On the Motion of the Heart and Blood in Animals

Overview

De Motu Cordis is seventy-two pages in its first printing, seventeen short chapters, published at Frankfurt in 1628 on cheap paper with so many misprints that Harvey complained about it. It is the shortest book on this shelf that overturned an entire science.

The system it overturned was Galen’s, and it is worth knowing what that was, because otherwise Harvey’s argument looks like an attack on nobody. In Galen’s account there are two kinds of blood and no circuit. Food becomes chyle in the gut, travels to the liver, and is made into venous blood, which seeps outward through the veins to the tissues and is consumed there. A portion passes through the right side of the heart, filters through invisible pores in the dividing wall, meets air from the lungs, and becomes a different, brighter arterial blood carrying vital spirit. The heart’s active phase is its expansion, drawing blood in the way a bellows draws air. Blood is made continuously and used up continuously. This had been taught for fourteen centuries.

Harvey studied at Padua under Fabricius, who had described the valves in the veins and concluded that they slowed the outward flow so that the limbs were not flooded. Harvey looked at the same structures and saw that they could only be one-way doors, pointing the wrong way for Galen’s system. He lectured on the circulation from about 1616 and waited twelve years to publish.

The argument

Chapters I to VII: what the heart actually does. Harvey begins by admitting that the motion of the heart is extremely difficult to observe: in a warm-blooded animal it is too fast to see. His solution is to slow it down, by working on cold-blooded animals, on newly hatched chicks, and on dogs at the end of a long vivisection when the beat has become slow and feeble. Watched that way, the sequence is unmistakable. The heart is active when it contracts, not when it dilates. It becomes hard and pale, it grows smaller, and it throws its contents out. The pulse in the artery is not the artery expanding of its own power but the impulse of blood arriving.

He then follows the blood through the right ventricle to the lungs and back to the left, a transit already described by others, and notes what it implies: the whole output of the right side must pass through the lungs, so the lungs are not merely cooling the blood.

Chapter VIII and IX: the calculation. This is the heart of the book. Harvey estimates how much blood the left ventricle holds, and how much of it is expelled at each stroke, and takes a deliberately low figure. He multiplies by the number of beats in half an hour. The answer is more blood than is in the entire body, several times over. Where would it come from, and where would it go? The liver cannot make it and the flesh cannot absorb it. No estimate cautious enough to save Galen is available, and Harvey invites the reader to try. Faced with a quantity that has to go somewhere and come back, he says he began to think there might be a motion as it were in a circle.

Chapters X to XIII: the proofs. Having stated the hypothesis he spends four chapters confirming it. Ligature experiments on a human arm are the most elegant. Tie the arm tightly and the pulse below vanishes and the arteries above swell: the flow in arteries is outward from the heart. Loosen the ligature so that the deeper arteries are open but the superficial veins are still compressed, and the arm below fills, the veins stand out and knot. Then, on a bound arm, press a finger along a swollen vein toward the heart and the vein empties and will not refill from above, because a valve stops it; release below and it fills instantly. Anyone can perform this, and Harvey tells the reader to.

Chapters XIV to XVII: the conclusion and its consequences. The statement of the circulation is one paragraph, placed after all the evidence rather than at the front. The rest gathers confirmations: how poisons and medicines spread, how bleeding from one small artery can empty the whole body, why the heart’s structure is shaped as it is.

One thing Harvey cannot show is how blood crosses from the smallest arteries to the smallest veins. He says it must, and that it happens through pores or passages he cannot observe. Marcello Malpighi saw the capillaries through a microscope in 1661, four years after Harvey’s death.

How to read it

Read the two dedications. The one to Charles I, comparing the heart of the animal to the king of the kingdom, tells you that Harvey is presenting a discovery to a court, and the one to the College of Physicians tells you he expects a fight.

Read chapters II to IV once for the method and then read chapter IX slowly, doing the arithmetic yourself with his figures. The whole revolution is in that page, and it is the only place in a seventeenth-century anatomy where a number does the work.

Then do the ligature experiment on your own arm. It takes a minute, it works, and it is a rare chance to repeat the decisive evidence of a classic text.

Skim the later chapters. They are accumulation rather than argument, and Harvey is answering objections that nobody now holds.

Watch his tone with Galen. Harvey is not a modern rebel against the ancients; he reveres Aristotle, quotes him constantly, and treats Galen as a great man who got something wrong. The book is a piece of careful persuasion aimed at people who had been taught the old system and would have to unlearn it.

Questions it raises

• Fabricius saw the valves and drew the wrong conclusion. What made the difference: better observation, or a different question asked of the same object?
• The quantitative argument rules out Galen without proving circulation. What else could Harvey’s number have meant?
• Harvey says the blood must pass from artery to vein through passages he cannot see. When is it legitimate to assert something you have not observed?
• He kept the discovery unpublished for about twelve years. What does that say about what it takes to overturn a system?
• The heart is called the sun of the microcosm and the prince of the body. Is the political analogy ornament, or is it doing argumentative work?
• Circulation explained nothing about what blood is for, and Harvey knew it. Does a discovery count as knowledge when it leaves the function unexplained?