CHAPTER IV. 

 HEARING. 



SEC. 1. ON THE GENERAL STRUCTURE OF THE EAR, 

 AND ON THE STRUCTURE AND FUNCTIONS OF THE 

 SUBSIDIARY AUDITORY APPARATUS. 



808. 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 construction 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 arrangement ; we may recognize on the one 

 hand a specially modified epithelium, which we may call the 

 auditory epithelium, so constituted that the vibrations of matter, 

 the rapidly alternating variations of pressure, which we call 

 " waves of sound," generate in the auditory nerve connected with 

 it, auditory impulses, developed in the brain into auditory sensa- 

 tions, and on the other hand an acoustic apparatus so arranged 

 that waves of sound are conducted in a proper manner to the 

 auditory epithelium. Just as visual impulses can be excited by 

 light only through the mediation of the retina, so auditory impulses 

 can be excited by sound only through the mediation of the audi- 

 tory epithelium ; but here the analogy between the optic auditory 

 nerves seems to end, for while as we have seen the optic nerve 

 conveys, so far as we know, visual impulses only, we have reason 

 to think ( 642) 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, 



