410 FUNCTIONS OF THE INTERNAL EAR. 
character, however, there can be no doubt, from its being 
divided, like the cochlea of Mammals and of Man, by a 
membranous partition on which the nerve is spread out. 
520. From the circumstance that in almost every instance 
in which the semicircular canals exist at all, they are three in 
number, and lie in three different directions, corresponding to 
those of the bottom and two adjoining sides of*a cube, it has 
been supposed (and with much probability) that they assist 
in producing the idea of the direction of sounds. It has been 
also supposed that the cochlea is the organ by which we judge 
of the pitch of sounds; and this would seem to be not im- 
probable, especially when we compare the development of the 
cochlea in different animals, with the variety in the pitch of ~ 
the sounds which it is important they should hear distinctly, 
especially the voices of their own kind. The compass of the 
voice (that is, the distance between its highest and its lowest 
tones) is much greater in Mammals than in Birds; as is also 
the length of the cochlea. In Reptiles, which have little true 
vocal power, the cochlea is reduced to its lowest form; and in — 
the Amphibia, it disappears altogether. 
521. That the Vestibule, and the passages proceeding from 
it, constitute even in Man the essential part of the organ of 
hearing, is evident from the fact, that when (as happens not 
unfrequently) the membrana tympani has been destroyed by 
disease, and the chain of bones has been lost, the faculty is 
not by any means abolished, though it is deadened. In this 
state, the vibrations of the air must act~at once upon the 
membrane of the fenestra ovalis, as in the lower animals which 
possess no external or middle ear; instead of striking the — 
membrane of the tympanum, and being transmitted along the — 
chain of bones. 
522. It- has been stated (§ 510) that the sensation of 
hearing is produced by the successive undulations or vibra- — 
tions communicated to the Ear from the sonorous body, either 
by the air, or by a liquid or solid medium. This is the case 
with all continwous sounds or tones ; but single momentary 
sounds, such as those produced by the discharge of a pistol, 
the blow of a hammer, the ticking of a watch, or the beat of 
a clock, make their impression on the ear by a single shock. 
All continuous tones are in fact caused by a succession of — 
such shocks, communicated to the ear with sufficient rapidity — 
