SEC. 3. ON THE DEVELOPMENT OF AUDITORY 



IMPULSES. 



§ 628. We may now turn for a little while to the obscure 

 question, How the vibrations of the perilymph give rise to audi- 

 tory impulses and so to auditory sensations. 



In speaking of the ossicles (§ 615) we gave reasons for 

 thinking that the vibrations of the tympanic membrane are 

 carried onward by the chain of ossicles swinging as a whole, 

 and not conveyed through the chain from molecule to molecule. 

 A similar argument may be applied to the perilymph. The 

 dimensions of the whole labyrinth compared with the length of 

 the waves of sound are so minute that molecular vibrations 

 may be neglected. Moreover the walls of the labyrinth may, 

 as a whole, be regarded as absolutely rigid so that, the peri- 

 lymph being incompressible, each blow given at the fenestra 

 ovalis is transmitted instantaneously through the whole mass 

 of perilymph ; the fluid driven in by the inward thrust of the 

 stapes has to find room for itself elsewhere, and that room is 

 furnished by the outward bulge of the membrane of the fenes- 

 tra rotunda, for we may neglect other means of escape such as 

 the lymph spaces around the endolymphatic duct, the nerves 

 and the blood vessels. Hence at each movement of the stapes 

 the whole mass of the perilymph swings bodily, the membrane 

 of fenestra rotunda moving outwards and inwards at the same 

 instant that the stapes moves inwards and outwards ; and each 

 such mass-vibration of the perilymph repeats the characters of 

 the vibration of the ossicles and tympanic membrane, of which 

 it is the continuation. 



As they sweep over the vestibule, these vibrations are com- 

 municated through the walls of the enclosed membranous laby- 

 rinth to the endolyiuph. The vibrations of the endolymph, or 

 of the walls themselves, affect in some way or other the audi- 

 tory epithelium of the three cristse and the two macular 



The vibrations also travel from the vestibule into the scala 

 vestibuli of the cochlea, ascending the spiral from below up- 

 wards. As they ascend they are transmitted across the mem- 



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