From The Surgeons Mate, John Woodall's guide for military and domestic surgery

Galen of Pergamon (129 - c.200 CE) was a prolific author who advanced medical humorism and developed many early surgical techniques. He was chiefly concerned with furthering anatomical and physiological theory, and his contributions to Western medicine were unmatched and largely unquestioned for centuries.

Galen was fascinated by human anatomy, but prohibitions against human dissections limited opportunities for such studies. He primarily dissected pigs, monkeys, and other animals. While Galen had hands-on experience as a surgeon to gladiators in Pergamon, and studied anatomy in Alexandria, much of his writing on human anatomy, including the existence of the rete mirabile, or a network of veins and arteries in humans, was based on animal dissections and other Greek texts. Blending several textual sources with his own experimentation, Galen outlined a theory of anatomy and physiology based on three components: their structure, action, and use.

Galen's theories of the body and clinical practice persisted throughout Byzantine and medieval Europe, sustained primarily by Arabic physicians and intellectuals. Galen was one of two Greek authorities whose works and traditions dominated medical theory well into the fifteenth century. The other was Aristotle, whose focus was on metaphysics and philosophy rather than clinical applications of medicine. This medical framework was easily compatible with an early Christian worldview, where man (the microcosm) mirrored the larger natural world (the macrocosm).