ffectively
compressed nothing is DRAWN to the member; the hand preserves its color;
nothing flows into it, neither is it distended; but when the pressure is
diminished, as it is with the bleeding fillet, it is manifest that the
blood is instantly thrown in with force, for then the hand begins to
swell; which is as much as to say that when the arteries pulsate the
blood is flowing through them, as it is when the moderately tight
ligature is applied; but when they do not pulsate, or when a tight
ligature is used, they cease from transmitting anything; they are only
distended above the part where the ligature is applied. The veins again
being compressed, nothing can flow through them; the certain indication
of which is that below the ligature they are much more tumid than above
it, and than they usually appear when there is no bandage upon the arm.

"It therefore plainly appears that the ligature prevents the return of
the blood through the veins to the parts above it, and maintains those
beneath it in a state of permanent distention. But the arteries, in
spite of the pressure, and under the force and impulse of the heart,
send on the blood from the internal parts of the body to the parts
beyond the bandage."(5)

This use of ligatures is very significant, because, as shown, a very
tight ligature stops circulation in both arteries and veins, while a
loose one, while checking the circulation in the veins, which lie nearer
the surface and are not so directly influenced by the force of the
heart, does not stop the passage of blood in the arteries, which are
usually deeply imbedded in the tissues, and not so easily influenced by
pressure from without.

The last step of Harvey's demonstration was to prove that the blood does
flow along the veins to the heart, aided by the valves that had been
the cause of so much discussion and dispute between the great
sixteenth-century anatomists. Harvey not only demonstrated the presence
of these valves, but showed conclusively, by simple experiments, what
their function was, thus completing his demonstration of the phenomena
of the circulation.

The final ocular demonstration of the passage of the blood from the
arteries to the veins was not to be made until four years after Harvey's
death. This process, which can be observed easily in the web of a frog's
foot by the aid of a low-power lens, was first demonstrated by Marcello
Malpighi (1628-1694) in 1661. By the aid of a lens he first