timaeus   

lesser weels he extended cords reaching all round to the extremities
of the network. All the interior of the net he made of fire, but the
lesser weels and their cavity, of air. The network he took and
spread over the newly-formed animal in the following manner:-He let
the lesser weels pass into the mouth; there were two of them, and
one he let down by the air-pipes into the lungs, the other by the side
of the air-pipes into the belly. The former he divided into two
branches, both of which he made to meet at the channels of the nose,
so that when the way through the mouth did not act, the streams of the
mouth as well were replenished through the nose. With the other cavity
(i.e. of the greater weel) he enveloped the hollow parts of the
body, and at one time he made all this to flow into the lesser
weels, quite gently, for they are composed of air, and at another time
he caused the lesser weels to flow back again; and the net he made
to find a way in and out through the pores of the body, and the rays
of fire which are bound fast within followed the passage of the air
either way, never at any time ceasing so long as the mortal being
holds together. This process, as we affirm, the name-giver named
inspiration and expiration. And all this movement, active as well as
passive, takes place in order that the body, being watered and cooled,
may receive nourishment and life; for when the respiration is going in
and out, and the fire, which is fast bound within, follows it, and
ever and anon moving to and fro, enters through the belly and
reaches the meat and drink, it dissolves them, and dividing them
into small portions and guiding them through the passages where it
goes, pumps them as from a fountain into the channels of the veins,
and makes the stream of the veins flow through the body as through a
conduit.
Let us once more consider the phenomena of respiration, and
enquire into the causes which have made it what it is. They are as
follows:-Seeing that there is no such thing as a vacuum into which any
of those things which are moved can enter, and the breath is carried
from us into the external air, the next point is, as will be dear to
every one, that it does not go into a vacant space, but pushes its
neighbour out of its place, and that which is thrust out in turn
drives out its neighbour; and in this everything of necessity at
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