622 



BIRDS. 



(c) The Skeleton.— Thtt rigidity of the greater part of the 

 backbone, due to fusion of vertebra;, 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 coracoids (which articulate 

 with 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 propor- 

 tion 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 bur- 

 rowing. 



(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 

 in development into 

 air spaces within the 

 bones 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 windpipe may for a time breathe. The whole 

 system of air-containing cavities is continuous, except in the 

 case of the skull bones, whose spaces receive air from the nasal 

 and Eustachian tubes. In view of these facts, it used to be 

 supposed that a bird with heated air in the sacs and spaces 



Fig. 220.- 



-Pectoral girdle and sternum 

 of Swan. 



A part of carina remo\-ed shows peculiar loop of 

 trachea (/?".); tV., clavicle; cor., coracoid ; sc. 

 scapula; ^/., glenoid ca\lty for head of humerus; 

 ;-., sternal parts of ribs. 



