FISHES. 323 



experiment has likewise countenanced the opinion. When 

 tlie air-bag oi" a lisli is punctured, the animal immediately 

 falls to the bottom, nor is it able, by any exertion of its fins, 

 to elevate itself again. When in a sound state, the exter- 

 nal skin of the air-bag (regarded as possessing strong mus- 

 cular power *) is supposed capable of contraction, so as to 

 condense the air, and enable the animal to sink, or of ex- 

 tension, so as to allow the air to expand, and aid the ani- 

 mal in rising in the water. 



The above theory fails in explaining all the phenomena. 

 The eel, which resides always at the bottom, is yet possess- 

 ed of an air-bag ; while the sharks, which roam about in all 

 depths, and the mackrel, which pursues its prey at the sur- 

 face, are destitute of this reputed organ of equilibrium. 



The air-bag of some fishes soon loses its muscular power, 

 in consequence of the air being expanded by the action of 

 the sun, when the fish has remained too long at the sur- 

 face. In this situation the fish continues at the surface. 

 When some fish are suddenly brought up from deep water, 

 the diminished pressure occasions the expansion of the air 

 contained in the bag. The organ sometimes bursts in such 

 cases, and the contents, rushing into the abdomen, push the 

 gullet out of the mouth of the fish. This effect is frequent- 

 ly produced in the cod-fish. 



Various opinions have been advanced with regard to the 

 manner in which this air-bag is filled. By some it has been 

 supposed, that a portion of the air, which fishes are capable 

 of abstracting from the water, is transmitted through the 

 gullet and stomach into the air-bag, when necessary, and 

 expelled and renewed at the pleasure of the animal. 

 Needham long ago considered that the air, or, as he term- 

 ed it, a vaporous exhalation contained in the air-bag, was 



• Phil. Trans. 1804, p. 19. 



x 2 



