SYSTEM.] ANATOMY 863 nates in a slight bulbous enlargement. The nerve fibre parts with its perineurial sheath after it enters the Paciuian corpuscle; and as it lies in the core it loses its medullary sub- etance, so that its terminal part consists only of the axial cylinder. Sometimes the nerve fibre divides into two branches within the corpuscle. Capillary blood-vessels are distributed to the concentric layers of the Pacinian corpuscle. elop- The mode of origin of the nervous tissue in the course [t f of development of the embryo is still involved in some obscurity. It is, however, believed that the nerve cells are derived from the embryo cells, which multiply, and the young cells then grow and assume characteristically granular and finely fibrillated contents. Processes or poles then appear at the periphery of the cells, which, according to the observations of Beale, connect adjacent cells together. As the groAvth of the part goes on, the cells are more widely separated from each other, and the anastomosing processes in consequence become considerably elongated, and form the axial cylinder of the nerve fibre. In the course of time the medullary sheath and the primitive membrane may form around this axial cylinder so as to insulate it. The exact mode of formation of the medullary sheath is not properly understood; but it is believed that the primi tive membrane, and the perineurial connective tissue, are derived from those surrounding embryonic cells which differentiate into connective tissue. Of the two originally contiguous cells from which the nerve fibre is, as it were, spun out, one, as Hensen conceived, may form a coll in a nerve centre, the other may differentiate into a peripheral end-organ. In the tail of the tadpole the formation and growth of nerve fibres have been studied by Kolliker and others, and it has been seen that the terminal part of a fibre may have fusiform or tri-radiate cells connected with it, the processes of which cells gradually differentiate into nerve fibres. The young cerebro-spinal nerve fibres are distinctly nucleated, and correspond in appearance and structural characters to the non-medullated nerve fibres of the adult. If in a young or adult person a nerve be cut across, its conducting power is destroyed; but after a time it reunites, and its function is restored. The part of the nerve which lies between the place of section and its peripheral extremity, undergoes, as Waller pointed out, degenerative changes. To how great an extent the de generation affects the various constituents of each fibre, it is difficult to determine; for whilst some experiments would seem to show that only the medullary sheath broke up into granular particles and was absorbed, in others both it and the axial cylinder disappeared. In process of time, however, these parts may be reproduced, and the nerve then recovers its functional activity. DESCRIPTIVE ANATOMY OP THE CEKEBKO-SPINAL NERVOUS SYSTEM. In this section the anatomy of the Brain and Spinal Cord, and of the numerous distributory Nerves which arise from them, will be described. The brain and spinal cord are the largest and most important of all the nerve centres. They occupy the cranial cavity and spinal canal, and are continuous with each other through the foramen magnum in the occipital bone. As the arrangement of the struc tures which compose the brain and spinal cord is extremely complex, and as the names applied to the several parts are numerous and often very arbitrary, it may be well, before commencing a detailed description, to make a few general observations on their mode of development. Development of the Cerebro- Spinal Nervous Axis. The brain and spinal cord are developed in the cranio-spinal groove of the embryo, and appear originally as a thin band extending along the whole length of this groove. About the time when the walls of the groove meet posteriorly to complete the cranio-spinal cavity, the margins of this band become elevated, bend backwards, and meet, so that the originally simple band becomes converted into a cylindri- Develop form cerebro-spinal title. In the walls of this tube the nervous uient of structures of the brain and spinal cord are formed, whilst the axis cerebro- of the tube forms a central canal. In the part which becomes the spinal Spinal Cord the central canal persists as the central canal of the system. spinal cord, and around it a layer of ciliated cylindrical endothelium is developed. Outside this layer a mass of grey matter containing nerve cells is formed, which is subsequently divided into two lateral crescent-shaped masses. Outside the grey matter white matter is produced, which ultimately becomes arranged in the form of longi tudinal columns of nerve fibres. With the formation and growth of these columns and of the internal grey matter, a longitudinal mesial fissure appears on the anterior and another on the posterior surface of the cord, which gradually increase in depth until the cord is almost completely divided into two lateral halves. At the bottom of the anterior median fissure the nerve fibres of the anterior commissure are developed, and at the bottom of the posterior median fissure those of the posterior commissure. These commissures unite the two halves of the cord together. The upper or cerebral end of the cerebro-spinal tube becomes the Enccphalon, or Brain. At first the cerebral part of the tube is uniform in appearance with the spinal part, but it soon expands into three vesicular dilatations the primary cerebral vesicles. These vesicles, named (from before backwards) anterior, middle, and posterior, are separated from each other by constrictions, and as the development progresses the vesicles bend on each other and on the upper end of the spinal cord. As each vesicle is an expan sion of the cerebro-spinal tube, it is necessarily hollow, and the space in its interior is continuous with the central canal of the spinal cord. In the walls of the vesicles the nervous structures are produced, which form the several subdivisions of the encephalon. The posterior cerebral vesicle bends first forwards from the upir end of the spinal cord, and then backwards; the part which bends forward becomes the medulla oblongata; that which bends backward the cerebellum, whilst the pons is developed at the angle where these two parts aro continuous with each other ; the central hollow forms the central canal of the medulla oblongata and the dilated space called the fourth ventricle. In the medulla oblongata shallow anterior and posterior median furrows then appear continuous with those in the cord, and each lateral half differen tiates into grey matter and into a longitudinal arrangement of nerve fibres continuous with the corresponding structures in the cord. A large proportion of these fibres are continued upwards through the pons as its longitudinal fibres. The cerebellum consists at first of a central lobe, and in the lower vertebrates ita development does not proceed beyond this stage; but in mammals, including man, a lateral lobe or hemisphere is superadded on each side, and with the growth of these lateral lobes numerous transverse fibres, which connect the two hemispheres together, are developed in the pons. The cerebellum is also connected below with the medulla oblongata by the pair of restiform bodies, or inferior peduncles, and above with the corpora quadrigemina by the parr of superior peduncles. The middle cerebral vesicle bends forwards from the posterior vesicle. In its roof the optic lobes are formed ; in its floor the crura cerebri; whilst the central hollow becomes the aqueduct of Sylvius. At first the optic lobes form a single structure, but about the sixth mouth of embryo life a median furrow divides this struc ture into two lateral halves (the corpora bigemina), and in the lower vertebrates the development does not proceed beyond this stage ; but in the seventh month of embryo life of the human foetus each lateral half is subdivided into two by a transverse fissure, so that four bodies (the corpora quadrigemina) are produced. Tho crura cerebri form the two cerebral peduncles, which, diverging from each other, pass upwards to the hemisphere of the cerebrum. They consist almost entirely of nerve fibres continuous with the longitudinal fibres of the pons, a few of which go to the corpora quadrigemina, but the greater number ascend to the cerebrum. The anterior cerebral vesicle bends downwards from the middle vesicle. The posterior part of this vesicle forms at first a simple hollow sac, but subsequently divides into the two optic thalami, one on each side of the central hollow, which hollow becomea the third ventricle. This ventricle is prolonged downwards into a funnel-shaped process, the infundibulum, which is connected with the pituitary body, or hypophysis cerebri, lodged in the pitui tary fossa in the sphenoid bone, whilst posteriorly it is continuous with the aqueduct of Sylvius. In its upper and posterior wall the pineal body, or epiphysis cerebri, is developed, and from this body two white peduncles run forwards on the sides of the optic thalami. Immediately belosv these peduncles the transverse fibres of the posterior commissure are developed, which pass between the two optic thalami. The anterior wall of this ventricle is closed in by the lamina cincrca or lamina tcrminalis, and behind it are formed the transverse nerve fibres of the anterior commissure, and the vertical fibres of the anterior pillars of the fornix. These forniz fibres pass to the base of the brain, and form the corpora albicantia, prior to entering the optic thalami. The posterior part of the anterior
vesicle gives oif from each side a flask-shaped prolongation, the
