874 



THE CENTRAL NERVOUS SYSTEM 



through the vestibular nerve, and these impulses ultimately reach the 

 cerebellum. 



The experimental evidence for these conclusions regarding the functions 

 of the semicircular canals is very strong. Thus, after destruction of all 

 the canals an operation which is particularly easy in the pigeon the 

 animal behaves very much the same as after -cerebellar destruction. After 

 some months these disorders disappear, because the cerebellum learns to 

 control the movements of the body from other proprioceptive impulses, 

 particularly those of sight. If such a recovered animal is bandaged, the 

 symptoms return in all severity. This compensation is furthermore an 

 educative process, for it does not occur when the cerebral centers as well 



Fig. 227. The semicircular canals of the ear, showing their arrangement in the three planes of 

 space. (From Howell's Physiology.) 



as the semicircular canals are removed, and it can be abolished in a re- 

 covered animal by removal of the cerebral cortex. 



Many observations of great interest have been made concerning these 

 labyrinthine sensations by Pike and others, but we can not discuss them 

 further here. One point of interest, however, is that forced movements 

 in definite planes are induced by removal of a canal. Removal of the 

 horizontal canals, for example, causes continued nodding movements of 

 the head in the plane of the injured canals. An experiment of great sig- 

 nificance was performed by Ewald to show the effect of causing a move- 

 ment of the fluid in one of the canals. For this purpose a bony canal was 

 opened at two places by a dental drill. Through the hole farther from the 

 ampulla, amalgam was introduced so as to block the backward movement 

 of fluid, and into that nearer the ampulla a fine tube was inserted con- 

 nected with a rubber bulb. By manipulating the bulb, the membranous 



