they have hesitated because they
did not perceive the route by which the blood is transferred from the
veins to the arteries, in consequence of the intimate connection between
the heart and lungs. And that this difficulty puzzled anatomists not a
little when in their dissections they found the pulmonary artery and
left ventricle full of black and clotted blood, plainly appears when
they felt themselves compelled to affirm that the blood made its way
from the right to the left ventricle by sweating through the septum of
the heart.

Had anatomists only been as conversant with the dissection of the lower
animals as they are with that of the human body, the matters that have
hitherto kept them in perplexity of doubt would, in my opinion, have met
them freed from every kind of difficulty. And first in fishes, in which
the heart consists of but a single ventricle, they having no lungs, the
thing is sufficiently manifest. Here the sac, which is situated at the
base of the heart, and is the part analogous to the auricle in man,
plainly throws the blood into the heart, and the heart in its turn
conspicuously transmits it by a pipe or artery, or vessel analogous to
an artery; these are facts which are confirmed by simple ocular
experiment. I have seen, farther, that the same thing obtained most
obviously.

And since we find that in the greater number of animals, in all indeed
at a certain period of their existence, the channels for the
transmission of the blood through the heart are so conspicuous, we have
still to inquire wherefore in some creatures--those, namely, that have
warm blood and that have attained to the adult age, man among the
number--we should not conclude that the same thing is accomplished
through the substance of the lungs, which, in the embryo, and at a time
when the functions of these organs is in abeyance, Nature effects by
direct passages, and which indeed she seems compelled to adopt through
want of a passage by the lungs; or wherefore it should be better (for
Nature always does that which is best) that she should close up the
various open routes which she had formerly made use of in the embryo,
and still uses in all other animals; not only opening up no new apparent
channels for the passage of the blood therefore, but even entirely
shutting up those which formerly existed in the embryos of those animals
that have lungs. For while the lungs are yet in a state of inaction,
Nature uses the two ventricles o