(born AD 129, Pergamum, Mysia, Anatoliadied c. 216) Greek physician, writer, and philosopher. He became chief physician to the gladiators in AD 157. Later, in Rome, he became a friend of Marcus Aurelius and physician to Commodus. Galen saw anatomy as fundamental and, based on animal experiments, described cranial nerves and heart valves and showed that arteries carry blood, not air. However, in extending his findings to human anatomy he was often in error. Following Hippocratic concepts (seeHippocrates), he believed in three connected body systemsbrain and nerves for sensation and thought, heart and arteries for life energy, and liver and veins for nutrition and growthand four humours (body fluids)blood, yellow bile, black bile, and phlegmthat needed to be in balance. Few had the skills to challenge his seductive physiological theory. He wrote about 300 works, of which about 150 survive. As they were translated, his influence spread to the Byzantine Empire, Arabia, and then western Europe. A revival of interest in the 16th century led to new anatomical investigations, which caused the overthrow of his ideas when Andreas Vesalius found anatomical errors and William Harvey correctly explained blood circulation.