Galen transmitted Hippocratic medicine all the way to the Renaissance. His On the Elements According to Hippocrates describes the philosopher’s system of four bodily humours, blood, yellow bile, black bile and phlegm, which were identified with the four classical elements, and in turn with the seasons. He created his own theories from those principles, and much of Galen’s work can be seen as building on the Hippocratic theories of the body, rather than being purely innovative. In turn, he mainly ignored Latin writings of Celsus, but accepted that the ancient works of Asclepiades had sound theory.
Amongst Galen’s own major works is a seventeen-volume On the Usefulness of the Parts of the Human Body. He also wrote about philosophy and philology, as well as extensively writing on anatomy. His collected works total twenty-two volumes, and he wrote a line a day for most of his life. Galen’s own theories, in accord with Plato’s, emphasized purposeful creation by a single Creator (“Nature” – Greek phusis) – a major reason why later Christian and Muslim scholars could accept his views. His fundamental principle of life was pneuma (air, breath) that later writers connected with the soul.
These writings on philosophy were a product of Galen’s well rounded education, and throughout his life Galen was keen to emphasise the philosophical element to medicine. Pneuma physicon (animal spirit) in the brain took care of movement, perception, and senses. Pneuma zoticon (vital spirit) in the heart controlled blood and body temperature. “Natural spirit” in the liver handled nutrition and metabolism. However, he did not agree with the Pneumatist theory that air passed through the veins rather than blood. Galen expanded his knowledge partly by experimenting with live animals.
One of his methods was to publicly dissect a living pig, cutting its nerve bundles one at a time. Eventually he would cut a laryngeal nerve (now also known as Galen’s Nerve) and the pig would stop squealing. He also tied the ureters of living animals to show that urine comes from the kidneys, and severed spinal cords to demonstrate paralysis. In addition to working with pigs, Galen also experimented with barbary apes and goats, though he emphasised that he practised on pigs due to the fact that, in some respects, they are quite anatomically similar to humans.
However, Galen was always keen to distance himself from the more manual, artisan elements of medicine. Public dissections were also a highly valuable way of disputing and disproving the biological theories of others, and were one of the main methods of academic medical learning in Rome. It was quite common for large numbers of medical students to attend these public gatherings, which would sometimes turn into debates. From the modern viewpoint, Galen’s theories were partially correct, partially flawed. He demonstrated that arteries carry blood, not air and made first studies about nerve functions, and the brain and heart.
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. 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. Since most of his knowledge of anatomy was based on dissection of pigs, dogs, and Barbary apes, he also assumed that rete mirabile, a blood vessel plexus of ungulates, also existed in the human body.
He also resisted the idea of tourniquets to stop bleeding and vigorously propagated blood letting as a treatment. Galen’s authority dominated medicine all the way to the 16th century. Experimenters’ disciples did not bother to experiment and studies of physiology and anatomy stopped – Galen had already written about everything. Blood letting became a standard medical procedure. Vesalius presented the first serious challenge to his hegemony. Much of medieval Islamic medicine drew on the works of the ancient Greeks, especially those elucidated by Galen, such as his expanded humoral theory.
Most of Galen’s Greek writings were first translated to the Syriac language by Nestorian monks in the university of Gundishapur, Persia. Muslim scholars primarily in Baghdad translated the Syriac manuscripts into Arabic, along with many other Greek classics. They became some of the main sources for Arabian scholars such as Avicenna, Rhazes, and Maimonides. Galen was known in Arabic as Jalinos, and many people with that name today are considered to be descended from him.