FLIGHT. 



615 



SC. 



amounting to nearly one-half in some pigeons. Buffon 

 noted that eagles disappeared from sight in about three 

 minutes, and a common rate of flight is about fifty feet per 

 second. In migration many birds fly at a rate of from 

 100 to 200 miles an hour. 



(c) The skeleton. — The rigidity of the dorsal part of 

 the backbone, due to 

 fusion of vertebrae, is 

 of advantage in afford- 

 ing a firm fulcrum 

 for the wing-strokes, 

 while the arched clavi- 

 cles (meeting in an 

 interclavicle and often 

 fused in front to the 

 sternum) and the 

 strong coracoids 

 (which articulate with tr. 

 the sternum) are 

 adapted to resist the 

 inward pressure of the 

 down-stroke. As the 

 keel of the breast- 

 bone serves in part 

 for the insertion of 

 the two chief muscles, 

 its size bears some 

 proportion to the 

 strength of flight. It 

 is absent in the run- 

 ning birds, such as 

 the ostriches, and has 

 degenerated in the 

 New Zealand parrot (Stringops), which has ceased to fly 

 and taken to burrowing. 



(d) Air-sacs and air-spaces. — The lungs of birds open 

 into a number of air-sacs, which have a larger cubic content 

 than the lungs, and in many cases these air-sacs are con- 

 tinued into the bones, among the viscera, and even under 

 the skin. From a broken bone it is possible to inflate the 

 air-sacs, and through a broken bone a bird with choked 



Fig. 270. — Pectoral girdle and sternum of 

 swan. 

 A part of carina removed shows peculiar loop of 

 trachea (tr.) ; ct., clavicle ; cor., coracoid ; sc, 

 scapula ; gl., glenoid cavity for head of humerus ; 

 r., parts of sternal ribs. 



