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 a character so novel and unheard-of 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 become a second nature. Doctrine once sown
strikes deep its root, and respect for antiquity influences all
men. Still the die is cast, and my trust is in my love of truth
and the candour of 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 study of 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 observing 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. But 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. This 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. By this arrangement are generations of
living things produced; and in like manner are tempests and
meteors engendered by the circular motion, and by the approach
and recession of the sun.
And similarly does it come to pass in the body, through the
motion of the blood, that the various parts are nourished,
cherished, quickened by the warmer, more perfect, vaporous,
spirituous, and, as I may say, alimentive blood; which, on the
other hand, owing to its contact with these parts, becomes
cooled, coagulated, and so to speak effete. It then 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 renews its fluidity, natural heat, and
becomes powerful, fervid, a kind of treasury of life, and
impregnated with spirits, it might be said with balsam. Thence it
is again dispersed. 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, and made nutrient, 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.
As the blood-vessels, therefore, are the canals and agents 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, not, 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 walls, but
solely in virtue of their distinct functions and uses. A vein and
an artery, both styled veins by the ancients, and that not
without reason, as Galen has remarked, for the artery is the
vessel which carries the blood from the heart to the body at
large, the 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.