Greek physician considered second only to Hippocrates of Cos in his importance to the development of medicine, Galen performed extensive dissections and vivisections on animals. Although human dissections had fallen into disrepute, he also performed and stressed to his students the importance of human dissections. He recommended that students practice dissection as often as possible. He studied the muscles, spinal cord, heart, urinary system, and proved that the arteries are full of blood. He believed that blood originated in the liver, and sloshed back and forth through the body, passing through the heart, where it was mixed with air, by pores in the septum. Galen also introduced the spirit system, consisting of natural spirit or "pneuma" (air he thought was found in the veins), vital spirit (blood mixed with air he believed to found in the arteries), and animal spirit (which he believed to be found in the nervous system). In On the Natural Facilities, Galen minutely described his experimentation on a living dog to investigate the bladder and flow of urine. It was Galen who first introduced the notion of experimentation to medicine.

Galen believed everything in nature has a purpose, and that nature uses a single object for more than one purpose whenever possible. He maintained that "the best doctor is also a philosopher," and so advocated that medical students be well-versed in philosophy, logic, physics, and ethics. Galen and his work On the Natural Faculties remained the authority on medicine until Vesalius in the sixteenth century, even though many of his views about human anatomy were false since he had performed his dissections on pigs, Barbary apes, and dogs. Galen mistakenly maintained, for instance, that humans have a five-lobed liver (which dogs do) and that the heart had only two chambers (it has four).

Hippocrates of Cos, Vesalius

Additional biographies: Greek and Roman Science and Technology