HOW ANIMALS BREATHE 



327 



consequence of which the air rushes in to prevent a 

 vacuum ; the latter, by the ascent of the diaphragm and 

 the descent of the ribs. 



As a rule, the more active and more muscular an 

 animal, the greater the demand for oxygen. Thus, 

 warm-blooded animals live fast, and their rapidly decay- 

 ing tissues call for rapid 

 respiration ; while in the 

 cold-blooded creatures the 

 waste is comparatively 

 slow. Respiration is most 

 active in birds, and least in 

 water-breathing animals. 

 The sluggish toad respires 

 more slowly than the busy 

 bee, the mollusk more 

 slowly than the fish. 

 But respirations, like 

 beats of the heart, are 

 fewer in large mammals 

 than in small ones. An 

 average man inhales about 

 300-400 cubic feet of air 

 per day of rest, and much 



* 

 more When at WOrk. 



. , i r 



Another reSUlt OI reS- 



piration, besides the puri- 

 fication of the blood, is 



the production of heat. The chemical combination of 

 the oxygen in the air with the carbon in the tissues 

 is a true combustion ; and, therefore, the more active 

 the animal and its breathing, the higher its tempera- 

 ture. Birds and mammals have a constant temperature, 

 which is usually higher than that of the atmosphere 

 (108 and 100 F. respectively). They are therefore 



/ 



FIG. 285. Human Thorax: a, vertebral col- 

 umn; b, b' , ribs, the lower ones false; c, 

 clavicle; e, intercostal muscles, removed on 

 the left side to show the diaphragm, d ; f, 

 pillars of the diaphragm attached to the lum- 



