THE SENSES. 685 



to the tympanum in three ways: 1, by causing the sonorous undulations, 

 entering directly from the atmosphere, to be transmitted by the air in 

 the passage immediately to the membrana tympani, and thus preventing 

 them from being dispersed ; 2, by the walls of the passage conducting 

 the sonorous undulations imparted to the external ear itself, by the 

 shortest path to the attachment of the membrana tympani, and so to this 

 membrane; 3, by the resonance of the column of air contained within the 

 passage ; 4, the external ear, especially when the tragus is provided with 

 hairs, is also, doubtless, of service in protecting the meatus and mem- 

 brana tympani against dust, insects, and the like. 



Regarding the cartilage of the external ear, therefore, as a conductor 

 of sonorous vibrations, all its inequalities, elevations, and depressions, 

 become of evident importance ; for those elevations and depressions upon 

 which the undulations fall perpendicularly, will be affected by them in 

 the most intense degree; and, in consequence of -the various form and 

 position of these inequalities, sonorous undulations, in whatever direc- 

 tion 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. 



In animals living in the atmosphere, the sonorous vibrations are con- 

 veyed 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 propaga- 

 tion 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 n 

 tense membrane exists between the air and the water, the sonorous vi- 

 brations are communicated from the former to the latter medium witli 

 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 communicat- 

 ing, 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 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 



