6 4 6 



TEXT-BOOK OF PHYSIOLOGY 



THE EIGHTH NERVE. THE ACOUSTIC 



The eighth cranial nerve, the acoustic, consists of the centrally coursing 

 axons of neurons which connect the essential organ of hearing with sensor 

 end-nuclei in the pons Varolii. This nerve consists of two portions: viz., 

 a cochlear or acoustic and a vestibular or equilibratory. 



Origin. The axons comprising the cochlear portion have their origin 

 in the bipolar nerve-cells of the spiral ganglion located in the spiral canal near 



the base of the osseous lamina spiralis 

 (Fig. 268). From this origin they pass 

 centrally into the central canal of the 

 modiolus, at the base of which they 

 emerge in well-defined bundles and 

 enter the internal auditory meatus. 

 Dendritic processes from these cells 

 pass peripherally to terminate on the 

 ciliated epithelial cells of the organ of 

 Corti. 



The axons comprising the vestibular 

 portion have their origin in the bipolar 

 nerve-cells of the ganglion of Scarpa 

 located in the internal auditory meatus. 

 From this origin they pass centrally in 

 connection with the cochlear portion. 

 Dendritic processes from these cells 

 pass peripherally into the internal ear, 

 where they terminate in the epithelial 

 cells situated on the inner surface of the 

 utricle and saccule and in the ampullae 

 of the semicircular canals. 



The common trunk of the acoustic 

 nerve, consisting of both cochlear and 

 vestibular divisions after emerging from 

 the internal auditory meatus, passes 

 backward, inward, and downward as 

 far as the lateral aspect of the pons 

 where the two divisions again separate. 

 The cochlear nerve, the external 

 root, passes to the outer side of the 

 res tiform .body and enters the lateral 

 acoustic nucleus and the ventral or accessory acoustic nucleus, around 

 the cells of which its end- tufts arborize. The vestibular nerve, the 

 internal root, passes on the inner side of the restiform body to the dorsal 

 portion of the pons, where, after bifurcating, the end-tufts of the axons 

 arborize around the dorso-internal nucleus (Bechterew's) and the dorso- 

 external nucleus (Deiters'). Some of the fibers of the vestibular branch 

 descend through the pons and medulla as far as the cuneate nucleus. 



Cortical Connections. From the nerve-cells of the ventral and lateral 

 acoustic nuclei axons arise, some of which cross the median plane to enter 

 the lateral lemniscus or fillet of the opposite side, while others enter the 



FIG. 268. ORIGIN AND TERMINATION 

 OF THE. ACOUSTIC NERVE, i. Cochlea. 

 2. Spiral ganglion (Corti). 3. Cochlear 

 nerve. 4. Ventral acoustic nucleus. 5. 

 Lateral acoustic nucleus. 6. Semi- 

 circular canals. 7. Ganglion of Scarpa. 

 8. Vestibular nerve. 9. Dorso-external 

 nucleus (Deiters). 10. Dorso-internal 

 nucleus. (After Moral and Doyon.) 



