Timaeus

Thereupon they arranged the irrigation on some such plan as this—a plan which we shall perceive more easily when we have first agreed upon the following postulates. All bodies composed of smaller particles shut in the larger, but those composed of larger particles cannot shut in the smaller; and fire, because of all the elements it has the smallest particles, passes through water and earth and air and all things composed thereof, and nothing can shut it in. We must conceive that the same law holds good of the action of our belly. Whenever foods and drinks flow into it it shuts them in, but air and fire, being of smaller particles than its own structure, it cannot shut in. These elements, therefore, God employed to provide irrigation from the belly to the veins, weaving out of air and fire a veil of mesh-work like unto a fish-weel, having two innerweels at its entrance; and one of these inner-weels He wove over again so as to make it bifurcated; and from the inner-weels He stretched as it were ropes all over it in a circle up to the extremities of the veil. Now the inward parts of the veil He constructed wholly of fire, but the inner-weels and the envelope of air; and taking this He placed it round about the living creature that was molded in the following manner. The part consisting of the inner-weels He let down into the mouth; and since this part was twofold, He let down one inner-weel by way of the windpipe into the lungs, and the other into the belly alongside the windpipe. And cleaving the former of these weels in two He gave to both sections a common outlet by way of the channels of the nose, so that when the first conduit by way of the mouth failed to act, its streams as well should be plenished from this. The rest of the enveloping mesh-work He made to grow round all the hollow part of our body; and He caused all this at one time to flow gently into the inner-weels, seeing they were of air, and at another time the weels to flow back into it. And inasmuch as the body was porous, He caused the veil to pass in through it and out again; and the inner rays of fire that were enclosed within it He made to follow the air as it moved in either direction; whence it comes that, so long as the mortal living creature preserves its structure, this process goes on unceasingly. And to this kind of process the Giver of Titles gave, as we say, the names of inspiration and expiration. And the whole of this mechanism and its effects have been created in order to secure nourishment and life for our body, by means of moistening and cooling.