

THE SENSES. 187 



3. The external auditory passage is important, inasmuch as the air 

 which it contains, like all insulated masses of air, increases the intensity 

 of sounds by resonance. 



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

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

 which are useless with regard to reflexion, become of evident importance; 

 for those elevations and depressions upon which the undulations fall per- 

 pendicularly, will be affected by them in the most intense degree; and, 

 in consequence of the various forms and positions of these inequalities, 

 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 atmos- 

 phere, 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 laby- 

 rinth. 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 sono- 

 rous 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 f enestra 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 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 fenestrae 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 mem- 

 brane, 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 



