SENSE OF HEARING. 263 



In addition to this, we find in many Fishes a communication with the air- 

 bladder ; which, indeed, seems to have in these but little other use. The 

 mode in which this increases by resonance the intensity of the sounds, will 

 appear from the following experimental conclusions. vn. When sonorous 

 vibrations are communicated from water to air enclosed in membranes or solid 

 bodies, a considerable increase in the intensity of the sound is produced, by 

 the resonance of the air thus circumscribed. vm. A body of air enclosed in 

 a membrane, and surrounded by water, also increases the intensity of the 

 sound by resonance, when the sonorous undulations are communicated to it by 

 a solid body. From these observations it .may be concluded, that the air- 

 bladder of Fishes, in addition to other uses, serves the purpose of increasing 

 by resonance the intensity of the sonorous undulations, communicated from 

 the water to the body of the Fish. Moreover, as the conducting and resonant 

 power of the air in the air-bladder is greater in proportion to its density, the 

 influence of this organ on the perception of sounds will, of course, be greater 

 in deep waters, where the pressure upon it is considerably increased. 



355. Most animals living in air, are provided with an opening into the ves- 

 tibule, covered by a thin membrane ; and, in the majority of cases, with the 

 tympanic apparatus also. The following experimental results bear upon the 

 manner in which the Ear of such animals is affected by sound. ix. Sonorous 

 undulations, in passing from air directly into water, suffer a considerable dimi- 

 nution in their strength; while, on the contrary, if a tense membrane exists 

 between the air and the water, the sonorous undulations are communicated 

 from the former to the latter medium with great intensity. x. The sonorous 

 vibrations are also communicated, without any perceptible loss of intensity, 

 from the air to the water; when, to the membrane forming the medium of 

 communication, there is attached a short solid body, which occupies the greater 

 part of its surface, and is alone in contact with the water. xi. A small solid 

 body, fixed in an opening by means of a border of membrane, so as to be 

 movable, communicates sonorous vibrations from air on one side to water or 

 the fluid of the labyrinth on the other, much better than solid media not so 

 constructed. But the propagation of sound to the fluid is rendered much 

 more perfect, if the solid conductor, thus occupying the opening, is by its other 

 end fixed to the middle of the tense membrane, which has atmospheric air on 

 both sides. The fact stated in ix. is evidently one of great importance in the 

 physiology of hearing ; . and fully explains the nature of the process in those 

 animals which receive the sonorous vibrations through air, but which have no 

 tympanic apparatus. In x. we have the elucidation of the action of the 

 fenestra ovalis, and of the movable plate of the stapes which occupies it, in 

 animals living in air but destitute of tympanic apparatus ; this is naturally the 

 case in many Amphibia ; and it may happen as the result of disease in the 

 human subject. In xi. we have a very interesting demonstration of the pur- 

 pose and action of the tympanum, in the more perfect forms of the auditory 

 apparatus. We are now prepared to inquire, in somewhat more of detail, into 

 the action of the different parts of this apparatus; and it will be better to com- 

 mence with that of the Internal Ear, the accessory organs being afterwards 

 considered. 



356. The object of the Membrana Tympani is evidently to receive the 

 sonorous undulations from the air, in such a manner as to be thrown by them 

 into a reciprocal vibration, which is to be communicated to the chain of bones. 

 This membrane is, in its usual state, rather lax than tense ; and this laxity is 

 found by experiment to be, for a small membrane, the best condition for the 

 propagation of ordinary sounds. This is easily rendered sensible in one's 

 own person ; for an increased tension may be given to the membrana tym- 

 pani, either by holding the breath and forcing air into the Eustachian tube, 



