HEARING. 387 



contained in the labyrinth, is affected by each ex- 

 ternal 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 extremities of the nervous filaments, which are 

 spread over the membranous labyrinth ; and these 

 impressions being conveyed to the brain, are imme- 

 diately followed by the sensation of sound. 



With regard to the precise purposes which are 

 answered by the winding passages of the semi- 

 circular canals, and cochlea, hardly any plausible 

 conjecture has been offered ; yet no doubt can be 

 entertained that the uses of all these parts are of 

 considerable importance, both as to delicacy and 

 correctness of hearing. There is an obvious cor- 

 respondence between the positions of the three 

 semicircular canals, (two of which are vertical and 

 one horizontal, and of which the planes are reci- 

 procally perpendicular to one another,) and the 

 three dimensions by which the geometrical relations 

 of space are estimated; and it might hence be con- 

 jectured that the object of this arrangement is to 

 allow of the transmission of vibrations of every kind, 

 in whatever direction they may arrive ; for as the 

 vibrations thus transmitted will produce the strongest 

 impression on different parts of the canals according 

 to their direction, the corresponding differences in 

 the sensations produced may indicate the position 

 of the external source of the sound.* It is not an 



* Autenrieth and Kerner are of opinion that the semicircular canals 

 contribute to the capacity of judging of the direction of sounds : and 

 they remark that the corresponding canals on the right and left sides 

 are not parallel to one another, and consequently do not receive 

 similar or equal impulses from the same sound : an inequality which 

 may lead to the discrimination of its direction. They also state that 



