he support and the extinction of flame; and he says that
if we could find out why flame is extinguished by absence of the air, we
might then know the nature of that substance which imparts warmth to the
blood during the process of respiration.

On another occasion he says that it is evidently the _quality_ and not
the _quantity_ of the air which is necessary to life. He further shows
that he recognized the analogy between respiration and combustion, by
comparing the lungs to a lamp, the heart to its wick, the blood to the
oil, and the animal heat to the flame.

From certain observations in various parts of his works, it appears
that, although ignorant of the doctrine of atmospheric pressure, he was
acquainted with some of its practical effects. Thus, he says, if you put
one end of an open tube under water and suck out the air with the other
end, you will draw up water into the mouth, and that it is in this way
that infants extract the milk from the mother's breast.

Again, Erasistratus supposed that the vapour of charcoal and of certain
pits and wells was fatal to life because _lighter_ than common air, but
Galen maintained it to be _heavier_.

He describes two kinds of respiration, one by the mouths of the arteries
of the lungs, and one by the mouths of the arteries of the skin. In each
case, he says, the surrounding air is drawn into the vessels during
their diastole, for the purpose of cooling the blood, and during their
systole the fuliginous particles derived from the blood and other fluids
of the body are forced out.

He considers the diaphragm to be the principal muscle of respiration,
but he makes a clear distinction between ordinary respiration, which he
calls a natural and involuntary effort, and that deliberate and forced
respiration which is obedient to the will; and he says that there are
different muscles for the two purposes. Elsewhere he particularly points
out the two sets of intercostal muscles and their mode of action, of
which, before his time, he asserts that anatomists were ignorant.

He describes various effects produced on respiration and on the voice by
the division of those nerves which are connected with the thorax; and
shows particularly the effect of dividing the recurrent branch of his
sixth pair of cerebral nerves (the pneumogastric of modern anatomy). He
explains how it happens that after division of the spinal cord, provided
that division be _beneath_ the lower termination of the ne