542 OF SENSATION. 



and to transmit them to that membrane, in such a manner that the 

 vibrations thus excited in the latter may be much more powerful, than 

 they would be if the air acted immediately upon it, as in the lower 

 Vertebrata. The usual condition of the Membrana Tympani appears 

 to be rather lax ; and, when in this condition, it vibrates in accordance 

 with grave or deep tones. By the action of the tensor tympani it may 

 be tightened, so as to vibrate in accordance with sharper or higher 

 tones ; but it will then be less able to receive the impressions of deeper 

 sounds. This state we may easily induce artificially, by holding the 

 breath, and forcing air from the throat into the Eustachian tube, so as 

 to make the membrane bulge out by pressure from within ; or by ex- 

 hausting the cavity by an effort at inspiration, with the mouth and 

 nostrils closed, which will cause the membrane to be pressed inwards 

 by the external air. In either case, the hearing is immediately found 

 to be imperfect ; but the deficiency relates only to grave sounds, acute 

 ones being heard even more plainly than before. There is a different 

 limit to the acuteness of the sounds, of which the ear can naturally 

 take cognizance, in different persons. If the sound be so high in pitch, 

 that the membrana tympani cannot vibrate in unison with it, the 

 individual will not hear it, although it may be loud ; and it has been 

 noticed, that certain individuals cannot hear the very shrill tones pro- 

 duced by particular Insects, or even Birds, which are distinctly audible 

 to others. 



952. Not only do we find the tympanic apparatus superadded, in 

 the higher forms of the organ of Hearing, but also the Semicircular 

 Canals, and the Cochlea. The former exist in all Vertebrata, save the 

 lowest Wishes ; and in nearly every case, they are three in number, 

 and lie in three different planes. Hence it has been supposed, with 

 some probability, that they assist in producing the idea of the direction 

 of sounds. The Cochlea does not exist at all in Fishes ; and in Reptiles 

 its condition is quite rudimentary. In Birds, this cavity is more com- 

 pletely formed, though the passage is nearly straight instead of spiral ; 

 of its real character, however, there can be no doubt, from its being 

 divided, like the cochlea of Man, by a membranous partition, on 

 which the ramifications of the auditory nerve are spread out. This 

 appendage has been supposed to be the organ that enables us to judge 

 of the pitch of sounds ; an idea which derives some confirmation from 

 the correspondence between the development of the cochlea in different 

 animals, and the variety in the pitch (or length of the scale) of the 

 sounds which it is important that they should hear distinctly, espe- 

 cially the voices of their own kind. That the Vestibule, with the 

 passages proceeding from it, constitutes the true organ of hearing, even 

 in Man, is evident from the fact, that when (as not unfrequently 

 happens) the tympanic apparatus has been entirely destroyed by 

 disease, so as to reduce the organ to the condition of that in which no 

 such apparatus exists, the faculty of Hearing is by no means abolished, 

 although it is deadened. 



953. The faculty of Hearing, like other senses, may be very much 

 increased in acuteness by cultivation ; but this improvement depends 

 rather upon the habit of attention to the faintest impressions made 



