298 



THE MECHANICAL FUNCTIONS. 



or sinking, without calling into action either the fins or the 

 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 im- 

 mediately 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 swhnming 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 end- 



wise, and communicating with each other by a narrow neck.* 

 When distended with air, it renders the whole fish specifi- 

 cally lighter than the surrounding water; and the fish is 

 thus buoyed up, and remains at the surface without any ef- 

 fort 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 de- 

 sign than the placing of this hydrostatic apparatus, acting 

 upon philosophical principles, in the interior of the organi- 

 zation, for a purpose so definite and unequivocal? 



In several tribes of fishes there is a canal (c d) establish- 

 ing a communication between this bladder and the stomach, 

 or the gullet (o;) so that by compressing the bladder, a quan- 

 tity of air may be forced out, and a very sudden increase of 



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

 different fishes. Sometimes it contains a large glandular body of a peculiar 

 structure, which has been conjectured to be an apparatus for secreting air 

 from the blood: but it is by no means very generally met with. 



