BIRDS, THEIR STRUCTURE AND ADAPTATIONS 285 



The Wing. The wing is almost as wonderful an organ as the 

 human hand, but although a modified arm, it has lost all power of 

 grasping and is adapted entirely for flight. The shoulder is strongly 

 braced by three bones, instead of two as in man, to withstand the 

 tremendous pull of the powerful muscles. There is the shoulder 

 blade, the collar bone (" wish bone "), and the coracoid bone ex- 

 tending to the sternum (breast bone). All three are devoted to 

 supporting the wing, using a sort of tripod arrangement, which is 

 very strong. The upper 



and lower arm bones are *. a/wii 



long, strong, and slender. 

 The wrist is lengthened as 

 are also the fingers; only 

 three are present, however, 

 the other two being sacri- 

 ficed for lightness. Thus 

 we have a long, three- 

 jointed lever, firmly at- 

 tached to the shoulder with 

 its leverage greatly in- 

 creased by the feathers. The problem now consists of providing 

 the necessary muscle to swing such an arm. 



Power Required. To illustrate the difficulty involved, we may 

 take as an example the pigeon. It weighs about a pound and has 

 a wing spread of about two feet. This would mean that a boy or 

 girl of ordinary weight would have to swing through the air a pair 

 of wings each from fifty to seventy-five feet long at the rate of 

 two hundred to five hundred strokes per minute. Try to swing 

 your own arm at this rate for a minute, and then imagine the power 

 needed for a wing as long as a building lot front. If we think of 

 keeping up this form of exercise for forty-eight hours without rest, 

 we will have some idea of the bird's problem, and the marvelous 

 way in which it has been solved. 



Muscles, competent for this task, could not be located on the 

 wing itself, as that would too greatly increase its weight, so we 

 find the breast bone enormously enlarged and attached to it, 



FIG. 97. Wing structure of bird. 



