nd "pneumatic" arterial
blood traversed the anastomotic channels, the existence of which was
assumed, and so reached the systemic veins, or whether, on the contrary,
some portion of the venous blood made its entrance by the same passages
into the arteries, depended upon circumstances. Sometimes the current
might set one way, sometimes the other.

In direct opposition to these universally received views Harvey asserts
that the natural course of the blood in the veins is from the peripheral
ramifications toward the main trunk; that the mass of the blood to be
found in the veins at any moment was a short time before contained in
the arteries, and has simply flowed out of the latter into the veins;
and, finally, that the stream of blood which runs from the arteries into
the veins is constant, continuous, and rapid.

According to the view of Harvey's predecessors, the veins may be
compared to larger and smaller canals, fed by a spring which trickles
into the chief canals, whence the water flows to the rest. The heart and
lungs represent an engine set up in the principal canal to aerate some
of the water and scatter it all over the garden. Whether any of this
identical water came back to the engine or not would be a matter of
chance, and it would certainly have no sensible effect on the motion of
the water in the canals. In Harvey's conception of the matter, on the
other hand, the garden is watered by channels so arranged as to form a
circle, two points of which are occupied by propulsive engines. The
water is kept moving in a continual round within its channels, as much
entering the engines on one side as leaves them on the other; and the
motion of the water is entirely due to the engines.

It is in conceiving the motion of the blood, as a whole, to be circular,
and in ascribing that circular motion simply and solely to the
contractions of the walls of the heart, that Harvey is so completely
original. Before him, no one, that I can discover, had ever so much as
dreamed that a given portion of blood, contained, for example, in the
right ventricle of the heart, may, by the mere mechanical operation of
the working of that organ, be made to return to the very place from
which it started, after a long journey through the lungs and through
the body generally. And it should be remembered that it is to this
complete circuit of the blood alone that the term "circulation" can, in
strictness, be applied. It is of the essence of a cir