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Galen was an anatomist and physiologist and probably the most influential theoretical physician of all time. His position as the leading authority in medical theory extended for at least fourteen hundred years.
Galen was born in Pergamum, studied in Smyrna, and Alexandria and was appointed surgeons of gladiators in Pergamum. In 161 he went to Rome where he gave lectures in the public theater and performed experiments before large audiences. Galen discovered that arteries contain blood and not air as had been taught up to his time.
Galen wrote more than 400 books and his influence on medical theory was dominant throughout the Middle Ages.
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From the Internet Encyclopedia of Philosophy