That the pulse is not produced by the arteries enlarging and so
filling, but by the arteries being filled with blood and so
enlarging.

We can now consider the method by which Harvey arrived at these results.
The work, "De Motu Cordis et Sanguinis," after giving an account of the
views of preceding physiologists, ancient and modern, commences with a
description of the heart as seen in a living animal when the chest has
been laid open and the pericardium removed. Three circumstances are
noted--

(_a_) The heart becomes erect, strikes the chest, and gives a beat;

(_b_) It is constricted in every direction;

(_c_) Grasped by the hand, it is felt to become harder during the
contraction.

From these circumstances it is inferred--

(1) That the action of the heart is essentially of the same nature as
that of voluntary muscles, which become hard and condensed when
they act;

(2) That, as the effect of this, the capacity of the cavities is
diminished, and the blood is expelled;

(3) That the intrinsic motion of the heart is the systole, and not the
diastole, as previously imagined.

The motions of the arteries are next shown to be dependent upon the
action of the heart, because the arteries are distended by the wave of
blood that is thrown into them, being filled like sacs or bladders, and
not expanding like bellows. These conclusions are confirmed by the
jerking way in which blood flows from a cut artery.

In the heart itself two distinct motions are observed--first of the
auricles, and then of the ventricles. These alternate contractions and
dilatations can have but one result, namely, to force the blood from the
auricle to the ventricle, and from the ventricle, on the right side, by
the pulmonary artery to the lungs, and on the left side by the aorta to
the system.

These considerations suggest to the mind of Harvey the idea of the
circulation. "I began to think," he says, "whether there might not be a
motion, as it were, in a circle." This is next established by proving
the three following propositions:--

(1) The blood is incessantly transmitted by the action of the heart
from the vena cava to the arteries in such quantity that it
cannot be supplied from the ingesta, and in such wise that the
whole mass must very quickly pass through the organ;

(2) The blood, under the influence of the arterial pulse, enters, and
is i