AUDITORY NERVES. 729 



the physiology of the auditory nerves. The auditory nerves conduct impres- 

 sions of sound, as the optic nerves conduct impressions of light ; but there 

 is an elaborate arrangement of parts by which the waves are collected, con- 

 veyed to a membrane capable of vibration, and finally carried to the nerves, 

 by which the intensity and the varied qualities of sound are appreciated. 



AUDITORY (EIGHTH NERVE). 



The origin of the auditory nerve can easily be traced to the floor of the 

 fourth ventricle, where it presents two roots. The external, or superficial 

 root, sometimes called the posterior root, can be seen usually without prepara- 

 tion. It consists of five to seven grayish filaments, which decussate in the 

 median line, and pass outward, winding from the fourth ventricle around 

 the restiform body. The deep root consists of a number of distinct filaments 

 arising from the gray matter of the fourth ventricle, two or three of which 

 pass to the median line, to decussate with corresponding filaments from the 

 opposite side. Filaments from this root have been traced to a gray nucleus 

 in the inferior peduncle of the cerebellum and thence to the white substance 

 of the cerebellum itself. The deep root passes around the restiform body 

 inward, so that this portion of the medulla is encircled by the two roots. 

 Passing from the superior and lateral portion of the medulla oblongata, 

 the trunk of the nerve is applied to the superior and anterior surface of 

 the facial. It then passes around the middle peduncle of the cerebellum, 

 and receives a process from the arachnoid membrane, which envelops it in 

 a common sheath with the facial. It finally penetrates the internal audi- 

 tory meatus. In its course it receives filaments from the restiform body 

 and possibly from the pons Varolii. Within the meatus the nerve divides 

 into an anterior and a posterior branch, the anterior being distributed to the 

 cochlea, and the posterior, to the vestibule and semicircular canals. The 

 distribution of these branches will be fully described in connection with the 

 anatomy of the internal ear. 



The auditory nerves are grayish in color, and their consistence is soft, 

 thus differing from the ordinary cerebro-spinal nerves, and resembling to a 

 certain extent the other nerves of special sense. On the external, or super- 

 ficial root, is a small, ganglioform enlargement, containing fusiform nerve- 

 cells. The filaments of the trunk of the nerve consist of very large axis- 

 cylinders, surrounded by a medullary sheath, but having no tubular mem- 

 brane. In the course of these fibres, are found small, nucleated, ganglionic 

 enlargements. 



General Properties of the Auditory Nerves. There can be no doubt,- as 

 regards the eighth, that it is the only nerve capable of receiving and convey- 

 ing to the brain the special impressions produced by waves of sound ; but it 

 is an important question to determine whether this nerve be endowed also 

 with general sensibility. Analogy with most of the other nerves of special 

 sense would indicate that the auditory nerves are insensible to ordinary 

 impressions; and this view has been sustained by direct experiments. In 

 experiments made by passing electric currents through the ears, some physi- 



