378 THE SPECIAL SENSES. 



circular canals constitute what might be called a muscle-tone organ, 

 and the obvious disturbances in motion caused by their injury are 

 due primarily to a diminution or loss in muscle tone, each canal 

 possibly being reflexly connected with special muscles. 



Summary. With reference to the kind of sensation mediated 

 by the nerves of the semicircular canals, it should be borne in mind 

 that these sensations are not distinctly recognized by consciousness ; 

 hence the difficulty of designating them by a specific name. Of 

 the many qualities of sensation or consciousness which we can 

 distinguish some have characteristics so clear that we recognize 

 them at once and give them distinctive names, such, for instance, 

 as the sensations of sight, hearing, taste, etc. Others, however, 

 produce a psychical reaction of such an indefinite character that 

 they escape recognition by mere introspection. The change in con- 

 sciousness is not sufficiently marked to make itself felt to the un- 

 trained mind. This condition prevails regarding the sensations 

 aroused through the semicircular canals; they 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 sensations. Our 

 perceptions or ideas of space and direction are doubtless founded 

 in part upon these reactions and in part upon the muscle sense, 

 vision, and tactile sensations. 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 general co-ordination or con- 

 trol seems to rest normally in the nervous mechanisms of the cere- 

 bellum and inasmuch as the vestibular nerves make end connections 

 with the cerebellum, together with the fibers of muscle sense, we 

 may assume that the cerebellum forms the brain center in which 

 the semicircular canal impulses exert their influence upon muscular 

 contractions and muscle tone, the cerebellum forms the nerve center 

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

 eral sense organ to 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 muscu- 

 lature is perhaps the most acceptable view. In regard to the 

 means by which these nerves are normally stimulated 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 constitute the immediate 

 cause of their excitation. Granting that changes in position or 



