cular motion that
that which moves returns to the place from whence it started. Hence the
discovery of the course of the blood from the right ventricle, through
the lungs, to the left ventricle was in no wise an anticipation of the
discovery of the circulation of the blood. For the blood which traverses
this part of its course no more describes a circle than the dweller in a
street who goes out of his own house and enters his next-door neighbor's
does so. Although there may be nothing but a party wall between him and
the room he has just left, it constitutes an efficient _defense de
circuler_. Thus, whatever they may have known of the so-called pulmonary
circulation, to say that Servetus or Columbus or Caesalpinus deserves any
share of the credit which attaches to Harvey appears to me to be to
mistake the question at issue.

It must further be borne in mind that the determination of the true
course taken by the whole mass of the blood is only the most conspicuous
of the discoveries of Harvey; and that his analysis of the mechanism by
which the circulation is brought about is far in advance of anything
which had previously been published. For the first time it is shown that
the walls of the heart are active only during its systole or
contraction, and that the dilatation of the heart, in the diastole, is
purely passive. Whence it follows that the impulse by which the blood is
propelled is a _vis a tergo_, and that the blood is not drawn into the
heart by any such inhalent or suctorial action as not only the
predecessors, but many of the successors, of Harvey imagined it to
possess.

Harvey is no less original in his view of the cause of the arterial
pulse. In contravention of Galen and of all other anatomists up to his
own time, he affirms that the stretching of the arteries which gives
rise to the pulse is not due to the active dilatation of their walls,
but to their passive distention by the blood which is forced into them
at each beat of the heart; reversing Galen's dictum, he says that they
dilate as bags and not as bellows. This point of fundamental, practical
as well as theoretical, importance is most admirably demonstrated, not
only by experiment, but by pathological illustrations.

One of the weightiest arguments in Harvey's demonstration of the
circulation is based upon the comparison of the quantity of blood driven
out of the heart, at each beat, with the total quantity of blood in the
body. This, so far as I