THE SENSE OF HEARING. 233 



a snail shell, consisting of two spiral canals, which wind 

 round a central pillar twice and a half, and also separated 

 by a spiral partition.! 



Fig. 9. 



Section of the Cochlea. 



14. The stapes then rests upon the membrane which is 

 placed across the opening into the labyrinth. When the 

 tympanum, or drum-head, is impressed by the vibrations of 

 the air, the vibration is communicated to the little bones, 

 and through them to the membrane of the labyrinth, which 

 causes corresponding tremws in the fluid which fills its 

 winding canals, and the spiral passages of the cochlea ; 

 upon which the auditory nerve is spread out like a fine 

 pulpy web, in the same manner as the retina is spread out 

 upon the choroid coat of the eye, or the olfactory nerve 

 upon the mucous membrane of the nose, and thus is sound 

 conveyed by the nerve to the brain. 



15. ffhe object of these spiral passages returning into 

 themselves is, doubtless, to furnish an extensive surface, 

 within a small space, for the expansion of the auditory 

 nerve, and also that the undulations of the fluid which fills 

 them after -passing from the oval fenestra along the winding 



20* 



