128 THE HUMAN BODY. 



ance (fig. 32, p. 124). These lines or folds are all of about 

 the same width, and are parallel through a portion of their 

 course, and then they form acute angles, and are collected 

 in fascicles, which point transversely downward or backward, 

 and divide the hemispheres into segments, which are divided 

 and subdivided into layers. 



The cerebellum is composed, like the cerebrum, of white 

 and gray substance, with the addition of a yellowish substance 

 interposed in layers between the two others. Each hemi- 

 sphere is formed of a central nucleus, around which the seg- 

 ments develop themselves. The layers of these segments 

 are in juxtaposition like the leaves of a book; the white sub- 

 stance is in the centre; then comes a layer of the yellow 

 matter, and over these the gray substance. If we make a 

 vertical section of the cerebellum, the alternating layers of 

 the three substances of which it is composed are seen forming 

 a series of ramifications which spring from a common trunk : 

 this has been named the arbor vita or tree of life (fig. 33, 

 p. 126). At the nucleus or knot, the peduncles of the cere- 

 bellum terminate ; they are three in number on either side, 

 and serve to attach it to the other parts of the encephalon. 

 Near the point where these different parts unite, there is a 

 cavity which is partly circumscribed by the peduncles of the 

 cerebellum; this is called the fourth ventricle, or ventricle of 

 the cerebellum. It communicates with the third ventricle of 

 the cerebrum by the canal of Sylvius. 



Isthmus of the encephalon. This is the term applied to 

 that portion of the encephalic mass which unites the cerebrum, 

 cerebellum, and the spinal bulb. It is the point of union of 

 the three great divisions of the nervous centre. It comprises 

 the pons Varolii, the peduncles of the cerebrum and cere- 

 bellum, the corpora quadrigemina, and the valve of Vieussens. 



At the base of the encephalon there is a convex projection 

 which surrounds the peduncles of the cerebrum and cerebel- 

 lum like a large ring, and which covers the expansions of the 

 spinal bulb toward these peduncles like a bridge. This is 

 the pons Varolii, or bridge of Varolius. This projection is 

 the centre of convergence or of emergence of the nervous 

 fascicles or bundles which it seems to cover. It is joined to 



