Galen's medical writings (comprising nearly a hundred treatises) became the standard source of medical knowledge for centuries. His experimental work was pioneering: he demonstrated the function of the nervous system by cutting animals' spinal cords and different points and observing their resulting paralysis. He was the first to consider the diagnostic value of taking a subject's pulse, and was the first to identify several muscles.
But although he was very able, Galen began from many faulty premises -- his De usu partium (On the Use of Parts) depends on the teleological theories of Aristotle, the tidal theory of the flow of the blood, and the humoral theory of disease -- and this led him to many inaccurate conclusions. His theories on the circulation of the blood, for instance, were not corrected until the sixteenth century and the work of William Harvey.