CEREBELLUM. 421 



bent pressure of the cerebrum by the tentorium cerebelli. Like the 

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

 cupying 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 down- 

 wards, and its borders being anterior, posterior, and lateral. In consi- 

 deration of its shape the cerebellum admits of a division into two he- 

 mispheres, into certain processes termed processes and lobules, and into 

 certain divisions of its substance called lobes, formed upon the hemi- 

 spheres 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 bor- 

 der of the organ there is a semi-lunar notch, indsura cerebelli anterior, 

 which encircles the corpora quadrigemina posteriorly. On the poste- 

 rior border there is another notch, indsura cerebelli posterior, which 

 receives the upper part of the falx cerebelli ; and upon the under surface 

 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 se- 

 parated by a sulcus, somewhat more strongly marked than the rest, 

 and extending deeper into the substance of the cerebrum ; 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 crus 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 

 vermiformis superior, and that which is lodged within the vallecula 

 being the processus vermiformis inferior. The superior vermiform pro- 

 cess is a prominent longitudinal ridge, extending from the incisura an- 

 terior to the incisura posterior cerebelli. In imitation of the hemi- 

 spheres it is divided into lobes of which three have received names 

 namely, the lobulus centralis, which is a small lobe situated in the in- 

 cisura anterior ; the monticidus cerebelli, a longer lobe, having its peak 

 and declivity ; and a small lobe near the incisura posterior, the com- 

 missura simplex. The lobes of the inferior vermiform process are four 

 in number, namely, the commissura brevis, situated in the incisura 



