THAT THERE IS A CIRCULATION OF THE BLOOD IS CONFIRMED FROM THE FIRST
PROPOSITION.
But lest any one should say that we give them words only, and make mere
specious assertions without any foundation, and desire to innovate
without sufficient cause, three points present themselves for
confirmation, which being stated, I conceive that the truth I contend
for will follow necessarily, and appear as a thing obvious to all.
First,--the blood is incessantly transmitted by the action of the heart
from the vena cava to the arteries in such quantity, that it cannot be
supplied from the ingesta, and in such wise that the whole mass must
very quickly pass through the organ; Second,--the blood under the
influence of the arterial pulse enters and is impelled in a continuous,
equable, and incessant stream through every part and member of the body,
in much larger quantity than were sufficient for nutrition, or than the
whole mass of fluids could supply; Third,--the veins in like manner
return this blood incessantly to the heart from all parts and members of
the body. These points proved, I conceive it will be manifest that the
blood circulates, revolves, propelled and then returning, from the heart
to the extremities, from the extremities to the heart, and thus that it
performs a kind of circular motion.
Let us assume either arbitrarily or from experiment, the quantity of
blood which the left ventricle of the heart will contain when distended
to be, say two ounces, three ounces, one ounce and a half--in the dead
body I have found it to hold upwards of two ounces. Let us assume
further, how much less the heart will hold in the contracted than in the
dilated state; and how much blood it will project into the aorta upon
each contraction;--and all the world allows that with the systole
something is always projected, a necessary consequence demonstrated in
the third chapter, and obvious from the structure of the valves; and let
us suppose as approaching the truth that the fourth, or fifth, or sixth,
or even but the eighth part of its charge is thrown into the artery at
each contraction; this would give either half an ounce, or three
drachms, or one drachm of blood as propelled by the heart at each pulse
into the aorta; which quantity, by reason of the valves at the root of
the vessel, can by no means return into the ventricle. Now in the course
of half an hour, the heart will have made more than one thousand beats,
in some as many as two, three, and even four thousand. Multiplying the
number of drachms propelled by the number of pulses, we shall have
either one thousand half ounces, or one thousand times three drachms, or
a like proportional quantity of blood, according to the amount which we
assume as propelled with each stroke of the heart, sent from this organ
into the artery; a larger quantity in every case than is contained in
the whole body! In the same way, in the sheep or dog, say that but a
single scruple of blood passes with each stroke of the heart, in one
half hour we should have one thousand scruples, or about three pounds
and a half of blood injected into the aorta; but the body of neither
animal contains above four pounds of blood, a fact which I have myself
ascertained in the case of the sheep.
Upon this supposition, therefore, assumed merely as a ground for
reasoning, we see the whole mass of blood passing through the heart,
from the veins to the arteries, and in like manner through the lungs.
But let it be said that this does not take place in half an hour, but in
an hour, or even in a day; any way it is still manifest that more blood
passes through the heart in consequence of its action, than can either
be supplied by the whole of the ingesta, or than can be contained in the
veins at the same moment.
Nor can it be allowed that the heart in contracting sometimes propels
and sometimes does not propel, or at most propels but very little, a
mere nothing, or an imaginary something: all this, indeed, has already
been refuted; and is, besides, contrary both to sense and reason. For if
it be a necessary effect of the dilatation of the heart that its
ventricles become filled with blood, it is equally so that, contracting,
these cavities should expel their contents; and this not in any trifling
measure, seeing that neither are the conduits small, nor the
contractions few in number, but frequent, and always in some certain
proportion, whether it be a third or a sixth, or an eighth, to the total
capacity of the ventricles, so that a like proportion of blood must be
expelled, and a like proportion received with each stroke of the heart,
the capacity of the ventricle contracted always bearing a certain
relation to the capacity of the ventricle when dilated. And since in
dilating, the ventricles cannot be supposed to get filled with nothing,
or with an imaginary something; so in contracting they never expel
nothing or aught imaginary, but always a certain something, viz. blood,
in proportion to the amount of the contraction. Whence it is to be
inferred, that if at one stroke the heart in man, the ox or the sheep,
ejects but a single drachm of blood, and there are one thousand strokes
in half an hour, in this interval there will have been ten pounds five
ounces expelled: were there with each stroke two drachms expelled, the
quantity would of course amount to twenty pounds and ten ounces; were
there half an ounce, the quantity would come to forty-one pounds and
eight ounces; and were there one ounce it would be as much as
eighty-three pounds and four ounces; the whole of which, in the course
of one half hour, would have been transfused from the veins to the
arteries. The actual quantity of blood expelled at each stroke of the
heart, and the circumstances under which it is either greater or less
than ordinary, I leave for particular determination afterwards, from
numerous observations which I have made on the subject.
Meantime this much I know, and would here proclaim to all that the blood
is transfused at one time in larger, at another in smaller quantity; and
that the circuit of the blood is accomplished now more rapidly, now more
slowly, according to the temperament, age, &c. of the individual,
to external and internal circumstances, to naturals and
non-naturals,--sleep, rest, food, exercise, affections of the mind, and
the like. But indeed, supposing even the smallest quantity of blood to
be passed through the heart and the lungs with each pulsation, a vastly
greater amount would still be thrown into the arteries and whole body,
than could by any possibility be supplied by the food consumed; in short
it could be furnished in no other way than by making a circuit and
returning.
This truth, indeed, presents itself obviously before us when we consider
what happens in the dissection of living animals; the great artery need
not be divided, but a very small branch only, (as Galen even proves in
regard to man,) to have the whole of the blood in the body, as well
that of the veins as of the arteries, drained away in the course of no
long time--some half hour or less. Butchers are well aware of the fact
and can bear witness to it; for, cutting the throat of an ox and so
dividing the vessels of the neck, in less than a quarter of an hour they
have all the vessels bloodless--the whole mass of blood has escaped. The
same thing also occasionally occurs with great rapidity in performing
amputations and removing tumours in the human subject.
Nor would this argument lose any of its force, did any one say that in
killing animals in the shambles, and performing amputations, the blood
escaped in equal, if not perchance in larger quantity by the veins than
by the arteries. The contrary of this statement, indeed, is certainly
the truth; the veins, in fact, collapsing, and being without any
propelling power, and further, because of the impediment of the valves,
as I shall show immediately, pour out but very little blood; whilst the
arteries spout it forth with force abundantly, impetuously, and as if it
were propelled by a syringe. And then the experiment is easily tried of
leaving the vein untouched, and only dividing the artery in the neck of
a sheep or dog, when it will be seen with what force, in what abundance,
and how quickly, the whole blood in the body, of the veins as well as of
the arteries, is emptied. But the arteries receive blood from the veins
in no other way than by transmission through the heart, as we have
already seen; so that if the aorta be tied at the base of the heart, and
the carotid or any other artery be opened, no one will now be surprised
to find it empty, and the veins only replete with blood.
And now the cause is manifest, wherefore in our dissections we usually
find so large a quantity of blood in the veins, so little in the
arteries; wherefore there is much in the right ventricle, little in the
left; circumstances which probably led the ancients to believe that the
arteries (as their name implies) contained nothing but spirits during
the life of an animal. The true cause of the difference is this perhaps:
that as there is no passage to the arteries, save through the lungs and
heart, when an animal has ceased to breathe and the lungs to move, the
blood in the pulmonary artery is prevented from passing into the
pulmonary veins, and from thence into the left ventricle of the heart;
just as we have already seen the same transit prevented in the embryo,
by the want of movement in the lungs and the alternate opening and
shutting of their minute orifices and invisible pores. But the heart not
ceasing to act at the same precise moment as the lungs, but surviving
them and continuing to pulsate for a time, the left ventricle and
arteries go on distributing their blood to the body at large and sending
it into the veins; receiving none from the lungs, however, they are soon
exhausted, and left, as it were, empty. But even this fact confirms our
views, in no trifling manner, seeing that it can be ascribed to no other
than the cause we have just assumed.
Moreover it appears from this that the more frequently or forcibly the
arteries pulsate, the more speedily will the body be exhausted in an
hemorrhagy. Hence, also, it happens, that in fainting fits and in states
of alarm, when the heart beats more languidly and with less force,
hemorrhages are diminished or arrested.
Still further, it is from this that after death, when the heart has
ceased to beat, it is impossible by dividing either the jugular or
femoral veins and arteries, by any effort to force out more than one
half of the whole mass of the blood. Neither could the butcher, did he
neglect to cut the throat of the ox which he has knocked on the head and
stunned, until the heart had ceased beating, ever bleed the carcass
effectually.
Finally, we are now in a condition to suspect wherefore it is that no
one has yet said anything to the purpose upon the anastomosis of the
veins and arteries, either as to where or how it is effected, or for
what purpose. I now enter upon the investigation of the subject.