THE NERVOUS SYSTEM 



1373 



mass of white matter in the centre is seen to send offshoots into 

 the lobules. From the sides of these offshoots secondary processes 

 are given off, and these in turn furnish tertiary processes, the white 

 matter in all cases being covered by grey matter. When the section 

 is made across the direction of the laminae or folia the appearance 

 presented is like the trunk and branches of a tree : hence the name 

 arbor vita cerebelli is applied to it. 



Nuclei. — The corpus dentatum is a collection of grey matter 

 which is situated within the white matter of each hemisphere, and 

 it is similar to the inferior olivary nucleus in the olivary' body of the 

 medulla oblongata. Tt is composed of a wavy grey lamina, disposed 

 in the form of a capsule, which encloses white matter. The capsule 

 presents an opening or hi him at its upper and inner part, and 

 through this a large number of the fibres of the superior cerebellar 

 peduncle emerge from the interior. 



There are three other nuclei on each side, as follows: (i) the 

 nucleus emboliformis, close to the inner side of the hilum of the 

 corpus dentatum; (2) the 

 nucleus globcsus, internal to 

 the preceding; and (3) the 

 nucleus fastigii, or roof-nucleus, 

 situated in the corpus trape- 

 zoides of the vermis, close to 

 the median line, and contiguous 

 to its fellow of the opposite side. 



Commissural and Associa- 

 tion Fibres. — The commissural 

 fibres pass from the white 

 matter of one hemisphere to 

 that of the opposite hemi- 

 sphere. They traverse the 

 corpus trapezoides of the ver- 

 mis in two sets, superior and 



inferior. The association fibres are confined to each side, and 

 they connect adjacent laminae, passing across the bottom of the 

 fissures which separate them. 



Medullary Vela. — These are thin laminae or curtains of white 

 matter, and are two in number, superior and inferior. 



The superior medullary velum, or valve of Vieussens, is continuous 

 with the corpus trapezoides or white matter of the vermis. It 

 extends between the converging superior cerebellar peduncles, 

 bridging over the interval between them, and becoming continuous 

 with their inner margins. Superiorly it extends to the inferior pair 

 of quadrigeminal bodies, and inferiorly it passes into the corpus 

 trapezoides or white matter of the vermis. It forms a large portion 

 of the roof of the upper part of the fourth ventricle, and its dorsal 

 surface supports the lingula of the superior vermis. From the 

 upper part of the superior medullary velum a band of white fibres, 

 called the frenulum veli, passes to the lower part of the median 

 longitudinal groove which separates the lateral pairs of quadri- 



FiG. 575. — Sagittal Section of a 

 Hemisphere of the Cerebellum, 

 SHOWING the Arbor Vit^. 



