GENERAL OUTLINES OF ICHTHYOLOGY. 19 



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portionate weight of brain in a Pike, as compared with its 

 body, is as 1 to 1300 ; in the Shark, as 1 to 2500 ; and in 

 the Tunny a remarkably stupid fish but as 1 to 3700. 



The Swimming- (or Air-} Bladder. 



Inhabiting an element which is of very nearly the same 

 specific gravity as their own bodies, fishes have no weight 

 to bear, but have merely to propel themselves through the 

 water; and their form, structure, and organs of motion 

 are all adapted to this one purpose, according to the 

 requirements of each species. Many families have, how- 

 ever, under the spine, in the abdomen, an air- or ' swim- 

 ming ' -bladder, varying in shape in various species, which 

 they can expand or contract at pleasure; and this is 

 believed to be for the purpose of enabling them to alter 

 their specific gravity to suit the densities, at different 

 depths, of the water in which they desire to swim or sus- 

 pend themselves. Many species, notwithstanding, though 

 wanting this apparatus, have very nearly the same habits 

 as those which are possessed of it ; and in some of these 

 latter there is no external passage by which the air in the 

 bladder can be inspired or expressed. 



The contents of these bladders have been found upon 

 analysis to consist sometimes of pure nitrogen, sometimes 

 of almost pure oxygen, sometimes of oxygen, nitrogen, 

 and carbonic acid combined in varying proportions, but 

 rarely of common atmospheric air. Some bladders are 

 composed of a single chamber or cavity ; others of two 



