410 THE SPECIAL SENSES. 



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. 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 

 at least it seems permissible to adopt the view that variations 

 in the pressure of the endolymph upon the hairs of the hair cells, 

 especially in movements of rotation, constitute the immediate 

 cause of their excitation. Granting that changes in position or 

 movement of the head may cause such variations in pressure the 

 theory offers a simple and satisfactory explanation of the mode 

 of excitation and the means by which the excitation may vary 

 appropriately under different conditions. While the endolymph 

 theory may be criticized easily, no other equally satisfactory theory 

 has been suggested to take its place. 



Functions of the Utriculus and Sacculus. — These small sacs 

 contain sensory hair cells similar in general structure to those found 

 in the cristse of the ampullary sacs. Each collection of hair cells, 

 together with the supporting cells, is designated as a macula. One 

 of these is found in the utriculus, the macula utriculi, and another 

 in the sacculus, the macula sacculi. Lying among the brirs of the 



