vein, or
cava, near the heart. This is called the portal circulation.

359. The ventricles of the heart contract, or the "pulse" beats, about
seventy-five times every minute; in adults; in infants, more than a
hundred times every minute; in old persons, less than seventy-five
times every minute. The energy of the contraction of this organ varies
in different individuals of the same age. It is likewise modified by
the health and tone of the system. It is difficult to estimate the
muscular power of the heart; but, comparing it with other muscles, and
judging from the force with which blood is ejected from a severed
artery, it must be very great.

_Observation._ The phenomenon known under the name of pulse, is the
motion caused by the pressure of the blood against the coats of the
arteries at each contraction of the ventricles.

360. The following experiment will demonstrate that the blood flows
from the heart. Apply the fingers upon the artery at the wrist, at
two different points, about two inches apart; if the pressure be
moderately made, the "pulse" will be felt at both points. Let the
point nearest the heart be pressed firmly, and there will be no
pulsation at the lower point; but make strong pressure upon the lower
point only, and the pulsation will continue at the upper point,
proving that the blood flows from the heart, in the arteries, to
different parts of the system.

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358. What is the course of the veins? What peculiarity is observable
in the veins of the liver? 359. How often does the heart contract, or
the pulse beat, in adults? In infants? In old persons? What is said of
the energy of its contraction in different persons? How is the pulse
produced? 360. Demonstrate by experiment that the blood flows from the
heart.

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361. There are several influences, either separately or combined that
propel the blood from the heart through the arteries, among which may
be named,--1st. The contraction of the muscular walls of the heart.
2d. The contractile and elastic middle coat of the arteries aids the
heart in impelling the blood to the minute vessels of the system. 3d.
The peculiar action of the minute capillary vessels is considered, by
some physiologists, as a motive power in the arterial circulation.
4th. The pressure of the muscles upon the arteries, when in a state of
contraction, is a powerful agent, particularly when they are in active
exercise.

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