HOW ANIMALS BREATHE 



325 



most quickly die for lack of it. In birds, respiration is 

 not confined to the lungs ; but, as in insects, extends 

 through a great part of the body. Air sacs connected 

 with the lungs exist in the abdomen and under the 

 skin of the neck, wings, and legs. Even the bones 

 are hollow for this purpose ; so that if the windpipe be 



FIG. 282. Lungs of a Frog; a, 

 hyoid apparatus; b, cartilaginous 

 ring at root of the lungs; c, pul- 

 monary vessels; d, pulmonary 

 sacs, having this peculiarity com- 

 mon to all cold-blooded air breath- 

 ers, that the trachea does not 

 divide into bronchial branches, but 

 terminates abruptly by orifices 

 which open at once into the gen- 

 eral cavity. A cartilaginous net- 

 work divides the space into little 

 sacs, on the walls of which the 

 capillaries are spread. 



FIG. 283. Distribution of Air Tubes in Mam- 

 malian Lungs: a, larynx; b t trachea; c, d, left 

 and right bronchial tubes: e,f. g, the ramifica- 

 tions. In Man the subdivision continues until 

 the ultimate tubes are one twenty-fifth of an 

 inch in diameter. Each lobule represents in 

 miniature the structure of the entire lung of a 

 Frog. 



tied, and an opening be made in the wing bone, the bird 

 will continue to respire. The right lung is usually the 

 larger ; in some snakes, the left is wanting entirely. 

 In most vertebrates, lungs are freely suspended ; in 

 birds, they are fastened to the back. 



The lungs communicate with the atmosphere by 

 means of the trachea, or windpipe, formed of a series of 

 cartilaginous rings, which keep it constantly open. It 



