THE BRAIN AND ITS MEMBRANES. 727 



its sides are much more curved, than those of the posterior. This notch is really 

 the deeply hollowed-out "anterior margin" of the cerebellum. It lies close to the 

 pons and'upper part of the medulla, while the upper edge of the notch extends to 

 or encircles the posterior pair of corpora quadrigemina of the mid-brain. This 

 edge can be raised, however, and then can be seen the superior cerebellar pedun- 

 cles and valve of Vieussens (see below). The posterior notch is free. When 

 within the cranium it contains the upper part of the falx cerebelli. The sides of 

 each notch are formed by the respective hemispheres, while the bottom of each 

 notch, or its centre, is the anterior and posterior extremity, respectively, of the 

 worm, 



The fissures of the cerebellum are very numerous and dip deeply into its 

 substance. Of these the largest and deepest is the great horizontal fissure. 

 This passes completely around the cerebellum, forming its circumference as it 

 were, and its plane is horizontal. As it crosses the median line, in front and 

 behind, it cuts into the respective extremities of the worm, and splits the sides 

 of each of the notches as well (see above). Hence this fissure divides each hemi- 

 sphere and the worm (the entire cerebellum) into an upper and a lower half. The 

 edges and sides of this fissure are everywhere in contact, and lined by gray mat- 

 ter, except where it runs across the anterior cerebellar notch, where its edges, 

 upper and lower, are separated by the passage between them of the u'hite matter 

 of the cerebellum. 



All the remaining fissures of the cerebellum are lined by gray matter ; their 

 edges are everywhere in contact, and they all terminate, by one extremity at least, 

 in the great horizontal fissure (see below for further details of these fissures). 



The Worm. This, as already stated, is the middle lobe of the cerebellum. It 

 has an upper and a lower surface, and two extremities, anterior and posterior. 

 The upper surface is called either the superior vermiform process or the upper 



'/> ; and the lower surface, either the inferior vermiform process or the lower 

 Its tides are attached directly to the mesial sides of the hemispheres, and 

 are not seen except on section. Each extremity is divided by that portion of the 

 great horizontal fissure which dips into the corresponding notch into an upper and 

 a lower half. Hence each of these anterior halves is the anterior extremity, 

 respectively, of the upper and lower worm ; and each posterior half is, similarly, 

 the posterior extremity of the corresponding worm. The horizontal fissure does 

 not dip into the extremities of the worm nearly so deeply as it does into the 

 im'! rgin of the hemispheres. 



Each surface of the worm, or the upper and lower worm respectively, is sub- 

 divided into lobules by transversely directed fissures which are continued laterally 

 into and across the corresponding surfaces of the hemisphere to the margin, where 

 they terminate in the great horizontal fissure. Hence any tAvo of these fissures 

 contain between them a lobule of the upper or lower worm in the middle, and, 

 laterally, a portion of the corresponding surfaces of the hemispheres. These 

 fissures are known as mler$0ftnl(ir fissures. 



The Hemispheres. Each hemisphere has a side, an upper surface, a lower sur- 

 face, and a margin. The side of each is directly attached to the corresponding 

 side of the worm. The margin is curved and extends around from the side of 

 the posterior extremity of the worm to the corresponding side of the anterior 

 extremity of the worm. Hence in the notches this margin is the same thing as 

 the side of the notch. The margins of both hemispheres, together with both 

 extremities of the worm, contain the great horizontal fissure /. e. the upper edge 

 of the Jissure is made up of the anterior extremity of the upper worm, the margin 

 of the upper half of one hemisphere, the posterior extremity of the upper worm, 

 the margin of the upper half of the other hemisphere. The lower edge of the 

 fissure is similarly made up. 



The surfaces, both lower and upper, are, like those of the worm, subdivided 

 into lobules by the lateral prolongations of the interlobular fissures, already men- 

 tioned. On the upper surface of each hemisphere these fissures are disposed 



