than that I have described. This is likewise evident, in
the common operation of blood letting: when the arm is tied, the vein
swells below the ligature, instead of above, and we do not make the
opening above the ligature, or on the side next the heart. If the
vein were opened above the ligature, it would not bleed. For it only
swells next the hand, which shows that the blood does not flow into
the vein downwards from the heart, but upwards from the hand.

If the ligature be too tight, the blood will not flow through the
opening in the vein. The reason of this, is, that the artery is
compressed, in this case, as well as the vein; and as the veins
derive their blood from the arteries, it follows that if the blood's
motion be obstructed in the latter, none can flow from them into the
former: when we wish to open an artery, the orifice must be made
above the ligature.

Another proof of the circulation being performed in this manner, is
derived from microscopic observations, on the transparent parts of
animals, in which the blood can be seen to move towards the
extremities, along the arteries, and return by the veins.

The blood, however, does not flow out of the heart into the arteries
in a continued stream, but by jets, or pulses; when the ventricles
are filled with blood from the auricles, this blood stimulates them,
and thereby causes them to contract; by such contraction, they force
the blood, which they contain, into the arteries; this contraction is
called the systole of the heart. As soon as they have finished their
contraction, they relax, till they are again filled with blood from
the auricles, and this state of relaxation of the heart, is called
the diastole.

This causes the pulsation or beating of the heart. The arteries must,
of course, have a similar pulsation, the blood being driven into them
only by starts; and accordingly we find it in the artery of the
wrist; this beating we call the pulse; the like may also be observed
in the arteries of the temples, and other parts of the body. The
veins, however, have no pulsation, for the blood flowing on, in an
uninterrupted course, from smaller tubes to wider, its pulse becomes
entirely destroyed.

The different cavities of the heart do not contract at the same time;
but the two auricles contract together, the ventricles being at that
time in a state of relaxation; these ventricles then contract
together, while the auricles become relaxed.

Both the arterie