6 74 



BIRDS. 



offers relatively little resistance. The attachment of the wings high 

 up on the thorax, the high position of such light organs as lungs and 

 air-sacs, the low position of the heavy muscles, the sternum, and the 

 digestive organs, the consequently low centre of gravity, are also 

 structural facts of importance. But it must be remembered that the 

 frictional resistance of the air is slight. 



(b) The muscles of flight. The pectoralis major brings the wing 

 downward^ forward^ and backward, keeping the bird up and carrying 



it onward. As it has 

 "X most work to do, it is 



^L. by far the largest. The 



pectoralis minor raises 

 the wing for the next 

 stroke. There are others 

 of minor importance. 

 On an average these 

 muscles weigh about one- 

 sixth of the whole bird, 

 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 mi- 

 gration many birds fly at 

 a rate of over loo miles 

 an hour. 



(c) The skeleton. The 

 rigidity of the dorsal part 

 of the backbone, due to 

 fusion of vertebrae, is of 

 advantage in affording a 

 firm fulcrum for the wing- 

 strokes, while the arched 

 clavicles (meeting in an 

 interclavicle and often 

 fused in front to the 

 sternum) and the strong 



FIG. 374. Pectoral girdle and sternum 

 of Bewick's swan. 



A part of carina removed shows peculiar loop of Sernum an e ron 



trachea (tr.)', cl. t clavicle; cor, coracoid ; sc., coracoids (which articu- 

 scapula ; /., glenoid cavity for head of humerus ; late with the sternum) 

 -r., parts of sternal ribs. are adapted to resist the 



inward pressure of the 



down-stroke. As the keel of the breast-bone serves in part for the in- 

 sertion of the two chief muscles, its size bears some proportion to the 

 strength of flight. It is absent in the running 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 continued into the bones, among the 



