system" to all the vessels which contain arterial blood.)

I have here (Figure 1) a purposely rough, but, so far as it goes,
accurate, diagram of the structure of the heart and the course of the
blood. The heart is supposed to be divided into two portions. It would
be possible, by very careful dissection, to split the heart down the
middle of a partition, or so-called 'septum', which exists in it, and to
divide it into the two portions which you see here represented; in which
case we should have a left heart and a right heart, quite distinct from
one another. You will observe that there is a portion of each heart
which is what is called the ventricle. Now the ancients applied the term
'heart' simply and solely to the ventricles. They did not count the rest
of the heart--what we now speak of as the 'auricles'--as any part of the
heart at all; but when they spoke of the heart they meant the left and
the right ventricles; and they described those great vessels, which we
now call the 'pulmonary veins' and the 'vena cava', as opening directly
into the heart itself.

What Erasistratus made out was that, at the roots of the aorta and
the pulmonary artery (Figure 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. Now that was a very
remarkable and striking discovery.

But it is not given to any man to be altogether right (that is a
reflection which it is very desirable for every man who has had the good
luck to be nearly right once, always to bear in mind); and Erasistratus,
while he made this capital and