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force than it does to return from the hand to the heart through the veins。
And since the latter current escapes from the arm by the opening made in
one of the veins； there must of necessity be certain passages below the
ligature； that is； towards the extremities of the arm through which it can
e thither from the arteries。 This physician likewise abundantly
establishes what he has advanced respecting the motion of the blood； from
the existence of certain pellicles； so disposed in various places along the
course of the veins； in the manner of small valves； as not to permit the
blood to pass from the middle of the body towards the extremities； but
only to return from the extremities to the heart； and farther； from
experience which shows that all the blood which is in the body may flow
out of it in a very short time through a single artery that has been cut； even
although this had been closely tied in the immediate neighborhood of the
heart and cut between the heart and the ligature； so as to prevent the
supposition that the blood flowing out of it could e from any other
quarter than the heart。
But there are many other circumstances which evince that what I have
alleged is the true cause of the motion of the blood： thus； in the first
place； the difference that is observed between the blood which flows
from the veins； and that from the arteries； can only arise from this； that
being rarefied； and； as it were； distilled by passing through the heart； it is
thinner； and more vivid； and warmer immediately after leaving the heart；
in other words； when in the arteries； than it was a short time before passing
into either； in other words； when it was in the veins； and if attention be
given； it will be found that this difference is very marked only in the
neighborhood of the heart； and is not so evident in parts more remote from
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DISCOURSE ON THE METHOD OF RIGHTLY CONDUCTING THE REASON； AND
SEEKING TRUTH IN THE SCIENCES
it。 In the next place； the consistency of the coats of which the arterial
vein and the great artery are posed； sufficiently shows that the blood
is impelled against them with more force than against the veins。 And
why should the left cavity of the heart and the great artery be wider and
larger than the right cavity and the arterial vein； were it not that the blood
of the venous artery； having only been in the lungs after it has passed
through the heart； is thinner； and rarefies more readily； and in a higher
degree； than the blood which proceeds immediately from the hollow vein？
And what can physicians conjecture from feeling the pulse unless they
know that according as the blood changes its nature it can be rarefied by
the warmth of the heart； in a higher or lower degree； and more or less
quickly than before？ And if it be inquired how this heat is municated
to the other members； must it not be admitted that this is effected by
means of the blood； which； passing through the heart； is there heated anew；
and thence diffused over all the body？ Whence it happens； that if the
blood be withdrawn from any part； the heat is likewise withdrawn by the
same means； and although the heart were as…hot as glowing iron； it would
not be capable of warming the feet and hands as at present； unless it
continually sent thither new blood。 We likewise perceive from this； that
the true use of respiration is to bring sufficient fresh air into the lungs； to
cause the blood which flows into them from the right ventricle of the heart；
where it has been rarefied and； as it were； changed into vapors； to bee
thick； and to convert it anew into blood； before it flows into the left cavity；
without which process it would be unfit for the nourishment of the fire that
is there。 This receives confirmation from the circumstance； that it is
observed of animals destitute of lungs that they have also but one cavity in
the heart； and that in children who cannot use them while in the womb；
there is a hole through which the blood flows from the hollow vein into
the left cavity of the heart； and a tube through which it passes from the
arterial vein into the grand artery without passing through the lung。 In
the next place； how could digestion be carried on in the stomach unless the
heart municated heat to it through the arteries； and along with this
certain of the more fluid parts of the blood； which assist in the dissolution
of the food that has been taken in？ Is not also the operation which
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DISCOURSE ON THE METHOD OF RIGHTLY CONDUCTING THE REASON； AND
SEEKING TRUTH IN THE SCIENCES
converts the juice of food into blood easily prehended； when it is
considered that it is distilled by passing and repassing through the heart
perhaps more than one or two hundred times in a day？ And what more
need be adduced to explain nutrition； and the production of the different
humors of the body； beyond saying； that the force with which the blood； in
being rarefied； passes from the heart towards the extremities of the arteries；
causes certain of its parts to remain in the members at which they arrive；
and there occupy the place of some others expelled by them； and that
according to the situation； shape； or smallness of the pores with which
they meet； some rather than others flow into certain parts； in the same
way that some sieves are observed to act； which； by being variously
perforated； serve to separate different species of grain？ And； in the last
place； what above all is here worthy of observation； is the generation of
the animal spirits； which are like a very subtle wind； or rather a very pure
and vivid flame which； continually ascending in great abundance from the
heart to the brain； thence penetrates through the nerves into the muscles；
and gives motion to all the members； so that to account for other parts of
the blood which； as most agitated and penetrating； are the fittest to
pose these spirits； proceeding towards the brain； it is not necessary to
suppose any other cause； than simply； that the arteries which carry them
thither proceed from the heart in the most direct lines； and that； according
to the rules of mechanics which are the same with those of nature； when
many objects tend at once to the same point where there is not sufficient
room for all （as is the case with the parts of the blood which flow forth
from the left cavity of the heart and tend towards the brain）； the weaker
and less agitated parts must necessarily be driven aside from that point by
the stronger which alone in this way reach it I had expounded all these
matters with sufficient minuteness in the treatise which I formerly thought
of publishing。 And after these； I had shown what must be the fabric of
the nerves and muscles of the human body to give the animal spirits
contained in it the power to