56 BIRDS ON THE WING. 



readily explained. Within the bird's body, con- 

 nected with the lungs, are at least nine small 

 air-sacs. These air-sacs, which are wonderful ana- 

 tomical contrivances, are also connected by air 

 passages in the best fliers, with many of the bones, 

 which aie hollow. When the bird wishes to rise 

 he inhales the air into his lungs, whence it rushes 

 into the air-sacs located in various parts of the 

 body, and thence into the cavities of the bones. 

 In this way every hollow space becomes quickly 

 filled with the light, expanding air, making the 

 plumed tilter more and more buoyant. Of course 

 the bird must mount upward like an inflated 

 balloon, especially if slightly assisted by the wings. 

 When he wishes to descend, he simply reverses 

 the process ; that is, by contracting the proper 

 muscles, he exhales the air from his body, and 

 down he glides gently and lightly, or swiftly, as he 

 chooses. By his outspread wings he holds himself 

 safely in check, so that he does not dash himself to 

 pieces on the ground. How wonderful is the law 

 of adaptation ! 



