HEARING. 507 



the endolymph ; and the terminal filaments of the auditory nerve 

 are distributed upon the membranous sac of the vestibule and upon 

 the ampullae, or membranous dilatations, at the commencement of 

 the three semicircular canals. The remaining portion of the audi- 

 tory nerve is distributed upon the septum between the two spiral 

 canals of the cochlea. 



Thus, the essential or fundamental portion of the auditory appa- 

 ratus is evidently the internal ear, a cavity, partly membranous and 

 partly bony, in which is distributed a nerve of special sense, the 

 auditory nerve, capable of appreciating sonorous impressions. The 

 accessory parts, on the other hand, are the chain of bones and the 

 membrane of the tympanum, which communicate the sonorous 

 vibrations directly to the internal ear ; and the meatus and external 

 ear, which collect them from the atmosphere. The reception of 



Fig. 165. 



HTM AX AUDITORY APPARATUS, showing external auditory meat us, tympanum, and laby- 

 rinth. 



sonorous impulses is therefore accomplished in a very indirect way. 

 For the sonorous body first communicates its vibrations to the 

 atmosphere. By the atmosphere these vibrations are communicated 

 to the membrana tympani. From the membrana tympani, they are 

 transmitted, through the chain of bones, to the membrane of the 

 foramen ovale ; thence to the perilymph, or fluid of the labyrinthic 

 cavity, and from the perilymph to the membranous parts of the 

 labyrinth and the nerves which are distributed upon them. 



The arrangement of the different parts composing the tympanum 

 is of the greatest importance for the perfect enjoyment of the sense 



