AUDITORY NERVES. 147 



paratus is by no means so complete as it is with regard to the 

 eye, nor do we as yet understand so clearly the physiological 

 relations of many points developed by late anatomical re- 

 searches ; and, for this reason, it does not seem desirable to 

 consider the structure of the ear as fully as we have the 

 anatomy of the eye, restricting ourselves, as we have done, 

 to the physiological anatomy of parts. With this end in 

 view, we shall take up fully the following points : 



1. The physiological anatomy and the general properties 

 of the auditory nerves 



2. The physiological anatomy of the parts essential to 

 the correct appreciation of sound. 



3. The laws of the propagation of sonorous vibrations, as 

 far as they are applicable to audition. 



4. The physiological action of different parts of the au- 

 ditory apparatus. 



Physiological Anatomy of the Auditory Nerves. The 

 auditory nerve constitutes the portio mollis of the seventh 

 pair of Willis. The origin of this 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 preparation. This 

 consists of from five to seven grayish filaments, which decus- 

 sate in the median line, and pass outward, winding from the 

 fourth ventricle around the restif orm body. The deep root 

 consists of numerous 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 of 

 the opposite side. This root passes around the restiform 

 body inward, so that this portion of the medulla is encircled, 

 as it were, 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 mem- 



