THE BIOLOGY OF BIRDS 



'AS 



Fig. 34. — Windpipe and lungs 

 of bird. T., tongue; hy„ 

 horns of hyoid ; gl., glottis ; 

 TR., trachea ; ST.M., sterno- 

 tracheal muscles attached 

 to breastbone ; SY., syrinx ; 

 BR., bronchial tubes ; l., 

 cavity of lung ; AS., meso- 

 bronchus leading into the 

 abdominal air-sac- 



mesothelium. Now the endoderm 

 of the pouch gives off numerous 

 hollow buds in a complicated way 

 and these form the " bronchial 

 tree " of the lung. They branch 

 and branch, but the image of a 

 tree is inaccurate since the branches 

 unite by a network of inter-com- 

 municating passages. Eventually 

 in the fully-developed lung no 

 bronchial twigs end blindly. Of 

 the complicated internal con- 

 figuration of the endodermic lin- 

 ing which forms something like 

 an irregular labyrinth, the meso- 

 dermic envelope of the lung gives 

 no indication whatsoever. 



To sum up, the special adapta- 

 tions of the bird's respiratory 

 system are — (i) the large internal 

 surface of the lungs, due to the 

 complex branching of the bronchi ; 

 (2) the great development of air- 

 sacs which increase the efficiency 

 of the lungs ; (3) the fixing of the 

 lungs to the ribs so that the move- 

 ments of these in flight helps the 

 expulsion and inrush of air. 



§ 6. Excretion 



All vital activity seems to in- 

 volve — whatever else it involves 

 — ^the metabolism of proteins, 

 namely, the nitrogenous carbon- 

 compounds which are invariably 

 present in living matter. Take 

 albumin as a sample. Every cell 



