FLIGHT. Zio 
perceptible only by the bubbles of air which infla- 
ted them; insomuch that, pressing the body of this 
bird, one heard a little noise’ like that produced by 
pressing the membranous parts of an animal which 
has been inflated. By the assistance of the probe 
and blowing, we discovered the communication of 
these membranes with the lungs.”* 
Besides the aircells filled from the lungs, there 
are others filled directly from the mouth and nos- 
trils through the windpipe. The latter, particularly 
in birds of prey and high-flying birds (Alauda, Cico- 
nia, §c.), are very large, ascending beneath the skin 
of the neck and shoulders and around the scull. 
“In the stork,” says Doctor Macartney, ‘‘ we find 
these cells large enough to admit the finger to pass 
a considerable way down upon the inside and back 
of the wing: they are also large in the owl.”’+ 
It is evident that all this extraordinary number 
and volume of aircells, as well as the very great 
strength observable in the bones of birds, is for the 
express purpose of rendering them light and lmoy- . 
ant, so that they may support themselves in the air 
with less effort. 
The following excellent remarks on the structure, 
which is so admirably adapted for rendering birds 
light and buoyant, are by Sir Charles Bell: 
* First,” he says, “it is necessary that birds, as 
they are buoyed in the air, be specifically lighter; 
secondly, the circumference of their thorax must be 
extended, and the motions of their ribs limited, that 
the muscles of the wings may have sufficient space 
and firmness for their attachment. Both these ob- 
jects are attained by a modification of the apparatus 
of breathing. ‘The lungs are highly vascular and 
spongy, but they are not distended with air. The 
air is drawn through their substance into the large 
* Hist. Géner. des Voyages, ix., 311. 
+ Reés’ Cyclopedia, Art. Birds, 
