576 HANDBOOK OF PHYSIOLOGY. 



and, in consequence of the various form and position of these inequali- 

 ties, sonorous undulations, in whatever direction they may come, must 

 fall perpendicularly upon the tangent of some one of them. This affords 

 an explanation of the extraordinary form given to this part. 



Functions of the Middle Ear. In animals living in the atmo- 

 sphere, the sonorous vibrations 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 vibrations 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 impreg- 

 nated with moisture. 



Sonorous vibrations are also communicated without any perceptible 

 loss of intensity from the air to the water, when to the membrane form- 

 ing the medium of communication, there is attached a short, solid body, 

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

 with the water. This fact elucidates the action of the fenestra ovalis, 

 and of the plate of the stapes which occupies it, and, with the preceding 

 fact, shows that both fenestrse that closed by membrane only, and that 

 with which the movable stapes is connected transmit very freely the 

 sonorous vibrations from the air to the fluid of the labyrinth. 



A small, solid body, fixed in an opening by means of a border of 

 membrane, so as to be movable, communicates sonorous vibrations from 

 air on the one side, to water, or the fluid of the labyrinth, on the other side, 

 much better than solid media not so constructed. But the propagation 

 of sound to the fluid is rendered much more perfect if the solid con- 

 ductor thus occupying the opening, or fenestra ovalis, is by its other end 

 fixed to the middle of a tense membrane, which has atmospheric air on 

 both sides. A tense membrane is a much better conductor of the vibra- 

 tions of air than any other solid body bounded by definite surfaces: and 

 the vibrations are also communicated very readily by tense membranes to 

 solid bodies in contact with them. Thus, then, the menibrana tympani 



