SENSE OF HEARING. 



435 



pagatc 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- 



[Fig. 193. 



[Fig. 194. 



A view of the Left Ear in its natural state ; 1. 2, 

 the origin and termination of the helix ; 3, the anti- 

 helix ; 4, the anti-tragus ; 5, the tragus; G, the Jobus 

 of the external ear; 7, points to the scapha and is on 

 the front and top of the pinna ; 8, the concha; 9, the 

 meatus auditorius externus.] 



An anterior view of the External Ear, as well 

 as of the Meatus Auditorius, Labyrinth, &c. ; 1, 

 the opening into the ear at the bottom of the con- 

 cha ; 2, the meatus auditorius externus or car- 

 tilaginous canal ; 3, the membrana tympani 

 stretching upon its ring ; 4, the malleus ; 5, the 

 stapes; 6. the labyrinth.] 



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 sufficiently evident ; the undulations which directly enter the passage, 

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

 be reflected from side to side, and at last will fall obliquely on the membrane, 

 thus perhaps contributing to the notion of direction. The power of the lining 

 of the meatus to conduct sound from the external ear, is made evident by the 

 fact, that, when both ears are closely stopped, the sound of a pipe having its 

 lower extremity covered by a membrane, is heard more distinctly, when it is 

 applied to the cartilage of the external ear itself, than when it is placed in con- 

 tact with the surface of the head. The resonant action of the air in the tube 

 is easily demonstrated, by lengthening the passage by the introduction of 



