outer net contains two lesser nets, one corresponding to
the stomach, the other to the lungs; and the entrance to the latter is
forked or divided into two passages which lead to the nostrils and to
the mouth. In the process of respiration the external net is said to
find a way in and out of the pores of the skin: while the interior of
it and the lesser nets move alternately into each other. The whole
description is figurative, as Plato himself implies when he speaks of a
'fountain of fire which we compare to the network of a creel.' He really
means by this what we should describe as a state of heat or temperature
in the interior of the body. The 'fountain of fire' or heat is also in a
figure the circulation of the blood. The passage is partly imagination,
partly fact.

He has a singular theory of respiration for which he accounts solely by
the movement of the air in and out of the body; he does not attribute
any part of the process to the action of the body itself. The air has
a double ingress and a double exit, through the mouth or nostrils, and
through the skin. When exhaled through the mouth or nostrils, it leaves
a vacuum which is filled up by other air finding a way in through the
pores, this air being thrust out of its place by the exhalation from the
mouth and nostrils. There is also a corresponding process of inhalation
through the mouth or nostrils, and of exhalation through the pores. The
inhalation through the pores appears to take place nearly at the same
time as the exhalation through the mouth; and conversely. The internal
fire is in either case the propelling cause outwards--the inhaled air,
when heated by it, having a natural tendency to move out of the body to
the place of fire; while the impossibility of a vacuum is the propelling
cause inwards.

Thus we see that this singular theory is dependent on two principles
largely employed by Plato in explaining the operations of nature, the
impossibility of a vacuum and the attraction of like to like. To these
there has to be added a third principle, which is the condition of
the action of the other two,--the interpenetration of particles in
proportion to their density or rarity. It is this which enables fire and
air to permeate the flesh.

Plato's account of digestion and the circulation of the blood is closely
connected with his theory of respiration. Digestion is supposed to be
effected by the action of the internal fire, which in the process of
respira