Aelius Galenus (Galen) was a Greek physician and philosopher in the Roman Empire who became the most influential medical researcher of antiquity. Because human dissection was illegal in Rome, he dissected animals (monkeys and pigs) to understand anatomy, which led to some errors but also groundbreaking discoveries about the circulatory and nervous systems. He proved that arteries carry blood, not air, and identified the functions of several cranial nerves. His theory of the 'four humors' (blood, phlegm, yellow bile, and black bile) dominated medical thought for over 1,500 years until the Renaissance. Galen served as the personal physician to several emperors, including Marcus Aurelius. His voluminous writings became the standard medical curriculum in both the Islamic world and Europe. Even though many of his anatomical assumptions were later corrected by Vesalius, Galen's systematic approach to observation and experimentation laid the foundation for modern physiology.