692 THE SENSE OF HEARING. 



exposing the nerve-fibres to the influence of sonorous 

 undulations by two media, is itself insulated by fluid 011 

 either side. 



The connection of the lamina spiralis with the solid 

 walls of the labyrinth, adapts the cochlea for the percep- 

 tion of the sonorous undulations propagated by the solid 

 parts of the head and the walls of the labyrinth. The 

 membranous labyrinth of the vestibule and semicircular 

 canals is suspended free in the perilymph, and is destined 

 more particularly for the perception of sounds through the 

 medium of that fluid, whether the sonorous undulations 

 be imparted to the fluid through the fenestrse, or by the 

 intervention of the cranial bones, as when sounding bodies : 

 are brought into communication with the head or teeth. 

 The spiral lamina? on which the nervous fibres are ex- 

 panded in the cochlea, is, on the contrary, continuous with 

 the solid walls of the labyrinth, and receives directly from 

 them the impulses which they transmit. This is an 

 important advantage ; for the impulses imparted by solid 

 bodies, have, cateris paribus, a greater absolute intensity 

 than those communicated by water. And, even when a 

 sound is excited in the water, the sonorous undulations 

 are more intense in the water near the surface of the 

 vessel containing it, than in other parts of the water 

 equally distant from the point of origin of the sound : thus 

 we may conclude that, cceteris paribus, the sonorous undula- 

 tions of solid bodies act with greater intensity than those 

 of water. Hence we perceive at once an important use of 

 the cochlea. 



This is not, however, the sole office of the cochlea ; the 

 spiral lamina, as well as the membranous labyrinth, 

 receives sonorous impulses through the medium of the 

 fluid of the labyrinth from the cavity of the vestibule, and 

 from the fenestra rotunda. The lamina spiralis is, indeed, 

 much better calculated to render the action of these undu- 

 lations upon the auditory nerve efficient, than the mem- 



