78 



STRUCTURE OF THE HUMAN EAR. 



fled by being conveyed from the tympanic membrane 

 to one which is twenty times smaller. Behind the 



Fig. '(6.— Diagram of human oar (after Bernstein). D, Auditory canal; E, moutli of 

 Eustacliian tabe ; tc, tympanic membvanr ; B, tympunic cavity ; o, fenestra ovalis ; 

 r, fenestra rotunda ^ s, semicircular canals; A, cuchlta. 



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



and on which the final 



Am. 



Am. 



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 begreatly 

 curtailed if it were not for 

 the second membrane, 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 tlie oar. H, Hammer ; 

 Am, anvil; Am. /,■, sliortcr proerj-s of the 

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

 S, stirrup ; Si, long process of the 

 hammer. 



