270 FACULTIES OF BIRDS. 



are relaxed, and the whole body not only becomes 

 specifically lighter, but the included air expands, and 

 buoys up the fish. The truth of this explanation 

 has been tried by the experiment of pricking 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 sur- 

 face, because it would appear, from the want of 

 voluntary compression, the swimming-bladder then 

 expands to its utmost dimensions ; at least we have 

 always, in dead fishes, observed it to be much dis- 

 tended. 



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." 

 Air in considerable volume is introduced into the 

 body, though it is not, as in fishes, contained in one 

 cavity, but is distributed into numerous cells in va- 

 rious parts of the body. The lungs, compared 

 with those of quadrupeds, are rather small, but the 

 aircells with which they communicate 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 

 body being always put in motion than in quadru- 



Besides these aircells, which fill the whole cavity 

 of the body from the neck downward, and serve the 

 double purpose of assisting in the assimilation of 

 nutriment by the supply of oxygen and the remo- 



* Ray in Phil. Trans., No. 114-15. 



