FUNCTIONS OF THE EXTERNAL EAR. 535 



As a conductor of undulations of air, the external 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 undulations which enter the pas- 

 sage obliquely are reflected by its parietes, and thus by reflec- 

 tion reach the membrana tympani. By reflection, also, the 

 external meatus receives the undulations which impinge upon 

 the concha of the external ear, when their angle of reflection 

 is such that they are thrown towards the tragus. Other sonor- 

 ous undulations again, which could not enter the meatus from 

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

 brought into it by inflection ; 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 accordance with the laws 

 of inflection, be bent into the external meatus by its margins. 

 But the action of those undulations which enter the meatus 

 directly are most intense ; and hence we are enabled to judge 

 of the point whence sound comes, by turning one ear in differ- 

 ent directions, till it is directed to the point whence the vibra- 

 tions may pass directly into the meatus, and produce the strong- 

 est 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 carti- 

 lage of the external ear itself, than when it is placed in con- 

 tact with the surface of the head. 



Lastly, 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. To convince 

 ourselves of this, we need only lengthen the passage by affix- 

 ing to it another tube : every sound that is heard, even the 

 sound of our own voice, is then much increased in intensity. 



The action of the cartilage of the external ear upon sonor- 

 ous vibrations is partly to reflect them, and partly to condense 

 and conduct them to the parietes of the external passage. 

 With respect to its reflecting action, the concha is the most 

 important part, since it directs the reflected undulations to- 

 wards the tragus, whence they are reflected into the auditory 

 passage. The other inequalities of the external ear do not 

 promote hearing by reflection ; and, if the conducting power 

 of the cartilage of the ear were left out of consideration, they 

 might be regarded as destined for no particular use ; but re- 



