CHAPTER IV. 



HEARING. 



SEC. 1. ON THE GENEEAL STRUCTUEE OF THE EAE, 

 AND OX THE STRUCTUEE AND FUNCTIONS OF THE 

 SURS1DIAEY AUDITOEY AFPAEATUS. 



§ 610. We have seen that the eye consists on the one hand 

 of the special modified epithelium, the retina, so constituted that 

 light falling upon it gives rise to visual impulses in the optic 

 nerve and thus to visual sensations in the brain, and on the 

 other hand of a special dioptric mechanism, into the construc- 

 tion of which several tissues enter and which is so arranged as 

 to cause the rays of light to fall in a proper manner on the 

 retina. In the ear we meet with a somewhat similar arrange- 

 ment ; we may recognize on the one hand a specially modified 

 epithelium, which we may call the auditory epithelium, so con- 

 stituted that the vibrations of matter, the rapidly alternating 

 variations of pressure, which Ave call " waves of sound," gener- 

 ate in the auditory nerve connected with it, auditory impulses, 

 developed in the brain into auditory sensations, and on the other 

 hand an acoustic apparatus so arranged that waves of sound are 

 conducted in a proper manner to the auditory epithelium. But 

 while, as we have seen, the optic nerve conveys, so far as we 

 know, visual impulses only, we have reason to think (§ 478) 

 that some fibres at least of the auditory nerve convey impulses 

 which do not give rise to auditory sensations, but enter in a 

 peculiar manner into the mechanism of coordinated movements. 



The retina as we have seen is developed out of the optic 

 vesicle, and the subsidiary dioptric mechanism is built up around 

 the optic vesicle ; and in a somewhat similar way the auditory 

 epithelium is developed into an otic vesicle, and the subsidiary 

 acoustic apparatus is built up around the otic vesicle. The 

 otic vesicle, like the optic vesicle, is lined by an epithelium 

 of epiblastic origin, but is not like that vessel budded off from 

 the medullary canal. It takes origin in an involution of the 



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