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the enjoyment of a highly developed civilisation。 But of what
knowledge they may have possessed beyond this we know nothing； and in
tracing back the springs of the origin of everything that we call
〃modern science〃 （which is not merely knowing； but knowing
systematically； and with the intention and endeavour to find out the
causal connection of things）I say that when we trace back the
different lines of all the modern sciences we come at length to one
epoch and to one countrythe epoch being about the fourth and fifth
centuries before Christ； and the country being ancient Greece。 It is
there that we find the commencement and the root of every branch of
physical science and of scientific method。 If we go back to that time
we have in the works attributed to Aristotle； who flourished between
300 and 400 years before Christ； a sort of encyclopaedia of the
scientific knowledge of that dayand a very marvellous collection of；
in many respects； accurate and precise knowledge it is。 But； so far as
regards this particular topic； Aristotle； it must be confessed； has not
got very far beyond common knowledge。 He knows a little about the
structure of the heart。 I do not think that his knowledge is so
inaccurate as many people fancy； but it does not amount to much。 A very
few years after his time； however； there was a Greek philosopher；
Erasistratus； who lived about three hundred years before Christ； and
who must have pursued anatomy with much care； for he made the important
discovery that there are membranous flaps； which are now called
〃valves；〃 at the origins of the great vessels； and that there are
certain other valves in the interior of the heart itself。
Fig。 1。The apparatus of the circulation； as at present known。 The
capillary vessels； which connect the arteries and veins； are omitted；
on account of their small size。 The shading of the 〃venous system〃 is
given to all the vessels which contain venous blood； that of the
〃arterial system〃 to all the vessels which contain arterial blood。
I have here （Fig。 1） a purposely rough； but； so far as it goes；
accurate； diagram of the structure of the heart and the course of the
blood。 The heart is supposed to be divided into two portions。 It
would be possible； by very careful dissection； to split the heart down
the middle of a partition； or so…called 'septum'； which exists in it；
and to divide it into the two portions which you see here represented；
in which case we should have a left heart and a right heart； quite
distinct from one another。 You will observe that there is a portion of
each heart which is what is called the ventricle。 Now the ancients
applied the term 'heart' simply and solely to the ventricles。 They did
not count the rest of the heartwhat we now speak 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 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