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.