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 continuous 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 



