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 arose from the brain and conveyed “animal spirits” to the body were different from the tendons or sinews ( ) which attach muscles to bones. First, then, the physicians of the time of Thomas Linacre knew that the blood is not stagnant in the body. So did Shakespeare and Homer, and every augur who inspected the entrails of a victim, and every village barber who breathed a vein. Plato even uses the expression to . But no one had a conception of a continuous stream returning to its source (a circulation in the true sense of the word) either in the system or in the lungs. If they used the word circulatio, as did Caesalpinus, it was as vaguely as the French policeman cries “Circulez.” The movements of the blood were in fact thought to be slow and irregular in direction as well as in speed, like the “circulation” of air in a house, or the circulation of a crowd in the streets of a city. Secondly, they supposed that one kind of blood flowed from the liver to the right ventricle of the heart, and thence to the lungs and the general system by the veins, and that another kind flowed from the left ventricle to the lungs and general system by the arteries. Thirdly, they supposed that the septum of the heart was pervious and allowed blood to pass directly from the right to the left side. Fourthly, they had no conception of the functions of the heart as the motor power of the movement of the blood. They doubted whether its substance was muscular; they supposed its pulsation to be due to expansion of the spirits it contained; they believed the only dynamic effect which it had on the blood to be 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.

Of the great anatomists of the 16th century, Sylvius (In Hipp. et Gal. phys. partem anatom. isagoge) described the valves of the veins; Vesalius (De humani corporis fabrica, 1542) ascertained that the septum between the right and left ventricles is complete, though he could not bring himself to deny the invisible pores which Galen’s system demanded. Servetus, in his Christianismi restitutio (1553), goes somewhat farther than his fellow-student Vesalius, and says: “Paries ille medius non est aptus ad communicationem et elaborationem illam; licet aliquid resudare possit”; and, from this anatomical fact and the large size of the pulmonary arteries he concludes that there is a communication in the lungs by which blood passes from the pulmonary artery to the pulmonary vein: “Eodem artificio quo in hepate fit transfusio a vena porta ad venam cavam propter sanguinem, fit etiam in pulmone transfusio a vena arteriosa ad arteriam venosam propter spiritum.” The natural spirit of the left side and the vital spirit of the right side of the heart were therefore, he concluded, practically the same, and hence two instead of three distinct spiritus should be admitted. It seems doubtful whether even Servetus rightly conceived of the entire mass of the blood passing through the pulmonary artery and the lungs. The transference of the spiritus naturalis to the lungs, and its return to the left ventricle as spiritus vitalis, was the function which he regarded as important. Indeed a true conception of the lesser circulation as a transference of the whole blood of the right side to the left was impossible until the corresponding transference in the greater or systematic circulation was discovered. Servetus, however, was the true predecessor of Harvey in physiology, and his claims to that honour are perfectly authentic and universally admitted. Realdo Columbus (De re anatomica, 1559) formally denies the muscularity of the heart, yet correctly teaches that blood and spirits pass from the right to the left ventricle, not through the septum but through the lungs, “quod nemo hactenus aut animadvertit aut scriptum reliquit.” The fact that Harvey quotes Columbus and not Servetus is explained by the almost entire destruction of the writings of the latter, which are now among the rarest curiosities. The great anatomist Fabricius, Harvey’s teacher at Padua, described the valves of the veins more perfectly than had Sylvius. Carlo Ruini, in his treatise on the Anatomy and Diseases of the Horse (1590), taught that the left ventricle sends blood and vital spirits to all parts of the body except the lungs—the ordinary Galenical doctrine. Yet on the strength of this phrase Professor J. B. Ercolani actually put up a tablet in the veterinary school at Bologna to Ruini as the discoverer of the circulation of the blood! The claims of Caesalpinus, a more plausible claimant to Harvey’s laurels, are scarcely better founded. In his Quaestiones peripateticae (1571) he followed Servetus and Columbus in describing what we now know as the pulmonary “circulation” under that name, and this is the only foundation for the assertion (first made in Bayle’s dictionary) that Caesalpinus knew “the circulation of the blood.” He is even behind Servetus, for he only allows part of the blood of the right ventricle to go round by this “circuit”; some, he conceives, passes through the hypothetical pores in the septum, and the rest by the superior cava to the head and arms, by the inferior to the rest of the body: “Hanc esse venarum utilitatem ut omnes partes corporis sanguinem pro nutrimento deferant. Ex dextro ventr° cordis vena cava sanguinem crassiorem, in quo calor intensus est magis, ex altero autern ventr°, sanguinem temperatissimum ac sincerissimum habente, egreditur aorta.” Caesalpinus seems to have had no original views on the subject; all that he writes is copied from Galen or from Servetus except some erroneous observations of his own. His greatest merit was as a botanist; and no claim to the “discovery of the circulation” was made by him or by his contemporaries. When it was made, Haller decided conclusively against it. The fact that an inscription has been placed on the bust of Caesalpinus at Rome, which states that he preceded others in recognizing and demonstrating “the general circulation of the blood,” is only a proof of the blindness of misplaced national vanity. The way then to Harvey’s great work had been paved by the discovery of the valves in the veins, and by that of the lesser circulation—the former due to Sylvius and Fabricius, the latter to Servetus—but the significance of the valves was unsuspected and the fact of even the pulmonary circulation was not generally admitted in its full meaning.

In his treatise Harvey proves (1) that it is the contraction, not the dilatation, of the heart which coincides with the pulse, and that the ventricles as true muscular sacs squeeze the blood which they contain into the aorta and pulmonary artery; (2) that the pulse is not produced by the arteries enlarging and so filling, but by the arteries being filled with blood and so enlarging; (3) that there are no pores in the septum of the heart, so that the whole blood in the right ventricle is sent to the lungs and round by the pulmonary veins to the left ventricle, and also that the whole blood in the left ventricle is again sent into the arteries, round by the smaller veins into the venae cavae, and by them to the right ventricle again—thus making a complete “circulation”; (4) that the blood in the arteries and that in the veins is the same blood; (5) that the action of the right and left sides of the heart, auricles, ventricles and valves, is the same, the mechanism in both being for reception and propulsion of liquid and not of air, since the blood on the right side, though mixed with air, is still blood; (6) that the blood sent through the arteries to the tissues is not all used, but that most of it runs through into the veins; (7) that there is no to and fro undulation in the veins, but a constant stream from the distant parts towards the heart; (8) that the dynamical starting-point of the blood is the heart and not the liver.

The method by which Harvey arrived at his complete and almost faultless solution of the most fundamental and difficult problem in physiology has been often discussed, and is well worthy of attention. He begins his treatise by pointing out the many inconsistencies and defects in the Galenical theory, quoting the writings of Galen himself, of Fabricius, Columbus and others, with great respect, but with unflinching criticism. For, in his own noble language, wise men must learn anatomy, not from the decrees of philosophers, but from the fabric of nature herself, “nec ita in verba jurare antiquitatis magistrae, ut veritatem amicam in apertis relinquant, et in conspectu omnium deserant.” He had, as we know, not only furnished himself with all the knowledge that books and the instructions of the best anatomists of Italy could give, but, by a long series of dissections, had gained a far more complete knowledge of the comparative anatomy of the heart and vessels than any contemporary—we may almost say than any successor—until the times of John Hunter and J. F. Meckel. Thus equipped, he tells us that he began his investigations into the movements of the heart and blood by looking at them—i.e. by seeing their action in living animals. After a modest preface, he heads his first chapter