CEREBELLUM. l8l 



are motor centers. Irritation by a weak galvanic current produces muscular 

 movements of the opposite side of the body; destruction of their substance by 

 a hemorrhage, as in apoplexy, is followed by a paralysis of motion of the 

 opposite side of the body, but there is no loss of sensation. When the hemor- 

 rhagic destruction involves the fibers of the anterior two thirds of the posterior 

 segment of the internal capsule, and thus separates them from their trophic 

 centers in the cortical motor region, a descending degeneration is established, 

 which involves the direct pyramidal tract of the same side and the crossed 

 pyramidal tract of the opposite side. 



Destruction of the posterior one third of the posterior segment of the inter- 

 nal capsule is followed by a loss of sensation on the opposite side of the body 

 and a loss of the senses of smell and vision on the same side (Charcot). The 

 precise function of the corpora striata is unknown, but they are in some 

 way connected with motion. 



The optic thalami receive the fibers of the tegmentum, the posterior portion of 

 the crura cerebri. They are insensible and inexci table to direct irritation. 

 Removal of one optic thalamus, or destruction of its substance by disease or 

 hemorrhage, is followed by a loss of sensibility of the opposite side of the body, 

 but there is no loss of motion; their precise function is also unknown, but they 

 are in some way connected with sensation. In both cases their action is 

 crossed. 



THE CEREBELLUM. 



The cerebellum is situated in the inferior fossae of the occipital bone, 

 beneath the posterior lobes of the cerebrum. It attains its maximum weight, 

 which is about five ounces, between the twenty-fifth and fortieth years, the 

 proportion between the cerebellum and cerebrum being as i to 8f. 



It is composed of two lateral hemispheres and a central elongated lobe, the 

 vermiform process; the two hemispheres are connected with each other by the 

 fibers of the middle peduncle, forming the superficial portion of the pons Varolii. 

 The cerebellum is brought into connection with the medulla oUongata and 

 spinal cord through the prolongation of the restiform bodies; with the cere- 

 brum, by fibers passing upward beneath the corpora quadrigemina and the 

 optic thalami, and then forming part of the diverging cerebral fibers. 



Structure. It is composed of both white and gray matter, the former 

 being internal, the latter external, and is convoluted, for economy of space. 



The white matter consists of a central stem, the interior of which is a den- 

 tated capsule of gray matter, the corpus dentatum. From the external sur- 

 face of the stem of white matter processes are given off, forming the lamina, 

 which are covered with gray matter. 



