, 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.

(FIGURE 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 between 50 and 60 pages of a small quarto in
Latin, and is as terse and concise as it possibly can be--when you come
to look at Harvey's work, you will find that he had long struggled with
the difficulties of the accepted doctrine of the circulation. He had
received from Fabricius, and from all the great authorities of the day,
the current view of the circulation of the blood. But he was a man
with that rarest of all qualities--intellectual honesty; and by dint of
cultivating that great faculty, which is more moral than intellectual,
it had become impossible for him to say he believed anything which he
did not clearly believe. This is a most uncomfortable peculiarity--for
it gets you into all sorts of difficulties with all sorts of
people--but, for scientific purposes, it is absolutely invaluable.
Harvey possessed this peculiarity in the highest degree, and so it was
impossible for him to accept what