684 THE SENSE OF HEARING. 



dicularly upon the tangent of some one of them, This 

 affords an explanation of the extraordinary form given to 

 this part. 



Functions of tlw Middle Ear ; the Tympanum, Ossicula, and 

 Fenestrce. 



In animals living in the atmosphere, the sonorous vibra- 

 tions are conveyed to the auditory nerve by three different 

 media in succession ; namely, the air, the solid parts of the 

 body of the animal and of the auditory apparatus, and 

 the fluid of the labyrinth. 



Sonorous vibrations are imparted too imperfectly from 

 air to solid bodies, for the propagation of sound to the 

 internal ear to be adequately effected by that means alone ; 

 yet already an instance of its being thus propagated has 

 been mentioned. 



In passing from air directly into water, sonorous vibra- 

 tions suffer also a considerable diminution of their strength ; 

 but if a tense membrane exists between the air and the 

 water, the sonorous vibrations are communicated from the 

 former to the latter medium with very great intensity. 

 This fact, of which Miiller gives experimental proof, 

 furnishes at once an explanation of the use of the fenestra 

 rotunda, and of the membrane closing it. They are the 

 means of communicating, in full intensity, the vibrations 

 of the air in the tympanum to the fluid of the labyrinth. 

 This peculiar property of membranes is the result, not of 

 their tenuity alone, but of the elasticity and capability of 

 displacement of their particles ; and it is not impaired 

 when, like the membrane of the fenestra rotunda, they are 

 not impregnated with moisture. 



Sonorous vibrations are also communicated without any 

 perceptible loss of intensity from the air to the water, 

 when to the membrane forming the medium of communi- 

 cation, there is attached a short, solid body, which occupies 

 the greater part of its surface, and is alone in contact 



