THE SENSES 



581 



Semicircular Canals and Vestibule. 



We have seen elsewhere (pp. 481, 522) that the eighth pair of 

 cranial nerves consists of two roots, dorsal and ventral, both dis- 

 tributed to the internal ear, but both are not concerned in the 

 sense of hearing. The dorsal root alone is distributed to the 

 cochlea, the ventral root passes to the vestibule. In consequence 

 the nerve has been distinguished as the cochlear and vestibular 

 division of the auditory nerve. The cochlear division is ex- 

 clusively devoted to the sense of hearing, the vestibular division 

 to the function of maintaining the equilibrium of the body. The 

 vestibular nerve is connected with the cerebellum, the nuclei 

 which control the muscles of the eyeball, and with motor cells 



Fig. 184. — The Semicircular Canals (Diagrammatic) (Ewald). 



H, Horizontal canal ; S, superior vertical canal ; P, posterior vertical canal. 

 The two horizontal canals are in the same plane. The superior vertical 

 canal of one side is parallel to the plane of the posterior vertical canal of 

 the opposite side. 



in the cord. It is not directly connected with the cerebrum. 

 In the internal ear it is connected with the semicircular canals 

 and the vestibule. 



The semicircular canals are three in number, and so arranged 

 that their three planes are placed at right angles to each other, 

 two being vertical and one horizontal (Fig. 184). Their positions 

 in the living animal are shown in Fig. 180. Within each bony 

 canal is a membranous counterpart separated by a fluid known 

 as peri-lymph, while within the membranous organ is another 

 fluid described as en do-lymph. Each canal terminates in a 

 swelling known as the ampulla. The semicircular canals open 

 into the vestibule by five openings — some say four (Sisson). 

 Within the vestibule are two membranous sacs, the utricle and 

 saccule ; the former communicates with the semicircular canals, 

 the latter with the utricle and cochlea. These sacs, together with 

 the ampullae, contain the hair-like processes representing the sen- 

 sory nerve organs of the vestibular division of the seventh nerve. 



