66 ANIMAL BIOLOGY 



are two bones. In the hand region, though the differences 



are more striking, the general plan of the two is the same. 



Unlike the bones of the human skeleton those of most bird^ 



are hollow and filled with air. 



Any one who has eaten a chicken's wing knows that the 



bones are covered by muscles ; these enable the bird to fold 



and unfold the 

 parts of the wing, 

 much as the human 

 arm is stretched out 

 or doubled up. On 

 the bird's body are 

 other powerful mus- 

 cles, which cause 

 the wing as a whole 

 to make the upward 

 and downward 

 strokes in fiight. 



Still another won- 

 derful adaptation of 

 the wing for flight 

 is evident in the 

 arrangement and 

 structure of the 

 feathers (Fig. 49). 

 The feathers fit over 

 each other in such 



Fig. 48. — A, skeleton of arm of a man ; B, skele- 

 ton of wing of an ostrich. (A. E. Rueff.) 



a way ^ that in the downward and backward stroke of the 

 wing a continuous surface is struck against the air, and 

 this propels the bird upward and forward. In the up- 



' Before assigning these paragraphs the structure of a feather and 

 the arrangement of the feathers on the wing of some bird (e.g. a 

 chicken) should be demonstrated to the class. 



