perennial stream. Two were cut down the back, along the back bone, where
the skin and flesh meet, one on the right and the other on 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