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 in which this point could be settled.

Fig.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