William Harvey and the Discovery of the Circulation of the Blood | Page 6

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 (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. 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 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
subject which was not improved upon for fully 1,300 years. I have
endeavoured, in Fig. 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 Fig. 1, only simplified. The same facts may be
looked upon by different people from different points of view. Galen
looked upon these facts from a very different point of view from that
which we ourselves occupy; but, so far as the facts are concerned, they
were the same for him as for