FISHES. 35 



bladder is entirely closed. A few fishes have a cellular swim- 

 ming-bladder, by which Lepidosteus in particular is distinguished. 

 The form of the bladder is usually elongate ; sometimes provided 

 at the anterior extremity with lateral blind appendages ; sometimes 

 with a row of such appendages extending the whole length on each 

 side. In some fishes it is constricted in the middle, and divided 

 into two chambers lying one behind the other. The blood-vessels 

 divide in a fan-like form into fine branches over the internal mucous 

 membrane, or form retia mirabilia at particular points, which have 

 been described as red glandular masses. Formerly it was thought 

 that such nets are absent in fishes whose swimming-bladder is pro- 

 vided with a duct; but this, however, is not a general rule 1 . 



The air contained in the swimming-bladder consists of a mix- 

 ture of oxygen and nitrogen, with sometimes a small quantity of 

 carbonic acid gas. In fishes of the same species chemical investi- 

 gation often indicates remarkable differences. According to BIOT 

 and DE LA ROCHE, the quantity of oxygen increases with the 

 depth in which the fishes live ; thus in general it is much more 

 abundant in marine than in fresh-water fishes ; in the last a con- 

 siderable quantity of nitrogen is always present, and in the carps it 

 has even been supposed that the air of the swimming-bladder con- 

 sists of this gas alone ; in marine fishes the air of the bladder may 

 contain even eighty per cent, of oxygen gas. This air is secreted 

 by the inner surface of the swimming-bladder, as appears from 

 those fishes that have the bladder entirely closed; but even in those 

 whose swimming-bladder has a duct, it is difficult to suppose, as 

 RATHKE has suggested, that the air penetrates into the bladder 

 from without. More probably the duct serves for the escape of 

 the air. 



Opinions differ respecting the use of the swimming-bladder in 

 the animal economy of fishes. The name that has been given to 

 it indicates the presumed connexion with swimming. This is the 

 common opinion, first advanced by BORELLI, according to which 

 the fish by compression or expansion of the bladder can make itself 

 specifically heavier or lighter, and alternately sink and rise in the 



1 For further particulars on these blood-glands of the swimming-bladder, see 

 J. MUELLER Abhandl. der Akad. der Wissensch. zu Berlin, a. dem Jdhre 1839, s. 262 

 271. 



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