120 



COMPARATIVE ZOOLOGY. 



bone and ribs) and a diaphragm, or muscular partition, 

 separating it from the abdomen. 68 Inspiration (or filling 

 the lungs) and expiration (or emptying the lungs) are 

 both accomplished by muscular exertion ; the former, by 

 raising the ribs and lowering the diaphragm, which en-, 



large the capacity of the 

 chest, and the air rushes 

 in to prevent a vacuum ; 

 the latter, by the ascent of 

 the diaphragm and the de- 

 . " & scent of the ribs. 



As a rule, the more ac- 

 tive and more muscular an 

 animal, the greater the de- 

 mand for oxygen. Thus, 

 warm-blooded animals live 

 fast, and their rapidly de- 

 caying 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 respi- 

 rations, like beats of the heart, are fewer in large Mam- 

 mals 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. 



Another result of respiration, besides the purification 

 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 



PIG. 88. Human Thorax: a, vertebral col- 

 umn ; &, &', 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 

 lumbar vertebrae ; g, muscles for elevating 

 the ribs ; h, sternum. 



