SEMICIRCULAR CANALS AND THE VESTIBULE. 411 



circular canals, are too indistinct to be recognized and named 

 by an appeal to consciousness, and it would seem to be wiser 

 to designate them after the analogy of the muscle sensations 

 simply as semicircular canal or labyrinthine sensations. Our 

 perceptions or ideas of space and direction are possibly founded 

 in part upon these reactions and in part upon the muscular, 

 visual, and tactile sensations. Our reasoning with regard to 

 the semicircular canal sensations would be more satisfactory 

 if it could be shown that the vestibular nerve, after ending in 

 the pons, was continued forward by sensory paths to the cortex 

 of the cerebrum. As a matter of fact, such paths have not 

 been demonstrated, and if we assume that conscious sensations 

 are mediated only through the cortex of the cerebrum we have 

 no anatomical proof that the semicircular canals give us any 

 reaction in consciousness. The vestibular nerve fibers end in the 

 nucleus of Deiters and the nucleus of Bechterew, through which 

 reflex connections are established with the motor centers of the 

 spinal and possibly the cranial nerves. There is a connection 

 also with the nucleus fastigii of the cerebellum and through 

 this possibly with the cerebellar cortex, although this latter 

 connection has not been actually demonstrated. With regard 

 to the influence of the nerve impulses from the semicircular canals 

 upon movements, all the facts known seem to indicate that they 

 play an important part in the regulation or co-ordination of the 

 movements of equilibrium and locomotion. Inasmuch as this gen- 

 eral co-ordination or control seems to rest normally in the nervous 

 mechanisms of the cerebellum and inasmuch as the vestibular 

 nerves probably make connections with the cerebellum, we may 

 assume that the cerebellum forms the brain center through which 

 the semicircular canal impulses exert their influence upon co-ordin- 

 ated muscular contractions the cerebellum forms the nerve center 

 for the semicircular canals, or the semicircular canals form a periph- 

 eral sense organ to the cerebellum. Some such hypothesis seems to 

 be necessary to account for the general similarity between the 

 effects of lesions of the canals and of the cerebellum. Whether the 

 impulses from the canals are excitatory or inhibitory or both, as 

 regards their effect upon muscular contractions, is not clearly 

 apparent from the experimental evidence so far furnished, but 

 Ewald's suggestion that they serve to maintain reflexly the tonus 

 of the body musculature is perhaps the most acceptable view, 

 especially when it is remembered that this tonicity may vary 

 in an adaptive way in different muscles according to the strength 

 of the stimuli coming from one or another of the canals. In 

 regard to the means by which these nerves are normally stim- 

 ulated there is also much room for conjecture, but provisionally 



