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
respiration moves into the stomach and minces the food. As the fire
returns to its place, it takes with it the minced food or blood; and in
this way the veins are replenished. Plato does not enquire how the blood
is separated from the faeces.

Of the anatomy and functions of the body he knew very little,--e.g. of the
uses of the nerves in conveying motion and sensation, which he supposed to
be communicated by the bones and veins; he was also ignorant of the
distinction between veins and arteries;--the latter term he applies to the
vessels which conduct air from the mouth to the lungs;--he supposes the
lung to be hollow and bloodless; the spinal marrow he conceives to be the
seed of generation; he confuses the parts of the body with the states of
the body--the network of fire and air is spoken of as a bodily organ; he
has absolutely no idea of the phenomena of respiration, which he attributes
to a law of equalization in nature, the air which is breathed out
displacing other air which finds a way in; he is wholly unacquainted with