OF THE QUANTITY OF BLOOD PASSING THROUGH THE HEART FROM THE VEINS TO
THE ARTERIES; AND OF THE CIRCULAR MOTION OF THE BLOOD
Thus far I have spoken of the passage of the blood from the veins
into the arteries, and of the manner in which it is transmitted and
distributed by the action of the heart; points to which some, moved
either by the authority of Galen or Columbus, or the reasonings of
others, will give in their adhesion. But what remains to be said upon
the quantity and source of the blood which thus passes, is of so novel
and unheard-of character, that I not only fear injury to myself from
the envy of a few, but I tremble lest I have mankind at large for my
enemies, so much doth wont and custom, that become as another nature,
and doctrine once sown and that hath struck deep root, and respect
for antiquity influence all men: Still the die is cast, and my trust
is in my love of truth, and the candour that inheres in cultivated
minds. And sooth to say, when I surveyed my mass of evidence, whether
derived from vivisections, and my various reflections on them, or from
the ventricles of the heart and the vessels that enter into and issue
from them, the symmetry and size of these conduits,--for nature doing
nothing in vain, would never have given them so large a relative size
without a purpose,--or from the arrangement and intimate structure
of the valves in particular, and of the other parts of the heart in
general, with many things besides, I frequently and seriously bethought
me, and long revolved in my mind, what might be the quantity of
blood which was transmitted, in how short a time its passage might be
effected, and the like; and not finding it possible that this could be
supplied by the juices of the ingested aliment without the veins on
the one hand becoming drained, and the arteries on the other getting
ruptured through the excessive charge of blood, unless the blood should
somehow find its way from the arteries into the veins, and so return
to the right side of the heart; I began to think whether there might
not be a A MOTION, AS IT WERE, IN A CIRCLE. Now this I afterwards found
to be true; and I finally saw that the blood, forced by the action of
the left ventricle into the arteries, was distributed to the body at
large, and its several parts, in the same manner as it is sent through
the lungs, impelled by the right ventricle into the pulmonary artery,
and that it then passed through the veins and along the vena cava, and
so round to the left ventricle in the manner already indicated. Which
motion we may be allowed to call circular, in the same way as Aristotle
says that the air and the rain emulate the circular motion of the
superior bodies; for the moist earth, warmed by the sun, evaporates;
the vapours drawn upwards are condensed, and descending in the form of
rain, moisten the earth again; and by this arrangement are generations
of living things produced; and in like manner too are tempests and
meteors engendered by the circular motion, and by the approach and
recession of the sun.
And so, in all likelihood, does it come to pass in the body, through
the motion of the blood; the various parts are nourished, cherished,
quickened by the warmer, more perfect, vaporous, spirituous, and, as
I may say, alimentive blood; which, on the contrary, in contact with
these parts becomes cooled, coagulated, and, so to speak, effete;
whence it returns to its sovereign the heart, as if to its source,
or to the inmost home of the body, there to recover its state of
excellence or perfection. Here it resumes its due fluidity and
receives an infusion of natural heat--powerful, fervid, a kind of
treasury of life, and is impregnated with spirits, and it might be said
with balsam; and thence it is again dispersed; and all this depends on
the motion and action of the heart.
The heart, consequently, is the beginning of life; the sun of the
microcosm, even as the sun in his turn might well be designated the
heart of the world; for it is the heart by whose virtue and pulse
the blood is moved, perfected, made apt to nourish, and is preserved
from corruption and coagulation; it is the household divinity which,
discharging its function, nourishes, cherishes, quickens the whole
body, and is indeed the foundation of life, the source of all action.
But of these things we shall speak more opportunely when we come to
speculate upon the final cause of this motion of the heart.
Hence, since the veins are the conduits and vessels that transport the
blood, they are of two kinds, the cava and the aorta; and this not by
reason of there being two sides of the body, as Aristotle has it, but
because of the difference of office; nor yet, as is commonly said, in
consequence of any diversity of structure, for in many animals, as I
have said, the vein does not differ from the artery in the thickness
of its tunics, but solely in virtue of their several destinies and
uses. A vein and an artery, both styled vein by the ancients, and that
not undeservedly, as Galen has remarked, because the one, the artery
to wit, is the vessel which carries the blood from the heart to the
body at large, the other or vein of the present day bringing it back
from the general system to the heart; the former is the conduit from,
the latter the channel to, the heart; the latter contains the cruder,
effete blood, rendered unfit for nutrition; the former transmits the
digested, perfect, peculiarly nutritive fluid.