SENSE OF HEARING. 



267 



Mammalia ; less in Birds ; and in Reptiles, which have little true vocal power, 

 the cochlea is reduced to its lowest form, disappearing entirely in the Amphibia. 

 That there should be an acoustic relation between the voice and ear of each 

 species of animals, cannot be regarded as improbable ; but the speculation of 

 M. Duges can at present only be received as a stimulus to further inquiry. 



360. We have now to consider the functions of the accessory parts, the 

 External Ear, and the Meatus. The Cartilage of the external ear may propa- 



[Fig. 58. 



[Fig. 5U. 



A view of the Left Ear in its natural state ; 1, 2, An anterior view of the External Ear, as well 



the origin and termination of the helix; 3, the as of the Meatus Auditorius, Labyrinth, &c.; l,the 



anti-helix; 4, the anti-tragus; 5, the tragus; 6, the opening into the ear at the bottom of the concha,; 



lobus of the external ear; 7, points to the%capha 2, the meatus auditorius externus or cartilaginous 



and is on the front and top of the pinna; 8, the canal; 3, the membrana tympani stretching upon 



concha; 9, the meatus auditorius externus.] its ring; 4, the malleus; 5, the stapes; 6, the laby- 

 rinth.] 



gate sonorous vibrations in two ways, by reflection and by conduction. In 

 reflection, the concha is the most important part, since it directs the reflected 

 undulations towards the tragus, whence they are thrown into the auditory 

 passage. The other inequalities of the external ear cannot promote hearing 

 by reflection ; and the purpose of the extension of its cartilage is evidently to 

 receive the sonorous vibrations from the air, and to conduct them to its point 

 of attachment. In this point of view, the inequalities become of importance ; 

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

 dicularly, will be affected by them in the most intense degree ; and in conse- 

 quence of the varied form and position of these inequalities, sonorous undula- 

 tions, in whatever direction they may come, must fall advantageously upon 

 some of them. The functions of the Meatus appear to be threefold. The 

 sonorous undulations entering from the atmosphere are propagated directly, 

 without dispersion, to the membrana tympani : the sonorous undulations 

 received on the external ear, are conveyed along the walls of the meatus to the 

 membrana tympani : the air which it contains, like all insulated masses of 

 air, increases the intensity of sounds by resonance. That, in ordinary hearing, 

 the direct transmission of atmospheric vibrations to the membrana tympani, is 

 the principal means of exciting the reciprocal vibrations of the latter, is suffi- 

 ciently evident ; the undulations which directly enter the passage, will pass 

 straight on to the membrane ; whilst those that enter obliquely will be reflected 



