WINGS. 431 



flight, the necessity for these peculiarities of struc- 

 ture is the more evident. The joints are very 

 movable, and are at the same time extremely firm 

 and well knit together. The wing is moved by 

 muscles placed on the bird's body as our arm by 

 muscles on the chest, shoulder, and back, and 

 these muscles are endowed with a surprising power 

 of enduring fatigue. This may be readily con- 

 ceived if we take a large fan in the hand and 

 imitate the action of a bird in flying. It will soon 

 be found that the fatigue induced by a little exer- 

 tion in this way is very great. It is far otherwise 

 in the bird, which can move its wings many thou- 

 sands of times, and yet experience but little 

 weariness. In long flight, however, such as in 

 migration from one country to another, the poor 

 bird's muscular efforts exhaust it, and it is often 

 glad to seek rest in the shrouds of ships, until 

 able to renew its journey. 



The breast-bone, or sternum, forms another im- 

 portant part of the bird's structure, for it is here 

 that the wings are attached, and this part, conse- 

 quently, requires to be peculiarly strengthened so 

 as to offer a firm point of action to the organs of 

 flight. In shape, the breast-bone of a bird may 



