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.
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