THE SWIMMING BLADDER. 83 



simple, as in perch, sometimes furnished with more 

 or less numerous appendages, as in some of the 

 haddock tribe, or branched, as in certain Science. 

 Occasionally we find it divided, as it were, into two 

 parts by a restriction, as in the genus Cyprinus, 

 several of the Salmonidse, and others. It is chiefly 

 among the abdominal fishes that we find it com- 

 municating by a tube or tunnel with the intestinal 

 canal, and either directly with the gullet, as in 

 Cyprmus, or with the base of the stomach, as in 

 the herring. That of the sturgeon opens into the 

 former portion by means of a conspicuous orifice. 

 The contents of the swimming bladder are usually 

 found to be azote, mingled with some fractional 

 parts of oxygen or carbonic acid. There is pro- 

 bably a want of uniformity in its composition, which 

 is of itself a proof that the air is secreted rather 

 than drawn in from the atmosphere in the ordinary 

 way. The gas in the carp was found by Fourcroy 

 to be nearly pure nitrogen, while other chemists 

 have found it composed of nitrogen, oxygen, and 

 carbonic acid the nitrogen in greater, the oxygen 

 in smaller proportion, than in atmospheric air. 

 Some physiologists seem to have regarded the 

 swimming bladder as a true lung, which both ad- 

 mitted and retained the external air ; but as we 

 have said, the connecting air-duct is in numerous 

 species entirely wanting, while in many others 

 which remain constantly at prodigious depths, the 

 quantity of oxygen in the swimming bladder is 

 greater than in those the abode of which is near 

 the surface. Indeed, the oxygen is said to increase 



