TBVmUXR ORGANTZATIONS. 141 



the fish, is contracted, which the fish probably possesses a 

 muscalar power of doings the bulk of the fish is contracted 

 along with it ; whereby since the absolute weight remains 

 the same, the specific gravity, which is the sinking force, 

 as increased, and the fish descends ; on the contrary, when 

 in consequence of the relaxation of the muscles, the elasti- 

 city of the enclosed, and now compressed air^ restores the 

 dimensions of the bladder, the tendency downwards be- 

 comes proportionably less than it was before, or is turned 

 into a contrary tendeecy. These are known properties of 

 t)odies immersed in a fluid. The enamelled figures, or 

 Jittle glass bubbles, in a jar of water,, are made to rise and 

 full by the same artifice. A diving machine might be 

 imade to ascend and descend upon the like principle ; name- 

 Jy, by introducing into the inside of it an air vessel, which 

 by its contraction would diminish^ and by its distension en- 

 large, the bulk of the machine itself^ and thus render it 

 specifically heavier^ or specifically lighter, than the water 

 which surrounds it Suppose this to be done ; and the ar- 

 tist to solicit a patent for his invention. The inspectors of 

 the model, whatever they might think of the use or value 

 of the contrivance, could, by no possibility, entertain a 

 ►question in their minds, whether it were a contrivance or 

 not. No reason has ever been assigned, no reason can be 

 assigned, why the conclusion is not as certain in the fish, 

 as in the machine ; why the argument is not as firm,, in one 

 case as the other. 



It would be very worthy of inquiry, if it were possible to 

 discover, by what method an animal, which lives constantly 

 in water, is able to supply a repository of air. The ex- 

 pedient, whatever it be, forms a part, and perhaps the most 

 curious part, of the provision.* Nothing similar to the air 

 bladder, is found in land animals : and a life in the water 

 has no natural tendency to produce a bag of air. Nothing 

 can be further from an acquired organization than this is. 



* Much obscurity still exists concerning- the exact purpose which 

 the air bag is intended to perform. But with regard to the manner in 

 which it is supplied with aiir, there seems no reason to doubt that it is 

 effected by a secretion from the blood. It is an established fact in 

 physiology, that many of the internal surfaces of the body have the 

 power of producing gases in this way. In the air-bag of many fishes 

 a very vascular organ is found which has been called the air-gland; 

 and in some species vessels have been discovered conveying the air 

 from this gland into the cavity^f the bag. Even where this gland dees 

 not exist, it is probable that the internal surface of the bag may per- 

 iform the same office. -^^ 



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