42 
COMPARATIVE ZOOLOGY. 
all animals, the functions are performed under conditions 
essentially the same. Thus, respiration in the Sponge, the 
Fish, and in Man has one object and one means, though 
the methods differ. A function, therefore, is a group of 
similar phenomena effected by analogous structures. 
The life of an animal consists in the accumulation and 
expenditure of force. The tissues are storehouses of 
power, which, as they waste, is given off in various forms. 
Thus, the nervous tissue generates nerve-force; the mus¬ 
cles, motion. If we contemplate the phenomena presented 
by a Dog, the most obvious fact is his power of moving 
from place to place, 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, hear¬ 
ing, 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 
muscles 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 un¬ 
der contribution to furnish a supply. Hence the feelings 
of hunger and thirst, impelling the creature to seek food. 
Only this 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 nourishing part is absorbed, and carried to the heart, 
which propels it through tubes, called blood-vessels, all 
over the body. In this process of digestion, certain fluids 
