ds
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 heart--what 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 flui