THE VESTIBULAR APPARATUS AND CEREBELLUM 189 



Fig. 88), and a much larger number leave by the superior pedun- 

 cle for the red nucleus (nucleus ruber, Fig. 75) and adjacent 

 parts of the brain stem, these fibers first crossing to the opposite 

 side of the brain in the cerebral peduncle under the aqueduct of 

 Sylvius. From the red nucleus fibers pass downward into the 

 spinal cord (rubro-spinal tract) and upward to the cerebral 

 cortex. 



The connections just described illustrate some of the pathways 

 by which the cerebellum is able to reinforce, coordinate, or 

 otherwise modify the somatic motor mechanisms. There is an 

 immense amount of potential nervous energy always available 

 in the neurons of the cerebellar cortex, and the cerebellum ap- 

 pears to be constantly exerting a stimulating or tonic effect upon 

 the body muscles. An injury to the cerebellum (especially an 

 unsymmetric lesion) produces motor incoordination, and the 

 total removal of the cerebellum results in loss of muscular tone 

 and great weakness, though there is no abolition of any particular 

 motor functions. The cerebellar cortex and the cerebral cortex 

 are very intimately connected by large fiber tracts, and each 

 apparently exerts an important physiological effect upon the 

 other. But the exact nature of this reciprocal control is still 

 obscure. 



The cerebellar cortex differs from the cerebral cortex in the 

 form and arrangement of its neurons and also, further, in that 

 it is structurally similar throughout its entire extent. The 

 cerebral cortex, on the other hand, shows differences in the forms 

 and arrangements of its neurons in different regions, and this 

 is correlated with a regional localization of diverse functions 

 (pp. 273, 281). There is some evidence that different parts 

 of the cerebellar cortex exert a dominant regulatory influence 

 over particular large groups of muscles; but this localization of 

 function is of a very general sort and is by no means so precise 

 as the localization of voluntary motor centers in the cerebral 

 cortex. Moreover, the physiological influence of the cerebellum 

 upon movement is of a very different sort from that of the cere- 

 bral cortex. 



The surface of the cerebellum is divided by deep fissures or 

 sulci into narrow leaf-like subdivisions termed folia or gyri, 

 so that when it is cut open across the median plane the cut sur- 



