What Erasistratus made out was that, at the roots of the aorta and the
pulmonary artery (Fig. 1) there were valves, which opened in the
direction indicated by the arrows; and, on the other hand, that at the
junction of what he called the veins with the heart there were other
valves, which also opened again in the direction indicated by the
arrows. This was a very capital discovery, because it proved that if
the heart was full of fluid, and if there were any means of causing
that fluid in the ventricles to move, then the fluid could move only in
one direction; for you will observe that, as soon as the fluid is
compressed, the two valves between the ventricles and the veins will be
shut, and the fluid will be obliged to move into the arteries; and, if
it tries to get back from them into the heart, it is prevented from
doing so by the valves at the origin of the arteries, which we now call
the semilunar valves (half-moon shaped valves); so that it is
impossible, if the fluid move at all, that it should move in any other
way than from the great veins into the arteries.
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