on the table. Observe in this instance that the
water is *all* pushed in the same direction. The movement of the water is
now like the effect produced on the blood in veins having valves when the
veins are compressed.

*To show the Position of the Valves in the Veins.*--Exercise the arm and
hand for a moment to increase the blood supply. Expose the forearm and
examine the veins on its surface. With a finger, stroke one of the veins
toward the heart, noting that, as the blood is pushed along on one side of
the finger the blood follows on the other side. Now stroke the vein toward
the hand. Places are found beyond which the blood does not follow the
finger. These mark the positions of valves.

*To show Effect of Exercise upon the Circulation.*--1. With a finger on the
"pulse" at the wrist or temple, count the number of heart beats during a
period of one minute under the following conditions: (_a_) when sitting;
(_b_) when standing; (_c_) after active exercise, as running. What
relation, if any, do these observations indicate between the general
activity of the body and the work of the heart?

2. Compare the size of the veins on the backs of the hands when they are
placed side by side on a table. Then exercise briskly the right hand and
arm, clenching and unclenching the fist and flexing the arm at the elbow.
Place the hands again side by side and, after waiting a minute, observe
the increase in the size of the veins in the hand exercised. How is this
accounted for?

*To Show the Effect of Gravity on the Circulation.*--Hold one hand high
above the head, at the same time letting the other hand hang loosely by
the side. Observe the difference in the color of the hands and the degree
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 bes