BRAIN.] ANATOMY 871 The grey matter of the medulla oblongata, which contains numerous multipolar nerve cells, is in part continuous with the grey matter of the Spinal cord, and in part consists of in dependent masses. As the grey matter of the cord enters the medulla it loses its crescentic arrangement. The posterior cornua are thrown outwards towards the surface, lose their pointed form, and, dilate into rounded masses named the grey tubercles of Rolando, whilst portions are prolonged into both the posterior pyramid and the restiform body. The grey matter of the anterior cornua and of the intermedio- lateral tracts loses its continuity, and becomes subdivided into numerous small masses, owing to being traversed by bundles of nerve fibres, which give rise to a network termed formatio reticularis, in the meshes of which the groups of nerve cells are contained. In the lower part of the medulla a central canal continuous with that of the cord exists, but when the restiform bodies and posterior pyramids on the opposite sides of the medulla diverge from each other, the central canal loses its posterior boundary, and dilates into the cavity of the 4th ventricle. The grey matter in the interior of the medulla appears, therefore, on the floor of the ventricle; that which corresponds to the anterior cornua being situated immediately on each side of the median fur row, whilst that which is continuous with the grey tubercles of Rolando and the posterior cornua is some distance ex ternal to it. This grey matter forms collections of nerve cells, which are the centres of origin of several important encephalic nerves. Of the independent masses of grey matter of the medulla, that which forms the corpus dentatum within the olivary body is the most important, and constitutes the nucleus of the inferior olive. It is folded on itself in a zig-zag or denticulated manner, and forms a sort of capsule open on the inner aspect, through which openings a bundle of nerve fibres from the interior of the capsule proceeds. These fibres aid in the formation of the olivary fasciculus, and as Deiters and Meynert have pointed out, in part arch across the mesial plane and join the restiform body on the opposite side, whilst some apparently join the posterior pyramid. The nerve cells of the olive are multipolar and flask-shaped, and in all probability give origin to the nerve fibres proceeding from the interior of the capsule. Separated from the inner part of the olive by a layer of reticular substance is a smaller grey mass, called by Stilling nucleus olivaris accessories. Crossing the anterior surface of the medulla oblongata, immediately below the pons, in the majority of mammals is a transverse arrangement of fibres forming the trapezium, which contains a grey nucleus, named by Van der Kolk the superior olive. In the human brain the trapezium is concealed by the lower transverse fibres of the pons, but when sections are made through it, as L. Clarke pointed out, the grey matter of the superior olive can be seen. Meynert states that its nerve cells give origin to some fibres, which run straight backwards to the restiform body of the same side, and to others which pass across the mesial plane to the opposite corpus restiforme. ons The PONS VAROLII or BRIDGE (PL XVIII. figs. 1, 2, 3,N) arolli. is cuboidal in form : its anterior surface rests upon the dorsum sellaa of the sphenoid, and is marked by a median longitudinal groove ; its inferior surface receives the pyra midal and olivary tracts of the medulla oblongata ; at its superior surface are the two crura cerebri ; each lateral surface is in relation to a hemisphere of the cerebellum, and a peduncle passes from the pons into the interior of each hemisphere ; the posterior surface forms in part the upper portion of the floor of the 4th ventricle, and in part is in contact with the corpora quadrigemina. The pons consists of white and grey matter : the nerve fibres of the white matter pass through the substance of the pons, either in a transverse or a longitudinal direction. The transverse fibres go from one hemisphere of the cerebellum to that of the opposite side ; some are situated on the anterior surface of the pons, and form its superficial transverse fibres, whilst others pass through its substance and form the deep transverse fibres. The transverse fibres of the pons constitute, therefore, the commissural or connecting arrangement by which the two hemispheres of the .cerebellum become anatomically continuous with each ether. The longitudinal fibres of the pons ascend or pass vertically upwards from the medulla oblongata, and consist of the fibres of the anterior pyramids, olivary fasciculi, fasciculi teretes, and posterior pyramids. They leave the pons. by emerging from its upper surface as fibres of the two crura cerebri. The pons possesses a median raphe continuous with that of the medulla oblongata, and formed like it by a decussation of fibres in the mesial plane. The grey matter of the pons is scattered irregularly through its substance, and appears on its posterior surface ; but not on the anterior surface, which is composed exclu sively of the superficial transverse fibres. It is traversed both by the longitudinal and deep transverse fibres, which form a well-defined formatio reticularis. To a portion of grey matter, containing nerve cells charged with dark pigment, the name of locus ccendeiis is applied. The locus lies on the floor of the 4th ventricle, close to the entrance to the aqueduct of Sylvius, and serves as the origin of the sensory root of the 5th, and perhaps of the posterior root of the 4th cranial nerve. The nerve cells of the pons are multi- polar and stellate. The pons acts as a conductor of impressions through its nerve fibres, and as a centre of origin of nerve fibres from nerve cells. Meynert states that some of the fibres of the crura cerebri end in the nerve cells of the pons, which cells again give origin to fibres that pass outwards to the cerebellum. The CEREBELLUM, LITTLE BRAIN, or AFTER BRAIN (PI. Cere- XVIII. fig. 2, c), occupies the inferior pair of occipital fossae, 1;>ellun and, along with the pons and medulla oblongata, lies below the plane of the tentorium cerebelli. It consists of two hemi spheres or lateral lobes, and of a median or central lobe, which in human anatomy is called the vermiform process. It is connected below with the medulla oblongata by the two resti form bodies which form its inferior peduncles, and above to the corpora quadrigemina of the cerebrum by two bands, which form its superior peduncles; whilst the two hemi spheres are connected together by the transverse fibres of the pons, which form the middle peduncles of the cerebellum. On the superior or tentorial surface of the cerebellum the median or vermiform lobe is a mere elevation, but on its inferior or occipital surface this lobe forms a well-defined inferior vermiform process, which lies at the bottom of a deep fossa or vallecula ; this fossa is prolonged to the posterior border of the cerebellum, and forms there a deep notch which separates the two hemispheres from each other; in this notch the falx cerebelli is lodged. Extending horizontally backwards from the middle cerebral peduncle, along the outer border of each hemisphere is the great horizontal fissure, which divides the hemisphere into its tentorial and occipital surfaces. Each of these surfaces is again sub divided by fissures into smaller lobes, of which the most important are the amygdala or tonsil, which forms the lateral boundary of the anterior part of the vallecula, and the flocculus, which is situated immediately behind the middle peduncle of the cerebellum. The inferior vermiform process is subdivided into a posterior part or pyramid ; an elevation or uvula, situated between the two tonsils ; and an anterior pointed process or nodule. Stretching between the two flocculi, and attached midway to the sides of the nodule, is a thin, white, semilunar-shaped plate of nervous matter, called the posterior medullary velum.
The whole outer surface of the cerebellum possesses a
