CHAPTEK VII. 



ATJDITOKY NEKVES TOPOGRAPHICAL ANATOMY OF THE EAR. 



Physiological anatomy of the auditory nerves General properties of the audi- 

 tory nerves Effects of galvanic currents passed through the ear Topo- 

 graphical anatomy of the parts essential to the appreciation of sound The , 

 external ear General arrangement of the parts composing the middle ear 

 Anatomy of the tympanum Arrangement of the ossicles of the ear 

 Muscles of the middle ear Mastoid cells Eustachian tube Muscles of 

 the Eustachian tube Mucous membrane of the middle ear and of the Eu- 

 stachian tube General arrangement of the bony labyrinth. 



THE general considerations introductory to the study of 

 vision are equally applicable to the physiology of hearing. 

 The impressions of sound are conveyed to the brain by spe- 

 cial nerves ; but, in order that these impressions shall reach 

 these nerves so as to be properly appreciated, a complex ac- 

 cessory apparatus is required, the integrity of which is essen- 

 tial to perfect audition. The study of the arrangement and 

 action of these accessory parts is even more important and is 

 far more intricate than the study of the auditory nerves. 

 The latter simply convey the impressions to the brain, by a 

 mechanism analogous to that of general nervous conduction, 

 the essential character of which is not fully understood. The 

 auditory nerves conduct impressions of sound, as the optic 

 nerves conduct impressions of light ; and this statement ex- 

 presses the extent of our positive knowledge ; but there is an 

 elaborate apparatus by which the waves are collected, con- 

 veyed to a membrane capable of vibration, and finally car- 

 ried to the nerves, by which we are enabled to appreciate 

 the intensity and the varied qualities of sound. 



Our positive and definite knowledge of the auditory ap- 



