THE SENSES. 575 



PHYSIOLOGY OF HEARING. 



All the acoustic contrivances of the organ of hearing are means for 

 conducting sound, just as the optical apparatus of the eye are media for 

 conducting light. Since all matter is capable of propagating sonorous 

 vibrations, the simplest conditions must be sufficient for mere hearing; 

 for all substances surrounding the auditory nerve would communicate 

 sound to it. The whole development of the organ of hearing, therefore, 

 can have for its object merely the rendering more perfect the propaga- 

 tion of the sonorous vibrations, and their multiplication by resonance; 

 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 propagation of sound to the tympanum in three ways: 

 1, by causing the sonorous undulations, entering directly from the atmo- 

 sphere, 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 im- 

 parted 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 membrana tympani against dust, 

 insects, and the like. 



1. As a conductor of undulations of air, the external auditory pas- 

 sage receives the direct undulations of the atmosphere, of which those 

 that enter in the direction of its axis produce the strongest impressions. 

 The undulations which enter the passage obliquely are reflected by its 

 parietes, and thus by reflexion reach the membrana tympani. 



2. The walls of the meatus are also solid conductors of sound; for 

 those vibrations which are communicated to the cartilage of the external 

 ear, and not reflected from it, are propagated by the shortest path through 

 the parietes of the passage to the membrana tympani. Hence, both ears 

 being close stopped, the sound of a pipe is heard more distinctly when 

 its lower extremity, covered with a membrane, is applied to the cartilage 

 of the external ear itself, than when it is placed in contact with the sur- 

 face of the head. 



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. 



Eegarding 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 

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



