hrough the other veins, to all parts of the body; that the blood of the
arteries takes a like course from the heart towards the periphery; and
that it is there, by means of the 'anastomoses', more or less mixed up
with the venous blood. It so happens, by a curious chance, that up to
the year 1625 there was at Padua, which was Harvey's own university, a
very distinguished professor, Spigelius, whose work is extant, and who
teaches exactly what I am now telling you. It is perfectly true
that, some time before, Harvey's master, Fabricius, had not only
re-discovered, but had drawn much attention to certain pouch-like
structures, which are called the valves of the veins, found in the
muscular parts of the body, all of which are directed towards the heart,
and consequently impede the flow of the blood in the opposite direction.
And you will find it stated by people who have not thought much about
the matter, that it was this discovery of the valves of the veins which
led Harvey to imagine the course of the circulation of the blood. Now
it did not lead Harvey to imagine anything of the kind. He had heard
all about it from his master, Fabricius, who made a great point of
these valves in the veins, and he had heard the theories which Fabricius
entertained upon the subject, whose impression as to the use of the
valves was simply this--that they tended to take off any excess of
pressure of the blood in passing from the heart to the extremities; for
Fabricius believed, with the rest of the world, that the blood in the
veins flowed from the heart towards the extremities. This, under the
circumstances, was as good a theory as any other, because the action of
the valves depends altogether upon the form and nature of the walls
of the structures in which they are attached; and without accurate
experiment, it was impossible to say whether the theory of Fabricius
was right or wrong. But we not only have the evidence of the facts
themselves that these could tell Harvey nothing about the circulation,
but we have his own distinct declaration as to the considerations which
led him to the true theory of the circulation of the blood, and amongst
these the valves of the veins are not mentioned.

Fig. 4.--The circulation of the blood as demonstrated by Harvey (A.D.
1628).

Now then we may come to Harvey himself. When you read Harvey's treatise,
which is one of the most remarkable scientific monographs with which I
am acquainted--it occupies betw