Pneuma and Vapor

All the arteries get power from the heart that makes them dilate and contract.

The arteries draw neighbouring matter into themselves when they dilate.

The arteries which reach the skin draw in the outer air when they dilate.

Those which anastomose at any point with the veins attract the thinnest and most vaporous part of the blood which these contain, and as for those arteries which are near the heart, it is on the heart itself that they exert their traction.

For, by virtue of the tendency by which a vacuum becomes refilled, the lightest and thinnest part obeys the tendency before that which is heavier and thicker.

The rank of the lightest and thinnest of anything in the body is:

1. Pneuma
2. Vapour
3. The blood which has been accurately elaborated and refined

The arteries draw these into themselves on every side.

Those arteries which reach the skin draw in the outer air 390 (this being near them and one of the lightest of things).

Those other arteries which pass up from the heart into the neck, and that which lies along the spine, as also such arteries as are near these—draw mostly from the heart itself.

Those which are further from the heart and skin necessarily draw the lightest part of the blood out of the veins.

So also the traction exercised by the diastole of the arteries which go to the stomach and intestines takes place at the expense of the heart itself and the numerous veins in its neighbourhood; for these arteries cannot get anything worth speaking of from the thick heavy nutriment contained in the intestines and stomach,391 since they first become filled with lighter elements.

For if you let down a tube into a vessel full of water and sand, and suck the air out of the tube with your mouth, the sand cannot come up to you before the water, for in accordance with the principle of the refilling of a vacuum the lighter matter is always the first to succeed to the evacuation.