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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 move the members； as when we see heads
shortly after they have been struck off still move and bite the earth；
although no longer animated； what changes must take place in the brain to
produce waking； sleep； and dreams； how light； sounds； odors； tastes； heat；
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DISCOURSE ON THE METHOD OF RIGHTLY CONDUCTING THE REASON； AND
SEEKING TRUTH IN THE SCIENCES
and all the other qualities of external objects impress it with different ideas
by means of the senses； how hunger； thirst； and the other internal
affections can likewise impress upon it divers ideas； what must be
understood by the mon sense （sensus munis） in which these ideas
are received； by the memory which retains them； by the fantasy which can
change them in various ways； and out of them pose new ideas； and
which； by the same means； distributing the animal spirits through the
muscles； can cause the members of such a body to move in as many
different ways； and in a manner as suited； whether to the objects that are
presented to its senses or to its internal affections； as can take place in our
own case apart from the guidance of the will。 Nor will this appear at all
strange to those who are acquainted with the variety of movements
performed by the different automata； or moving machines fabricated by
human industry； and that with help of but few pieces pared with the
great multitude of bones； muscles； nerves； arteries； veins； and other parts
that are found in the body of each animal。 Such persons will look upon
this body as a machine made by the hands of God； which is inparably
better arranged； and adequate to movements more admirabl