Galen

Greek physician and anatomist whose ideas dominated Western medicine for almost 1,500 years. Central to his thinking were the threefold circulation of the blood and the theory of humours (blood, phlegm, choler/yellow bile, and melancholy/black bile) that contributed to mental and physical state. His On Anatomical Procedures, a detailed description of animal dissections when work on human corpses was forbidden, became a standard text on anatomy when rediscovered in Western Europe in the 16th century. He remained the highest medical authority until Andreas Vesalius and William Harvey exposed the fundamental errors of his system.

Galen postulated a circulation system in which the liver produced the natural spirit, the heart the vital spirit, and the brain the animal spirit. He also wrote about philosophy and believed that Nature expressed a divine purpose, a belief that became increasingly popular with the rise of Christianity (Galen himself was not a Christian). This helped to account for the enormous influence of his ideas.