192 HAND-BOOK OF PHYSIOLOGY. 



sense, possibly by the constant alterations of the pressure of the fluid 

 within them; the change in the pressure of the fluid in each canal 

 which takes place on any movement of the head, producing sensations 

 which aid in forming an exact judgment of the alteration of position 

 which has occurred. 



Functions of the Cochlea. 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 

 labyrinth 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 laby- 

 rinth, 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 

 perception of sounds through the medium of that fluid, whether the sono- 

 rous undulations be imparted to the fluid through the fenestrae, 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, con- 

 tinuous 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, cceteris par-ibus, 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 undulations 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 undulations upon the audi- 

 tory nerve efficient, than the membranous labyrinth is; for as a solid body 

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



The rods of Corti are probably arranged so that each is set to vibrate 

 in unison with a particular tone, and thus strike a particular note, the 

 sensation of which is carried to the brain by those filaments of the audi- 

 tory nerve with which the little vibrating rod is connected. The distinc- 

 tive function, therefore, of these minute bodies is, probably, to render 



