FLIGHT. 233 



weight may be considered the same, or, as mathema- 

 ticians say, a constant quantity, the specific gravity, 

 upon which the buoyancy depends, must increase, 

 and the fish must accordingly sink. On the other 

 hand, by the effort to distend the swimming-bladder, 

 the muscles are relaxed, and the whole body not only 

 becomes specifically lighter, but the included air ex- 

 pands, and buoys up the fish. The truth of this 

 explanation has been tried by the experiment of prick- 

 ing the bladder and allowing the air to escape, when 

 the fish sinks and cannot rise above the bottom of the 

 water*; but when a fish dies it floats to the surface, 

 because it would appear, from the want of voluntary 

 eompression, the swimming-bladder then expands to 

 its utmost dimensions; at least we have always in 

 dead fishes observed it to be much distended f. 



The contrivance for rendering birds buoyant in the 

 air is considerably different from either of these, and 

 was first discovered by the celebrated Harvey; at 

 least, he says, he does " not remember it to have been 

 previously observed by anybody J." Air in consider- 

 able volume is introduced into the body, though it is 

 not, as in fishes, contained in one cavity, but is distri- 

 buted into numerous cells in various parts of the body. 

 The lungs, compared with those of quadrupeds, are 

 rather small, but the air-cells with which they com- 

 municate occupy a considerable extent of the chest 

 and belly. These cells are much divided by partitions, 

 furnished, as has been observed in large birds, with 

 muscular fibres, supposed to be employed in sending 

 the air back to the lungs, as is done by the diaphragm 

 in other animals, and which is wanting in birds. This 

 is no doubt the reason why birds appear to pant so 

 much in breathing, a much greater portion of the 







Ray in Phil. Trans. No. 114-15. t J. R. 



De Generatione Animal, p. 4, 4to. London, 1651. 

 Willis, de Anim. Brutorum, i. 3. 



