20. Internally, however, the closeness of the connection of the passages is not maintained throughout; for the heart intervening between the base of the two, infuses in the one the powers for respiration, and in the other for nutriment. Now the fiery element is naturally inclined to seek for the material which serves as fuel, and this necessarily happens with regard to the receptacle of nourishment; for the more it becomes penetrated by fire through the neighbouring warmth, the more it draws to itself what nourishes the heat. And this sort of impulse we call appetite.