INTERIOR OF CEREBELLUM. 215 



MEDULLARY CENTRE. In the centre of each cerebellar hemisphere is 

 a large white mass, containing in its substance a dentate body. From its 

 surface offsets are furnished to the different laminae. And from the ante- 

 rior part proceeds a large stalk-like process, the crus cerebelli, which is 

 subdivided into three pieces or peduncles, an upper for the cerebrum, a 

 middle piece for the pons, and a lower one for the medulla oblongata. 



The superior peduncle (processsus ad cerebrum) is directed forwards 

 towards the corpora quadrigemina (fig. 65, l ). It is rather flattened in 

 shape, and forms part of the roof of the fourth ventricle : between the pro- 

 cesses of opposite sides the valve of Vieussens is situate. Its fibres, con- 

 tinuous behind with the inferior vermiform process, receive an offset from 

 the interior of the corpus dentatum ; and passing beneath the band of the 

 fillet and the pair of the corpora quadrigemina of the same side, enter 

 the optic thalamus, and are applied to the fibres of the crus cerebri (p. 

 209). 



Beneath the corpora quadrigemina the internal fibres of the peduncle 

 are directed across the middle line, through the bundle prolonged from the 

 fasciculus teres. 1 In this way the fibres of each peduncle end partly in 

 the same, and partly in the opposite hemisphere of the cerebrum. 



Between the superior peduncles is a thin, translucent, white layer the 

 valve of Vieussens (vellum medullare anterius), which enters into the roof 

 of the fourth ventricle (fig. 68, k). It is thin and pointed anteriorly, but 

 widens behind, where it is connected with the under part of the vermiform 

 process. Near the corpora quadrigemina the fourth nerve ( 4 ) is attached 

 to the surface of the valve ; and close to the cerebellum the surface is 

 marked by some gray transverse ridges. 



The middle peduncle (processus ad pontem), commonly named the crus 

 cerebelli (fig. 65, 2 ), is the largest of the three peduncular processes. Its 

 fibres begin in the lateral part of the cerebellum, and are directed forwards 

 to the pons, of which they form the transverse fibres, and unite with the 

 peduncle of the opposite side. This peduncle is supposed to serve as a 

 commissural or connecting band between the halves of the cerebellum. 



The inferior peduncle (fig. 65, 3 ) (processus ad medullam) passes down- 

 wards to the medulla oblongata, and gives rise to the restiform body. Its 

 fibres begin chiefly in the lamina? of the upper surface of the cerebellum. 

 It will be better seen when the fourth ventricle has been opened. 



The fibres in the peduncles connect one cerebellar hemisphere with the 

 cerebrum; witli its fellow; and with the medulla oblongata of the same- 

 side, in the manner mentioned above. 



The dentate body (corpus dentatum) is contained in the white mass of 

 the cerebellum, and resembles the like part in the corpus olivare of the 

 medulla oblongata. This body measures nearly an inch from before back, 

 and is situate near the inner part of the white centre. It consists of a 

 small plicated capsule, which, when cut across, appears as a thin, wavy, 

 grayish-yellow line ; the bag is open at the interior part, and incloses \\. 

 nucleus of whitish substance. Through its aperture issues a band of fibres 

 from the nucleus to join the superior peduncle. 



Dissection. One other section (fig. 68) must be made to show the fourth 

 ventricle, and the structure of the vermiform process. The cerebellum still 



1 This intercommunication was known to Rcil, and was named " ansa" by him, 

 but the decussation has been since noticed by Stilling, Ueber den Ban dcs lltrnk- 

 notens: 1846. 



