OP SENSATION, AND THE ORGANS OF THE SENSES. 



V. Sonorous undulations, propagated through water, are partially reflected by 

 the surfaces of solid bodies. vi. Thin membranes conduct sound in water 

 without any loss of its intensity, whether they be tense or lax. From in., 

 IV., and vi., we learn the mode in which the sound is conducted to the ear, in 

 aquatic animals not breathing atmospheric air. The labyrinth of such is 

 either entirely inclosed within the bones of the head, as in the Cephalopoda, and 

 in the Cyclostome and Osseous Fishes ; or, its cavity being prolonged to the 

 surface of the body, it is there brought into communication with the conducting 

 medium, by means of a membrane, besides receiving the vibrations through 

 the 'medium of the solids of the body, as is the case in Cartilaginous Fishes 

 and Crustacea. It would seem as if, in the Osseous Fishes, the resonance of 

 the cranial bones, in which the labyrinth is imbedded, were sufficient to give the 

 requisite increase of intensity to the sound ; whilst in the Cartilaginous orders, 

 the softness of these bones renders some other means necessary. 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 follow- 

 ing experimental conclusions. VII. When sonorous vibrations are communi- 

 cated from water, to air inclosed 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 inclosed in a membrane, and sur- 

 rounded 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 percep- 

 tion of sounds will, of course, be greater in deep waters, where the pressure 

 upon it is considerably increased. 



901. Most animals living in air are provided with an opening into the Vesti- 

 bule, covered by a thin membrane ; and, in the majority of cases, with a Tym- 

 panic apparatus also. The following experimental results bear upon the man- 

 ner 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 exist 

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

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

 tions are also communicated, without any perceptible loss of intensity, from the 

 air to the water ; when, to the membrane forming the medium of communica- 

 tion, 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, commu- 

 nicates 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 appa- 

 ratus. 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 Am- 

 phibia ; and it may happen as the result of disease in the Human subject. la 



