Galen: On the Natural Faculties

Galen was a Greek doctor who lived in the Roman Empire about 1,850 years ago. As a young man he looked after gladiators, the fighters of the Roman games, and later he became doctor to a Roman emperor.

Galen wanted to know how the body does its everyday jobs: how food becomes part of us, how we grow, and how the body gets rid of what it doesn't need. He thought every part of the body has powers (he called them faculties): a power to draw in what it needs, to hold on to it, to change it, and to push out what it doesn't need.

Some people in Galen's day thought the bladder filled up the way a sponge soaks up mist. Galen tied off the tubes that run from the kidneys to the bladder and showed that the bladder stayed empty. Urine is made in the kidneys and flows down those tubes. He was right.

Who, when and why

Galen (AD 129–c. 216) was born in Pergamon, a Greek city in what is now western Turkey. He studied medicine there and in Smyrna and Alexandria, served as physician to the gladiators of Pergamon, and moved to Rome in 162, where he became physician to the emperor Marcus Aurelius and later to his son Commodus. He wrote more than any other ancient author whose works survive, on anatomy, medicine, drugs and philosophy.

On the Natural Faculties takes on one question: how does a living body grow, take in food and get rid of waste? In Galen's day two rival schools gave answers he thought wrong. Followers of Erasistratus explained the body by tubes and pressure, like plumbing. Asclepiades explained it by tiny particles and pores, with nothing directing them. Galen argued that living parts have active powers of their own.

The four faculties

Galen admits he does not know what a faculty is. A faculty is the name for the unknown cause of an observed effect. He is careful about this: he will not claim more knowledge than he has, but he insists that the effects are real and need a cause.

The ureter experiment

Asclepiades said urine reaches the bladder as vapor, soaking through. Galen answered with an experiment on a living animal. He tied the ureters, the tubes from the kidneys to the bladder. The bladder stayed empty, and the ureters swelled above the ties. When he loosened them, the bladder filled. Urine comes from the kidneys through the ureters, and nowhere else.

Galen uses the magnet as his favorite example: here is something that plainly attracts at a distance, so why deny that a kidney can attract? The comparison is clever. It is also a warning: naming a power is not the same as explaining it, as later scientists who studied magnets (Gilbert, then Faraday, both on this ladder) would show.

Blood, the liver and the heart

Galen thought food, once digested, went to the liver, which turned it into blood. Blood then spread through the veins and was used up by the body parts as food, so new blood had to be made all the time. Some blood, he thought, passed from the right side of the heart to the left through tiny pores in the wall between them. This picture lasted 1,400 years, until William Harvey (1628, later on this ladder) measured how much blood the heart pumps and showed it must go round and round.

What later medicine kept, and what it changed

• Kept: experiment on living things, the idea that each organ has its own function (the root of physiology), and the result that the kidneys make urine. Galen also showed, in another book, that arteries carry blood, not air, as Erasistratus had taught.
• Changed: the faculties became processes we can explain: filtration in the kidney, muscular movement of the gut, chemical digestion by enzymes. Galen's anatomy was corrected by Vesalius and his blood by Harvey; the four humors were given up in the 1800s.

Purpose in nature

Galen believed that nature, which he sometimes calls an artist, does nothing in vain: every part is shaped for its use. He takes this view from Aristotle and Hippocrates. It led him to ask the right question about each organ, what is it for?, and also to accept answers too quickly when an answer seemed fitting. Biologists after Darwin ask the same question in a different way: what did this part do that helped its owners survive and reproduce?

An authority too long trusted

For more than a thousand years, in the Islamic world and in Europe, Galen was "the physician". His books were translated into Syriac, Arabic (by Hunayn ibn Ishaq in the 800s) and Latin. Some later physicians did find errors: Ibn al-Nafis, in Cairo in the 1200s, argued that blood cannot pass through the wall of the heart and must go through the lungs. But most readers took Galen's word over their own eyes. Galen himself had urged doctors to test claims, including old ones. His followers often forgot that part of his teaching.

Big questions

• Is it fair to call a power a "faculty" when you don't know what causes it? When does naming help, and when does it hide ignorance?
• Galen tested his ideas on animals. People today still argue about when experiments on animals are justified; what do you think the main arguments are on each side?
• Why do people go on trusting a famous expert even after some of their claims are shown to be wrong?