Galen, AD 129 – 199 or 217
Galen of Pergamum, was a prominent Roman physician and philosopher of Greek origin, and probably the most accomplished medical researcher of the Roman period. His theories dominated and influenced Western medical science for well over a millennium. His account of medical anatomy was based on monkeys as human dissection was not permitted in his time, but it was unsurpassed until the printed description and illustrations of human dissections by Andreas Vesalius in 1543. Galen's account of the activities of the heart, arteries and veins endured until William Harvey established that the blood circulates with the heart acting as a pump in 1628. In the 19th century, student physicians would still read Galen to learn some concepts.
Galen developed many nerve ligation experiments that supported the theory, which is still believed today, that the brain controls all the motions of the muscles by means of the cranial and peripheral nervous systems. Galen wrote a small work called "That the Best Physician is also a Philosopher", and he saw himself as being both, which meant grounding medical practice in theoretically sound knowledge or "philosophy" as it was called in his time. Galen was very interested in the dispute between Rationalist and Empiricist medical sects, and his use of direct observation, dissection and vivisection in medical training and as a way to ground medical practice can be understood as considering both of those perspectives and constructing a more complex and nuanced middle ground that avoided problems with each position.