FLIGHT. 273 



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, 4-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."f 



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 buoy- 

 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. G6ner. des Voyages, he., 311. 

 t Rees' Cyclopaedia, Art. Birds. 



