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 (Figure 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 in which
this point could be settled.

(FIGURE 2. The Course of the Blood according to Galen (A.D. 170).)

And hence we are indebted, for the correction of the error of
Erasistratus, to one of the greatest experimenters of ancient or modern
times, Claudius Galenus, who lived in the second century after Christ. I
say it was to this man more than any one else, because he knew that the
only way of solving physiological problems was to examine into the facts
in the living animal. And because Galen was a skilful anatomist, and
a skilful experimenter, he was able to show in what particulars
Erasistratus had erred, and to build up a system of thought upon this
subject which was not improved upon for fully 1,300 years. I have
endeavoured, in Figure 2, to make clear to you exactly what it was he
tried to establish. You will observe that this diagram is practically
the same as that given in Figure 1, only simplified. The same facts may