Today, we meet the man who showed us how blood flows. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

The old Roman physician Galen told us that blood is generated in our liver, that it passes only once through the heart, and that it's consumed in the outlying body tissues. Food is converted to blood, Galen said. Then it carries nutrients to the far corners of the body.

Doctors still believed those old Galenic doctrines in 1600, but one thing puzzled them. The left side of the heart supplies fresh red blood to the extremities. The right side pumps dark blood into the lungs. But where does that blood come from? According to Galen, blood leaked through microscopic pores in the septum to get into the right side. The right side was no more than a secondary supply system for the lungs.

The fresh winds of modern science were just gathering in northern Italy. The University of Padua was on the cutting edge of the science of anatomy. And it was there that a young Englishman named William Harvey went to study. He completed his doctorate on blood flow in 1602.

He published a little 72-page book 26 years later. It was compact and beautifully illustrated. With wonderful economy Harvey quite literally went to the heart of the matter. He gave us all a fine lesson on using our head.

First he demolished the Galenic idea that each gram of blood passes through the heart only once. Harvey counted the pulse and estimated the outflow of each stroke of the heart. He found that it puts out three times our body weight each hour. Imagine eating food fast enough to replenish that much blood.

And the right side of the heart? It receives the used blood from the extremities and sends it to the lungs to be refreshed. Harvey didn't know about oxygen, but he saw that the blood flowed from one side of the heart to the other, not through the septum, but through the lungs, where it was recycled.

Harvey had to add an unproven doctrine of his own to complete a new blood-flow theory. How did blood get from the tiny outlying arteries back into the veins? He guessed that a structure of capillaries made the connection. He couldn't see any capillaries, but his guess fared better than Galen's. The new microscope lenses found his capillaries four years after he died.

Harvey was a great genius in an age rich with genius. Galen's doctrines, grooved into the collective conscious of medicine, were a tough hurdle. To get around them, Harvey had to show us more than a clear head and keen insight. He had to give us all a lesson in dancing to our own drum.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Nuland, S.B., Doctors: The Biography of Medicine. New York, Vintage Books, 1988.