Circulation of the Blood

Summary

For more than fourteen centuries, European medicine taught that blood was made continuously in the liver, flowed outward to the organs as they needed it, and was consumed. No one thought it circulated. In 1628, the English physician William Harvey published Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus and showed that the heart is a pump, that blood moves through the lungs from the right side of the heart to the left, and that it returns to the heart through the veins in a continuous circuit. This was the central fact of physiology that Galen’s system had hidden from view. Harvey arrived at it through a combination of vivisection, comparative anatomy, quantitative argument, and a distinctly Aristotelian research program, not by mechanistic speculation. The reception of his doctrine was contested, uneven, and philosophically complex, taking more than a generation to consolidate. The missing anatomical link, the capillaries connecting arteries to veins, was supplied by Marcello Malpighi in 1661 through microscopic observation.

The Galenic Physiology Harvey Was Overturning

The core feature of Galen’s physiological system was a tripartite architecture organized around three organs, the liver, heart, and brain, each the center of a distinct bodily economy.(Nutton, 2023) Mattern, working closely from Galen’s own treatises, describes the system as follows: the liver was the central organ of nutrition, receiving food from the stomach and manufacturing venous blood, which flowed outward through the veins to nourish the body’s parts.(Nutton, 2023)(Mattern, 2013) The heart was the center of vitality, producing arterial blood and vital pneuma, while the brain was the center of sensation and movement, producing psychic pneuma distributed by nerves.(Mattern, 2013)(Nutton, 2023)

Rocca traces the mechanics of this pneumatic chain in close detail. Inspired air entered the lungs, was converted into a “pneuma-like substance,” and passed into the left ventricle of the heart, where innate heat elaborated it into vital pneuma. Vital pneuma then traveled through the arteries to the brain, where the retiform plexus and choroid plexuses completed its transformation into psychic pneuma, the medium of thought, sensation, and voluntary motion.(Rocca, 2003) Galen’s errors here were compounded by his use of animal anatomy: the retiform plexus he described at the base of the brain is present in cattle and sheep but absent in humans, as well as a bicornuate uterus (finding in animals rather than humans) and the belief that blood passes through septal perforations.(Mattern, 2013)

Underlying this system was a vitalist framework.(Nutton, 2023) Galen held that every living being was provided by its Creator with four natural faculties, attraction, assimilation, excretion, and growth, which governed all physiological processes.(Nutton, 2023) This explicitly contrasted with the mechanistic models of Erasistratus and Asclepiades, who had tried to explain bodily function through purely physical principles.(Nutton, 2023) [GAP: Specific physiological examples (blood movement toward hungry organs, liver drawing nutriment from the gut, arteries pulsing with vital heat) are not present in the cited card.]

Hall’s account in The Scientific Revolution captures the most important feature of this scheme: the movement of blood was not circular but tidal, “not constant, but rather an ebb-and-flow alternating motion, by which the humours were uniformly distributed about the body.”(Hall, A. Rupert, 1954) Blood was manufactured continuously in the liver from digested food, moved slowly outward as the tissues consumed it, and was not expected to return. The heart was not a pump in any meaningful sense. In Galen’s physiology it was an organ of pneumatic elaboration, warming the blood and infusing it with vital spirit.

Two anatomical claims held this system together that Harvey would have to demolish. The first was the perforations in the interventricular septum. Nutton notes that Galen was fully aware that the pulmonary artery carried blood from the right heart into the lungs, but he “seems to have regarded this means of transit as secondary to that via the septum.”(Nutton, 2023) He asserted the pores existed even though he never demonstrated them, and no one after him succeeded in demonstrating them either. Harvey later showed that the large pulmonary vessels, what he called the “arterial vein and venous artery”, provided the actual route of blood transit through the lungs, and that Galen’s picture of the cardiac anatomy “was confused by the presence of two large vessels… which leave the heart and disappear, obliterated, into the lungs.”(French, 1994) The second was Galen’s belief that the pulse was not caused by the heart’s pumping action but originated in the heart and was transmitted through the walls of the arteries themselves. Mattern documents that a vivisection Galen designed to prove this, inserting a reed into an artery and compressing the coating against it, “must have failed; or his observations were biased toward his own view.”(Mattern, 2013)

Galen also made himself the most forceful advocate of phlebotomy, therapeutic bloodletting, as a regular medical procedure, a stance that put him at odds with Erasistratean competitors who rejected it. He wrote treatises both defending and then qualifying its use.(Nutton, 2023) A circulation, if true, would undermine the rationale for the profession’s most widely used treatment.(French, 2003) This is why French argues that Harvey’s opponents “reasonably pointed out that the doctrine of circulation would destroy not only the theory of medicine but also the major therapeutic technique of selective blood-letting.”(French, 2003)

Precursors: Ibn al-Nafis, Servetus, Colombo

The full systemic circulation was Harvey’s achievement, but the pulmonary transit, the passage of blood from the right side of the heart through the lungs to the left side, had been glimpsed or asserted before him.

Hall notes that the lesser (pulmonary) circulation was accurately stated by Ibn al-Nafis al-Qurashi, an Egyptian or Syrian physician of the thirteenth century, in a commentary on Avicenna’s Canon. He “deduced it specifically from the impermeability of the septum”, reasoning that since no blood could pass through the wall between the chambers, it must traverse the lungs by another route.(Hall, A. Rupert, 1954) There is, however, no evidence this work was known in sixteenth-century Europe. The later discussions appear to be entirely independent of it.

In Europe, the first printed account of the pulmonary circuit was Michael Servetus’s in 1553, buried in a theological work, Christianismi Restitutio. Hall describes his account: “He denied that there was any communication through the septum of the heart: instead, ‘the subtle blood, by a great artifice, passes along a duct through the lungs; prepared by the lungs, it is made bright, and transfused from the pulmonary artery to the pulmonary vein.’”(Hall, A. Rupert, 1954) Servetus understood this as a preparation of blood for its union with inspired air, his physiological conceptions remained Galenic, and he did not connect this observation to any wider circulation. Nearly all copies of the book were destroyed when he was burned for heresy the same year.

Realdo Colombo, Harvey’s anatomical predecessor at Padua, performed vivisectional experiments that touched on the pulmonary transit. French argues that Colombo punctured the pulmonary vein to discover whether it contained blood or spirit, finding only blood. Harvey knew Colombo’s work and learned from his vivisectional methods, though French is careful to warn against treating Colombo’s observation as a “contribution” or stepping-stone to Harvey’s discovery: “Not too much should be made of the idea that Harvey’s adoption of Colombo’s notion that blood crossed the lungs was a sort of ‘contribution’… Harvey’s purposes were different from Colombo’s.”(French, 1994) Colombo was interested in where vital spirits were generated; Harvey was interested in what the heart actually does.

None of these predecessors performed the decisive integrative act that Harvey performed: recognizing that blood moving from arteries to veins in the organs, combined with the venous valves pointing uniformly toward the heart, and the sheer quantity of blood the heart expels every hour, demanded a continuous circuit.

Harvey’s De Motu Cordis (1628)

Origins in Anatomy Lectures and Disputation

Harvey’s path to the circulation began not with a dramatic experimental revelation but with the ordinary work of anatomy lecturing. He was appointed to deliver the Lumleian Lectures at the College of Physicians in London, and his notes from those lectures, composed in 1616 and surviving as the Prelectiones Anatomiae Universalis, contain the earliest written trace of his developing views on the motion of the heart.(Keynes, Geoffrey, 1978) Those notes were not yet a theory of circulation, but they show Harvey working through the problem of cardiac motion in an academic setting.

French’s reconstruction of how the discovery emerged places disputation, not solitary experiment, at its center. Harvey faced a specific obstacle during his anatomy lectures: attempting to describe the motion of the heart to an audience, he could not determine by sensory observation alone which motion, the elevation or the relaxation, was diastole and which was systole. The difficulty was compounded because Galen had taught that diastole, the expansion of the heart, was its vigorous proper motion, the phase in which it drew blood from the vena cava. Harvey’s opponents, operating from this framework, naturally interpreted the heart’s visible elevation as diastole, not systole.(French, 1994) He turned to Aristotelian theory of propria motio (proper motion) to resolve it, reasoning that every organ has a characteristic action by which it achieves its function.(French, 1994) Observing dying animals, he watched the heart become motionless in its relaxed, not its elevated, position, and concluded that elevation was the heart’s proper motion, that systole was active ejection, not passive filling.(French, 1994)

Once Harvey had committed to the forceful systole, he faced an objection in disputation that French argues was decisive. Critics raised what French calls the “boiling milk” objection: that blood in the heart, heated, simply expanded like milk in a pan and occupied more volume without any actual ejection. Answering this objection, showing that it was untenable, “compelled Harvey to recognise that the blood circulated.”(French, 1994) The three keys to the discovery, in French’s account, were lectures, vivisection, and disputation together.

De motu cordis itself was structured around two sequential theses, with strict logical priority. The first was the forceful systole, the claim that the heart actively and forcibly ejects blood at each contraction. Only once that was established did Harvey introduce his second thesis: the circuit of blood from arteries back to veins.(French, 1994) French notes that the first seven chapters of the book do not mention the circulation at all; they establish what Harvey had already demonstrated publicly in the anatomy lectures, “obsignatis tabulis”, with formally attested evidence.(French, 1994) The circulation argument depended logically on the forceful systole being already in place.

Harvey’s Natural Philosophy and Method

William Harvey’s natural philosophy was assembled during his education at Cambridge and Padua, and as French argues in his exhaustive study, it “contained ways of structuring knowledge, formulating questions and arriving at answers that directed the programme of work in which he discovered the circulation of the blood.”(French, 1994) The key intellectual framework was Aristotelian, not Cartesian or mechanistic. Harvey derived his research program from Aristotle’s animal books, the Historia animalium, De partibus animalium, and De generatione animalium, which represented, in French’s reading, “a programme of acquiring knowledge” rather than a fixed system.(French, 1994) His teacher Fabricius of Aquapendente had developed this into what became known as the “Aristotle project” at Padua: studying anatomy not merely as structure but as historia, actio, and usus, structure, action, and purpose, across multiple animal species.(French, 1994) This program directly responded to a limitation in Vesalius’s great reformation of anatomy. Vesalius’s De humani corporis fabrica (1543) had been driven by the belief that Galen described simian rather than human anatomy; correcting Galen’s structural errors required staying close to Galen’s descriptions, so Vesalius’s work became almost entirely historia, an account of the fabric of the body, with little attention to function.(French, 1994) Fabricius and Harvey went beyond the anatomical reforma­tion to ask what the parts actually did.

French’s broader historical argument is that the seventeenth-century revolution Harvey participated in was not primarily about science but about experiment and the status of natural knowledge.(French, 1994) Harvey’s contribution was the experimental method itself.(French, 1994)

The core method was what Harvey called the Rule of Socrates: searching for a universal functional definition across many animals by comparison.(French, 1994) French explains: Harvey investigated the heart in fish (which have only one ventricle and no lungs), in amphibians, in embryonic animals, and in dying animals, looking for what all hearts share in their action across this diversity.(French, 1994)(French, 1994)(French, 1994) In fish, “the auricle is clearly seen sending blood into the ventricle which in turn can be seen (by puncturing the artery) to send the blood into the artery.”(French, 1994) In embryos, the foramen ovale and ductus arteriosus exist because embryonic animals cannot use their lungs, so their hearts route blood differently, the form varying with function, exactly as the Rule of Socrates predicted.(French, 1994) Harvey also criticized investigators who relied on dead human bodies rather than living animals, arguing this method “amounted to an attempt to syllogise about a universal form from a particular proposition, like a man who pretended to know about agriculture because he knew a single field.”(French, 1994) By watching hearts in dying animals become motionless in the relaxed rather than the elevated position, Harvey concluded that cardiac elevation, systole, was the heart’s proper motion.(French, 1994)

The Quantitative Argument and Its Limits

French reconstructs how the discovery of the full circulation likely emerged. Harvey had established to his own satisfaction that the heart forcefully ejected blood at each systole, the first thesis of De motu cordis. He then faced a crisis: if the heart ejects blood continuously, where does it all go? French cites the autobiographical passage from chapter 8 of De motu cordis that describes this moment: “at last I perceived that the veins should be quite emptied, and the arteries on the other side be burst with too much intrusion of blood, unless the blood did pass back again some way out of the veins into the arteries and return into the right ventricle of the heart. I began to bethink myself if it might not have a circular motion.”(French, 1994) Harvey told Robert Boyle that the venous valves, structures his teacher Fabricius had described but misinterpreted as slowing the descent of blood, were what first directed his thinking toward circulation. Since all the valves pointed toward the heart, blood in the veins must move toward the heart from the periphery.(French, 1994)

The quantitative argument was central to Harvey’s published case but was already implicit in his work before he could fully articulate it.(French, 1994) French argues that Harvey made a rough estimate of blood volume before he had settled on the circulation hypothesis, and that this calculation lay historically at the center of his discovery.(French, 1994) He estimated that if even one-eighth of the left ventricle’s contents, approximately one drachm, was ejected at each heartbeat, then after 1,000 beats (half an hour at a normal pulse rate), 1,000 drachms of blood would have left the heart.(French, 1994) Hall’s account is similar: “by calculation he proved that, on the lowest estimate of the change in volume of the ventricles, all the blood in the body passed through the heart more than once in half an hour. Even this was a generous underestimate.”(Hall, A. Rupert, 1954) Porter confirms the simple force of the point: “the amount of blood forced out of the heart in an hour far exceeded its volume in the whole animal”, and this excess, by itself, made any non-circulatory theory untenable.(Porter, 1997)

But French is careful to resist the historiographic tendency to make Harvey a proto-quantitative scientist. Modern translators have read their own conceptions of inductive or quantitative method into Harvey’s Latin, “anachronistically portraying him as doing inductive or quantitative experiments.”(French, 1994) The quantitative argument was, French insists, “not an experiment or even an observation, but a mode of argument, or rhetoric, far less precise than the quantitative work of Sanctorius and van Helmont.”(French, 1994) Contemporaries who were not convinced said as much: Caspar Hofmann called it “an accountant’s trick, incapable of true demonstration,” while James Primrose called it a “likely conjecture” rather than the direct visual proof, autopsia, that a proper anatomical demonstration required.(French, 1994) This objection was not simply obscurantist. Many contemporaries, following Aristotle, held that mathematics described the appearance of things but did not deal with essences and causes, and therefore had no place in natural philosophy.(French, 1994) To them, Harvey was doing a tradesman’s calculation when he should have been using his eyes.

Whether or not his quantitative argument satisfied philosophical standards for proof, it did convince contemporaries.(French, 1994) French observes that the quantitative argument was considered convincing, but argues that quantification was not a special part of Harvey’s natural philosophy.(French, 1994)

The Structure and Rhetoric of De Motu Cordis

Harvey presented De motu cordis as an academic exercitatio, not a report of experiments but a sustained academic argument defending a disputable proposition, with objections raised and met in proper scholastic form.(French, 1994) Its “logic” was not demonstrative in the Aristotelian syllogistic sense but something French calls “judicial” or “deliberative”, the kind of presentation familiar from Cambridge declamation practice, combining what anatomists called “physical logic” with a historical narrative of how Harvey had arrived at his conclusions.(French, 1994)

The two experimental proofs Harvey considered most decisive were the ligature demonstrations. A tight ligature on an arm showed that the artery proximal to the ligature swelled and pulsed violently while the arm distal to it became cold and pulseless, demonstrating that arterial blood flows outward from the heart. A moderate ligature allowed arterial blood through but blocked the deeper veins, causing the superficial veins below the ligature to swell; this could only be explained by blood returning from the extremities through the veins.(French, 1994) Harvey then demonstrated venous valves using a second quantitative argument: blood squeezed past a valve toward the heart cannot return. If done repeatedly, the quantity moved exceeds any possible local supply.(French, 1994) The deeper lesson Harvey drew from the valves was a form-function analysis: all venous valves, in the jugular, renal, mesenteric, and limb veins alike, share the common function of imposing unidirectional flow toward the heart. Fabricius and others who had described the valves had been wrong, Harvey concluded, in deciding that their function was to delay the descent of blood.(French, 1994) Hall records Robert Boyle’s recollection that it was precisely this observation, that nature would not have placed so many valves without design, that first induced Harvey to think of the circulation.(Hall, A. Rupert, 1954)

Harvey used Aristotle’s meteorological analogy of the water cycle to frame the circulation rhetorically as a natural cosmic cycle: “the circulating blood is restored and perfected in the heart, which is the treasure-house of life, the sun of the microcosm, the fountain of the body and its domestic deity.”(French, 1994) Porter confirms that Harvey’s conceptual framework “was not a machine, but was moved by vital forces. In discussing the circulation, his terms and ideas were Aristotelian.”(Porter, 1997) This Aristotelian framing, which later historians have sometimes found incongruous with what they take to be Harvey’s “scientific” achievement, was not incidental. It was the philosophical language in which he actually thought.

Reception and Resistance

De Motu Cordis was published at Frankfurt in 1628 as a small quarto of 72 pages.(Keynes, Geoffrey, 1978) Keynes notes that Harvey’s first published critic, James Primrose, wrote his refutation in fourteen days and “appealed not once to experiment as a means of investigation.”(Keynes, Geoffrey, 1978) This response, French argues, was methodologically revealing: opponents of Harvey typically relied on textual authority and Galenic doctrinal commitments, while supporters almost invariably appealed to vivisectional observation and ligature experiments.

French identifies two linked reasons for resistance. First, many physicians who grudgingly accepted that blood circulated could not accept Harvey’s account of the forceful systole, they could not follow him on the basic claim about how the heart moves. French documents Roger Drake’s position, representative of many early adopters: Drake accepted the circulation but explained the heart’s motion as blood converted to spirit that “forcibly expands the heart and arteries, causing the pulse”, a compromise position that preserved some Galenic pneumatics.(French, 1994) Second, and more principled, was the position of Jean Riolan, anatomist at Paris, who accepted a partial circulation through major vessels only, while maintaining that blood moved the traditional way in the branches and through septal pores, a position Glisson diagnosed as driven by Riolan’s unwillingness to accept the destruction of humoral medicine that a full circulation would require.(French, 1994) Keynes records that Harvey finally broke his characteristic silence and replied to Riolan in two letters published simultaneously at Cambridge and Rotterdam in 1649, demolishing Riolan’s position that “while a Harveian circulation took place in the large vessels, the blood moved in the opposite direction in their branches.”(Keynes, Geoffrey, 1978)

Descartes accepted Harvey’s circulation in 1632, but disagreed fundamentally on the mechanism of the heartbeat. Keynes documents the disagreement: “Descartes was unable to entertain the idea of the heart acting as an involuntary muscle” and instead proposed the heart acted as a furnace, its heat causing blood to ferment and expand, driving the pulse.(Keynes, Geoffrey, 1978) Harvey considered this interpretation wrong and contested it explicitly in his second letter to Riolan (1649). Harvey was, as French shows, on principle averse to controversy. Keynes quotes him: “Perish my thoughts if they are empty and my experiments if they are wrong… if I am right, sometime, in the end the human race will not disdain the truth.”(Keynes, Geoffrey, 1978)

The Cambridge Circle: Glisson as Transmitter

Harvey’s own university, Cambridge, became one of the primary channels through which his doctrine spread in England. The Cambridge that had taught Harvey the philosophy of Aristotle was an important centre from which his views were subsequently disseminated.(French, 1994) Francis Glisson gave lectures and conducted disputations in Cambridge, demonstrated the circulation as Goulstonian Lecturer at the College of Physicians from 1639 onward, and as one of the country’s leading teachers played an active part in the growth of a consensus.(French, 1994)

Glisson could not accept Harvey’s account without significant modification. He believed matter was innately mobile, and this meant he could not agree that the heart alone caused circulation.(French, 1994) His alternative was that both the arteries and the blood actively participated in the circuit.(French, 1994) On the question of arterial blood, Harvey had concluded that arterial blood differed from venous not by the addition of spirits but by being the same blood restored.(French, 1994) The innate mobility of the blood distanced his views further from the Cartesian model in which heat alone drove the circulation.(French, 1994)

The Low Countries and the Role of Experiment

Walaeus demonstrated the force of circulation by cutting the tip of a ventricle and watching blood spurt four feet, estimating that the full mass of blood circulated in about a quarter of an hour.(French, 1994) Descartes, watching the controversy from Paris, believed Harvey was using Walaeus as an agent to spread his doctrines through university channels, so prominent had Walaeus become as the practical demonstrator of the theory.(French, 1994) What made Walaeus’s experimental program distinctive was its scope: he extended Harveian vivisection technique to the controversy over chyle movement in the lacteals, linking the circulation controversy to a broader physiological programme.(French, 1994)

The opposition of John Primrose raised an epistemological objection French finds interesting: Primrose argued that vivisections were unnatural and therefore epistemologically invalid, the violence of ligating an artery introduced conditions not present in an intact living animal, so the results could not be taken to show what actually happened in nature.(French, 1994) Walaeus’s response was direct: Primrose never performed even the simplest experiment, and his insistence that “the mind grasps things more surely than the eye can see them” represented an anti-empirical position that the growing consensus of seventeenth-century natural philosophy was rejecting.(French, 1994) By mid-century, experiment had become so integral to natural philosophy that a purely verbal treatment was felt to be inadequate. Drake upbraided Primrose for never doing even the simplest ligature experiment: “in the absence of experiment, even to speak as though from an oracle is no way to convince people.”(French, 1994)

Machines, Mathematics, and Mechanical Philosophy

Seventeenth-century medical thinkers deployed mechanical devices, clocks, pumps, syringes, bellows, cupping-glasses, both as rhetorical analogies to explain how the body worked and as actual experimental apparatus to test claims about attraction, ejection, and blood pressure.(French, 1994) Jean Pecquet used physico-mathematical experiments on vacua, drawing on Torricellian pneumatics, to deny that the heart’s diastole attracted blood by suction, arguing instead that blood was impelled into the heart by the weight and spring of the air.(French, 1994) Pecquet’s Experimenta nova anatomica of 1652 marked what French calls a watershed in the controversy: it symbolized both the beginning of general acceptance of Harvey’s doctrines and the spread of a mechanical vision of the body.(French, 1994)

Harvey’s quantitative argument continued to generate disagreement even among those who accepted the circulation. Hofmann’s dismissal of it as “an accountant’s arithmetic” captured a genuine philosophical objection: many still held with Aristotle that mathematics could not reveal essences or causes, and therefore had no demonstrative force in natural philosophy.(French, 1994) Yet the argument did its persuasive work. French’s account of consensus formation shows it proceeding in stages: first the individual, convinced by teachers, reading, disputations, or experiments; then the group, professional colleges, university faculties, clubs, and hospitals, whose members came to share a belief; and finally national and generational dynamics that locked in a new orthodoxy.(French, 1994)

In Sweden, Olaus Rudbeck’s 1652 Uppsala disputation defended Harvey’s forceful systole and muscular heart contraction against Cartesian alternatives, and French notes that the Harveian experimental method pursued there was energetic enough to lead Rudbeck directly to the discovery of the lymphatics.(French, 1994)

French’s central argument is that Harvey’s truth had “of itself no power” and that people were persuaded by some other active thing.(French, 1994) Thomas Bartholin, for example, accepted the circulation only in 1651 after it became clear that a consensus had formed, incorporating it into his anatomy textbook while taking a cautious “middle way” on contested details.(French, 1994) By 1654, Adrien Auzout treated the circulation as established history, writing that it was “old, and as clear as light itself,” and asserted that everything pertaining to it would be explained on mechanical principles.(French, 1994)

In Catholic Europe, resistance lasted far longer, as France pronounced the death penalty in 1624 for departure from approved authors, and Spain had introduced it in 1558 for importing foreign books.(French, 2003) As late as the 1680s, Juan de Cabriada was still attempting to persuade Galenists in Madrid to accept that the blood circulated.(French, 2003) French’s conclusion is characteristically measured: “The new doctrines were also a minority opinion, promulgated by a handful of people limited largely to two European countries, England and Holland. Elsewhere… if they finally absorbed the new doctrines, it was not until well into the eighteenth century.”(French, 2003)

Experimental Verification and the Missing Link

In his second letter to Riolan (1649), Harvey came close to describing capillaries, noting that “the terminal threads of the arteries going to the umbilical vein are lost in the coats of that vessel.”(Keynes, Geoffrey, 1978) The letter also notes that no experimental evidence was called upon to strengthen his case until the penultimate paragraph.(Keynes, Geoffrey, 1978)

Harvey had demonstrated the circulation in person when the opportunity arose. During the Arundel embassy of 1636, traveling through Germany, he performed a public vivisection at Altdorf near Nuremberg for Caspar Hofmann, the very critic who had called his argument an accountant’s trick. Keynes records Harvey writing that he was “as certain that the blood is diffused through the arteries into the body as I am certain that our Thames falls into the sea.”(Keynes, Geoffrey, 1978) Hofmann remained unconvinced, but the demonstration illustrated Harvey’s willingness to stake his case on direct observation rather than argument alone.

The anatomical demonstration of capillaries came in 1661 when Marcello Malpighi, using an early microscope, observed blood moving through capillary vessels in the lungs of a living frog. Hall describes this as “the final link that clinched Harvey’s theory of circular blood motion.”(Hall, A. Rupert, 1954) Porter confirms: Malpighi’s De pulmonibus “described this fine texture of blood vessels, the missing link in Harvey’s theory, and thus provided decisive confirmation of the blood’s circulation.”(Porter, 1997)

Richard Lower, working in 1669, demonstrated that the change in blood colour from venous (dark) to arterial (bright) occurred in the lungs through the action of a “nitrous spirit” taken up from inspired air, an important step toward understanding respiration before the chemical revolution.(Hall, A. Rupert, 1954)

Harvey analyzed the vascular system hydraulically, “considering the heart as a pump, the veins and arteries as pipes, the valves as mechanical valves, the blood itself simply as a fluid”, and for this purpose he disregarded ‘spirits’ altogether.(Hall, A. Rupert, 1954) His great merit was to integrate known but ineffective facts into a new and comprehensive generalization; the individual observations had all been available, but what was new was the organizing principle.(Hall, A. Rupert, 1954)

Significance for Medicine and Natural Philosophy

The Epistemological Legacy

French argues that Harvey’s natural philosophy was fundamentally experimental from the start. His first discovery, the forceful systole, had been found experimentally and had to be defended experimentally; for the presentation of circulation in De motu cordis and the subsequent battle for its acceptance, “Harvey relied on the force of direct experimental demonstration.”(French, 1994) Yet vivisectional experiments could not claim Aristotelian demonstrative proof in the strict sense, which required linking the presence or absence of organs to modes of life and displaying causal relationships. What Harvey’s program achieved, French argues, was different: it helped form a consensus that experimental results stood apart from both rationalist systems and philosophical demonstrative truth, constituting their own category of reliable knowledge.(French, 1994)

French’s conclusion is that the most important event in the Harvey affair was not that his doctrines were true, “but that it came to be thought that truth, or the best possible approximation to it, could be discerned by experiment.”(French, 1994)

The Physiological Legacy

This had consequences throughout physiology. Porter documents that by the late seventeenth century, anatomists had “laid bare the glandular structures of the digestive system, destroying the food-chyle-blood scheme so basic to Galenic physiology which centred on a blood-making liver.”(Porter, 1997) The discovery of lacteals, the thoracic duct, the lymphatic system, and the pancreatic duct all followed from the experimental methods that Harvey’s program licensed. Each discovery eroded another pillar of the Galenic system. By 1700, the tripartite architecture of liver, heart, and brain, with its three pneumas and three blood types, had been replaced by a single unified cardiovascular system.

The Philosophical Tangle

Harvey’s discovery of blood circulation was made possible by his deep acquaintance with Aristotle’s methods, theories, and logic.(Nutton, 2023) Glisson could not accept that the heart alone caused circulation because he believed matter was innately mobile; his alternative was that both arteries and blood actively participated.(French, 1994) The first consensus on Harvey was actually a consensus around a misunderstanding: agreement that venous blood returned to the heart, defining Harvey’s topic as “the circulation.”(French, 1994)

Nutton’s final verdict places Harvey in the long tradition he both extended and overturned: “Galen is an ambiguous figure… who left a legacy that variously inspired, daunted and constricted his successors. After him, mediaeval learned physicians… strove to synthesise what they knew of ancient medicine through him. The authority was not fundamentally challenged until Vesalius in anatomy and Harvey in physiology.”(Nutton, 2023) In that sentence is the full scope of the problem. Vesalius challenged the structure; Harvey challenged the function. Both challenges built on precisely the tradition they dismantled.

See Also

• william-harvey
• galen
• galenic-physiology
• experimental-method
• humoral-theory
• bloodletting
• anatomy
• pneuma
• ibn-al-nafis
• de-motu-cordis
• padua-and-medicine
• scientific-revolution

HUMAN-NOTES