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last comes round to that place from whence the breath came forth；
and enters in there； and following the breath； fills up the vacant
space； and this goes on like the rotation of a wheel； because there
can be no such thing as a vacuum。 Wherefore also the breast and the
lungs； when they emit the breath； are replenished by the air which
surrounds the body and which enters in through the pores of the
flesh and is driven round in a circle； and again； the air which is
sent away and passes out through the body forces the breath inwards
through the passage of the mouth and the nostrils。 Now the origin of
this movement may be supposed to be as follows。 In the interior of
every animal the hottest part is that which is around the blood and
veins； it is in a manner on internal fountain of fire； which we
compare to the network of a creel； being woven all of fire and
extended through the centre of the body； while the…outer parts are
composed of air。 Now we must admit that heat naturally proceeds
outward to its own place and to its kindred element； and as there
are two exits for the heat； the out through the body； and the other
through the mouth and nostrils； when it moves towards the one； it
drives round the air at the other； and that which is driven round
falls into the fire and becomes warm； and that which goes forth is
cooled。 But when the heat changes its place； and the particles at
the other exit grow warmer； the hotter air inclining in that direction
and carried towards its native element； fire； pushes round the air
at the other； and this being affected in the same way and
communicating the same impulse； a circular motion swaying to and
from is produced by the double process； which we call inspiration
and expiration。
The phenomena of medical cupping…glasses and of the swallowing of
drink and of the projection of bodies； whether discharged in the air
or bowled along the ground； are to be investigated on a similar
principle； and swift and slow sounds； which appear to be high and low；
and are sometimes discordant on account of their inequality； and
then again harmonical on account of the equality of the motion which
they excite in us。 For when the motions of the antecedent swifter
sounds begin to pause and the two are equalised； the slower sounds
overtake the swifter and then propel them。 When they overtake them
they do not intrude a new and discordant motion； but introduce the
beginnings of a slower； which answers to the swifter as it dies
away； thus producing a single mixed expression out of high and low；
whence arises a pleasure which even the unwise feel； and which to
the wise becomes a higher sort of delight； being an imitation of
divine harmony in mortal motions。 Moreover； as to the flowing of
water； the fall of the thunderbolt； and the marvels that are
observed about the attraction of amber and the Heraclean stones；…in
none of these cases is there any attraction； but he who investigates
rightly； will find that such wonderful phenomena are attributable to
the combination of certain conditions…the non…existence of a vacuum；
the fact that objects push one another round； and that they change
places； passing severally into their proper positions as they are
divided or combined
Such as we have seen； is the nature and such are the causes of
respiration…the subject in which this discussion originated。 For the
fire cuts the food and following the breath surges up within； fire and
breath rising together and filling the veins by drawing up out of
the belly and pouring into them the cut portions of the food； and so
the streams of food are kept flowing through the whole body in all
animals。 And fresh cuttings from kindred substances； whether the
fruits of the earth or herb of the field； which God planted to be
our daily food； acquire all sorts of colours by their inter…mixture；
but red is the most pervading of them； being created by the cutting
action of fire and by the impression which it makes on a moist
substance； and hence the liquid which circulates in the body has a
colour such as we have described。 The liquid itself we call blood；
which nourishes the flesh and the whole body； whence all parts are
watered and empty places filled。
Now the process of repletion and evacuation is effected after the
manner of the universal motion by which all kindred substances are
drawn towards one another。 For the external elements which surround us
are always causing us to consume away； and distributing and sending
off like to like； the particles of blood； too； which are divided and
contained within the frame of the animal as in a sort of heaven； are
compelled to imitate the motion of the universe。 Each； therefore； of
the divided parts within us； being carried to its kindred nature；
replenishes the void。 When more is taken away than flows in； then we
decay； and when less； we grow and increase。
The frame of the entire creature when young has the triangles of
each kind new； and may be compared to the keel of a vessel which is
just off the stocks； they are locked firmly together and yet the whole
mass is soft and delicate； being freshly formed of marrow and nurtured
on milk。 Now when the triangles out of which meats and drinks are
composed come in from without； and are comprehended in the body； being
older and weaker than the triangles already there； the frame of the
body gets the better of them and its newer triangles cut them up；
and so the animal grows great； being nourished by a multitude of
similar particles。 But when the roots of the triangles are loosened by
having undergone many conflicts with many things in the course of
time； they are no longer able to cut or assimilate the food which
enters； but are themselves easily divided by the bodies which come
in from without。 In this way every animal is overcome and decays；
and this affection is called old age。 And at last； when the bonds by
which the triangles of the marrow are united no longer hold； and are
parted by the strain of existence； they in turn loosen the bonds of
the soul； and she； obtaining a natural release； flies away with joy。
For that which takes place according to nature is pleasant； but that
which is contrary to nature is painful。 And thus death； if caused by
disease or produced by wounds； is painful and violent； but that sort
of death which comes with old age and fulfils the debt of nature is
the easiest of deaths； and is accompanied with pleasure rather than
with pain。
Now every one can see whence diseases arise。 There are four
natures out of which the body is compacted； earth and fire and water
and air； and the unnatural excess or defect of these； or the change of
any of them from its own natural place into another； or…since there
are more kinds than one of fire and of the other elements…the
assumption by any of these of a wrong kind； or any similar
irregularity； produces disorders and diseases； for when any of them is
produced or changed in a manner contrary to nature； the parts which
were previously cool grow warm； and those which were dry become moist；
and the ligh