274 PHYSIOLOGY FOR DENTAL STUDENTS. 



volves some of the centers concerned in the higher control of 

 the muscles which are used in speech, and very commonly it is 

 situated in the left side of the cerebrum. In all three forms of 

 aphasia there is more or less decrease in the mental powers. 



Cerebellum. 



'The afferent impulses set up by stimulation of the nerves of 

 the skin in a spinal animal, and due therefore to changes in 

 the environment, after entering the spinal cord travel to the 

 various centers in the cord. Although complicated movements 

 may result (e.g., the scratch reflex), there is an entire absence 

 of the power of maintaining bodily equilibrium, and the animal 

 cannot stand because the muscles are not kept in the degree of 

 tone which is necessary to keep the joints properly stiffened 

 A similar inability to maintain the center of gravity of the 

 body results from removal of the cerebellum, or small brain, 

 which it will be remembered is situated dorsal to the medulla 

 and pons, with which it is connected by three peduncles. The 

 cerebellum consists of two lateral hemispheres and a median 

 lobe called the vermis. The remarkable infolding of the grey 

 matter which composes its surface, and the large number of 

 nuclei which lie embedded in its central white matter are struc- 

 tural peculiarities of the cerebellum. 



The immediate results of removal of the cerebellum consist in 

 extreme restlessness and inco-ordination of movements. The 

 animal is constantly throwing itself about in so violent a man- 

 ner that unless controlled it may dash itself to death. Gradually 

 the excitement gets less until after several weeks all that is 

 noticed is that there is a condition of muscular weakness and 

 tremor, and difficulty in maintaining the body equilibrium. 

 Quite similar symptoms occur when the cerebellum is diseased 

 in man (as by the growth of a tumor), the condition being 

 called cerebellar ataxia, and being characterized by the uncer- 

 tain gait which is like that of a drunken man. 



These observations indicate that the function of the cerebellum 

 is to harmonize the actions of the various muscular groups, so 



