128 



THE BIOLOGY OF BIRDS 



advantage. When the cavity of the thorax and abdomen 



is lessened by a raising of the sternum, and it may be a 

 lowering of the fused dorsal region of the vertebral column 

 if the bird is flying, the air is driven out of the lungs. But 



this also implies that non- 

 vitiated air passes into the 

 lungs from the air-sacs. 

 When the rebound comes 

 and the cavity of the body 

 increases again, air rushes 

 into the lungs and some 

 of it fills the air-sacs. 

 Thus in every complete re- 

 spiratory movement there 

 is a double tide of air into 

 and out of the lungs. Or, 

 to put it in another way, 

 the tidal air is renewed 

 twice in every respiratory 

 act. 



Another function of 

 the air-sacs is to promote 

 internal perspiration and 

 thus help in regulating the 

 high temperature of the 

 body. Most birds take 

 much water, but there is 

 little water in their urine, 

 which is semi-solid, and 



Fig. 33. — The system of air-sacs 

 bird. L., the right lung ; c, a cervical 

 air-sac ; i.ci.., the interclavicular ; a.s. 

 an outgrowth into the humerus (h.) ; xhcy have nO SWCat-glands 

 A.TH., anterior thoracic air-sac ; p.th., ■' ° 



the posterior thoracic; abd., the ab- 

 dominal air-sac ; tr. 

 windpipe. 



like mammals. Hence the 

 the trachea or advantage of the internal 

 surfaces of the air-sacs, 

 for water-vapour diffuses from the blood into the cavities 

 and passes out by the lungs. 



Some investigators of the air-sacs of birds think that 

 their importance in connection with respiration has been 

 over-rated. Thus B. IMiiller (1907) maintains that the 



