ADAPTATIONS TO FLIGHT 



757 



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 

 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 

 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. The air must 

 lessen the specific gravity 

 of the bird, but a few 

 mouthfuls of food are 

 sufficient to counteract 

 the lightening. More- 

 over, in many small 

 birds of powerful flight, 

 all the large bones, or 

 all except the humerus, 

 contain marrow, and are 

 therefore not "pneu- 

 matic " ; and the horn- 

 bill, which has no great 

 power of flight, is one 

 of the most pneumatic 

 of birds. It is certain 

 that in ordinary flight 



the lightest of birds has 



to keep itself from falling 



by constant effort. The 



air-sacs increase the 



bird's respiratory con- 

 tent, secure more perfect 



aeration of the lungs, 



and assist in internal 



perspiration, thus help- 

 ing in the regulation of 



the body temperature. 

 To carry the weight of 



the bird, the wings strike 



vertically; to carry the 



bird onwards, they strike 



obliquely. Sometimes 



the direction of the 



stroke is more vertical, 



and then the bird mounts upward ; sometimes it is more oblique, and 



then the bird speeds onwards ; usually both directions are combined. 



The raising of the wing after each stroke requires relatively little 



effort, the resistance to be overcome being very slight. In steering, 



the feathers of the tail often bear to the wings a relation comparable 



to that between rudder and sail. 



Modes of flight. — There are three chief modes of flight : — 



I. By gliding or skimming, during which the bird has its wings 



spread, but does not flap them, depending for its movement on the 



Fig. 454. — Pectoral girdle and sternum 

 of Bewick's swan. 



A part of carina removed shows peculiar loop of 

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

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

 r., parts of ster7ial ribs. 



