HEARING. 433 



circular canals, and cochlea, hardly any plaus- 

 ible conjecture has been offered ; yet no doubt 

 can be entertained that the uses of all these parts 

 are of considerable importance, both as to deli- 

 cacy and correctness of hearing. There is an 

 obvious correspondence between 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 ano- 

 ther,) and the three dimensions by which the geo- 

 metrical relations of space are estimated ; and it 

 might hence be conjectured that the object of 

 this arrangement is to allow of the transmission 

 of vibrations of every kind, in whatever direction 

 they may arrive. It is not an improbable sup- 

 position that the return into the vestibule, of 

 undulations which have passed through these 

 canals, has the effect of at once putting a 

 stop to all further motion of the fluid, and pre- 

 venting 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 the membrane 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 produce but a single mo- 



VOL. II, F F 



