To grasp the magnitude of his reconceptualization, it is worth recalling the ancient model of the blood system was still authoritative around 1600. In traditional Galenic physiology there were two types of blood, the venous and the arterial, with distinct pathways and functions, relating to the three chief body centres: the liver (responsible for nutrition and growth), the heart (vitality), and the brain (sensation and reason). Nourishment and growth were secured by the venous blood originating in the liver, while vitality was a conveyed to the body parts by the arterial blood originating in the heart. Arterial (life-giving) blood contained pneuma (spirituous air) and blood; and fed the body when needed, being there used up; it did not return to the heart. Given the quite distinct functions of venous and arterial blood, their different sources, and the belief that they were expended, there was no question of the blood circulating around the body. The heart did not even drive blood through the arteries for Galen the active phase of the heart’s motion was diastole (dilation), when, like a bladder filling, it sucked blood in. The heart did not pump blood out; the blood’s movement through the arteries was explained by an innate ‘pulsative faculty’ in the arteries themselves.

One awkwardness in Galen’s cardiovascular system lay in explaining how the ingredient of arterial blood (that is, venous blood and air) got into the left ventricle of the heart where they were changed into the arterial blood. Evidently venous blood had to pass from the right side of the heart to the left. Galen had stated that this occurred by the blood seeping through hidden pores in the interventricular septum, the fleshy wall separating the ventricles of the heart. He had no conception of the pulmonary transit of the blood: passing from the right to the left side of the heart by looping through the lungs (through the pulmonary artery into the lungs, then into the pulmonary vein, and finally into the heart). Hardly any blood (just enough to nourish the lungs) left the right side of the heart, he believed.

If air was necessary for making arterial blood, how did it get to the heart? Galenists believed the pulmonary vein’s function was to convey air from the lungs to the left side of the heart. The end result of the mixture of pneuma and blood in the left ventricle was arterial blood thinner and brighter than the dark-red venous blood. A by-product of this process were ‘sooty vapours’, which travelled back to the lungs along the same pulmonary vein and were then exhaled. The pulmonary vein was a two-way street: air went down into the heart, and sooty vapours returned up to the lungs. With hindsight, the lack of a sound observational basis for many of these particulars (the permeable septum, the paucity of blood in the pulmonary vein, and the sooty vapours there) appears problematic, but the Galenic cardiovascular system scored because it explained so much so coherently.

Renaissance anatomists questioned aspects of Galen’s model. In the Fabrica, Vesalius had observed that the vena cava did not, after all, originate in the liver, and in the second edition he denied the septum’s permeability. Realdo Colombo brought to light the pulmonary transit, and vivisection experiments led him to conclude that the active phase of the heart was when it constricted (systole), not when it dilated (diastole). The heart discharged blood into the arteries when it constricted and the arteries then felt full a discovery crucial for Harvey, as it led him to ponder the quantity of blood flowing out of the heart at every constriction.

Other anatomists also contributed observations that did not square with Galen: Servetus conceived of the pulmonary transit; Andrea Cesalpino (1519-1603) supported the pulmonary circulation, described the heart’s valve action, and began to use the term circulation; and in 1603, Fabricius published his description of venous valves. The culmination of such developments came in 1628, when Harvey published his Exercitatio anatomica de motu cordis et sanguinis in animalibus [An Anatomical Essay Concerning the Movement of the Heart and the Blood in Animals].

Born at Folkestone, William Harvey (1578-1657) attended Caius College, Cambridge. From 1600 he studied at Padua under Fabricius, from whom he absorbed Aristotelian approaches to the study of nature, especially in comparative anatomy and embryology. Like his teacher, he dissected animals to discover how particular organs worked, relating structure to function, concerned with the ‘action, function and purpose’ of the parts.

In 1602 he returned to England. A swarthy man who habitually wore a dagger, he was elected a Fellow of the College of Physicians in 1607 and two years later physician to St. Bartholomew’s Hospital. He swiftly established himself; in 1615 he was appointed by the College its Lumleian lecturer, charged with lecturing on anatomy and conducting public dissections. By 1618 he was one of the royal physicians.

Harvey’s Paduan experiences set him at the forefront of learned medicine, where neo-Aristotelian ideas were challenging Galenic ones. His 1616 Lumleian lectures significantly begin with the statement that anatomy deals with ‘the uses and actions of the parts [of the body] by eyesight inspection and by dissection’ a clear sign that he had adopted Fabricius’s Aristotelianism. He had also taken initial steps towards probing the blood system. Confirming Colombo’s work on the pulmonary transit, he concluded that the heart worked as a muscle, with the ventricles contracting and expelling blood in systolic contractions rather than sucking it in during diastole (relaxation); the arteries pulsated because of the shockwave from the beating of the heart they did not pulsate of their own intrinsic ‘pulsative virtue’.

De motu cordis falls into two parts. Harvey first pointed out Galen’s flaws. How could the air and sooty vapours be kept distinct in the pulmonary vein? When it was opened, neither the said air nor the vapours could be seen in that vein, but blood alone. Discussing the action of the auricles and the ventricles of the heart, he showed the reality of the pulmonary transit of the blood, pointing to the vivisections he ahd performed on frogs whose hearts were simpler and beat more slowly than those of warm-blooded animals, thereby permitting slow-motion experiments.

He then turned to novel matters, announcing his discovery of the circulation. Experiments showed that so much blood left the heart in a minute that it could not conceivably be absorbed by the body and continually replaced by blood made in the liver from chyle. Like no one before him, Harvey noted that the amount of blood forced out of the heart in an hour far exceeded its volume in the whole animal. This quantitative evidence established that the blood must constantly move in a circuit, otherwise the arteries and body would explode under the pressure:

Since all things, both argument and ocular demonstration, show that the blood passes through the lungs and heart by the force of the ventricles, and is sent for distribution to all parts of the body, where it makes its way into the veins and porosites of the flesh, and then flows by the veins from the circumference on every side to the centre, from the lesser to the greater veins, and is by them finally discharged into the vena cava and right auricle of the heart, and this in such a quantity or in such a flux and reflux thither by the arteries, hither by the veins, as cannot possibly be supplied by the ingesta, and is much greater than can be required for mere purposes of nutrition; 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.

What Harvey could not achieve was to display the complete pathways of the circular movement. He could not see with his eyes the minute connexions the capillaries between the arteries and the veins, and did not attempt to do so with the newly developed microscope. By means of a simple experiment, however, he showed that a connexion, albeit unknown, must exist. He ligated a forearm extremely tightly so that no arterial blood could flow below the ligature down the arm. He then loosened it so that arterial blood flowed down the arm, though it remained tight enough to stop venous blood moving back above the ligature. With the ligature very tight, the veins in the arm below it had appeared normal, but now they became swollen, showing that blood had poured down the arteries and then back up the arm within the veins; thus there had to be as yet undiscovered pathways at the extremities for the blood to pass from arteries to veins. Finally, he showed that the valves in the veins always directed blood back to the heart; pace Fabricius, they did not act to prevent the lower parts of the body from flooding with blood. Having displayed the circulation and its passages through the one-way valve system, Harvey was then able to explain, by means of the circulation theory, such previously puzzling phenomena as the rapid spread of poisons through the body.

Harvey’s work appears very modern: he experimented and obeyed the injunction of the Paduan anatomists to see for oneself. ‘I profess to learn and teach anatomy not from books but from dissections,’ he declared, ‘not from the tenets for Philosophers but from the fabric of Nature.’ But that is only half true; certainly he looked for himself (and without the aid of the microscope), but he saw through Aristotelian spectacles. Harvey did not, as sometimes supposed, conceive of the body in a ‘modern’ mechanical fashion: it was to a machine, but was moved by vital forces. In discussing the circulation, he wrote, drawing on traditional macrocosm/microcosm correlations, was to transport life-giving blood to the periphery and then to return it to the heart where it could be re-enlivened:

So in all likelihood it comes to pass in the body, that all the parts are nourished, cherished, and quickened with blood, which is warm, perfect, vaporous, full of spirit, and , that I may so say, alimentative: in the parts the blood is refrigerated, coagulated, and made as it were barren, from thence it returns to the heart, as to the fountain or dwelling-house of the body, to recover its perfection, and there again by natural heat, powerful, and vehement, it is melted, and is dispensed again through the body from thence, being fraught with spirits, as with balsam, and that all the things do depend upon the motional pulsation of the heart: To the heart is the beginning of life, the Sun of the Microcosm, as proportionally the Sun deserves to be called the heart of the world, by whose virtue, and pulsation, the blood is moved perfected, made vegetable, and is defended from corruption, and mattering; and this familiar household-god doth his duty to the whole body, by nourishing, cherishing, and vegetating, being the foundation of life, and author of all. [From the 1653 translation].

It was not, in other words, from the ‘new philosophy’ that Harvey drew his inspiration; indeed, according to John Aubrey’s gossipy Brief Lives, Harvey sighted Francis Bacon (‘he writes Philosophy like a Lord Chancellour’), while ‘shitt-breeches’ was his put-down for the ‘neoteriques’ (the Paracelsans). With Aristotle he shared a teleological view of the body and the belief that its workings depended upon the distinctive soul.

It is somewhat ironic, therefore, that the fiercest attack on Harvey came from an ultra-conservative, Jean Riolan the Younger (1580-1657), the leading Galenist in the Paris faculty. Riolan had studied medicine under his distinguished father, continuing his war against the Paracelsans and attaining formidable erudition in classical literature and philosophy. He grasped that Harvey’s doctrine of the circulation had the potential to explode Galenic physiology. It would mean, for instance, that that liver was no longer the blood-making organ, and once the liver’s function was questioned, what else would not be questioned? Even Galenic therapeutics would be challenged, because the rationale for bleeding had been undermined: what price all the old rules about the correct places to bleed if the same blood were streaming round the body?

In his Opuscula anatomica (1649) [Little Anatomical Work], Riolan fired salvoes while offering a few concessions: the blood still followed the old Galenic pathways and did not generally circulate, but he conceded a minor circulation in the aorta and vena cava. Harvey retaliated in his Exercitatio anatomica de circulatione sanguinis (1649) [Anatomical Exercise on the Circulation of the Blood], insisting that Riolan’s position made observational nonsense, for the blood in all the arteries moved with considerable force and in great quantities, which pointed clearly to the circulation.