634 THE EAR 



and extending beyond the heads of the inner pillar cells. These 

 are followed by the interdigitating phalanges of the cells of Deiters, 

 which enter into the formation of the reticular membrane, through 

 the fenestra of which the cilia of the three to five rows of outer 

 hair cells project. This cuticular membrane is continued outward, 

 and beneath it are successively seen the ends of the cells of Hensen, 

 and of the cells of Claudius. 



THE AUDITORY NERVE 



The eighth cranial nerve presents two distinct divisions both of 

 which are sensory, but which, as we have already seen, differ greatly 

 as regards their central termination (see Chapter XXVI). They 

 likewise differ in their peripheral distribution. Within the inter- 

 nal auditory meatus the nerve divides, each branch consisting of 

 numerous bundles. The vestibular (superior or anterior) division 

 is supplied with a ganglion of considerable size, the ganglion of 

 Scarpa, beyond which the nerve separates into three branches 

 which supply, respectively, the macula of the utricle, and the 

 cristae of the superior and external semicircular canals, in the neuro- 

 epithelium of each of which their terminal fibrils end in relation 

 with the bases of the hair cells (Figs. 452 and 453). The remain- 

 ing nerve fibres which are distributed to the vestibule are derived 

 from a branch of the cochlear (inferior or posterior) division, and 

 they supply in a similar manner the macula of the saccule and the 

 crista of the posterior semicircular canal. 



The cochlear branch proper, cochlear nerve, enters the modiolus, 

 where it becomes abruptly narrowed by giving off numerous fine 

 branches which pass outward between the layers of the bony spiral 

 lamina. Here they form a continuous spiral succession of small 

 nerve trunks, supplied with many ganglion cells, which collectively 

 form the spiral ganglion (Fig. 456). They penetrate the margin of 

 the bony sulcus through the foramina nervosa, a succession of per- 

 forations, in the tympanic lip of the sulcus. Here the nerve fibres 

 lose their medullary sheath and come almost at once into relation 

 with the inner hair cells. From this point the path of the non- 

 medullated fibres varies, most of them passing for some distance 

 along a spiral course through the organ of Corti. One such spiral 

 bundle is found on the inner, and another on the outer side of the 

 inner pillars, the latter lying within the canal of Corti. Still other 

 fibres, the tunnel fibres, cross the canal of Corti to form a spiral 

 plexus beneath the outer hair cells and the cells of Deiters. Ter- 



