680 THE SENSE OF HEARING. 



All the acoustic contrivances of the organ of hearing- are 

 means for conducting the sound, just as the optical appa- 

 ratus of the e}*e are a media for conducting the light. Since 

 all matter is capable of propagating sonorous vibrations, 

 the simplest conditions must be sufficient for mere hear- 

 ing; for all substances surrounding the auditory nerve 

 would communicate sound to it. In the eye a certain 

 construction was required for directing the rays or undula- 

 tions of light in such a manner that they should fall upon 

 the optic nerve with the same relative disposition as that 

 with which they issued from the object. In the sense of 

 hearing this is not requisite. Sonorous vibrations, having 

 the most various direction and the most unequal rate of 

 succession, are transmitted by all media without modifica- 

 tion, however manifold their decussations ; and, wherever 

 these vibrations or undulations fall upon the organ of 

 hearing and the auditory nerves, they must cause the 

 sensation of corresponding sounds. The whole develop- 

 ment of the organ of hearing, therefore, can have for its- 

 object merely the rendering more perfect the propagation of 

 the sonorous vibrations, and their multiplication by re- 

 sonance ; and, in fact, all the acoustic apparatus of the organ 

 may be shown to have reference to these two principles. 



Functions of the External Ear. 



The external auditory passage influences the propaga- 

 tion of sound to the tympanum in three ways : I, 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 pre- 

 venting 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 attach- 

 ment of the membrana tympani, and so to this membrane ; 

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

 the passage. 



