Greek physician and anatomist who carried forward the systematic investigation of anatomy begun by Herophilus in Alexandria. He described the cerebrum and cerebellum of the brain, studied nerves and the valves of the heart. He distinguished between veins and arteries (which he believed to be full of air). He proposed mechanical explanations for many bodily processes, including digestion. He believed in a three part system of humors consisting of nervous spirit (nerves), animal spirit (arteries), and blood (veins). The use of dissection and study of anatomy declined after his work.