Erasistratus of Iulis on *Ceos (about 315–240 bce?) is the only scientist other than *Herophilus to whom ancient sources attribute systematic scientific dissections of human cadavers. *Cornelius Celsus claims that Erasistratus, like Herophilus, also vivisected convicted criminals (see vivisection). The extant evidence leaves little doubt that he performed vivisectory experiments on animals. Often taking a functional approach to his anatomical discoveries, he combined detailed descriptions of parts with explanations of their physiological roles. Thus he not only gave the first reasonably accurate description of the heart valves but also demonstrated that their function is to ensure the irreversibility of the flow through the valves.Three consistent features of Erasistratus' approach are his use of mechanistic principles to explain bodily processes, an Aristotelian teleological perspective, and the verification of an *hypothesis by means of *experiment. His major mechanistic principle is that matter naturally moves by means of ‘following toward what is being emptied’ (πρὸς τὸ κενούμενον ἀκολουθία), i.