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 wing 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 stil] more 
