printed by M. Flourens in his "History." Nothing can be less equivocal than the description given by Servetus of the passage of the blood from the heart to the lungs, "where it is agitated, prepared, changes its colour, and is poured from the pulmonary artery into the pulmonary vein."*

This idea was as novel as it was true; but we cannot agree with M. Flourens in regarding this as a discovery, in the strict sense of the term; for although Servetus had some notion of the anatomical evidence furnished by the large calibre of the pulmonary artery, which enables it to carry a far greater quantity of blood than could be needed for the nutrition of the lungs, yet we have only to read the passages in which he describes this pulmonary circulation to perceive that he had no accurate idea of it: he speaks confidently of the nerves being continuations of the arteries, and describes with grave precision how the air passes from the nose into the ventricles of the brain, and how the devil takes the same route to lay siege to the soul. All we can say is, that Servetus made one lucky guess among his numerous guesses by no means of the lucky kind. He announced the fact of the pulmonary circulation, and may receive from History the whole credit of such priority; he also guessed rightly that in the lungs, and not in the liver, the blood received its elaboration, passing from venous into arterial. But whatever merit may be assigned to Servetus, no influence can be attributed to his discovery, since both he and his treatise were roasted by Calvin, and no one heard of the pulmonary circulation. Six years afterwards, Realdo Colombo rediscovered the pulmonary circulation; and that the discovery was ready to be made on all sides, is

seen in the fact of its being also made by Casalpinus, the great botanist, who does not seem to have been aware of what Colombo had written, since he makes no mention of him, and as M. Flourens observes, le grand mérite est toujours probe. Caesalpinus, moreover, was the first to pronounce the phrase "circulation of the blood." I

Here the reader may ask, What further remained for Harvey to discover? and it may surprise him to hear the answer: Everything! Such is pretty nearly the fact. The pulmonary circulation takes place only through a small arc of the great circle traversed by the blood. Besides this arc, there is the other greater arc, through which the systemic, or general circulation, takes its course; and of this no one except Cæsalpinus had even a suspicion. Every one supposed that the veins carried the blood to the tissues, and nourished them; no one suspected that this function was reserved for the arteries, and that the veins carried the blood only to the heart. It was thought that the arteries had their origin in the heart, and the veins in the liver; from the liver the veins carried the blood to every part. A single fact, familiar to every surgeon, and to every barber who ever opened a vein, ought to have revealed the error; since every time a ligature was applied, the operator must have seen that the vein swelled below the ligature, and not above it, from which the deduction seems obvious that the blood in the vein flowed to the heart, and not from it. But here, as in so many other cases, familiar facts were not observed; they were seen, but not interpreted. Caesalpinus was the first who observed it; but he has only the merit of having suspected the cause to be the current setting towards the heart.§ His suspicion was not a demonstration; and we are

surprised to find De Blainville saying, "that the circulation was known to Casalpinus, although he had not demonstrated it." In science, the difference between a guess and a demonstration is as great as that between the fame which an unwritten poem may achieve, and the fame which a great poen has achieved. If guesses counted as achievements, the temple of fame would be thronged with the statues of heroes. De Blainville says, that the reason why Haller and others have denied the claim of Cæsalpinus, is because they did not read what he had said in his work On Plants.* Now, if we turn to the passage in question, we shall see how far the writer really was from the truth, and yet how near his guess went to the truth." In animals, we see the food carried by the veins to the heart, as to a centre of innate heat; and there, having acquired its final perfection, it is distributed over the whole body through the arteries, by the agency of the spirit, which is engendered in the heart by this same food." Easy as it is for us to read into this passage almost all that we understand by the circulation, a close historical criticism detects in it nothing but a guess; and, as Bérard remarks, we ought not to confound two such vague statements as these, themselves requiring demonstration, and by Casalpinus himself subsequently contradicted, with the clear ideas, and imposing proofs, on which Harvey established his discovery. The convincing evidence of Harvey's originality is, that not only was this guess advanced by Casalpinus without any influence on the theories of that day, in spite of his deserved authority; but when Harvey promulgated his theory, he found, all over Europe, the greatest difficulty in getting it accepted. Bérard maintains, that so far from any one before

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Harvey having had a clear idea of the true theory, no one had even accurately conceived the true theory of pulmonary circulation; for although Servetus, Colombo, and Casalpinus knew that the blood passed through the lungs, they fancied only so much passed as was necessary for the reception of the "vital spirits," quantity which their predecessors fancied took its course through the perforated septum of the heart. But they had no conception of the entire mass of blood traversing the lungs ; and even had they known so much, they would have been wholly at a loss to say whence it came, and whither it went. It was necessary to understand the whole circulation before any part of it could accurately be understood.

The discovery that the veins had valves, opening and closing like doors, brought the discovery of the circulation within compass. It was made in 1574 by Fabrice d'Acquapendente, under whom Harvey studied at Padua. These valves, preventing any flow from the heart, but admitting the flow to the heart, ought to have suggested to their discoverer the true interpretation of their use. But five-and-forty years elapsed before any one arose, who had the sagacity to perceive the real value of this anatomical structure in respect to the blood-currents; and during these five-and-forty years, everything that had been discovered or surmised respecting the circulation, was familiar to every anatomist of the great Paduan school in which Harvey studied: nevertheless, when Harvey promulgated his theory, it was vehemently opposed. In 1619 he first publicly taught what he had discovered; and in 1628 he published, for the benefit of Europe, his celebrated treatise, Exercitatio Anatomica de Motu Cordis et Sanguinis, which may justly be called the basis

i. 44.

of modern physiology. That the theory was new, and would be opposed as a heresy, no one more clearly divined than he did.* The greatness of the discovery, and the force of genius required to make it, can only be appreciated by those who, familiar with the state of opinion in those days, read the evidence and arguments by which Harvey established his doctrine. It is true that he appeared at a peculiar epoch, when the confluence of various discoveries rendered his discovery possible; but that a man of genius was necessary to interpret and co-ordinate those discoveries, is evident in the fact, that no one except Harvey had, for nearly half a century, seen the significance of the facts.

Here, however, a caution must be interposed. The importance of the valves has been greatly exaggerated, and their real bearing on Harvey's discovery misconceived. They are thought to have rendered the discovery facile, because, inasmuch as they prevent the blood from flowing backwards, while permitting it to flow outwards, the idea of the circulation, it is said, must necessarily have emerged from the contemplation of them. Against this supposition there is one decisive fact: no one did deduce the conclusion, which is thus said to lie so ready. Moreover, in many cases, circulation takes place entirely without their aid. There are no valves in the veins of the Invertebrata,t none in the veins of fishes and reptiles, and very few in birds; yet the circulation is as complete in these animals as in man. Nay, even in man the chief veins are destitute of valves, although writers on Natural Theology, and even better-informed physiologists, are in the habit of speaking of them as if they were universal and indispensable; it may, therefore, be useful to mention, that there are no valves whatever in the great venous trunks-the venæ cavæ, and portal veins-none in the hepatic, renal, and uterine veins-none in the

brachio - cephalic, spinal, and iliac veins, and they are rarely present in the azygos and intercostal veins.

M. Flourens says, that when Harvey appeared everything had been suspected or indicated, but nothing established. This seems to us even less than the truth, for we cannot persuade ourselves that any one had the slightest conception of the process. Acquapendente could make nothing of the valves he had detected. He thought their office was simply to prevent a too great accumulation of blood in the lower parts of the body and a diminution from the upper parts. Colombo thought with his contemporaries, that the veins had their origin in the liver, and carried blood to the tissues. Casalpinus, in spite of his recognition of pulmonary circulation, thought the blood also passed from the right chamber of the heart to the left. But Harvey not only conceived a clear idea of the process, he described it minutely and accurately. He noticed the successive contractions of each auricle and ventricle, which forced the blood into the ventricle, when the auricle contracted, and forced it from the ventricle into the lungs, when the ventricle contracted: a process repeated on the left side with the aerated blood. And at each passage of the blood from one cavity to another, there were the valves, or "little doors" (ostiola) opening to let the current pass, and closing to prevent its reflux. He described the course of the blood along the arteries, which he attributed to the pulsations of the heart; and in this, instead of in Galen's "pulsific virtue," he recognised the cause of the blood's movement.

By Harvey the overthrow of ancient authority was completed. Men dared no longer swear by Galen-they swore by Harvey, who had discov ered the greatest fact in the animal economy, a fact totally unknown and unsuspected by Galen, or any other ancient. The new era had com

menced. It was not in the nature of things for the old system quietly to accept the new; accordingly, the opposition was loud and vehement. Like many other parts of this history, and like most oppositions to new doctrines, it has been immensely exaggerated by historians, and by writers who have chosen it as a theme for rhetoric. It is true that the Faculty rejected the new doctrine. But it is no less true that eminent men accepted it with enthusiasm. If Guy Patin was caustic in opposition, Molière laughed at Guy Patin's prejudice; and Boileau ridiculed the Faculty. Some anatomists accepted the doctrine, and the great Descartes warmly espoused it.* Swammerdamm and Malpighi, two of the great names of the century, speak of Harvey with reverence; and soon no one spoke of him in any other tone.

It is impossible to read Harvey's work without the highest admiration for the scientific genius it displays, and the conviction that here the circulation was not only demonstrated, but for the first time conceived. The experiments and arguments by which he establishes the fact, are still worthy of study, as models of investigation. But there were necessary gaps in his doctrine. The course of the circulation was not known to him, could not indeed have been discovered by any instruments at his disposal. He supposed the blood passed from arteries to veins by two paths, either through anastomosis (that is to say, the arteries opening directly into veins), or through the porosities of the parts-aut porositates carnis et partium solidarum pervias sanguini. He thought it necessary that so much of the blood as was required by the tissues for their nutrition, should remain behind in the tissues, and the rest be carried onwards to the heart. The error is considerable;

and its bearing on the theory of the circulation will be appreciated by any one who reflects on the fact that venous and arterial blood being so obviously distinguished, it is necessary that the passage of the one into the other should be demonstratednot surmised-before the theory of the circulation can be accepted as complete; for any one might reasonably assume that the blood in the veins is altogether another fluid from that in the arteries, and not merely another state of that fluid. This, indeed, was assumed by the adversaries of Harvey, and has found supporters even in our own day. Burdach cites two German physiologists-Willbrand and Runge-whom he thinks worthy of refutation, and who maintained that arterial blood was transformed in a mass into the tissues, and that venous blood was the re-transformed tissues. Unless the passage of the blood into the veins be clearly traced, there can be no reason against supposing that the veins simply absorb from the tissues, in the same way as the lymphatics and lacteals absorb their fluid. To prove that the blood makes a circuit, that circuit must be traced; and Harvey plainly declares that, with all his diligence, he could not succeed in tracing any connection between arteries and veins; in only three places did he find them presenting anything like an anastomosis; in every other place he imagined porosities.

Nor, with the means at his disposal, could Harvey have traced the complete course of the blood. The Microscope was needed; and the first to employ the microscope in such researches was Malpighi, who, four years after Harvey's death, in 1661, detected those capillaries which form the channel of communication between arteries and veins. He says

that, at first, he thought the blood poured out from the minute arteries in streams, without detecting any vessels for these streams; but afterwards he detected the distinct walls of these vessels; and he describes the modes of examining them in the lung of the frog. Their reticular arrangement on the pulmonary cells is well described by him. Nevertheless, in 1668, Leewenhoek describes them as if previously they had been quite unknown. I used every means I could devise" he says, “to see the complete circulation of the blood -namely, that one of the smallest of those vessels which we call veins, arose from another which is called an artery, and afterwards conveyed its contents to a larger vein; but I found this to be impossible, for when I followed the course of the artery until it became so small as only to admit of one or two globules to pass through it at a time, I then lost sight of it." This was in the wing of a bat; but subsequently he was more fortunate with the tail of a tadpole "a sight presented itself more delightful than any that my eyes had ever beheld; for here I discovered more than fifty circulations of the blood in different places. I saw that not only the blood in many places was conveyed through exceedingly minute vessels, from the middle of the tail towards the edges, but that each of these vessels had a curve or turning, and carried the blood back towards the middle of the tail, in order to be conveyed to the heart. Hereby it appeared plainly to me that the blood-vessels I now saw in this animal, and which bear the names of arteries and veins, are, in fact, one and the same-that is to say, that they are properly termed arteries so long as they convey the blood to the farthest extremities of its vessels, and veins when they bring it back towards the heart." Thus, then, was the demonstration of the course of the blood completed; and we must confess that it is with surprise we find all historians over

looking the great gap in the doctrine which had been left by Harvey, a gap only filled up by Malpighi and Leewenhoek in their discovery of these capillaries forming the true passage of arterial to venous blood.

It is necessary to bear in mind that the capillaries are a distinct set of vessels, differing from the arteries and veins which they connect, in their anatomical structure, and in their arrangement as a network. Bichat was the first who systematically conceived them as a distinct system; but their structure was not known until investigated by Henle, in 1841, and by subsequent histologists. The existence of these vessels is not only important to the theory of the circulation, but is even more important to the theory of nutrition; since it assures us that not only does the blood truly circulate, but circulates in a system of closed vessels, so that only by oozing through the walls of those vessels can it reach the tissues, and nourish them. Indeed, those who imagined that the blood was poured on to the tissues, were not aware that blood under such circumstances would act like a foreign substance: instead of nourishing, it would destroy. If the reader feels any difficulty in understanding how the blood can ooze through the walls of the vessels in sufficient quantities for the purposes of nutrition, he is referred to any work on Chemistry, which will explain to him the laws of Endosmosis, or the passage of fluids through animal membranes; and having there learned with what facility this passage takes place when the membrane separates two fluids of unequal density, he will be prepared to understand how it takes place even in the blood-vessels of the Slug, which have a continuous layer of chalk forming their middle tunic -a fact which considerably surprised us when we first observed it. In other animals the walls of the blood-vessels are more delicate, and the capillaries form a network of vessels, each being about the