274 THE NERVOUS SYSTEM 



A difference in intellectual vigor may be present in per- 

 sons whose brains have the same weight and even the same 

 amount of gray matter. A difference in the quality of the 

 gray substance may in such cases account for the varying 

 results. It is a matter of common observation that mental 

 exercise increases mental vigor and capacity, just as muscu- 

 lar exercise develops muscular strength. It is difficult to 

 reach a conclusion as to whether there is an increase in the 

 amount of gray substance or whether that already present is 

 endowed with additional power. 



The Cerebellum. 



Anatomy. The cerebellum, or little brain (see Fig. 79), 

 is situated beneath the occipital lobes of the cerebrum, 

 weighs some 5*4 ounces in the male to 4^ ounces in the fe- 

 male, and consists of a central and two lateral lobes. It is 

 composed of white and gray matter, the latter being, with 

 the exception of the corpora dentata in the lateral lobes, sit- 

 uated externally. The convolutions on its surface are much 

 finer than are those on the cerebral surface. It is separated 

 from the parts above by the tentorium cerebelli, a pro- 

 cess of the dura mater. 



Fibers. The fibers passing away from the cerebellum are 

 collected into three bundles on each side, known as the su- 

 perior, middle and inferior peduncles. The superior pe- 

 duncle has a direction forward and upward to reach the crus 

 and optic thalamus ; fibers in it connect the cerebellum with 

 the cerebrum. Certain of these decussate underneath the 

 corpora quadrigemina with corresponding fibers from the 

 opposite side, so that each side of the cerebellum is connected 

 with both sides of the cerebrum. Attention has been called 

 to fibers passing down from the cerebrum through the pons 

 to the cerebellum. Fibers in the middle peduncle connect 

 the two lateral halves of the cerebellum through the pons. 

 Fibers in the inferior peduncle are continuous below with 



