682 THE SENSE OF HEARING. 



auditory passage receives the direct undulations of the 

 atmosphere, of which those that enter in the direction of 

 its axis produce the strongest impressions. The undula- 

 tions which enter the passage obliquely are reflected by 

 its parieties, and thus by reflection reach the membrana 

 tympani. By reflexion, also, the external meatus receives 

 the undulations which impinge upon the concha of the 

 external ear, when their angle of reflexion is such that 

 they are thrown towards the tragus. Other sonorous 

 undulations, again, which could not enter the meatus from 

 the external air either directly or by reflexion, may still be 

 brought into it by inflexion ; undulations, for instance, 

 whose direction is that of the long axis of the head, and 

 which pass over the surface of the ear, must, in accord- 

 ance with the laws of inflexion, be bent into the external 

 meatus by its margins. But the action of those undula- 

 tions which enter the meatus directly are most intense ; and 

 hence we are enabled to judge of the point whence sound 

 conies, by turning one ear in different directions, till it is 

 directed to the point whence the vibrations may pass directly 

 into the meatus, and produce the strongest impressions. 



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 surface of 

 the head. 



Lastly, the external auditory passage is important, inas- 

 much as the air which it contains, like all insulated masses 

 of air, increases the intensity of sounds by resonance. 

 To convince ourselves of this, we need only lengthen the 

 passage by affixing to it another tube : every sound that is 





