AUDITORY NERVES 333 



Nerves of the internal ear. There are two distinct mech- 

 anisms in the internal ear; one for the sense of hearing, the other 

 to serve as an organ of equilibration. So there are two separate 

 nerves, included under the one name auditory. Both are 

 auditory in the sense of being connected with the ear; both are 

 purely sensory and both are called into action by the stimulus of 

 mechanical vibration, but here the likeness ends. They differ 

 in their terminals and their central connections. We shall speak of 

 them as the cochlear nerve and the vestibular nerve. 



The cochlear nerve is the true nerve of hearing. Its terminal 

 fibers are found in highly specialized epithelial cells in the mem- 

 branous cochlea (and one ampulla) ; there they receive impres- 

 sions transmitted by the vibrating chain of bones. 



By these fibers the cochlear nerve 



is formed. It passes through the llIC utf^Mr UMALLEUS 



internal auditory canal into the 

 cranial cavity and disappears in the 

 medulla. Its fibers end in nuclei 

 situated in the medulla, from which STAPES 

 the impressions brought by them 

 are conveyed finally to the auditory 

 area in the temporal lobe (superior FIG. 213. BONES OF THE EAR. 

 temporal convolution). 



The terminal fibers of the vestibular nerve are found in 

 the special cells within the membranous vestibule and ampulla of 

 the semicircular canals. From thence they are gathered to form 

 the nerve; it leaves the petrous bone in company with the cochlear 

 nerve and enters the medulla. The cortical centers for this nerve 

 are in the cerebellum. It is not concerned in hearing but is neces- 

 sary to the power of preserving equilibrium in standing, walking, 

 etc. The person in whom this nerve has been destroyed cannot 

 walk steadily and is not subject to seasickness. 



The vestibular nerve filaments are thought to be stimulated 

 when vibrations of the endolymph are caused by changes in the 

 position of the head. This seems to be established clinically. 

 Through wide association with other nerves, changes in the 

 position of the body also affect this nerve. The vestibular nerve 

 fibers end in nuclei situated in the medulla, from which impressions 

 brought by them are conveyed to the centers in the cerebellum. 



