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- 
eds. 
« 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. 
