CEREBELLUM. 381 



gether with a layer of pia mater which forms its inferior boundary. In 

 this manner a perfect communication is established between all the ven- 

 tricles, with the exception of the fifth, which has its own proper membrane. 

 It is this membrane which gives them their polished surface, and transudes 

 the secretion which moistens their interior. When the fluid accumulates 

 to an unnatural degree, it may then break down this layer and the layer 

 of pia mater at the bottom of the fourth ventricle, and thus make its way 

 into the sub-arachnoidean space ; but in the normal condition it is doubt- 

 ful whether a communication exists between the interior of the ventricles 

 and the serous cavity of the sub-arachnoidean space. 



CEREBELLUM. 



The Cerebellum (figs. 171, 172, 173), seven times smaller than the 

 cerebrum, is situated beneath the posterior lobes of the latter, being lodged 

 in the posterior fossa of the base of the cranium, and protected from the 

 superincumbent pressure of the cerebrum by the tentorium cerebelli. Like 

 the cerebrum, it is composed of grey and white substance, the former 

 occupying the surface, the latter the interior, and its surface is formed of 

 parallel lamellae separated by sulci, and here and there by deeper sulci. 

 In form, the cerebellum is oblong and flattened, its greater diameter being 

 from side to side, its two surfaces looking upwards and downwards, and 

 its borders being anterior, posterior, and lateral. In consideration of its 

 shape the cerebellum admits of a division into two hemispheres, into cer- 

 tain prominences termed processes and lobules, and into certain divisions 

 of its substance called lobes, formed upon the hemispheres by the deeper 

 sulci above referred to. The two hemispheres are separated from each 

 other on the upper surface of the cerebellum by a longitudinal ridge which 

 is termed the superior vermiform process, and which forms a commissure 

 between them. On the anterior border of the organ there is a semilunar 

 notch, indsura cerebelli anterior, which encircles the corpora quadrigemina 

 posteriorly. On the posterior border there is another notch, incisura cere- 

 belli posterior, which receives the upper part of the falx cerebelli ; and on 

 the under surface of the cerebellum is a deep fissure corresponding with 

 the medulla oblongata, and termed the vallecula (valley). 



Each hemisphere of the cerebellum is divided by means of a fissure 

 (sulcus horizontalis) which runs along its free border, into an upper and a 

 lower portion, and upon each of these portions certain lobes are marked 

 out. Thus on the upper portion there are two such lobes separated by a 

 sulcus, somewhat more strongly marked than the rest, and extending 

 deeper into the substance of the cerebellum ; they are the lobus superior 

 anterior and lobus superior posterior. Upon the under portion of the 

 hemisphere there are three such lobes, namely, lobus inferior anterior, 

 medius, and posterior, and two additional ones of peculiar form, the lobus 

 inferior internus or tonsil, and the flocculus. The tonsil (amygdala) is 

 situated on the side of the vallecula, and projects into the fourth ventricle. 

 The flocculus or pneumogastric lobule, long and slender, extends from 

 the side of the vallecula around the corpus restiforme to the cms cerebelli, 

 lying behind the filaments of the eighth pair of nerves. 



The commissure between the two hemispheres is termed the worm 

 'vermis), that portion of the worm which occupies the upper surface of 

 the cerebellum as far back as the horizontal fissure being the processus 



