18S SENSOR1AL FUNCTIONS. 



PHYSIOLOGY OF AUDITION IN MAN. 



581. The uses of several parts oi the complex appara- 

 tus above described, remain unknown The following, 

 however, appears to be the manner in which hearing is 

 performed. 



582. The sonorous vibrations, being transmitted through 

 the air to the external ear, are collected by its sinuous ca- 

 nals, and directed through the auditory orifice to the ear- 

 drum, which is thereby made to vibrate. The action of 

 the tympanum as a muscle, is undoubtedly concerned, 

 in this effect, probably becoming more or less tense as the 

 sound is less or more audible. The vibrations of the tym- 

 panum are communicated through the chain of bones to 

 the fenestra ovalis, which being a membrane covering a 

 part of the labyrinth, the motion is communicated to the 

 fluid which the labyrinth contains. The undulations of 

 the fluid thus excited, produce auditory impressions on 

 the nerves of the ear, which are spread over the inside 

 of the membrane lining the labyrinth, and by them are 

 conveyed to the brain, thus giving the sensation called 

 sound. 



583. The tympanum undoubtedly becomes more tense, 

 by the stimulus of sound, and hence in some persons 

 where this part is naturally lax, or has become so by dis- 

 ease, there is a difficulty of hearing low sounds, except 

 when the drum is excited by louder ones. Thus we know 

 a person who can distinguish ordinary conversation when 

 walking in the sound of the surf on the seashore, but who 

 hears with much difficulty even a loud voice, in a silent 

 place. 



584. With regard to the purposes which are answered 

 by the semi-circular canals, and the cochlea, hardly any 

 plausible conjectures can be offered; yet no doubt can 

 be entertained of the importance of all these parts in au- 

 dition ; for, we find that when w T e are able fully to un- 

 derstand the uses of any piece of natural mechanism, 

 every part in one way or another, serves to make the 



Are the uses of all the parts of the internal ear understood ? What is 

 .he use of the external ear ? What is the use of the tympanum ? How 

 does the tympanum probably act as a muscle ? How are the vibrations of 

 the ear-drum communicated to the vestibule? 



