872 ANATOMY [NERVOUS SYSTEM- characteristic foliated or laminated appearance, due to its subdivision into multitudes of thin plates of lamellae by numerous fissures. The cerebellum consists both of grey and white matter. The grey matter forms the exterior or cortex of the lamellae, and passes from one to the other across the bottoms of the several fissures. The white matter lies in the interior of the organ, and extends into the core of each lamella. "When a vertical section is made through the organ, the prolongations of white matter branching off into the interior of the several lamellae give to the section an arborescent appearance, known by the fanciful name of arbor vitce(Pl. XVIII. fig. 3, c). Independent masses of grey matter are, however, found in the interior of the cerebellum. If the hemisphere be cut through a little to the outer side of the median lobe, a zig-zag arrange ment of grey matter, similar in appearance and structure to the nucleus of the olivary body in the medulla oblongata, and known as the corpus dentatum of the cerebellum, is seen ; it lies in the midst of the white core of the hemi sphere, and encloses white fibres, which leave the interior of the corpus at its inner and lower side. Stilling has de scribed, in connection with the anterior end of the in ferior vermiform process, which projects forwards into the valve of Vieussens, and aids in the formation of the roof of the 4th ventricle, two grey masses, named roof nuclei. They possess flask-shaped nerve cells like those of the corpus dentatum. The white matter is more abundant in the hemispheres than in the median lobe, and is for the most part directly continuous with the fibres of the peduncles of the cerebellum. Thus the restiform or inferior peduncles pass from below upwards through the white core, to end in the grey matter of the tentorial surface of the cerebellum, more especially in that of the central lobe ; on their way they are connected both with the grey matter of the corpus dentatum and of the roof nuclei. The superior peduncles, which descend from the corpora quadrigemina of the cerebrum, reach the grey cortical matter, more especially on the inferior surface of the cerebellum, though they also form connections with the corpus dentatum. The middle peduncles form a large proportion of the white core, and their fibres terminate in the grey matter of the foliated cortex of the hemispheres. But, in addition to these peduncular fibres, which connect the cerebellum to other subdivisions of the encephalon, its white matter contains fibres proper to the cerebellum itself. The fibrce proprice have been especially described by Stilling ; some, which he has termed the median fasciculi, lie near the mesial plane, and connect the grey matter on the tentorial aspect of the middle lobe with that of the inferior vermiform process, whilst others cross directly the mesial plane to unite opposite and symmetrical regions of the hemispheres. Further, the auditoiy -nerve was said by Foville to derive some of its fibres of origin from the cerebellum ; the connection of this nerve with the cerebellum has been strongly insisted on by Meynert, and this anatomist has also ascribed a cerebellar origin to a portion of the sensory root of the 5th cranial nerve. The grey matter of the cortex is divided into two well- defined layers, an external grey, and an inner rust coloured layer of about equal thickness. The rust coloured layer is distinguished by containing multitudes of so called " gran ules," the well-defined nucleus in which, as described by Strachan, is invested by a small quantity of branched protoplasm. These "granules" are, therefore, minute stellate cells. Where the rust coloured layer joins the grey layer the characteristic nerve cells of the cerebel lum, named the corpuscles of Purkinje, are situated. A slender central process arising from each cell enters the rust coloured layer, and, as the observations of Hadlich and Koscheanikoff show, becomes continuous with the axial cylinder of a medullated nerve fibre ; for the nerve fibres of the white core enter this layer, divide into minute fibres, and ramify amidst the granules. From the oppo site aspect of each cell two peripheral processes arise, and ramify in an antler-like manner in the external grey layer. Obersteiner and Hadlich maintain that the finer branches of these processes curve back towards the rust coloured layer, where, according to Boll, they form a net work of extreme minuteness, from which it is believed that nerve fibres may arise. The substratum of the grey layer, in which the branched processes of the cells of Purkiuje lie, consists of a very delicate neuroglia, in which scattered corpuscles are imbedded ; but, in the outer part of this layer, delicate supporting connective tissue-like fibres are also met with. The Fourth Ventricle is the dilated upper end of the cen- Fourth tral canal of the medulla oblongata. Its shapo is like an"Ventri< heraldic lozenge. Its floor is formed by the gioy matter of FIG. 68. Floor of tho fourth Tentricle and adjacent structures. 1, pinea gland; 2, the nates, and 3, tile testes of the corpora quadriRemina ; 4, 4, middle peduncles, 5, 5, superior peduncles, 9, 9, inferior peduncles of the cerebellum; 6, C, valve of Vieussens divided; 7, 7, fasciculi teretes; 8, 8, roots of the auditory nerves; 9 , corpus dentatum; 10, 10, posterior pyramids; 11, calamus scriptorius. the posterior surfaces of the medulla oblongata and pons; its roof partly by the inferior vermiform process of the cere bellum, the nodule of which projects into its cavity, and partly by a thin layer, called valve of Vieussens, or anterior medullary velum ; its lower lateral boundaries, by the diver gent restiform bodies and posterior pyramids ; its upper lateral boundaries, by the superior peduncles of the cere bellum ; the reflection of the arachnoid membrane from the back of the medulla to the inferior vermiform process closes it in below, but allows of a communication between its cavity and the sub-arachnoid space ; above, it communicates with the aqueduct of Sylvius, which is tunnelled through the sub stance of the corpora quadrigemina. Along the centre of the floor is the median furrow, which terminates below in a pen-shaped form, the so-called calamus scriptorius. Situated on its floor are the fasciculi teretes, striae acous- ticse, and deposits of grey matter described in connection with the medulla oblongata. Its endothelial lining is con tinuous with that of the central canal. The CEREBRUM or GREAT BRAIN lies above the plane of the tentorium, and forms much the largest division of the encephalon. It is customary in human anatomy to include under the name of cerebrum, not only the convo lutions, the corpora striata, and the optic thalami, developed in the anterior cerebral vesicle, but also the corpora quadri gemina and crura cerebri developed in the middle cerebral vesicle,. The cerebrum is ovoid in shape, and presents
superiorly, anteriorly, and posteriorly a deep median longitu-
