49 COMPARATIVE ZOOLOGY. 
a power produced by the interplay of muscles and bones. 
We observe, also, that his motions are neither mechanical 
nor irregular; there is method in his movement. He has 
the power of willing, seeing, hearing, feeling, etc.; and 
these functions are accomplished by a delicate apparatus 
of nerves. 
But the Dog does not exhibit perpetual motion. Sooner 
or later he becomes exhausted, and rest is necessary. Sleep 
gives only temporary relief. In every exercise of the mus- 
cles and nerves there is a consumption or waste of their 
substance. The blood restores the organs, but in time the 
blood itself needs renewal. If not renewed, the animal 
becomes emaciated, for the whole body is laid under con- 
tribution to furnish a supply. Hence the feelings of hun- 
ger and thirst, impelling the creature to seek food. This 
alone will maintain the balance between waste and repair. 
We notice, therefore, an entirely different set of functions, 
involving, however, the use of motion and will. The Dog 
seizes a piece of meat, grinds it between its teeth, sends it 
into the stomach, where it is digested, and then into the 
intestine, where it is further changed; there the nourish- 
ing part is absorbed, and carried to the heart, which 
propels it through little tubes, called blood - vessels, all 
over the body. In this process of digestion, certain fluids 
are required, as saliva, gastric juice, and bile: these are 
secreted by special organs, called glands. Moreover, since 
all the food eaten is not fitted to make blood, and as the 
blood itself, in going around the body, acts like a scaven- 
ger, picking up worn-out particles, we have another funce- 
tion, that of excretion, or removal of useless matter from 
the system. The kidneys and lungs do much of this; but 
the lungs do something else. They expose the blood to 
the air, and introduce oxygen, which, we shall find, is es- 
sential to the life of every animal. 
These centripetal and centrifugal movements in the 
