The importance of Harvey's discovery of the circulation of the blood can only be properly estimated by bearing in mind what was done by his predecessors in the same field of inquiry. Aristotle had taught that in man and in the higher brutes the blood was elaborated from the food in the liver, conveyed to the heart, and thence distributed by it through the veins to the whole body. Erasistratus and Herophilus held that, while the veins carried blood from the heart to the members, the arteries carried a subtle kind of air or spirit. Galen discovered that the arteries were not merely air-pipes, but that they contained blood as well as vital air or spirit. Sylvius, the teacher of Vesalius, was aware of the presence of valves in the veins; and Fabricius, Harvey's teacher at Padua, described them much more accurately than Sylvius had done; but neither of these men had a true idea of the significance of the structures of which they wrote. Servetus, the friend and contemporary of Vesalius, writing in 1533, correctly described the course of the lesser circulation in the following words: "This communication (i.e. between the right and left sides of the heart) does not take place through the partition of the heart, as is generally believed; but by another admirable contrivance, whereby from the right ventricle the subtle blood is agitated in a lengthened course through the lungs, wherein prepared, it becomes of a crimson colour, and from the vena arterialis (pulmonary artery) is transferred into the arteria venalis (pulmonary vein). Mingled with the inspired air in the arteria venalis, freed by respiration from fuliginous matter, and become a suitable home of the vital spirit, it is attracted at length into the left ventricle of the heart by the diastole of the organ." But when Servetus comes to speak of the systemic circulation, what he has to say is as old as Galen.
The opinions, therefore, on the subject of the blood and its distribution which were prevalent at the end of the sixteenth century prove—
(1) That although the blood was not regarded as stagnant, yet its circulation, such as is nowadays recognized, was unknown;
(2) That one kind of blood was thought to flow from the liver to the right ventricle, and thence to the lungs and general system by the veins, while another kind flowed from the left ventricle to the lungs and general system by the arteries;
(3) That the septum of the heart was regarded as admitting of the passage of blood directly from the right to the left side;
(4) That there was no conception of the functions of the heart as the motor power of the movement of the blood, for biologists of that day doubted whether the substance of the heart were really muscular; they supposed the pulsations to be due to expansion of the spirits it contained; they believed the only dynamic effect which it had on the blood to be that of sucking it in during its active diastole, and they supposed the chief use of its constant movements to be the due mixture of blood and spirits.
This was the state of knowledge before Harvey's time. By his great work he established—
(1) That the blood flows continuously in a circuit through the whole body, the force propelling it in this unwearied round being the rhythmical contractions of the muscular walls of the heart;
(2) That a portion only of the blood is expended in nutrition each time that it circulates;
(3) That the blood conveyed in the systemic arteries communicates heat as well as nourishment throughout the body, instead of exerting a cooling influence, as was vulgarly supposed; and
(4) That the pulse is not produced by the arteries enlarging and so filling, but by the arteries being filled with blood and so enlarging.
We can now consider the method by which Harvey arrived at these results. The work, "De Motu Cordis et Sanguinis," after giving an account of the views of preceding physiologists, ancient and modern, commences with a description of the heart as seen in a living animal when the chest has been laid open and the pericardium removed. Three circumstances are noted—
(a) The heart becomes erect, strikes the chest, and gives a beat;
(b) It is constricted in every direction;
(c) Grasped by the hand, it is felt to become harder during the contraction.
From these circumstances it is inferred—
(1) That the action of the heart is essentially of the same nature as that of voluntary muscles, which become hard and condensed when they act;
(2) That, as the effect of this, the capacity of the cavities is diminished, and the blood is expelled;
(3) That the intrinsic motion of the heart is the systole, and not the diastole, as previously imagined.
The motions of the arteries are next shown to be dependent upon the action of the heart, because the arteries are distended by the wave of blood that is thrown into them, being filled like sacs or bladders, and not expanding like bellows. These conclusions are confirmed by the jerking way in which blood flows from a cut artery.
In the heart itself two distinct motions are observed—first of the auricles, and then of the ventricles. These alternate contractions and dilatations can have but one result, namely, to force the blood from the auricle to the ventricle, and from the ventricle, on the right side, by the pulmonary artery to the lungs, and on the left side by the aorta to the system.
These considerations suggest to the mind of Harvey the idea of the circulation. "I began to think," he says, "whether there might not be a motion, as it were, in a circle." This is next established by proving the three following propositions:—
(1) The blood is incessantly transmitted by the action of the heart from the vena cava to the arteries in such quantity that it cannot be supplied from the ingesta, and in such wise that the whole mass must very quickly pass through the organ;
(2) The blood, under the influence of the arterial pulse, enters, and is impelled in a continuous, equable, and incessant stream through every part and member of the body, in much larger quantity than were sufficient for nutrition, or than the whole mass of fluids could supply;
(3) The veins in like manner return this blood incessantly to the heart from all parts and members of the body.
As to the first proposition Harvey says, "Did the heart eject but two drachms of blood on each contraction, and the beats in half an hour were a thousand, the quantity expelled in that time would amount to twenty pounds and ten ounces; and were the quantity an ounce, it would be as much as eighty pounds and four ounces. Such quantities, it is certain, could not be supplied by any possible amount of meat and drink consumed within the time specified. It is the same blood, consequently, that is now flowing out by the arteries, now returning by the veins; and it is simply matter of necessity that the blood should perform a circuit, or return to the place from whence it went forth."
Demonstration of the second proposition—that the blood enters a limb by the arteries and returns from it by the veins—is afforded by the effects of a ligature. For if the upper part of the arm be tightly bound, the arteries below will not pulsate, while those above will throb violently. The hand under such circumstances will retain its natural colour and appearance, although, if the bandage be kept on for a minute or two, it will begin to look livid and to fall in temperature. But if the bandage be now slackened a little, the hand and the arm will immediately become suffused, and the superficial veins show themselves tumid and knotted, the pulse at the wrist in the same instant beginning to beat as it did before the application of the bandage. The tight bandage not only compresses the veins, but the arteries also, so that blood cannot flow through either. The slacker ligature obstructs the veins only, for the arteries lie deeper and have firmer coats. "Seeing, then," says Harvey, "that the moderately tight ligature renders the veins turgid, and the whole hand full of blood, I ask, Whence is this? Does the blood accumulate below the ligature coming through the veins, or through the arteries, or passing by certain secret pores? Through the veins it cannot come; still less can it come by any system of invisible pores; it must needs, then, arrive by the arteries."
The third position to be proved is that the veins return the blood to the heart from all parts of the body. That such is the case might be inferred from the presence and disposition of the valves in the veins; for the office of the valves is by no means explained by the theory that they are to hinder the blood from flowing into inferior parts by gravitation, since the valves do not always look upwards, but always towards the trunks of the veins, invariably towards the seat of the heart. The action of the valves is then demonstrated experimentally on the arm bound as for blood-letting. The point of a finger being kept on a vein, the blood from the space above may be streaked upwards till it passes the valve, when that portion of the vein between the valve and the point of pressure will not only be emptied of its contents, but will remain empty as long as the pressure is continued. If the pressure be now removed, the empty part of the vein will fill instantly and look as turgid as before.
Other confirmatory evidence is then added, e.g. the absorption of animal poisons and of medicines applied externally, the muscular structure of the heart and the necessary working of its valves.
William Harvey, the illustrious physiologist, anatomist, and physician, to whom this discovery is due, was the eldest son of a Kentish yeoman, and was born in April, 1578. At the age of ten he entered the Canterbury Grammar School, where he appears to have remained for some years. At sixteen he passed to Caius-Gonvil College, Cambridge, and three years afterwards took his B.A. degree and quitted the university. Like most students of medicine of that day, he found it necessary to seek the principal part of his professional education abroad. He travelled to Italy, selected Padua as his place of study, and there continued to reside for four years, having as one of his teachers the famous Fabricius of Aquapendente. On his return to England, in 1602, he took his doctor's degree at Cambridge, and entered on the practice of his profession.
In 1604 he joined the College of Physicians, and three years later was elected a Fellow of that learned body. Two years afterwards he applied for the post of physician to St. Bartholomew's Hospital; and his application being supported by letters of recommendation to the governor, from the king and from the president of the College of Physicians, he was duly elected to the office in the same year, as soon as a vacancy occurred.
In 1615, when thirty-seven years of age, Harvey was chosen to deliver the lectures on surgery and anatomy to the College of Physicians, and it is possible that at this time he gave an exposition of his views on the circulation. He continued to lecture on the same subject for many years afterwards, although he did not publish his views until 1628, when they appeared in the work "De Motu Cordis."
Some few years after his appointment as lecturer to the college, he was chosen one of the physicians extraordinary to King James I., and about five or six years after the accession of Charles I. he became physician in ordinary to that unfortunate monarch. The physiologist's investigations seem to have interested King Charles, for he had several exhibitions made of the punctum saliens in the embryo chick, and also witnessed dissections from time to time.
When, in 1630, the young Duke of Lennox made a journey on the Continent, Harvey was chosen to travel with him, and probably remained abroad about two years. During this time Harvey most likely visited Venice. Of this tour the doctor speaks in the following terms in a letter written at the time: "I can only complayne that by the waye we could scarce see a dogg, crow, kite, raven, or any bird or any thing to anatomise; only sum few miserable poeple the reliques of the war and the plauge, where famine had made anatomies before I came."
Six years after this, in April, 1636, he accompanied the Earl of Arundel in his embassy to the emperor. Having to visit the principal cities of Germany, he was thus afforded an opportunity of meeting the leading biologists of the time, and at Nuremberg he probably met Caspar Hoffmann, and made that public demonstration of the circulation of the blood which he had promised in his letter dated from that city, and which convinced every one present except Hoffmann himself. Hollar, the artist, informs us that Harvey's enthusiasm in his search for specimens often led him into danger, and caused grave anxiety to the Earl of Arundel. "For he would still be making of excursions into the woods, making observations of strange trees, plants, earths, etc., and sometimes like to be lost; so that my lord ambassador would be really angry with him, for there was not only danger of wild beasts, but of thieves."
Soon after his return to England, as court physician, his movements became seriously restricted by the fortunes of the king. Aubrey says, "When King Charles I., by reason of the tumults, left London, Harvey attended him, and was at the fight of Edgehill with him; and during the fight the Prince and the Duke of York were committed to his care. He told me that he withdrew with them under a hedge, and tooke out of his pockett a booke and read; but he had not read very long before a bullet of a great gun grazed on the ground neare him, which made him remove his station.... I first sawe him at Oxford, 1642, after Edgehill fight, but was then too young to be acquainted with so great a doctor. I remember he came severall times to our Coll. (Trin.) to George Bathurst, B.D., who had a hen to hatch egges in his chamber, which they dayly opened to see the progress and way of generation."
In 1645, Charles, after the execution of Archbishop Laud, took upon himself the functions of visitor of Merton College, and having removed Sir Nathaniel Brent from the office of warden for having joined "the Rebells now in armes against" him, he directed the Fellows to take the necessary steps for the election of a successor. This course consisted in giving in three names to the visitor, in order that one of the three (the one named first, probably) should be appointed. Harvey was so named by five out of the seven Fellows voting, and was accordingly duly elected. A couple of days after his admission he summoned the Fellows into the hall and made a speech to them, in which he pointed out that it was likely enough that some of his predecessors had sought the office in order to enrich themselves, but that his intentions were quite of another kind, wishing as he did to increase the wealth and prosperity of the college; and he finished by exhorting them to cherish mutual concord and amity. After the surrender of Oxford, July, 1646, Harvey retired from the court. He was in his sixty-ninth year, and doubtless found the hardships and inconveniences which the miserable war entailed far from conducive to health. The rest and seclusion to be had at the residence of one or other of his brothers offered him the much-needed opportunity of renewing his inquiries into the subject of generation, and it is of this time that Dr. Ent speaks in the preface to the published work on that subject which appeared in 1651. "Harassed with anxious and in the end not much availing cares, about Christmas last, I sought to rid my spirit of the cloud that oppressed it, by a visit to that great man, the chief honour and ornament of our college, Dr. William Harvey, then dwelling not far from the city. I found him, Democritus-like, busy with the study of natural things, his countenance cheerful, his mind serene, embracing all within its sphere. I forthwith saluted him, and asked if all were well with him. 'How can it,' said he, 'whilst the Commonwealth is full of distractions, and I myself am still in the open sea? And truly,' he continued, 'did I not find solace in my studies, and a balm for my spirit in the memory of my observations of former years, I should feel little desire for longer life. But so it has been, that this life of obscurity, this vacation from public business, which causes tedium and disgust to so many, has proved a sovereign remedy to me.'"
Harvey died in June, 1657. Aubrey, his contemporary, says, "On the morning of his death, about ten o'clock, he went to speake, and found he had the dead palsey in his tongue; then he sawe what was to become of him, he knew there was then no hopes of his recovery, so presently sends for his young nephews to come up to him, to whom he gives one his watch, to another another remembrance, etc.; made sign to Sambroke his Apothecary to lett him blood in the tongue, which did little or no good, and so he ended his dayes.... The palsey did give him an easie passeport.... He lies buried in a vault at Hempsted in Essex, which his brother Eliab Harvey built; he is lapt in lead, and on his brest, in great letters, 'Dr. William Harvey.' I was at his Funerall, and helpt to carry him into the vault."
The publication of Harvey's views on the movement of the blood excited great surprise and opposition. The theory of a complete circulation was at any rate novel, but novelty was far from being a recommendation in those days. According to Aubrey, the author was thought to be crackbrained, and lost much of his practice in consequence. He himself complains that contumelious epithets were levelled at the doctrine and its author. It was not until after many years had elapsed, and the facts had become familiar, that men were struck with the simplicity of the theory, and tried to prove that the idea was not new after all, and that it was to be found in Hippocrates, or in Galen, or in Servetus, or in Cæsalpinus—anywhere, in fact, except where alone it existed, namely, in the work, "De Motu Cordis et Sanguinis." No one seems to have denied, while Harvey lived, that he was the discoverer of the circulation of the blood; indeed, Hobbes of Malmesbury, his contemporary, said of him, "He is the only man, perhaps, that ever lived to see his own doctrine established in his lifetime."
In one important respect Harvey's account of the circulation was incomplete. He knew nothing of the vessels which we now speak of as capillaries. Writing to Paul Marquard Slegel, of Hamburg, in 1651, he says, "When I perceived that the blood is transferred from the veins into the arteries through the medium of the heart, by a grand mechanism and exquisite apparatus of valves, I judged that in like manner, wherever transudation does not take place through the pores of the flesh, the blood is returned from the arteries to the veins, not without some other admirable artifice" (non sine artificio quodam admirabili). It was this artificium admirabile of which Harvey was unable to give a description. On account of the minuteness of their structure, the capillaries were beyond his sight, aided as it was by a magnifying glass merely. He indeed demonstrated physiologically the existence of some such passages; but it remained for a later observer, with improved appliances, to verify the fact. This was done by Malpighi in 1661, who saw in the lung of a frog, which was so mounted in a frame as to be viewed by transmitted light, the network of capillaries which connect the last ramifications of the arteries with the radicles of the veins.
Harvey rightly denied that the arteries possessed any pulsific power of their own, and maintained that their pulse is owing solely to the sudden distension of their walls by the blood thrown into them at each contraction of the ventricles. But the remission which succeeds the pulse was regarded by him as caused simply by collapse of the walls of the arteries due to elastic reaction. Knowing nothing of the muscular coat of the arteries, he was unaware of the fact that the elastic reaction of the arteries, after their distension, is aided by the tonic contractility of their walls; the two forces, physical and vital, acting in concert with each other—the former converting the intermittent flow from the heart into an even stream in the capillaries and veins; the latter, through the vaso-motor system, regulating the flow of blood to particular parts in order to meet changing requirements.
It is somewhat surprising to find that such an accurate observer as Harvey should have failed to recognize the significance and importance of the system of lacteal vessels. But such was the case. Eustachius, in the sixteenth century, had discovered the thoracic duct in the horse, although he seems to have thought that it was peculiar to that animal. Aselli, while dissecting the body of a dog in 1622, accidentally discovered the lacteals, and thought at first that they were nerves; but upon puncturing one of them, and seeing the milky fluid which escaped, found them to be vessels. He, however, failed to trace them to the thoracic duct, and believed them to terminate in the liver. Pecquet of Dieppe followed them from the intestines to the mesenteric glands, and from these into a common sac or reservoir, which he designated receptaculum chyli, and thence to their entry by a single slender conduit into the venous system at the junction of the jugular and subclavian veins. The existence of the lacteals had not entirely escaped Harvey, however. He had himself noticed them in the course of his dissections before Aselli's book was published, but "for various reasons" could not bring himself to believe that they contained chyle. The smallness of the thoracic duct seemed to him a difficulty, and as it was a demonstrated fact that the gastric veins were largely absorptive, the lacteals appeared to him superfluous. He is not "obstinately wedded to his own opinion," and does not doubt "but that many things, now hidden in the well of Democritus, will by-and-by be drawn up into day by the ceaseless industry of a coming age."
Late in the author's life, as we have seen, the work on the "Generation of Animals" appeared; but neither physiological nor microscopical science was sufficiently advanced to admit of the production of an enduring work on a subject necessarily so abstruse as that of generation. It was impossible, however, for so shrewd and able an investigator as Harvey to work at a subject even as difficult as this without leaving the impress of his original genius. He first announced the general truth, "Omne animal ex ovo," and clearly proved that the essential part of the egg, that in which the reproductive processes begin, was not the chalazæ, but the cicatricula. This Fabricius had looked upon as a blemish, a scar left by a broken peduncle. Harvey described this little cicatricula as expanding under the influence of incubation into a wider structure, which he called the eye of the egg, and at the same time separating into a clear and transparent part, in which later on, according to him, there appeared, as the first rudiment of the embryo, the heart, or punctum saliens, together with the blood-vessels. He was clearly of opinion that the embryo arose by successive formation of parts out of the homogeneous and nearly liquid mass. This was the doctrine of epigenesis, which, notwithstanding its temporary overthrow by the erroneous theory of evolution, is, with modifications, the doctrine now held.
Of Harvey's scholarship and culture we are not left in ignorance. Bishop Pearson, writing about seven years after the doctor's death, and Aubrey have told us of his appreciation of the works of Aristotle, and in his own writings he refers more frequently to the Stagirite than to any other individual. Sir William Temple has also put it on record that the famous Dr. Harvey was a great admirer of Virgil, whose works were frequently in his hands. His store of individual knowledge must have been great; and he seems never to have flagged in his anxiety to learn more. He made himself master of Oughtred's "Clavis Mathematica" in his old age, according to Aubrey, who found him "perusing it and working problems not long before he dyed."
Nor should it be forgotten that this illustrious physiologist and scholar was also the first English comparative anatomist. Of his knowledge of the lower animals he makes frequent use, and he says (in his work on the heart), "Had anatomists only been as conversant with the dissection of the lower animals as they are with that of the human body, many matters that have hitherto kept them in a perplexity of doubt, would, in my opinion, have met them freed from every kind of difficulty." Aubrey says that Harvey often told him "that of all the losses he sustained, no grief was so crucifying to him as the loss of his papers (containing notes of his dissections of the frog, toad, and other animals), which, together with his goods in his lodgings at Whitehall, were plundered at the beginning of the rebellion."
 According to the theory of evolution, the egg contained from the first an excessively minute, but complete animal, and the changes which took place during incubation consisted not in a formation of parts, but in a growth, i.e. in an expansion of the already existing embryo (see p. 40).
 See p. lxxxii. of "Life," by Dr. Willis.
 "Miscellanies:" Part II. on Poetry, p. 314.