THE FIRST POSITION: OF THE QUANTITY OF BLOOD PASSING FROM THE VEINS TO
THE ARTERIES. AND THAT THERE IS A CIRCUIT OF THE BLOOD, FREED FROM
OBJECTIONS, AND FARTHER CONFIRMED BY EXPERIMENT.
So far our first position is confirmed, whether the thing be referred to
calculation or to experiment and dissection, viz., that the blood is
incessantly infused into the arteries in larger quantities than it can
be supplied by the food; so that the whole passing over in a short
space of time, it is matter of necessity that the blood perform a
circuit, that it return to whence it set out.
But if any one shall here object that a large quantity may pass through
and yet no necessity be found for a circulation, that all may come from
the meat and drink consumed, and quote as an illustration the abundant
supply of milk in the mammæ--for a cow will give three, four, and even
seven gallons and more in a day, and a woman two or three pints whilst
nursing a child or twins, which must manifestly be derived from the food
consumed; it may be answered, that the heart by computation does as much
and more in the course of an hour or two.
And if not yet convinced, he shall still insist, that when an artery is
divided a preternatural route is, as it were, opened, and that so the
blood escapes in torrents, but that the same thing does not happen in
the healthy and uninjured body when no outlet is made; and that in
arteries filled, or in their natural state, so large a quantity of blood
cannot pass in so short a space of time as to make any return
necessary;--to all this it may be answered, that from the calculation
already made, and the reasons assigned, it appears, that by so much as
the heart in its dilated state contains in addition to its contents in
the state of constriction, so much in a general way must it emit upon
each pulsation, and in such quantity must the blood pass, the body being
healthy and naturally constituted.
But in serpents, and several fishes, by tying the veins some way below
the heart, you will perceive a space between the ligature and the heart
speedily to become empty; so that, unless you would deny the evidence of
your senses, you must needs admit the return of the blood to the heart.
The same thing will also plainly appear when we come to discuss our
second position.
Let us here conclude with a single example, confirming all that has been
said, and from which every one may obtain conviction through the
testimony of his own eyes.
If a live snake be laid open, the heart will be seen pulsating quietly,
distinctly, for more than an hour, moving like a worm, contracting in
its longitudinal dimensions, (for it is of an oblong shape,) and
propelling its contents; becoming of a paler colour in the systole, of a
deeper tint in the diastole; and almost all things else by which I have
already said that the truth I contend for is established, only that
here everything takes place more slowly, and is more distinct. This
point in particular may be observed more clearly than the noon-day sun:
the vena cava enters the heart at its lower part, the artery quits it at
the superior part; the vein being now seized either with forceps or
between the finger and thumb, and the course of the blood for some space
below the heart interrupted, you will perceive the part that intervenes
between the fingers and the heart almost immediately to become empty,
the blood being exhausted by the action of the heart; at the same time
the heart will become of a much paler colour, even in its state of
dilatation, than it was before; it is also smaller than at first, from
wanting blood; and then it begins to beat more slowly, so that it seems
at length as if it were about to die. But the impediment to the flow of
blood being removed, instantly the colour and the size of the heart are
restored.
If, on the contrary, the artery instead of the vein be compressed or
tied, you will observe the part between the obstacle and the heart, and
the heart itself, to become inordinately distended, to assume a deep
purple or even livid colour, and at length to be so much oppressed with
blood that you will believe it about to be choked; but the obstacle
removed, all things immediately return to their pristine state--the
heart to its colour, size, stroke, &c.
Here then we have evidence of two kinds of death: extinction from
deficiency, and suffocation from excess. Examples of both have now been
set before you, and you have had opportunity of viewing the truth
contended for with your own eyes in the heart.