In 1628 a short, badly printed book of seventy two pages appeared in Frankfurt. Its author was the physician to the King of England, and its argument was that the accepted account of what blood does in the body, taught without serious challenge for fourteen centuries, was impossible on arithmetical grounds. It is one of two occasions when a piece of ancient biology was demolished by counting, the other being Pasteur’s flasks two centuries later.
William Harvey’s De Motu Cordis is usually described as the founding text of modern physiology. What makes it remarkable is the method. Harvey did not overturn Galen by dissecting better than his predecessors, although he did. He overturned him by multiplying.
The Theory He Inherited
Galenic physiology, in place since the second century, described a system with no circulation in it.
Food was converted in the liver into venous blood, which carried nourishment outward through the veins to the tissues, where it was consumed. The blood was made continuously and used up continuously. A portion passed to the right side of the heart, and from there seeped through invisible pores in the septum into the left side, where it mixed with air from the lungs and acquired vital spirit before being distributed through the arteries.
The heart was understood mainly as a source of heat, and the arterial pulse as an active expansion of the arteries themselves, drawing blood in rather than receiving it under pressure.
The system had already been damaged before Harvey. Vesalius had looked for the pores in the septum in 1543 and reported that he could not find them. Michael Servetus and later Realdo Colombo described the pulmonary transit, in which blood passes from the right side of the heart through the lungs to the left, which removes the need for the pores. Harvey’s teacher at Padua, Girolamo Fabricius, published a careful account of the valves in the veins in 1603.
Each of these findings was a local repair. None of the authors drew the obvious conclusion, and Fabricius in particular described the venous valves without understanding what they were for.
The Calculation
Harvey studied at Padua from 1599 to 1602 and returned to England, becoming physician at St Bartholomew’s Hospital and later to James I and Charles I. He worked on the motion of the heart for roughly twenty years before publishing. His argument was arithmetical rather than anatomical, which puts it in the same family as the map John Snow drew of a cholera outbreak in 1854.
His decisive argument is set out in Chapter 9 and takes barely a page. He estimated the capacity of the left ventricle and how much of it is expelled at each beat. Taking a conservative figure of about two ounces per contraction and a pulse of roughly seventy two beats per minute, the heart pushes out around eight and a half pounds of blood every minute.
That is more than five hundred pounds an hour. It is several times the weight of the entire body, produced from food, every hour, and consumed in the tissues at the same rate.
The conclusion is unavoidable. No animal can manufacture and destroy that quantity of blood. The same blood must be passing through the heart again and again in a closed loop.
This is worth pausing on. Harvey did not need a microscope, a new dissection technique, or an instrument nobody else had. He needed to treat the heart as a pump with a measurable output and to multiply that output by the number of beats in an hour. Physiology had been a descriptive science, and in one page it became a quantitative one.
The Experiments
Harvey supported the calculation with a series of demonstrations, several of which anyone can still perform.
The best known involves a bandage on the upper arm. Tighten it enough to stop the arterial flow and the arm below goes pale and cool, with no pulse beyond the ligature. Loosen it so that the deeper arteries can push blood in but the surface veins remain compressed, and the veins below swell up and stand out. Blood is clearly arriving from the arteries and being blocked on its way back through the veins.
He then pressed a finger along a swollen vein to empty a section of it and showed that it refills only from the direction of the extremities, never from the direction of the heart. Combined with Fabricius’s valves, which permit flow in only one direction, this establishes that venous blood travels inward.
He also worked on cold blooded animals, whose hearts beat slowly enough that the sequence of contraction can be followed by eye, and on dying mammals whose heartbeat had weakened. Both let him establish that the heart contracts actively during systole and that the arterial pulse is the consequence of that contraction, not an independent action of the artery.
The Missing Link
Harvey’s system had one unobserved component. Blood leaves the heart through arteries and returns through veins, so somewhere in the tissues the two must connect. He had never seen the connection and said so, suggesting either direct anastomoses or a general seepage through the flesh.
The gap was closed by Marcello Malpighi in 1661, four years after Harvey’s death, using a microscope on a frog’s lung. He observed capillaries linking arteries to veins, and later, Antonie van Leeuwenhoek watched blood cells moving through them in the tail of a tadpole.
The instrument makers of the same generation, including Robert Hooke, were building the tools that turned Harvey’s necessary inference into a direct observation.
The Reception
The book was attacked immediately, most persistently by Jean Riolan the Younger in Paris, who defended Galen and argued that if Harvey’s account were correct, all of medicine would need rewriting. He was right about that, which was part of the objection.
The practical stakes were considerable. Bloodletting was the central therapy of European medicine, and where and how much to bleed depended on the Galenic map of which vessels served which organs. A closed circulation makes that map meaningless.
Harvey’s biographer John Aubrey reports that his medical practice suffered and that he was called a circulator, a Latin word that meant both one who circulates and a quack. Harvey largely declined to argue in public, remarking that the anatomy of the heart is best learned from dissection rather than from books, including his own.
Acceptance came within his lifetime, helped by the fact that the demonstrations are easy to repeat. Descartes endorsed the circulation in 1637, though he disagreed with Harvey about the mechanism of the heartbeat.
The Rest of His Life
Harvey remained physician to Charles I through the English Civil War and was present at the battle of Edgehill in 1642, where by one account he sat under a hedge reading a book while looking after the royal princes. His London lodgings were ransacked and a substantial quantity of his notes and observations was lost, including work on the generation of insects that he described as the greatest loss of his career.
He published one further major work, De Generatione Animalium, in 1651, on embryology and reproduction, containing the principle that all living things come from an egg.
He died in 1657, wealthy, and left money to the Royal College of Physicians for an annual oration that is still delivered.
Why the Book Matters
The circulation itself is now a fact taught to children. The interesting part of the story is how it was established.
Harvey worked in a discipline where the accepted framework had been assembled by an authority of enormous prestige, was internally coherent, and had absorbed a series of anatomical corrections without collapsing. What broke it was not another correction but a change of question. Instead of asking what the parts look like, Harvey asked how much passes through them per hour, and the answer was incompatible with the entire structure.
His illustrations, showing bandaged arms with the swollen veins numbered for reference, are among the plainest scientific images of the century, and they were included because the argument depends on the reader repeating the demonstration rather than taking his word for it. The history of that relationship between scientific images and scientific persuasion is the subject of Portraying Science, which gathers the pictures through which science has explained and presented itself.
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