f the heart as if they formed but one
for the transmission of the blood. The condition of the embryos of those
animals which have lungs is the same as that of those animals which have
no lungs.

Thus, by studying the structure of the animals who are nearer to and
further from ourselves in their modes of life and in the construction of
their bodies, we can prepare ourselves to understand the nature of the
pulmonary circulation in ourselves, and of the systemic circulation
also.

_II.--Systemic Circulation_

What remains to be said is of so novel and unheard of a character that I
not only fear injury to myself from the envy of a few, but I tremble
lest I have mankind at large for my enemies, so much do wont and custom
that become as another nature, and doctrine once sown that hath struck
deep root, and respect for antiquity, influence all men.

And, sooth to say, when I surveyed my mass of evidence, whether derived
from vivisections and my previous reflections on them, or from the
ventricles of the heart and the vessels that enter into and issue from
them, the symmetry and size of these conduits--for Nature, doing nothing
in vain, would never have given them so large a relative size without a
purpose; or from the arrangement and intimate structure of the valves in
particular and of the many other parts of the heart in general, with
many things besides; and frequently and seriously bethought me and long
revolved in my mind what might be the quantity of blood which was
transmitted, in how short a time its passage might be effected and the
like; and not finding it possible that this could be supplied by the
juices of the ingested aliment without the veins on the one hand
becoming drained, and the arteries on the other getting ruptured through
the excessive charge of blood, unless the blood should somehow find its
way from the arteries into the veins, and so return to the right side of
the heart; when I say, I surveyed all this evidence, I began to think
whether there might not be _a motion as it were in a circle_.

Now this I afterwards found to be true; and I finally saw that the
blood, forced by the action of the left ventricle into the arteries, was
distributed to the body at large, and its several parts, in the same
manner as it is sent through the lungs, impelled by the right ventricle
into the pulmonary artery; and that it then passed through the veins and
along the _vena cava_, and so round to the left ventricl