t
results. It should be attached to a thin board which has an opening in one
end over which the web of the foot may be stretched. Threads should extend
from two of the toes to pins driven into the board to secure the necessary
tension of the web, and the foot and lower leg should be kept moist. Using
a two-thirds-inch objective, observe the branching of the small arteries
into the capillaries and the union of the capillaries to form the small
veins. The appearance is truly wonderful, but allowance must be made for
the fact that the _motion_ of the blood is magnified, as well as the
different structures, and that it appears to move much faster than it
really does. With a still higher power, the movements of the corpuscles
through the capillaries may be studied.

NOTE.--To perform this experiment without destroying the brain, the frog is
first carefully wrapped with strips of wet cloth and securely tied to the
board. The wrapping, while preventing movements of the frog, must not
interfere with the circulation.

CHAPTER VI - THE LYMPH AND ITS MOVEMENT THROUGH THE BODY

[Fig. 27]

Fig. 27--*Diagram showing position of the lymph* with reference to the
blood and the cells. The central tube is a capillary. The arrows indicate
the direction of slight movements in the lymph.

The blood, it will be remembered, moves everywhere through the body in a
system of _closed_ tubes. These keep it from coming in contact with any of
the cells of the body except those lining the tubes themselves. The
capillaries, to be sure, bring the blood very near the cells of the
different tissues; still, there is need of a liquid to fill the space
between the capillaries and the cells and to transfer materials from one
to the other. The lymph occupies this position and does this work. The
position of the lymph with reference to the capillaries and the cells is
shown in Fig. 27.

*Origin of the Lymph.*--The chief source of the lymph is the plasma of the
blood. As before described, the walls of the capillaries consist of a
single layer of flat cells placed edge to edge. Partly on account of the
pressure upon the blood and partly on account of the natural tendency of
liquids to pass through animal membranes, a considerable portion of the
plasma penetrates the thin walls and enters the spaces occupied by the
lymph.

The cells themselves also help to form the lymph, since the water and
wastes