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.