526 THE CEREBELLUM. 



truncated at its upper part. Below, it is bounded by the diverging posterior 

 pyramids and restiform bodies surmounted by the ligula. It has already 

 been sufficiently described in connection with the medulla oblongata. 



The lining membrane of the ventricle is continuous with that of the 

 ventricles in the interior of the cerebrum, through the aqueduct of Sylvius, 

 in which situation it is marked by delicate rugse, oblique or longitudinal in 

 direction. At the sides it is reflected from the medulla to the cerebellum, 

 and extends for a considerable distance outwards between the flocculus and 

 the seventh and eighth nerves. At the lower end of the ventricle, there is, 

 as was ascertained by Magendie, a narrow orifice in the membrane by which 

 the cavity communicates with the subarachnoid space. 



Projecting into the fourth ventricle at each side, and passing from the 

 point of the inferior vermiform process outwards and upwards to the outer 

 border of the restiform bodies, are two small vascular processes, which have 

 been named the choroid plexuses of the fourth ventricle. 



Fig. 358. Fig. 358. OUTLINE SKETCH OP A VER- 



TICAL SECTION OF THE CEREBELLUM TO 

 SHOW THE CORPUS DENTATUM IN ITS 

 MEDULLARY STEM. 



The section has been carried through the 

 left lateral part of the pons so as to divide 

 the superior peduncle and pass nearly 

 through the middle of the left cerebellar 

 hemisphere. The olivary body has also 

 been divided longitudinally so as to expose 

 in section its corpus dentatum. 



cr, crus cerebri ; /, fillet ; q, corpora 

 quadrigemina ; sp, superior peduncle of 

 the cerebellum divided; m p, middle 

 peduncle or lateral part of the pons Varolii, with fibres passing from it into the white 

 stem ; av, continuation of the white stem radiating towards the arbor vitse of the folia ; 

 cd, corpus dentatum ; o, olivary body with its corpus dentatum; p, anterior pyramid. 



Internal structure of the cerebellum. The central part is composed of 

 white matter, which sends out spreading and gradually thinning layers into 

 the interior of all the laminse, larger and smaller, of the grey substance 

 which form a continuous covering on the surface. In consequence of this 

 arrangement of the white and grey substances, sections of the cerebellum 

 crossing the laminae, and dividing the grey and white substance together, 

 present a beautifully foliated or arborescent appearance, named arbor vitce. 

 This appearance is seen in any vertical section, but it is most perfect in that 

 which passes through the median plane, where the relative quantity of the 

 central white matter is small. The foliations are arranged somewhat pin- 

 nately, the section of each primary lamina having those of secondary laminse 

 clustered round it like leaflets 011 a stalk. 



In the lateral hemispheres, where the peduncles enter, the white matter 

 is more abundant ; and, if a section be made through either hemisphere half 

 way between its centre and the middle of the vermiform process, it will 

 display a nucleus of grey matter, which is named the corpus dentatum of the 

 cerebellum. This structure, very similar to that already described in the 

 olivary body of the medulla oblongata, presents the appearance of a 

 waved line of compact yellowish brown matter, surrounded by white sub- 

 stance and containing whitish matter within. This line is interrupted at 



