68 THE STORY OF BIRD-LIFE. 



technical to explain here. The mystery, how- 

 ever, is even yet not quite cleared up. The 

 subject is one well worth the attention of those 

 fond of puzzles. 



Most of us probably regard a bird's wings as 

 the only parts of its body concerned in its flight. 

 As a matter of fact, flight has two powerful auxi- 

 liaries in the shape of its lungs and air-sacs. 



The former, which are very large, are peculiar 

 in more ways than one. In the first place, they 

 are not more or less freely suspended in the 

 body cavity, as in other animals, bat fitted closely 

 to the back. So much so, that when dissected 

 and taken out of the body they are found to be 

 marked by deep grooves or tunnels indicating 

 ' the shape of the ribs, in between which they had 

 forced themselves. In the second they are tra- 

 versed by numerous tubes which open eventually 

 into large thin-walled bags placed along each 

 side of the body, and called air-sacs. These air- 

 sacs were believed at one time to be for the 

 purpose of making the bird lighter when in the 

 air; but the amount of difference which they 

 make in this direction would be inappreciable. 



They probably serve as a reservoir upon which 

 the bird can draw during the violent exertions of 

 flight, or singing, and for regulating the body 

 temperature. 



In many birds even the bones of the skeleton 

 are hollow and filled with air from the air-sacs. 

 This again was supposed to be a powerful 

 auxiliary to flight ; but, when we recollect that 

 some of the most powerful fliers, like the swift, 

 the swallow, and the albatross, have marrow in 



