prevent new blood from coming forward through the arteries, because these
are situated below the veins, and their coverings, from their greater
consistency, are more difficult to compress; and also that the blood which
comes from the heart tends to pass through them to the hand with greater
force than it does to return from the hand to the heart through the veins.
And since the latter current escapes from the arm by the opening made in
one of the veins, there must of necessity be certain passages below the
ligature, that is, towards the extremities of the arm through which it can
come thither from the arteries. This physician likewise abundantly
establishes what he has advanced respecting the motion of the blood, from
the existence of certain pellicles, so disposed in various places along
the course of the veins, in the manner of small valves, as not to permit
the blood to pass from the middle of the body towards the extremities, but
only to return from the extremities to the heart; and farther, from
experience which shows that all the blood which is in the body may flow
out of it in a very short time through a single artery that has been cut,
even although this had been closely tied in the immediate neighborhood of
the heart and cut between the heart and the ligature, so as to prevent the
supposition that the blood flowing out of it could come from any other
quarter than the heart.

But there are many other circumstances which evince that what I have
alleged is the true cause of the motion of the blood: thus, in the first
place, the difference that is observed between the blood which flows from
the veins, and that from the arteries, can only arise from this, that
being rarefied, and, as it were, distilled by passing through the heart,
it is thinner, and more vivid, and warmer immediately after leaving the
heart, in other words, when in the arteries, than it was a short time
before passing into either, in other words, when it was in the veins; and
if attention be given, it will be found that this difference is very