740 EVOLUTION OF ANIMALS Part V 



breastbone furnishes the attachment for the great flight muscles. The wing 

 socket is formed at the junction of the shoulder blade or scapula, the coracoid 

 that connects with the sternum, and the spread ends of the collarbones or 

 wishbone. The spread of the wishbone helps to keep the shoulders sprung 

 apart when the wings are raised. The keel of the breastbone, familar to any- 

 body who has carved a chicken, is the attachment for the great flight muscles 

 (pectorals). In ostriches, as in other flightless birds, the breastbone is a simple 

 shield without a keel. 



A bird's hips are mainly broad plates that form attachment places for the 

 great leg muscles and a saddle above the otherwise unprotected vital organs. 

 The presence of a pelvic saddle of bones fused together and to the vertebrae 

 instead of a pelvic girdle allows the passage of the large hard-shelled egg 

 (Fig. 36.18). 



Wings and Flight (Figs. 36.11, 36.12). When a bird folds its wings the 

 elbows point backward like human elbows. At the same time, a bird folds its 

 "hands" backward in a jackknife bend with the wrists in a sharp point forward, 

 impossible for the human wrist. A bird's "hand" is small and rigid, reduced to 

 three fingers from the five of its reptilian ancestors. The inner stub next to the 

 ulna corresponds to the index finger, the outer stub and the bones fused to- 

 gether at the tip of the wing also represent fingers. In reptiles, the fingers end 



Fig. 36.11. The take-off of an American egret. The bird leaps into the air, 

 raises its wings and stretches out its neck, thrusts the feet down. In the air as in 

 this picture it draws the head back; the legs balance the neck; the wings go into 

 the down stroke. (Photograph by Allan D. Cruickshank. Courtesy, National 

 Audubon Society.) 



