CHAPTER XVIII. 



SPECIAL SENSES (Continued.) 



THE SENSE OF HEARING. 



BY means of the special sense of hearing the individual gains 

 knowledge of a kind differing from the just-mentioned senses. It 

 does not tell him what is going on in the outer world by actual con- 

 tact, as in touch or taste; nor yet by particles of matter impinging 

 upon the exposed end of nerve-filaments, as in the sense of smell. 

 In the special sense of hearing the impressions conveyed to the cen- 

 tral nervous system are produced by wavelike vibrations in the sur- 

 rounding air. For the reception of these vibrations, so that they 

 may be properly interpreted and the corresponding impressions con- 

 veyed to the brain, it becomes necessary to have a special sense- 

 organ: the ear. 



The Ear. 



The organ of hearing in its greatest simplicity may be repre- 

 sented by a small membrane stretched like a drumhead over the 

 bottom of a funnel-shaped tube. The tube opens upon the surface 

 of the body so that it is in direct communication with the enveloping 

 atmosphere. The membrane is so disposed that it is readily thrown 

 into vibrations when the external air becomes undulatory as the 

 result of vibrations of some body. Its vibrations are communicated 

 to an inner vesicle that is filled with a liquid. The liquid is like- 

 wise thrown into waves whose undulations stimulate the ramifica- 

 tions of the auditory nerve which are spread out upon the walls of 

 the vibrating vesicle. 



Anatomy. The apparatus for hearing is composed of three 

 parts : external ear, middle ear, and internal ear. 



EXTERNAL EAR. The external ear is composed of the auricle 

 and external auditory meatus. 



The auricle has the form of an irregularly shaped shell. It is 

 composed of yellow, elastic cartilage which is covered over with skin. 

 From its shape one might readily believe that the function of the 

 auricle is to collect and reflect sound-waves into the auricle : that is, 

 to behave in the capacity of an ear-trumpet. But it is found that 



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