scientific knowledge of that day--and 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 heart--what we now speak of as the