Almost everything we consider common knowledge today was once a total mystery. Around the second century AD, no one in the western world knew that the arteries, veins, and heart were filled with blood. Most thought they were filled with air. Here's how one man disproved that theory.

Do you know what you'll find if you cut open the heart of a corpse? Not much. The circulatory system is a series of tubes, and in order to keep liquid moving through all of those tubes, something needs to keep the pressure on. Once the heart stops, the liquid in the veins and arteries drains away - usually to the back of the body, since most corpses are placed on their back. The arteries and the chambers of the heart empty of blood and fill with air.

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It made sense. Having done all their anatomical research on dead bodies, most people thought that being filled with air was an artery's natural state. And everyone knew that people needed air. It was perfectly logical to think that the veins and arteries were the air-delivery system for the body, and the pulse was the contraction of those arteries, helping the air circulate. Meanwhile, blood seeped out from any cut made in the body, even in areas where there weren't any blood vessels. Blood was clearly all over the place, so it probably just squished around a person's tissues, oozing from one place to another.

Famed physician Galen, who treated gladiators coming back from their fights, thought the veins might contain something other than air — and considering the injuries he saw, he would know. He set out to do a series of experiments to show what emerged from veins and arteries when they were opened, and how they did and did not work.

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First he got some hollow tubes and inserted them into the arteries and veins of various unlucky animals. Guess what came out of the tubes? The pulsing blood was a complete surprise to some people. He then proved that the pulse originates in the heart, rather than in the arteries, by simply wrapping a tight piece of cord around the center femoral artery in the legs of pigs and dogs, putting a length of artery on either side of the cord. The length of artery on the side nearest to the heart pulsed away. The length of artery on the far side of the cord, away from the heart, stopped pulsing until he released the cord. It was only then, around 150 AD, that what any elementary school kid today knows first became accepted by the leading physicians and scholars at the time.

Galen didn't get it exactly right. He believed that different types of blood went to different parts of the body, and that instead of circulating, blood is used up by the organs. No one would correct the misconception for over a thousand years. Then again, no one gets it exactly right. And no scientific discovery comes without work.

Image: Horst Sturm

[Sources: William Harvey and the Mechanics of the Heart, The Man Who Touched His Own Heart.]

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