Galen
Does cutting open an animal teach you about a human?
That time, that place
Pergamon was a city of gladiatorial games, and Galen began as the physician who patched the gladiators up. Torn and punctured wounds were, for him, a textbook of the human body opening daily. Later he went to Rome and became physician to the emperor.
Why this question
One problem stood in the way. The brief Alexandrian window had closed, and cutting open a human body was forbidden again. So how do you learn what lies beyond the wound?
Galen's answer: if you cannot open a human, open an animal. He dissected apes and pigs.
What was found
He did not merely look; he experimented. Cutting nerves one by one in a living pig, he checked what each controlled. Sever the nerve running to the throat and the pig fell silent — evidence that the voice comes from the brain, not the throat. He also established that arteries carry blood, not air.
The method was excellent. The conclusions were the problem. He transcribed what he saw in apes directly onto humans, and every difference became an error. He wrote of a pore in the human heart that is not there, and described the liver as the maker of blood.
Arteries carry air
Dissection of apes and pigs, and nerve experiments on living animals
His books were medicine's final authority for fourteen centuries. Vesalius found over two hundred errors in the actual human body in 1543