e heart to the lungs, and four pulmonary veins, which bring it back
again; then the aorta or large artery, which carries the blood from
the left ventricle of the heart to all parts of the body; the two
venae cavae, and the coronary vein of the heart, which bring it back
again.

At the beginning of both arteries, where they leave the heart, are
placed valves, which allow the blood to flow freely from the heart
into the arteries, but which prevent its return to the heart. There
are likewise valves between the auricles and ventricles, which permit
the blood to flow from the former into the latter, but prevent its
return into the auricles. The veins are likewise furnished with
valves, which allow the blood to flow from their minute branches
along the larger toward the heart, but prevent its returning to these
minute branches.

The blood being brought back from all parts of the body into the
right auricle of the heart, distends this cavity, and thus causes it
to contract; this auricle, by contracting, forces the blood into the
right ventricle; this muscular cavity being distended and irritated
by the blood, contracts, and propels the blood through the pulmonary
artery into the lungs: from hence it is brought back by the pulmonary
veins, to the left auricle of the heart, by whose contraction it is
forced into the left ventricle. The contraction of this ventricle
propels the blood, with great force, into the aorta, through the
innumerable ramifications of which, it is carried to every part of
the body, and brought back by veins, which accompany these arterial
ramifications, and form the venae cavae, which conduct the blood into
the right auricle of the heart, from whence it is again sent into the
right ventricle, which sends it through the pulmonary artery, to the
lungs; the pulmonary veins bring it back again to the heart, from
whence it is propelled through the aorta, to all parts of the body:
thus running a perpetual round, called the circulation of the blood.

Thus then we see, that the circulation consists of two circles or
stages, one through the lungs, which may be called the pulmonary, or
lesser circle, and the other through all parts of the body, which may
be termed the aortal, or greater circle.

That the blood circulates in this manner, is evident, from the valves
placed at the origin of the arteries, and in the large branches of
the veins, which prevent the return of the blood to the heart, in any
other manner