I 90 



The Bird 



breast muscle, which makes possible the all-important 

 downward sweep of the wings, weighs one-fifth as much 

 as the entire bird, bones and all. This arrangement of 

 a great weight of muscle hung below the point of attach- 



FiG. 139. — Wing and breast of Pigeon, showing immense pectoral muscles, and 

 tendons of wing used in Hight. 



ment of the wings is, for mechanical reasons, the only 

 one possible in a bird of flight; since any excess of weight 

 above the wings w^ould instantly overbalance the bird. 

 If we remove the skin from the upper arm of a bird, 



