128 FISHES AND FISHING. 



of the gaseous contents of this bladder are increased 

 or diminished by secretion, or absorption, according 

 to the wants or desires of the fish. 



According to Biot, fish which inhabit very deep 

 waters have, in this bladder, 87 per cent, of oxygen ; 

 and Foureroy asserts, that in those which are found 

 in more shallow streams, such as carp, roach, and 

 others of that species, there is an excess of nitrogen. 

 The extended experiments of Humboldt, and M 

 Provenzal, during many months in hot climates, 

 proved that the composition of the gas in this bladder 

 varies in the same animal, under the circumstances 

 I have mentioned, and that the sword-fish which 

 inhabits the lower strata of the sea, has been found 

 under the tropics with its air-vessel entirely filled 

 with oxygen. 



Some fish have two processes which go from the 

 air-bladder, and terminate in the fauces; salmon have 

 an opening from the same vessel into the oesophagus, 

 which is controlled by muscular fibre ; trout, also, 

 have a similar conformation, and when they rise to 

 the surface in a dull stream, they emit a bubble of 

 air. The whole of this part of the anatomy of fish 

 more fully and impressively illustrates the truth, 

 that consummate wisdom alone could have devised 

 any arrangement so wonderful in design, or so 

 beautifully useful in effect. 



