118 NATURAL HISTORY. 



200. The amount of muscular power required for flight 

 in the air is not commonly appreciated. If we look at 

 the breadth of wing in a bird, as compared with the size 

 of the animal when stripped of its feathers, we can have 

 some idea of the extent of whig which a man would need 

 to enable him to fly. And to work efficiently such enor- 

 mous wings as he would require, he must have enormous 

 muscles. Those which move the arms of the most broad- 

 chested and brawny man are far from being large enough 

 to enable him to fly, even if he had wings. To do this, 

 he must have the keel on the breast-bone, like the bird, 

 to afford an attachment for a thick mass of muscle. We 

 see, then, why it is that all the attempts which men have 

 made to fly have proved failures. It is not that the wings 

 have not been properly made, but that there was not suf- 

 ficient muscle to work them. 



201. As flying requires such strong exertion, it is im- 

 portant that the Bird should be as light as possible. 

 There is a singular contrivance for this purpose. The 

 air taken into the lungs does not all stop there, but some 

 of it passes thence into cells or sacs in different parts of 

 the body, and also into many of the bones, which are hol- 

 low for this purpose. This air apparatus is in extent pro- 

 portionate to the powers of flight. Thus, in the Eagle, 

 the air goes into all the bones, while in the Ostrich and 

 the Penguin it goes only into the thigh-bones. 



202. The digestive organs of the Bird are very pecul- 

 iar. They are the only animals that have a gizzard. This 

 organ is a stomach, which has on its inside a lining as 

 tough and hard as leather. This is for the purpose of 

 bruising and rubbing the food, which is done by the ac- 

 tion of very stout muscles. These constitute the bulk of 

 the gizzard ; and they are so arranged that they squeeze 

 and rub two opposite surfaces of the inside lining against 

 each other. The food is therefore ground in the same 

 manner as grain is between the millstones of a flour-mill. 

 The power of this grinding apparatus is made still more 



