Timaeus

For as the respiration goes in and out the inward fire attached thereto follows it; and whenever in its constant oscillations this fire enters in through the belly and lays hold on the meats and drinks, it dissolves them, and dividing them into small particles it disperses them through the outlets by which it passes and draws them off to the veins, like water drawn into channels from a spring; and thus it causes the streams of the veins to flow through the body as through a pipe. Once again let us consider the process of respiration, and the causes in virtue of which it has come to be such as it now is. This, then, is the way of it. Inasmuch as no void exists into which any of the moving bodies could enter, while the breath from us moves outwards, what follows is plain to everyone— namely, that the breath does not enter a void but pushes the adjacent body from its seat; and the body thus displaced drives out in turn the next; and by this law of necessity every such body is driven round towards the seat from which the breath went out and enters therein, filling it up and following the breath; and all this takes place as one simultaneous process, like a revolving wheel, because that no void exists. Wherefore the region of the chest and that of the lungs when they let out the breath become filled again by the air surrounding the body, which filters in through the porous flesh and circulates round. And again, when the air is repelled and passes out through the body it pushes the inspired air round and in by way of the passages of the mouth and of the nostrils. The originating cause of these processes we must assume to be this. Every living creature has its inward parts round the blood and the veins extremely hot, as it were a fount of fire residing within it; and this region we have, in fact, likened to the envelope of the fish-weel, saying that all that was extended at its middle was woven of fire, whereas all the other and outward parts were of air. Now we must agree that heat, by Nature’s law, goes out into its own region to its kindred substance; and inasmuch as there are two outlets, the one out by way of the body, the other by way of the mouth and the nose, whenever the fire rushes in one direction it propels the air round to the other, and the air which is thus propelled round becomes heated by streaming into the fire, whereas the air which passes out becomes cooled. And as the heat changes its situation and the particles about the other outlet become hotter, the hotter body in its turn tends in that direction, and moving towards its own substance propels round the air which is at the former outlet; and thus the air, by continually undergoing and transmitting the same affections, causes inspiration and expiration to come about as a result of this double process, as it were a wheel that oscillates backwards and forwards.