CHAPTER III. 

 HEARING, SMELL, AND TASTE. 



SEC. 1. HEAEING. 



As in the eye, so in the ear, we have to deal first with a nerve of 

 special sense, the stimulation of which gives rise to a special sensa- 

 tion; secondly with terminal organs through which the physical 

 changes proper to the special sense are enabled to act on the nerve ; 

 and thirdly with subsidiary apparatus, by which the usefulness of 

 the sense is increased. The central connections of the auditory 

 nerve are such that whenever the auditory fibres are stimulated, 

 whether by means of the terminal organs in the usual way or by 

 the direct application of stimuli, electrical, mechanical, &c., the 

 result is always a sensation of sound. Just as stimulation of the 

 optic fibres produces no other sensation than that of light, so 

 stimulation of the auditory fibres produces no other sensation than 

 that of sound 1 . The terminal organs of the auditory nerve are of 

 two kinds : the complicated organ of Corti in the cochlea, and the 

 epithelial arrangements of the maculae and cristse acusticse in other 

 parts of the labyrinth. Waves of sound falling on the auditory 

 nerve itself produce no effect whatever; it is only when by the 

 medium of the endolymph they are brought to bear on the delicate 

 and peculiar epithelium cells which constitute the peripheral ter- 

 minations of the nerve, that sensations of sound arise. Such 

 delicate structures are for the sake of protection naturally with- 

 drawn from the surface of the body where they would be subject to 

 injury. Hence the necessity of an acoustic apparatus, forming the 

 middle and external ear, by which the waves of sound are most 

 advantageously conveyed to the terminal organs. 



1 It will be seen later on that there are reasons for thinking that impulses 

 passing along the auditory nerve may give rise to other effects than auditory 

 sensations. 



