contract, as do also the arteries, because the blood that has entered them
has cooled, and the six small valves close, and the five of the hollow
vein and of the venous artery open anew and allow a passage to other two
drops of blood, which cause the heart and the arteries again to expand as
before.  And, because the blood which thus enters into the heart passes
through these two pouches called auricles, it thence happens that their
motion is the contrary of that of the heart, and that when it expands they
contract.  But lest those who are ignorant of the force of mathematical
demonstrations and who are not accustomed to distinguish true reasons from
mere verisimilitudes, should venture.  without examination, to deny what
has been said, I wish it to be considered that the motion which I have now
explained follows as necessarily from the very arrangement of the parts,
which may be observed in the heart by the eye alone, and from the heat
which may be felt with the fingers, and from the nature of the blood as
learned from experience, as does the motion of a clock from the power, the
situation, and shape of its counterweights and wheels.

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 become 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 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 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
marked only in the neighborhood of the heart; and is not so evident in
parts more remote from it.  In the next place, the consistency of the coats
of which the arterial vein and the great artery are  composed,
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 communicated 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.  We likewise perceive from this, that the true use of respiration is
to bring sufficient fresh air into the lungs, to cause the blood which
flows into them from the right ventricle of the heart, where it has been
rarefied and, as it were, changed into vapors, to become thick, and to
convert it anew into blood, before it flows into the left cavity, without
which process it would be unfit for the nourishment of the fire that is
there.  This receives confirmation from the circumstance, that it is
observed of animals destitute of lungs that they have also but one cavity
in the heart, and that in children who cannot use them while in the womb,
there is a hole through which the blood flows from the hollow vein into
the left cavity of the heart, and a tube through which it passes from the
arterial vein into the grand artery without passing through the lung.  In
the next place, how could digestion be carried on in the stomach unless
the heart communicated heat to it through the arteries, and along with
this certain of the more fluid parts of the blood, which assist in the