382 



THE MECHANICAL FUNCTIONS. 



tail. Such is precisely the object of a peculiar 

 mechanism, which nature has provided in the 

 interior of the body of the fish. A large bladder, 

 filled with air, has been placed immediately under 

 the spine, in the middle of the back and above 

 the centre of gravity. This is known by the name 

 of the air-bladder, or the sivimmmg -bladder, and in 

 the cod-fish it is called the sound. It frequently, as 

 in the Carp, consists of two bladders (a, b, Fig. 195) 



joined endwise, and communicating with each 

 other by a narrow neck.* When distended with 

 air, it renders the whole fish specifically lighter 

 than the surrounding water; and the fish is thus 

 buoyed up, and remains at the surface without any 

 effort of its own. On compressing the bladder, by 

 the action of the surrounding muscles, the included 

 air is condensed, the specific gravity of the whole 

 body is increased, and the fish sinks to the bottom. 

 On relaxing the same muscles, the air recovers its 

 former dimensions, and the fish is again rendered 

 buoyant. Can there be stronger evidence of design 

 than the placing of this hydrostatic apparatus, 

 acting upon philosophical principles, in the interior 



* There is great variety in the form and structure of the air- 

 bladder in different fishes. Sometimes it contains a large glandular 

 body of a pecuHar structure, which has been conjectured to be an 

 apparatus for secreting air from the blood : but this is by no means 

 very generally met with. 



