Erasistratus, together with Herophilus, founds the first school of systematic anatomy in Alexandria, with exceptional access to dissection of human cadavers under Ptolemaic patronage. He precisely describes the tricuspid and mitral heart valves and their unidirectional-flow function, the morphological difference between arteries and veins, the cerebellum and cerebral ventricles, and the distinction between motor and sensory nerves. He rejects Hippocratic humoral theory and proposes mechanistic explanations of bodily processes. He attributes disease to an excess of blood (plethora) rather than the humors, anticipating circulatory concepts Harvey would formalize 1,900 years later. His works were completely lost; knowledge of his work comes only through citations, often polemical, by Galen.