suggest the Double Work of Glands.*--Prepare a simple filter by fitting
a piece of porous paper into a glass funnel. Through this pass pure water
and also water having salt dissolved in it and containing some sediment,
as sand. The water and the dissolved salt pass through, while the sediment
remains on the filter. Now substitute a fresh piece of paper in the funnel
and drop on its surface a little solid coloring matter, such as cochineal.
Again pass the liquid through the funnel. This time it comes through
colored, the color being added by the filter. Compare the filter and
materials filtered to the gland and the materials concerned in secretion
(blood, the liquid secreted, substances added by the gland, etc.).

[Fig. 92]

Fig. 92--*The physiological scheme.* Diagram suggesting the essential
relation of the bodily activities. See Summary of Part I, page 215, and
Summary of Part II, page 413.

SUMMARY OF PART I

The body is an organization of different kinds of cells; it grows through
the growth and reproduction of these cells; and its life as a whole is
maintained by providing such conditions as will enable the cells to keep
alive. Of chief importance in the work of the body is a nutrient fluid
which supplies the cells with food and oxygen and relieves them of waste.
A moving portion of this fluid, called the blood, serves as a transporting
agent, while another portion, called the lymph, passes the materials
between the blood and the cells. Through their effects upon the blood and
the lymph, the organs of circulation, respiration, digestion, and
excretion minister in different ways to the cells, and aid in the
maintenance of life. By their combined action two distinct movements are
kept up in the body, as follows:

1. An _inward_ movement which carries materials from the outside of the
body toward the cells.

2. An _outward_ movement which carries materials from the cells to the
outside of the body.

Passing _inward_ are the oxygen and food materials _in a condition to
unite with each other_ and thereby change their potential into kinetic
energy. Passing _outward_ are the oxygen and the elements that formed the
food materials _after having united_ at the cells and liberated their
energy.

As a final and all-important result, there is kept up a _continuous series
of chemical changes_ in the cells. These liberate the energy, provide
special substa