, showing the valve in a closed condition.

*To illustrate the Action of a Ventricle.*--Procure a syringe bulb with an
opening at each end. Connect a rubber tube with each opening, letting the
tubes reach into two tumblers containing water. By alternately compressing
and releasing the bulb, water is pumped from one vessel into the other.
The bulb may be taken to represent one of the ventricles. What action of
the ventricle is represented by compressing the bulb? By releasing the
pressure? Show by a sectional drawing the arrangement of the valves in the
syringe bulb.

[Fig. 25]

Fig. 25--Illustrating elasticity of arteries.

*To show the Advantage of the Elasticity of Arteries.*--Connect the syringe
bulb used in the last experiment with a rubber tube three or four feet in
length and having rather thin walls. In the opposite end of the rubber
tube insert a short glass tube which has been drawn (by heating) to a fine
point (Fig. 25). Pump water into the rubber tube, observing:

1. The swelling of the tube (pulse) as the water is forced into it. (This
is best observed by placing the fingers on the tube.)

2. The forcing of water from the pointed tubs during the interval when no
pressure is being applied from the bulb. Compare with the action of the
arteries when blood is forced into them from the ventricles.

Repeat the experiment, using a long glass tube terminating in a point
instead of the rubber tube. (In fitting the glass tube to the bulb use a
very short rubber tube.) Observe and account for the differences in the
flow of water through the inelastic tube.

*To show the Advantage of Valves in the Veins.*--Attach an open glass tube
one foot in length to each end of the rubber tube used in the preceding
experiment and fill with water (by sucking) to within about six inches of
the end. Lay on the table with the glass tubes secured in an upright
position (Fig. 26). Now compress the tube with the hand, noting that the
water rises in both tubes, being pushed in both directions. This effect is
similar to that produced on the blood when a vein having no valves is
compressed.

[Fig. 26]

Fig. 26.--*Simple apparatus* for showing advantage of valves in veins.

Now imitate the action of a valve by clamping the tube at one point, or by
closing it by pressure from the finger, and then compressing with the hand
some portion of the tube