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But if it be asked how it happens that the blood in the veins； flowing in
this way continually into the heart； is not exhausted； and why the arteries
do not bee too full； since all the blood which passes through the heart
flows into them； I need only mention in reply what has been written by a
physician 1 of England； who has the honor of having broken the ice on
this subject； and of having been the first to teach that there are many small
passages at the extremities of the arteries； through which the blood
received by them from the heart passes into the small branches of the
veins； whence it again returns to the heart； so that its course amounts
precisely to a perpetual circulation。 Of this we have abundant proof in
the ordinary experience of surgeons； who； by binding the arm with a tie of
moderate straitness above the part where they open the vein； cause the
blood to flow more copiously than it would have done without any ligature；
whereas quite the contrary would happen were they to bind it below； that
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DISCOURSE ON THE METHOD OF RIGHTLY CONDUCTING THE REASON； AND
SEEKING TRUTH IN THE SCIENCES
is； between the hand and the opening； or were to make the ligature above
the opening very tight。 For it is manifest that the tie； moderately
straightened； while adequate to hinder the blood already in the arm from
returning towards the heart by the veins； cannot on that account prevent
new blood from ing forward through the arteries； because these are
situated below the veins； and their coverings； from their greater
consistency； are more difficult to press； and also that the blood which
es 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
e 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 e 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 marked only in the
neighborhood of the heart； and is not so evident in parts more remote from
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DISCOURSE ON THE METHOD OF RIGHTLY CONDUCTING THE REASON； AND
SEEKING TRUTH IN THE SCIENCES
it。 In the next place； the consistency of the coats of which the arterial
vein and the great artery are posed； sufficiently shows that the blood
is impelled against them with more force than against the veins。 And
why should the left cavity of the heart and the great artery be wider and
larger than the right cavity and the arterial vein； were it not that the blood
of the venous artery； having only been in the lungs after it has passed
through the heart； is thinner； and rarefies more readily； and in a higher
degree； than the blood which proceeds immediately from the hollow vein？
And what can physicians conjecture from feeling the pulse unless they
know that according as the blood changes its nature it can be rarefied by
the warmth of the heart； in a higher or lower degree； and more or less
quickly than before？ And if it be inquired how this heat is municated
to the other members； must it not be admitted that this is effected by
means of the blood； which； passing through the heart； is there heated anew；
and thence diffused over all the body？ Whence it happens； that if the
blood be withdrawn from any part； the heat is likewise withdrawn by the
same means； and although the heart were as…hot as glowing iron； it would
not be capable of warming the feet and hands as at present； unless it
continually sent thither new blood。