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 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.