SWIMMING BLADDER OF FISHES. 429 



sess the means of rising or sinking, without 

 calling into action either the fins or the tail. 

 Such is precisely the object of a peculiar me- 

 chanism, which nature has provided in the 

 interior of the body of the fish. A large blad- 

 der, 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 swimming 

 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 



D 



195 



communicating with each other by a narrow 

 neck.* When distended with air, it renders the 

 whole fish specifically lighter than the surround- 

 ing 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 



* 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 peculiar structure, which has been conjec- 

 tured to be an apparatus for secreting air from the blood : but 

 it is by no means very generally met with. 



