BREATHING OF BIRDS 121 



The working out of these .capacities in each group 

 is a problem of great interest. In birds the lungs are 

 small, the windpipe long, and the organ of voice 

 at its end, not in the throat at its commencement. 

 Birds, therefore, are ventriloquists. From their lungs 

 spring a number of air-sacs which buoy up the viscera 

 as on cushions, line the skin, and often penetrate the 

 very bones and feathers. To inflate this system a 

 dual mechanism is provided. When stationary, the 

 bird may raise and lower its ribs ; when flying stiff- 

 breasted, it lifts and depresses its flexible back. In 

 either case two results follow. Compression involves 

 not only expulsion of air from the lungs, but from 

 the air-sacs through the lungs. Expansion entails 

 an inrush through the lungs into the air-sacs. Thus 

 the air in the lungs is wholly tidal. The expiration of 

 combustion gas and the inspiration of fresh air are 

 thoroughly carried out, in which processes the function 

 of the air-sacs is not to act as respiratory organs 

 directly, but to subserve this ventilation of the lungs 

 by increasing the inward and outward wash of air and 

 carbonic acid. 



This rapid rushing breathing stirs the blood. 

 The muscles, fanned as by a draught, precipitate 

 their activity in a series of superbly adjusted move- 

 ments. The tissues burn well. Fuel has to be con- 

 tinuously supplied. Food is imperiously needful, 

 and the unappeasable, proverbial appetite of a bird 

 becomes intelligible to us. Heat, too, increases. The 

 feathers check its radiation and conduction. A nervous 



