THE BRAIN AND ITS MEMBRANES. 



725 



FIG. 424. Diagram showing development 

 of cerebellum, e: A, a. simple thickening of 

 the roof; B, more fully advanced. (Gegen- 

 baur.) 



fourth ventricle (Fig. 424). of which the ventral boundaries are, as already 

 described, parts of the dorsal surfaces of the 

 pons and medulla (after-brain). 



The Cerebellum is contained in the inferior 

 occipital fossae. It is situated beneath the oc- 

 cipital lobes of the cerebrum, from which it is 

 separated by the tentorium. In form the cere- 

 bellum is oblong, and flattened from above 

 downward, its great diameter being from side 

 to side. It measures from three and a half to 

 four inches (10 centimetres) transversely, and 

 from two to two and a half inches from before 

 backward, being about two inches thick in the centre and about six lines at the 

 circumference, which is the thinnest part. It consists of gray and white matter : 

 the former, darker than that of the cerebrum, occupies the surface ; the latter, 

 the interior. The surface of the cerebellum is not convoluted like the cerebrum, 

 but traversed by numerous curved furrows or sulci, which vary in depth at dif- 

 ferent parts, and separate the laminae of which its exterior is composed. 



Weight of the Cerebellum. Its average weight in the male is 5 ozs. 4 drs. It 

 attains its maximum weight between the twenty-fifth and fortieth year, its increase 

 in weight after the fourteenth year being relatively greater in the female than in 

 the male. The proportion between the cerebellum and cerebrum is, in the male, 

 as 1 to Si. and in the female, as 1 to 8J. In the infant the cerebellum is propor- 

 tionately much smaller than in the adult, the relation between it and the cerebrum 

 being, according to Chaussier, between 1 to 13 and 1 to 26 ; by Cruveilhier the 

 proportion was found to be 1 to 20. 



Main Lobes of the Cerebellum. The cerebellum is divided into three large 

 lobes, a middle and two lateral. The middle lobe is the worm, and the two lateral 

 are the hemispheres. These lobes are not separable from each other, being joined 

 together by their sides. Hence the upper surface of the cerebellum, as a whole, 



Dadfee 



Lobuspott. 

 in/eiw 



FIG. 425. Upper surface of cerebellum, x i- (Gegenbaur.) 



is the upper surfaces of the two hemispheres connected in the middle line by the 

 upper surface of the u-orm, which last appears as a slightly elevated narrow ridge, 

 about 1 centimetre wide, from which the upper surfaces of the hemispheres slope 

 away laterally and posteriorly, and are therefore flattened or slightly concave 

 (Fig. 425). *The inferior surfaces (Fig. 426) of the hemispheres are markedly 

 convex both from before backward and from side to side. In the middle line they 

 partially overlap and conceal the inferior surface of the tvorm ; but on separating 

 them which can be done to a certain degree without tearing any tissue the 

 entire inferior surface of the worm is brought into view. This is far more dis- 



