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