430 ON SENSATION, AND THE ORGANS OF THE SENSES. 



is conducted to the ear, in aquatic animals not breathing atmospheric air. 

 The labyrinth of such is either entirely inclosed witbin 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, be- 

 sides 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 laby- 

 rinth 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 following experimental con- 

 clusions. vii. When sonorous vibrations are communicated from water to 

 air inclosed in membranes or solid bodies, a considerable increase in the in- 

 tensity of the sound is produced, by the resonance of the air thus circum- 

 scribed. vin. A body of air inclosed in a membrane, and surrounded by 

 water, also increases the intensity of the sound by resonance, when the sono- 

 rous undulations are communicated to it by a solid body. From these ob- 

 servations, 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 per- 

 ception of sounds will, of course, be greater in deep waters, where the pres- 

 sure upon it is considerably increased. 



562. 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. Sono- 

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

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

 exists between the air and the water, the sonorous undulations are communi- 

 cated from the former to the latter medium with great intensity. x. The so- 

 norous vibrations are also communicated without any perceptible loss of in- 

 tensity, from the air to the water ; when, to the membrane forming the me- 

 dium 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 atmo- 

 spheric air on both sides. The fact stated in ix. is evidently one of great im- 

 portance 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 demonstra- 

 tion of the purpose and action of the tympanum, in the more perfect forms of 

 the auditory apparatus. We are now prepared to inquire, in somewhat more 



