190 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 flight. 
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 would instantly overbalance the bird. 
If we remove the skin from the upper arm of a bird, 
