542 THE SENSE OF HEARING. 



these membranes and the nerves vibratory impulses of greater 

 intensity than the fluid of the labyrinth can impart. This ap- 

 pears to be the office of the otoconia. Sonorous undulations 

 in water are not perceived by the hand itself immersed in the 

 water, but are felt distinctly through the medium of a rod held 

 in the hand. The fine hair-like prolongations from the epi- 

 thelial cells of the ampullae have, probably, the same function. 



The cochlea seems to be constructed for the spreading out of 

 the nerve fibres over a wide extent of surface, upon a solid 

 lamina which communicates with the solid walls of the laby- 

 rinth and cranium, at the same time that it is in contact with 

 the fluid of the labyrinth, and which, besides exposing the 

 nerve-fibres to the influence of sonorous undulations by two 

 media, is itself insulated by fluid on either side. 



The connection of the lamina spiralis with the solid walls of 

 the labyrinth, adapts the cochlea for the perception 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 per- 

 ception 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 

 expanded 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 advan- 

 tage; for the impulses imparted by solid bodies, have, cceteris 

 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 undu- 

 lations 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 sonor- 

 ous 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 undulations upon the auditory nerve effi- 

 cient, than the membranous labyrinth is ; for, as a solid body 

 insulated by a different medium, it is capable of resonance. 



