432 THE SENSORIAL FUNCTIONS. 



Great uncertainty prevails with regard to the 

 real functions performed by the several parts of 

 this very complex apparatus. It is most pro- 

 bable, however, that the sonorous vibrations of 

 the air which reach the external ear, are directed 

 down the meatus, and striking against the ear- 

 drum which closes the passage, throw that mem- 

 brane into vibrations of the same frequency ; to 

 which the action of its muscles, which appear in- 

 tended to regulate its tension, may also contribute. 

 The vibrations of the ear-drum, no doubt, excite 

 corresponding motions in the air contained in 

 the cavity of the tympanum ; which, again, com- 

 municates them to the membrane of the fenestra 

 rotunda ; while, on the other hand, the mem- 

 brane closing the fenestra ovalis, receives similar 

 impressions from the stapes, conveyed through 

 the chain of tympanic ossicula, which appear to 

 serve as solid conductors of the same vibrations. 

 Thus the perilymph, or fluid contained in the 

 labyrinth, is affected by each external sound, both 

 through the medium of the air in the tympanum, 

 and by means of the ossicula : the undulations 

 thus excited produce impressions on the extre- 

 mities of the nervous filaments, which are spread 

 over the membranous labyrinth ; and these im- 

 pressions being conveyed to the brain, are imme- 

 diately followed by the sensation of sound. 



With regard to the purposes which are an- 

 swered by the winding passages of the semi- 



