388 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, 

 if any, aroused through the semicircular canals; they are too 

 indistinct to be recognized and named by an appeal to conscious- 

 ness, 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 

 possibly founded in part upon these reactions and in part upon 

 the muscle sense, 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 brain 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 make possible end connections with the cerebellum, to- 

 gether with the fibers of muscle sense, we may assume that the 



