BIRDS. 
203 
assisting in spreading the wing. There are only three 
fingers, and the bones of these and of the palm and 
wrist are so grown together that it is difficult to distin¬ 
guish them. Connected with the wing are two clavi¬ 
cles, or collar-bones, which unite at their ends to form 
the wish-bone. These, with the scapulars or shoulder- 
blades and the coracoid bones, form the pectoral girdle 
holding the wings in place. 
While the chief organ of flight is the wing, it must 
be remembered that the whole body aids in aerial navi- 
Fig. 159.—A Swallow Feeding Her Young. 
gation. The sternum or breast-bone is large and 
strong and fastened firmly to the ribs. Its surface is 
often increased by a ridge of bone along the middle 
called the keel (Fig. 162). The muscles of the breast 
attached to this sternum move the wings. These 
muscles are dark colored and tough in birds of great 
wing-power and white and tender in barn-yard fowls 
which have lost the power of continuous flight. 
