62. The following experiments will demonstrate that the blood from
every part of the system flows to the heart by the agency of the
veins. 1st. Press firmly on one of the veins upon the back of the
hand, carrying the pressure toward the fingers; for a moment, the vein
will disappear. On removing the pressure of the finger, it will
reappear, from the blood rushing in from below.

2d. If a tape be tied around the arm above the elbow, the veins below
will become larger and more prominent, and also a greater number will
be brought in view, while the veins above the tape are less distended.
At this time, apply the finger at the wrist, and the pulsation of the
arteries still continues, showing that the blood is constantly flowing
from the heart through the arteries, into the veins; and the
increased size of the veins shows that the pressure of the tape
prevents its flowing back to the heart.

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361. State the influences that propel the blood from the heart. 362.
Demonstrate by the first experiment that the blood flows to the heart.
By the second experiment.

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363. The influences that return the blood to the heart through the
veins, are not so easily understood as those that act on the blood in
the arteries. Some physiologists have imputed an active propulsive
power to the capillary vessels in carrying the blood through the
veins. This is not easily explained, and perhaps it is as difficult to
understand. An influence upon which others have dwelt, is the suction
power of the heart in active dilatation, acting as a _vis a fronte_
(power in front) in drawing blood to it.

364. Another influence that aids the venous circulation is attributed
to the propulsive power of the heart. It is not easy to comprehend how
this power of the heart can be extended through the capillary vessels
to the blood in the veins. Again, an important agency has been found,
by some physiologists, in the inspiratory movements, which are
supposed to draw the blood of the veins into the chest, in order to
supply the vacuum which is created there by the elevation of the ribs
and the descent of the diaphragm.

365. One of the most powerful causes which influence the venous
circulation, is the frequently-recurring action of the muscles upon
the venous trunks. When the muscles are contracted, they compress that
portion of the veins which lie beneath the swell, and thus force the
blood from one valve t