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, byj 
raising the ribs and lowering the diaphragm, which en¬ 
large the capacity of the 
chest, and the air rushes 
b 
9 
a 
/ 
\ 
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- 
"~ h tive and more muscular an 
" d animal, the greater the de- 
('" b mand for oxygen. Thus, 
1-6’ warm-blooded animals live 
fast, and their rapidly de¬ 
caying tissues call for rapid 
v b> respiration; while in the 
\ b , cold-blooded creatures the 
waste is comparatively 
/ 
/ 
a 
Fig. SS.—Human Thorax: a, vertebral col- slow. Respiration is IllOSt 
nirm • Ti TrS riho tlio Imirot* rrnoD foloo • /» 
the ribs ; h, sternum. 
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 
