SEC. 4. ON THE DEVELOPMENT OF AUDITORY 



IMPULSES. 



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

 question, How the vibrations of the perilymph give rise to auditory 

 impulses and so to auditory sensations. 



In speaking of the ossicles ( 817) 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 Avhole, be regarded as 

 absolutely rigid so that, the perilymph 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 fenestra 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 mem- 

 brane, 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 endolymph. The vibrations of the endolymph, or of 

 the walls themselves, affect in some way or other the auditory 

 epithelium of the three cristae and the two maculae. 



The vibrations also travel from the vestibule into the scala 

 vestibuli of the cochlea, ascending the spiral from below upwards. 

 As they ascend they are transmitted across the membrane of 



