Therefore, as moisture turned into vapour by the sun's heat is, when it has ascended to the upper regions, cooled by the coldness of the latter, and becoming condensed, is carried downwards, and turned into water once more; just so the waste exhalation, when carried up by the heat to the region of the brain, is condensed into phlegm (which explains why catarrhs are seen to proceed from the head); while that exhalation which is nutrient and not unwholesome, becoming condensed, descends and cools the hot.