Plutarch's Platonic questions (7.3)

As to cupping-glasses, the case is thus: the air next to the flesh being comprehended and inflamed by the heat, and being made more rare than the pores of the brass, does not go into a vacuum (for there is no such thing), but into the air that is without the cupping-glass, and has an impulse upon it. This air drives that before it; and each, as it gives way, strives to succeed into the place which was vacuated by the cession of the first. And so the air approaching the flesh comprehended by the cupping-glass, and exciting it, draws the humors into the cupping-glass.