospinalis, 330 solitary (solitarius), 132, 181, 330 spinal, of N. V., 132, 136, 144 of spinal cord, 94-112, 110 spinocerebellar, dorsal, 100, 314 ventral, 100, 313 spino-olivary, 105 spinotectal, 105, 145 INDEX 395 Tract, spinothalamic, 1 15, 163, 219, 307 lateral, 102 ventral, 101, 305 Btrionigral, 1<>1, 263 sulcomarginal, 108 tei tobulbar, 161, 167 tectocerebellar, 206 tectospinal, ill, 145, 161, 167 tegmentospinal. Sec Tract, reticulospinal. temporoponl ine, 164, 261 i halamocon ical, 26 1 i halamo-olivai j , 115, 219 t halamospinal, 21 ( > transverse peduncular, 369 trigeminothalamic, 183, 185 ventral spinocerebellar, 100, 144, 145, 157,206 vestibulocerebellar, 190, 206 vestibulospinal, 111, 190, M { > of Vicq d'Azyr. Sec Tract, mamillothalamic. Trapezium. Sec Trapezoid body. Trapezoid body, 121, 150, 186 Triangle of Gombault ami Philippe, 107 Trigone (or trigonum) acustici. See Area acusl ica. collateral, 248 habenulae, 220 hypoglossi, 127 interpedunculare. See Fossa interpeduncula- ris. olfactory, 266 vagi. See Ala cinerea. Trophic unity of neuron, 51 Truncus corporis callosi, 244 Trunk, sympathetic, 335, 346, 347, 348 Tuber vermis, 198, 201 Tubercle (or tuberculum) acusticum. See Nu- cleus, dorsal cochlear. anterior, of thalamus, 213 cinereum, 122, 280 cuneate, 121, 137 olfactorium, 268, 282 cf Rolando. See Tuberculum cinereum. Tufted cells, 276 Tiirck's bundle. See Tract, ventral cortico- spinal. Uncus, 240, 269, 277 Utricle, 193 Uvula vermis, 198 Vallecula of cerebt Hum, 197 \ alve of Vieusscns. Set l elum, anterior med- nll.ii \ . Velum, anterior medullary, 125, 128, 155 am I. urn. See Velum, antei ior medullai interpositum. See Tela chorioidea ol third venl ricle. medullare, anterius, 125 inferius. See Velum medullare, p rius. posterius, 2<)2 superius. See Velum, anterior medullary, transversum, 29, -il Vena terminalis, 214 Ventrit le (oi ventricles) of the brain, 25, 26, 27, 117 development, 26, 33, 34 in the dogfish, 27, 28, 30, .-51 fourth, 26, 118, 125, 126, 127, 128 lateral, 26, 118, 246 third, 26, IIS, 223 Ventriculus lateralis, 26, 246 terminalis, 81 tertius. See Ventricle, third. Vermis, inferior, 197, 198 superior, 167 Vesicles, cerebral, primary, 24, 25 optic, 225 Vestibular apparatus, 188, 189, 190 Vicq d'Azyr, bundle of. See Tract, mamillo- thalamic. Yieussens, valve of. See Velum, anterior med- ullary. Visceral innervation, 335 Visual apparatus, 225 receptive center, 292 Yisuo-psyehic area, 293, 294 Vomiting, mechanism of, 331 Wallerian degeneration, 105, 106, 107 Weight of brain, 301 Worms, nervous system of, 19, 20, 21, 22 Wrisberg, nerve of. See Nervus intermedius. Zone, cortical. See Center, cortical, ependymal, 37 mantle, 37, 42, 196 marginal, 37, 42, 196 COLUMBIA UNIVERSITY LIBRARIES This book is due on the date indicated below or at thp IT,? °H n ; f K a V "^ Peri ° d after the dat * ofborrowing as provided by the rules of the Library or by specLT If rangement with the Librarian in charge ^£j_iafii >9S€ QM 451 Sanson c 1