78 



STRUCTURE OF THE HUMAN EAR. 



fied by being conveyed from the tympanic membrane 

 to one which is twenty times smaller. Behind the 



Fig. 46, —Diagram of human ear (after Berusteiu). I), Auditory canal; E, mouth of 

 Eustachian tube ; cc, tympanic membrane ; B, tympanic cavity ; o, fenestra ovalis ; 

 r, fenestra rotunda ; s, semicircular canals ; A, cochlea. 



fenestra ovalis is the labyrinth, which is filled with fluid, 



and on which the final 

 filaments of the auditory 

 nerve are distributed. 

 This fluid is thrown into 

 vibrations by those of the 

 stirrup, but as it is en- 

 closed in a bony case, the 

 vibrations would be greatly 

 curtailed if it were not for 

 the second meaibrane, or 

 fenestra rotunda. This 

 round membrane, there- 

 fore, acts as a counter 

 opening, for if the fluid is 

 compressed in one place, it must claim more room in 

 another. The labyrinth consists mainly of two parts, 



Fig. 47.— Ossicles of the ear. H, Hammer ; 

 Am, anvil ; Am. k, shorter process of the 

 anvil; Am. I, longer process of the anvil ; 

 S, stirrup ; St, long process of the 

 hammer. 



