nt Fluid in its Relations to the Cells.*--The maintenance of
life within the cells requires, as we have seen, that they be supplied
with water, food, and oxygen, and that they be relieved of such wastes as
they form. This double purpose is accomplished through the agency of an
internal nutrient fluid, a portion of which has already been referred to
as the lymph. Not only does this fluid supply the means for keeping the
cells alive, but, through the cells, it is also the means of preserving
the life of the body as a whole.

The cells, however, rapidly exhaust the nutrient fluid. They take from it
food and oxygen and they put into it their wastes. To prevent its becoming
unfit for supplying their needs, food and oxygen must be continually added
to this fluid, and waste materials must be continually removed. This is
not an easy task. As a matter of fact, the preparation, distribution, and
purification of the nutrient fluid requires the direct or indirect aid of
practically all parts of the body. It supplies for this reason a broad
basis for the division of labor on the part of the cells.

*Relation of the Body to its Environment.*--While life is directly
dependent upon the internal nutrient fluid, it is indirectly dependent
upon the physical surroundings of the body. Herein lies the need of the
_external_ organs--the feet and legs for moving about, the hands for
handling things, the eyes for directing movements, etc. That the great
needs of the body are supplied from its surroundings are facts of common
experience. Food, shelter, air, clothing, water, and the means of
protection are external to the body and form a part of its environment. In
making the things about him contribute to his needs, man encounters a
problem which taxes all his powers. Only by toil and hardship, "by the
sweat of his brow," has he been able to wrest from his surroundings the
means of his sustenance.

*The Main Physiological Problems.*--The study of the body is thus seen to
resolve itself naturally into the consideration of two main problems:

1. _That of maintaining in the body a nutrient fluid for the cells._

2. _That of bringing the body into such relations with its surroundings as
will enable it to secure materials for the nutrient fluid and satisfy its
other needs._

The first problem is _internal_ and includes the so-called vital
processes, known as digestion, circulation, respiration, and excretion.
The second problem is _external_, as