46 COMPARATIVE ZOOLOGY. 
1. The food must be procured, and swallowed. (Inges: 
tion.) 
2. The food must be dissolved, and the nutritious parts 
separated into a fluid. (Digestion.) 
3. The nutritive fluid must be carefully taken up, and 
then distributed all over the body. (Absorption and Cir- 
culation.) 
4. The nutritive fluid, now called blood, must be ex- 
posed to the air, to absorb oxygen and liberate carbonic 
acid. (Respiration.) 
5. The tissues must repair their parts wasted by use, 
by transforming particles of blood into living matter like 
themselves. (Assimilation.) 
6. Certain matters must be strained from the blood, 
some to serve a purpose, others to be @gst out of the sys- 
tem. (Secretion and Excretion.) 
The mechanism to accomplish all this in the lowest 
forms of life is exceedingly simple, a single cavity per- 
forming all the functions. But in the majority of ani- 
mals the apparatus is very complicated: there is a set of 
organs for the prehension of food; another, for digestion ; 
a third, for absorption; a fourth, for distribution; and a 
fifth, for purification. 
CHAPTER VII. 
THE FOOD OF ANIMALS. 
Tur term food includes all substances which contribute 
to nutrition, whether they simply assist in the process, or 
are actually appropriated, and become tissue. With the 
food is usually combined more or less indigestible matter, 
which is separated in digestion. 
