the
left, having the marrow of generation between them. In the next place,
they divided the veins about the head and interlaced them with each
other in order that they might form an additional link between the head
and the body, and that the sensations from both sides might be diffused
throughout the body. In the third place, they contrived the passage
of liquids, which may be explained in this way:--Finer bodies retain
coarser, but not the coarser the finer, and the belly is capable of
retaining food, but not fire and air. God therefore formed a network of
fire and air to irrigate the veins, having within it two lesser nets,
and stretched cords reaching from both the lesser nets to the extremity
of the outer net. The inner parts of the net were made by him of fire,
the lesser nets and their cavities of air. The two latter he made to
pass into the mouth; the one ascending by the air-pipes from the lungs,
the other by the side of the air-pipes from the belly. The entrance to
the first he divided into two parts, both of which he made to meet at
the channels of the nose, that when the mouth was closed the passage
connected with it might still be fed with air. The cavity of the network
he spread around the hollows of the body, making the entire receptacle
to flow into and out of the lesser nets and the lesser nets into and out
of it, while the outer net found a way into and out of the pores of the
body, and the internal heat followed the air to and fro. These, as we
affirm, are the phenomena of respiration. And all this process takes
place in order that the body may be watered and cooled and nourished,
and the meat and drink digested and liquefied and carried into the
veins.

The causes of respiration have now to be considered. The exhalation of
the breath through the mouth and nostrils displaces the external air,
and at the same time leaves a vacuum into which through the pores the
air which is displaced enters. Also the vacuum which is made when the
air is exhaled through the pores is filled up by the inhalation of
breath through the mouth and nostrils. The explanation of this double
phenomenon is as follows:--Elements move towards their natural places.
Now as every animal has within him a fountain of fire, the air which
is inhaled through the mouth and nostrils, on coming into contact
with this, is heated; and when heated, in accordance with the law of
attraction, it escapes by the way it entered toward the place of fir