42 ANATOMY AND PHYSIOLOGY 



AIR BLADDER. 



Only such fishes as swim with facility, rising and 

 falling, as circumstances require, possess this cu- 

 rious and extraordinary organ. Flounders, and 

 indeed all the flat fish, together with many of the 

 eels, are without it ; — therefore, as scavengers of 

 the ocean, they generally remain at the bottom. 

 In common parlance, this air sac is called the sound. 



These diagrams are representations of the different forms 

 which the sound or swimming bladders have, in different 

 families of fishes. The short threads at the extremities, are 

 air ducts which communicate with some organ that is sup- 

 posed to secrete the air, and through them it passes into 

 the sac. 



In domestic economy, sounds become an article of 

 merchandise, — being sold in barrels, for food. 

 The physiology of this apparently simple bladder 

 of air, is not well understood. The fish undoubt- 

 edly secretes the air in it, and has, also, the pow- 

 er of allowing it to be diminished or increased in 

 quantity, unless taken by surprise. 



