views respecting the structure of the organs concerned were, for
the most part, as accurate as the means of anatomical analysis at his
command permitted; and that he had exact and consistent, though by no
means equally just, notions of the actions of these organs and of the
movements of the blood.

Starting from the fundamental facts established by Erasistratus
respecting the structure of the heart and the working of its valves,
Galen's great service was the proof, by the only evidence which could
possess demonstrative value; namely, by that derived from experiments
upon living animals, that the arteries are as much full of blood during
life as the veins are, and that the left cavity of the heart, like the
right, is also filled with blood.

Galen, moreover, correctly asserted--though the means of investigation
at his disposition did not allow him to prove the fact--that the
ramifications of the vena arteriosa in the substance of the lungs
communicate with those of the arteria venosa, by direct, though
invisible, passages, which he terms anastomoses; and that, by means of
these communications, a certain portion of the blood of the right
ventricle of the heart passes through the lungs into the left ventricle.
In fact, Galen is quite clear as to the existence of a current of blood
through the lungs, though not of such a current as we now know traverses
them. For, while he believed that a part of the blood of the right
ventricle passes through the lungs, and even, as I shall show, described
at length the mechanical arrangements by which he supposes this passage
to be effected, he considered that the greater part of the blood in the
right ventricle passes directly, through certain pores in the septum,
into the left ventricle. And this was where Galen got upon his wrong
track, without which divergence a man of his scientific insight must
infallibly have discovered the true character of the pulmonary current,
and not improbably have been led to anticipate Harvey.

The best evidence of the state of knowledge respecting the motions of
the heart and blood in Harvey's time is afforded by those works of his
contemporaries which immediately preceded the publication of the
_Exercitatio Anatomica_, in 1628. And none can be more fitly cited for
this purpose than the _de Humani Corporis Fabrica_, Book X, of Adrian
van den Spieghel, who, like Harvey, was a pupil of Fabricius of
Aquapendente, and was of such distinguished ability and le