humours within proper bounds. I think indeed
this very pressure doth not a little contribute to
keep on the circulation of the blood.—For as the
whole body is continually compressed by the incumbent
atmosphere, it must force the blood
through the veins towards the heart, seeing it
cannot run back by a retrograde motion into the
arteries on account of the valves, that are found
up and down in the veins. So little indeed of the
original motion, impressed by the heart on the
blood, remains in the larger veins, that it would
scarce mount from the feet to the heart, unless to
preserve, as it were, an equilibrium in the veins
and arteries, which, being contiguous, form a kind
of inverted syphon, whose legs are of equal altitude.
So slow truly is the progress of the blood in the
veins of the limbs, that many valves are placed
in them to take off the weight of the superincumbent
blood, lest it should slide back towards the
extremities. It seems therefore by its mere motion
in the veins not capable of overcoming the constrictive
power of the heart and forcing into the
ventricle, so as sufficiently to dilate it.—The perpetual
pressure therefore of the atmosphere adds to
the momentum of the blood, acting as a kind of
antagonist against the innate and strongly constrictive
power of the heart, which is more or less
the natural faculty of every sphincter-muscle. For
as soon as the lungs, expanded by inspiration,
make room for the blood issuing out of the heart
into the empty blood-vessels of the lungs (a kind
of momentary vacuum being made in them by the
expanse of the lungs) the heavy atmosphere, constantly
compressing the whole habit of the body,
forces as much blood into the right ventricle of
the heart as it can well receive, whilst at the same
time the elastic air, rushing through all the lungs
and pressing on all its blood-vessels, adds no small
degree of force to the very rapid currents of blood,
which are observed to be here vastly much swifter
than in any of the branches of the aorta ; so that
by this means the momentum of the blood is so
much increased as to force into the left ventricle
of the heart. But enough of this, as it is not my
design in this little work to give a complete account
of the circulation of the blood ; I only would here
by the way take notice, that the pressure of the
atmosphere is one, out of many, and that too, not
a contemptible one, of the causes of the circulation
of the blood, and to hint at the same time how