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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 us。 Well then； the first thing that Galen
did was to make out experimentally that； during life； the arteries are
not full of air； but that they are full of blood。 And he describes a
great variety of experiments which he made upon living animals with the
view of proving this point； which he did prove effectually and for all
time； and that you will observe was the only way of settling the
matter。 Furthermore； he demonstrated that the cavities of the left
side of the heartwhat we now call the left auricle and the left
ventricleare； like the arteries； full of blood during life； and that
that blood was of the scarlet kindarterialised； or as he called it
〃pneumatised；〃 blood。 It was known before； that the pulmonary artery；
the right ventricle； and the veins； contain the darker kind of blood；
which was thence called venous。 Having proved that the whole of the
left side of the heart； during life； is full of scarlet arterial blood；
Galen's next point was to inquire into the mode of communication
between the arteries and veins。 It was known before his time that both
arteries and veins branched out。 Galen maintained； though he could not
prove the fact； that the ultimate branches of the arteries and veins
communicated together somehow or other； by what he called
'anastomoses'； and that these 'anastomoses' existed not only in the body
in general but also in the lungs。 In the next place； Galen maintained
that all the veins of the body arise from the liver； that they draw the
blood thence and distribute it over the body。 People laugh at that
notion now…a…days； but if anybody will look at the facts he will see
that it is a very probable supposition。 There is a great vein （hepatic
veinFig。 1） which rises out of the liver； and that vein goes straight
into the 'vena cava' （Fig。 1） which passes to the heart； being there
joined by the other veins of the body。 The liver itself is fed by a
very large vein （portal veinFig。 1）； which comes from the alimentary
canal。 The way the ancients looked at this matter was； that the food；
after being received into the alimentary canal； was then taken up by the
branches of this great vein； which are called the 'vena portae'； just
as the roots of a plant suck up nourishment from the soil in which it
lives； that then it was carried to the liver； there to be what was
called 〃concocted；〃 which was their phrase for its conversion into
substances more fitted for nutrition than previously existed in it。
They then supposed that the next thing to be done was to distribute
this fluid through the body； and Galen like his predecessors； imagined
that the 〃concocted〃 blood； having entered the great 'vena cava'； was
distributed by its ramifications all over the body。 So that； in his
view （Fig。 2）； the course of the blood was from the intestine to the
liver； and from the liver into the great 'vena cava'； including what we
now call the right auricle of the heart； whence it was distributed by
the branches of the veins。 But the whole of the blood was not thus
disposed of。 Part of the blood； it was supposed； went through what we
now call the pulmonary arteries （Fig。 1）； and； branching out there； gave
exit to certain 〃fuliginous〃 products； and at the same time took in
from the air a something which Galen calls the 'pneuma'。 He does not
know anything about what we call oxygen； but it is astonishing how very
easy it would be to turn his language into the equivalent of modern
chemical theory。 The old philosopher had so just a suspicion of the
real state of affairs that you could make use of his language in many
cases； if you substituted the word 〃oxygen；〃 which we now…a…days use；
for the word 'pneuma'。 Then he imagined that the blood； further
concocted or altered by contact with the 'pneuma'； passed to a certain
extent to the left side of the heart。 So that Galen believed that
there was such a thing as what is now called the pulmonary
circulation。 He believed； as much as we do； that the blood passed
through the right side of the heart； through the artery which goes to
the lungs； through the lungs themselves； and back by what we call the
pulmonary veins to the left side of the heart。 But he thought it was
only a very small portion of the blood which passes to the right side of
the heart in this way； the rest of the blood； he thought； passed
through the partition which separates the two ventricles of the heart。
He describes a number of small pits； which really exist there； as
holes； and he supposed that the greater part of the blood passed
through these holes from the right to the left ventricle （Fig 2）。
It is of great importance you should clearly understand these teachings
of Galen； because； as I said just now； they sum up all that anybody
knew until the revival of learning； and they come to thisthat the
blood having passed from the stomach and intestines through the liver；
and having entered the great veins； was by them distributed to every
part of the body； that part of the blood； thus distributed； entered the
arterial system by the 'anastomoses'； as Galen called them； in the
lungs； that a very small portion of it entered the arteries by the
'anastomoses' in the body generally； but that the greater part of it
passed through the septum of the heart； and so entered the left side
and mingled with the pneumatised blood； which had been subjected to the
air in the lungs； and was then distributed by the arteries； and
eventually mixed with the currents of blood； coming the other way；
through the veins。
Yet one other point about the views of Galen。 He thought that both the
contractions and dilatations of the heartwhat we call the 'systole'
or contraction of the heart； and the 'diastole' or dilatationGalen
thought that these were both active movements； that the heart actively
dilated； so that it had a sort of sucking power upon the fluids which
had access to it。 And again； with respect to the movements of the
pulse； which anybody can feel at the wrist and elsewhere； Galen was of
opinion that the walls of the arteries partook of that which he
supposed to be the nat