On the Motion of the Heart and Blood in Animals
He proved the blood travels one endless loop — and the heart is the pump that drives it.
For 1,400 years doctors believed the body was always making fresh blood and burning it up. One careful sum proved that was impossible.
The big idea
Before Harvey, medicine followed the ancient Greek physician Galen: blood was made in the liver, flowed out to the body, and was used up like fuel — a one-way tide, made and consumed. Harvey showed instead that blood travels in a loop. The heart is a pump; it pushes the same blood out through the arteries and receives it back through the veins, around and around, ceaselessly.
His clinching move wasn't a new instrument — it was arithmetic. He worked out how much blood the heart pushes out with each beat, multiplied by how many beats happen in just half an hour, and got a quantity far larger than all the blood in the body. There was no way the body could make that much that fast. So it had to be the same blood, going round.
How it came about
Harvey studied at Padua, the great anatomy school in Italy, under Fabricius — who had noticed little valves inside the veins but never grasped what they were for. Back in London, physician to two kings, Harvey spent years dissecting living animals, watching hearts beat and slow and stop, and feeling pulses keep time with each squeeze.
He saw the valves all pointed one way — toward the heart. He pressed on the veins in his own arm and watched them refuse to refill from the wrong direction. And he did the sum. In 1628 he published it in a thin Latin book, “De Motu Cordis.” It was attacked for years; some peers thought him “crack-brained.” He lived to see it accepted.
Why it mattered
Once you know blood circulates, everything downstream becomes possible: measuring blood pressure, understanding shock and heart failure, transfusion, open-heart surgery, the heart–lung machine. But the deeper revolution was in method. Harvey didn't win by quoting older authorities; he won by measuring, and letting the number overrule fifteen centuries of belief. That is how modern medicine learned to think.
A way to picture it
Picture a city's water supply. Galen imagined a spring the city drinks dry — water made at the source, used up at the taps, forever needing more. Harvey saw the truth: it's a closed plumbing loop with a pump at its heart, the same water circulating past every house and returning. The one-way valves are the check-valves that stop it running backward.
Where it sits
Harvey's book (1628) stands at the start of the modern scientific revolution, a generation before Newton's “Principia” (1687, also in this Library) made the same move in physics: replace inherited authority with measurement and mechanism. In medicine it opened the line that runs through Pasteur and Fleming (whose penicillin is in this Library) — the slow turning of healing into a science. The last piece of Harvey's own loop, the capillaries, was found by Malpighi in 1661 with the new microscope.
Dedication · to Charles I
The heart of animals is the foundation of their life, the sovereign of everything within them, the sun of their microcosm, that upon which all growth depends, from which all power proceeds.
Ch. VIII · A motion in a circle
I began to think whether there might not be a motion, as it were, in a circle. Now this I afterwards found to be true; and I finally saw that the blood, forced by the action of the left ventricle into the arteries, was distributed to the body at large, and its several parts, in the same manner as it is sent through the lungs, impelled by the right ventricle into the pulmonary artery, and that it then passed through the veins and along the vena cava, and so round to the left ventricle in the manner already indicated.
Ch. IX · The quantitative argument
Whence it is to be inferred, that if at one stroke the heart in man, the ox or the sheep, ejects but a single drachm of blood, and there are one thousand strokes in half an hour, in this interval there will have been ten pounds five ounces expelled: were there with each stroke two drachms expelled, the quantity would of course amount to twenty pounds and ten ounces; were there half an ounce, the quantity would come to forty-one pounds and eight ounces; and were there one ounce it would be as much as eighty-three pounds and four ounces; the whole of which, in the course of one half hour, would have been transfused from the veins to the arteries.
Ch. XIV · The conclusion
It is absolutely necessary to conclude that the blood in the animal body is impelled in a circle, and is in a state of ceaseless motion; that this is the act or function which the heart performs by means of its pulse; and that it is the sole and only end of the motion and contraction of the heart.