THAT THERE IS A CIRCULATION OF THE BLOOD IS SHOWN FROM THE SECOND
POSITION DEMONSTRATED
If these things be so, another point which I have already
referred to, viz., the continual passage of the blood through the
heart will also be confirmed. We have seen, that the blood passes
from the arteries into the veins, not from the veins into the
arteries; we have seen, farther, that almost the whole of the
blood may be withdrawn from a puncture made in one of the
cutaneous veins of the arm if a bandage properly applied be used;
we have seen, still farther, that the blood flows so freely and
rapidly that not only is the whole quantity which was contained
in the arm beyond the ligature, and before the puncture was made,
discharged, but the whole which is contained in the body, both
that of the arteries and that of the veins.
Whence we must admit, first, that the blood is sent along with an
impulse, and that it is urged with force below the ligature; for
it escapes with force, which force it receives from the pulse and
power of the heart; for the force and motion of the blood are
derived from the heart alone. Second, that the afflux proceeds
from the heart, and through the heart by a course from the great
veins; for it gets into the parts below the ligature through the
arteries, not through the veins; and the arteries nowhere receive
blood from the veins, nowhere receive blood save and except from
the left ventricle of the heart. Nor could so large a quantity of
blood be drawn from one vein (a ligature having been duly
applied), nor with such impetuousity, such readiness, such
celerity, unless through the medium of the impelling power of the
heart.
But if all things be as they are now represented, we shall feel
ourselves at liberty to calculate the quantity of the blood, and
to reason on its circular motion. Should anyone, for instance,
performing phlebotomy, suffer the blood to flow in the manner it
usually does, with force and freely, for some half hour or so, no
question but that the greatest part of the blood being
abstracted, faintings and syncopes would ensue, and that not only
would the arteries but the great veins also be nearly emptied of
their contents. It is only consonant with reason to conclude that
in the course of the half hour hinted at, so much as has escaped
has also passed from the great veins through the heart into the
aorta. And further, if we calculate how many ounces flow through
one arm, or how many pass in twenty or thirty pulsations under
the medium ligature, we shall have some grounds for estimating
how much passes through the other arm in the same space of time:
how much through both lower extremities, how much through the
neck on either side, and through all the other arteries and veins
of the body, all of which have been supplied with fresh blood,
and as this blood must have passed through the lungs and
ventricles of the heart, and must have come from the great veins,
we shall perceive that a circulation is absolutely necessary,
seeing that the quantities hinted at cannot be supplied
immediately from the ingesta, and are vastly more than can be
requisite for the mere nutrition of the parts.
It is still further to be observed, that in practising phlebotomy
the truths contended for are sometimes confirmed in another way;
for having tied up the arm properly, and made the puncture duly,
still, if from alarm or any other causes, a state of faintness
supervenes, in which the heart always pulsates more languidly,
the blood does not flow freely, but distils by drops only. The
reason is, that with a somewhat greater than usual resistance
offered to the transit of the blood by the bandage, coupled with
the weaker action of the heart, and its diminished impelling
power, the stream cannot make its way under the ligature; and
farther, owing to the weak and languishing state of the heart,
the blood is not transferred in such quantity as wont from the
veins to the arteries through the sinuses of that organ. So also,
and for the same reasons, are the menstrual fluxes of women, and
indeed hemorrhages of every kind, controlled. And now, a contrary
state of things occurring, the patient getting rid of his fear
and recovering his courage, the pulse strength is increased, the
arteries begin again to beat with greater force, and to drive the
blood even into the part that is bound; so that the blood now
springs from the puncture in the vein, and flows in continuous
stream.