d, 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 important discovery, made a very capital
and important error in another direction, although it was a very natural
error. If, in any animal which is recently killed, you open one of those
pulsating trunks which I referred to a short time ago, you will find, as
a general rule, that it either contains no blood at all or next to none;
but that, on the contrary, it is full of air. Very naturally, therefore,
Erasistratus came to the conclusion that this was the normal and natural
state of the arteries, and that they contained air. We are apt to think
this a very gross blunder; but, to anybody who is acquainted with
the facts of the case, it is, at first sight, an exceedingly natural
conclusion. Not only so, but Erasistratus might have very justly
imagined that he had seen his way to the meaning of the connection of
the left side of the heart with the lungs; for we find that what we now
call the pulmonary vein is connected with the lungs, and branches out in
them (Fig. 1). Finding that the greater part of this system of vessels
was filled with air after death, this ancient thinker very shrewdly
concluded that its real business was to receive air from the lungs, and
to distribute that air all through the body, so as to get rid of the
grosser humours and purify the blood. That was a very natural and very
obvious suggestion, and a highly ingenious one, though it happened to be
a great error. You will observe that the only way of correcting it was
to experiment upon living animals, for there is no other way