to which the large veins are filled. Repeat the experiment, reversing the
position of the hands. What results are observed? In what parts of the
body does gravity aid in the return of the blood to the heart? In what
parts does it hinder? Where fainting is caused by lack of blood in the
brain (the usual cause), is it better to let the patient lie down flat or
to force him into a sitting posture?

*To study the Circulation in a Frog’s Foot* (Optional).—A compound
microscope is needed in this study and for extended examination it is best
to destroy the frog’s brain. This is done by inserting some blunt-pointed
instrument into the skull cavity from the neck and moving it about. A
small frog, on account of the thinness of its webs, gives the best
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.