192 SENSORIAL FUNCTIONS. 



PHYSIOLOGY OF AUDITION IN MAN. 



The uses of several parts of the complex apparatus 

 above described, remain unknown. The following, 

 however, appears to be the manner in which hearing is 

 performed. 



The sonorous vibrations being transmitted through 

 the air to the external Ear, are collected by its sinuous 

 canals, 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 vibra- 

 tions of the tympanum are communicated through the 

 chain of bones to the fenistra ovalis, which being a mem- 

 brane 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 audito- 

 ry 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. 



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 disease, 

 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 sea-shore, 

 but who hears with much difficulty even a loud voice in 

 a silent place. 



With regard to the purposes which are answered by 

 the semi-circular canals, and the cocklea, hardly any 

 plausible conjectures can be offered ; yet no doubt can 

 be entertained of the importance of all these parts in 

 audition ; for, we find that when we are able fully to 

 understand 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 

 the use of the external ear 1 What is the use of the tympanum ? How 

 does the tympanum probably act as a muscle 7 How are the vibrations 

 of the ear-drum communicated to vestibule? 



