CHAPTER XXIX 



AUDITION 



Auditory (eighth nerve) General properties of the auditory nerves Topographical anatomy of 

 the parts essential to the appreciation of sound The external ear General arrangement 

 of parts composing the middle ear General arrangement of the bony labyrinth Physics 

 of sound Pitch of musical sounds Musical scale Quality of musical sounds Har- 

 monics, or overtones Harmony Discords and dissonance Tones by influence 

 Uses of different parts of the middle ear Structure of the membrana tympani Uses of 

 the membrana tympani Mechanism of the ossicles of the ear Physiological anatomy of 

 the internal ear General arrangement of the membranous labyrinth Liquids of the 

 labyrinth Distribution of the nerves in the labyrinth Organ of Corti Uses of different 

 parts of the internal ear Uses of the semicircular canals Uses of the parts contained in 

 the cochlea Centres for audition. 



IMPRESSIONS of sound are conveyed to the brain by special nerves ; 

 but in order that these impressions shall reach these nerves so as to 

 be properly appreciated, a complex accessory apparatus is required, the 

 integrity of which is essential to normal audition. The study of the 

 arrangement and action of these accessory parts is even more important 

 and is far more intricate than the physiology of the auditory nerves. 

 The auditory nerves conduct impressions of sound, as the optic nerves 

 conduct impressions of light; but there is an elaborate arrangement of 

 parts by which the waves are collected, conveyed 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 preparation. 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 



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