Claudius Galenus of Pergamum (129-200 AD), better known in English as Galen, was an ancient Greek physician. His views dominated European medicine for over a thousand years. From the modern viewpoint, Galen's theories were partially correct and partially flawed: he demonstrated that arteries carry blood rather than air, and conducted the first studies of nerve, brain, and heart function. He also argued that the mind was in the brain, not in the heart as Aristotle had claimed.

However, much of Galen's understanding is flawed from the modern point of view. For example, he did not recognize blood circulation and thought that venous and arterial systems were separate. This view did not change until William Harvey's work in the 17th century.