HEARING. 307 



duce impressions on the extremities of the nervous filaments, 

 which are spread over the membranous labyrinth; and these 

 impressions being conveyed to the brain, are immediately 

 followed by the sensation of sound. 



With regard to the purposes which are answered by the 

 winding passages of the semicircular canals, and cochlea, 

 hardly any plausible conjecture has been oflered; yet no 

 doubt can be entertained that the uses of all these parts are 

 of considerable importance, both as to delicacy and correct- 

 -ness of hearing. There is an obvious correspondence be- 

 tween the positions of the three semicircular canals, (two of 

 which are vertical, and one horizontal, and of which the 

 planes are reciprocally perpendicular to one another,) and the 

 three dimensions by which the geometrical relations of space 

 are estimated; and it might hence be conjectured that the ob- 

 ject of this arrangement is to allow of the transmission of vi- 

 brations of every kind, in whatever direction they may ar- 

 rive. It is not an improbable supposition tliat tiie return 

 into the vestibule, of undulations which have passed through 

 these canals, has the effect of at once putting a stop to all 

 farther motion of the fluid, and preventing the continuance 

 of the impression which has been already made on the 

 nerves. The same use may be assigned to the double spiral 

 convolutions of the tubes of the cochlea: for the undulations 

 of the fluid in the tympanic tube, received from tlie mem- 

 brane of the fenestra rotunda, will meet those proceeding 

 along the vestibular tube, derived from the membrane of the 

 fenestra ovalis, and like two opposing waves, will tend to 

 destroy one another. Thus each external sound will pro- 

 duce but a single momentary impression; the prolongation 

 of the undulations of the fluid of the labyrinth being pre- 

 vented by their mutual collision and neutralization.*' 



* The preliminary steps in the process above described are not absolutely 

 essential to hearing-, for many instances have occuiTed in which the power of 

 bearing has been perfectly retained after the membrane of the ear-drum, and 

 also the ossicula had been destroyed by disease. A small aperture in the 

 membrane does not interfere with its power of vibration; but if the whole 



