120 COMPARATIVE ZOOLOGY. 
bone and ribs) and the 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, thus enlarging the 
capacity of the chest, in 
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 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 
w r aste is comparatively 
slow 7 . 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 
Fig. S8 .—Human Thorax: a , vertebral col¬ 
umn ; &, ribs, the lower ones false ; c, 
clavicle; c, intercostal muscles, removed 
on the left side to show the diaphragm, d; 
/, pillars of the diaphragm attached to the 
lumbar vertebrae; g, muscles for elevating 
the ribs ; h , sternum. 
