BIRDS. 



229 



that sends it 

 through the 

 great aorta in- 

 to numerous 

 branches that 

 lead all over 

 the body. 



Respiration. 

 The breath- 

 ing organs are 

 shown in Fig. 

 269, /,/. The 

 lungs, /, /, are 

 two in number, 

 spongy in con- 

 sistency, and 

 are attached to 

 the walls of the 

 cavity in which 

 they are placed. 



The large air-passages of the lungs end in air-sacs, that 

 in turn connect with the cavities of the bones, so that air 

 taken in passes down the trachea, /, enters the lungs, /, /, 

 aerating the blood there, also passing into the 

 air-sacs and penetrating the hol- 

 low bones in every part of the body ; 

 thus the bird can alter its specific 

 gravity at will. The air-sacs are 

 nine in number ; two are placed in 

 the abdomen, four in the thorax, 

 and three near the wish-bone. 



Nervous System. The brain 

 (Fig. 270, i) is larger than in the 

 reptiles, and has no convolutions. 



Covering. The birds differ from all other animals in 

 being covered with feathers (Fig. 271), that are modified 



FIG. 269. Parts of a bird, i, digestive apparatus ; 

 c, crop ; g, gizzard ; ^, trachea ; , , bronchial 

 tubes ; /, /, lungs ; 2, bones of the wings. 



FIG. 270. i, Brain of a 

 bird ; 2, eye, showing nic- 

 titating membrane. 



