two-thousandth of an inch in calibre.*

Harvey did not live to hear of the Capillaries; but there is another system of vessels, of which indeed he heard, although he failed to appreciate their significance. We allude to the Lymphatics, or Absorbents. They are minute vessels, abounding in all the viscera, rare in the muscles, and not yet detected in the nervous centres; when they rise from the alimentary canal, they are called Lacteals, or chyliferous vessels, and as such were, until quite recently, supposed to be the channels for the conveyance of the chyle to the blood. We know now that this chyle is not the quintessence of the digested food, and that the lacteals differ from all other lymphatics in carrying a larger proportion of fat, which gives their fluid its milky aspect during digestion. Lymph is blood without the cells -plasma, in short. Chyle is lymph with the addition of fat, and perhaps also the addition of some other products of digestion. Lymph contains albumen, fibrine, fat, salts, and extractive matters, like the blood: and, like the blood, it coagulates at certain temperatures. The chief differences between lymph and blood are the absence of the cells, and the presence of a greater proportion of water. By absorbing the water from the tissues, lymph leaves the plasma in a more concentrated state. Lymph is one of the streams which sets towards the heart, and joins the venous current.

The discovery of the Lymphatics is due to Aselli, Pecquet, Rudbeck, and Bartholin. Anatomists taught that there were three kinds of vessels in the body: the veins, which carried blood; the arteries, which carried spirituous blood; and the nerves, which carried "animal spirits." To the surprise of all, the news came that an Italian anatomist, Aselli, had discovered a fourth kind which carried the chyle. This discovery was announced in 1622-three years after Harvey first announced his discovery, but six years before his book was

published. Aselli was dissecting a dog, and, to his surprise, on opening the abdomen he saw a network of delicate white vessels. What could they be? Did they contain the chyle? He pricked one, and, in a transport of delight, exclaimed Eureka as he saw a milky fluid flowing out. But on opening another dog, he was greatly discouraged, for there was not a vessel of the kind to be seen. Had he been deceived? Was his joy premature ? In this perplexity it occurred to him that the first dog had been fed a little while before he was opened; whereas the second dog was fasting. With the insight of genius, Aselli detected here the clue which might lead him to the truth. He fed another dog; four hours afterwards he opened it, and had the intense satisfaction of once more seeing the milky vessels.

But although Aselli made Europe aware of a new system of vessels, which he named Lacteals, he failed to trace their issue. He thought they conducted the chyle to the liver. In 1648, a French anatomist, Pecquet, distrusting the conclusions of that "mute and frigid science," as he calls the dissection of dead bodies, determined to seek the truth in the living organism, and began a series of vivisections. He was rewarded by the discovery of the course taken by the chyle in the lacteals, as it passes into the reservoir, still named after him, and along the thoracic duct, to be poured into the subclavian vein, and thence, mingled with the blood, into the heart. In 1650, Rudbeck, a young Swede, discovered the lymphatics in the liver, and their connection with the reservoir of the chyle. In 1652, Bartholin, another Frenchman, completed this discovery by finding the lymphatics in the viscera and limbs, and by tracing them into a common trunk. These discoveries, following in such quick succession, greatly disturbed the equanimity of the Faculty, whose members swore by Galen, and could not tolerate the idea of the ancients being supposed to have overlooked

anything. "Un chacun invente à present!" was the indignant sarcasm of Riolan, the most renowned teacher of that day-the only adversary whom Harvey condescended to answer, and of whom he stood in such awe, that, even when answering his attacks, he declares the book which contains those attacks, "will live for ever, and when marble shall have mouldered, will proclaim to posterity the glory which belongs to your name.'

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Harvey knew indeed of the exist ence of the lacteals, and says that he had observed them before Aselli published his book. But he denied that they contained the chyle, and he is twitted by Riolan for his disbelief. In consequence of this, a tradition has come down, that Harvey showed the same spirit of opposition towards the novelties of others, as was shown by others towards the novelties he advocated; but this is a mistake. If the reader will turn to Harvey's letter to Dr Morison of Paris, he will see a very circumstantial and temperate exposition of the objections which Harvey felt against the notion of the lacteals conveying the chyle: many of these objections are of great force, and still remain unshaken. His chief error is, in supposing that the fluid in the lacteals is milk. But he insists very properly that it should be demonstrated that this fluid is really chyle brought from the intestines, and that it supplies nourishment to the whole body; "for unless we are agreed upon this point, all discussion is vain." Subsequent investigations have cleared up what was obscure, and have shown that the lacteals convey lymph mingled with fat, and that the lymphatics convey whatever they can absorb from the tissues, in which they are imbedded like the roots of a tree in the earth. The lacteal and lymphatic streams are confluent with the great venous streams, and thus form parts of the circulation.

From this exposition of the march of discoveries, it is clear that Harvey did not discover what we mean by the circulation of the blood; but he discovered the great fact, that the

blood does circulate, propelled from the heart along the arteries, and back to the heart and lungs through the veins. Having thus shown that he did not discover the real course of the circulation, we have now to show that he did not discover its real cause. Before doing so, it will be interesting to remark, that in one important respect he was behind Servetus, for he thought that the blood returned to the heart, as to a fountain, there to receive the additions necessary to its perfection-tanquam ad fontem sive ad lares corporis, perfectionis recuperandæ causa reverti. Servetus correctly stated that it was in the lungs, and not in the heart, that this perfection was attained. Instead of perceiving that arterial blood acquired its bright colour by its exposure to the air, Harvey maintained that this colour was owing to the lighter part being "strained" from the heavier in its passage through minute openings. In blood-letting, when the blood forcibly escapes to a distance in a full stream, it is thicker and darker; but when flowing from a small orifice, drop by drop, it is brighter because it is then strained, and the thinner portion alone escapes." We know that the difference is owing to a more perfect exposure of the whole mass to the air when it issues slowly in drops, and that is why the blood is then of a brighter hue. Harvey, having no suspicion of this atmospheric influence, is driven to ingenious devices to explain the change of colour. "In the lungs," he says, "it is more florid than in the arteries, because it is strained through the pulmonary tissue." His notion of the office of the lungs was, that they served to cool the blood, "and prevent it from boiling up."

What is it which causes the blood to circulate? The heart, answers an unhesitating reader. That the heart pumps blood incessantly into the arteries, and that this pumping must drive the stream onwards with great force, there is no doubt; but although one puissant agent in the circulation, the heart is not the sole agent; and the more we study this

our

difficult question, the more doubts gather round the explanation. Let a few of the difficulties be stated. There have been cases of men and animals born without a heart; these "acardiac monsters" did not live, indeed could not live; but they had grown and developed in the womb, and consequently their blood must have circulated. In most of these cases there has been a twin embryo, which was perfect; and the circulation in both was formerly attributed to the heart of the one; but it has been fully established that this is not the case. Further, "it has occasionally been noticed that a degeneration in the structure of the heart has taken place, during life, to such an extent that scarcely any muscular tissue could at last be detected in it, but without any such interruption to the circulation as must have been anticipated if this organ furnishes the sole impelling force." On the other hand, an influence acting on the capillaries will give a complete check to the action of the heart, although that organ is itself perfectly healthy and vigorous. The circulation in the liver is peculiar, since a complete circuit is there performed by the blood, yet no heart, or pulsating organ, is found in that circle; the small vessels which belong to the intestine converge into a large trunk, and this portal vein on entering the liver ramifies over it in the manner of an artery that is to say, it branches and branches into the minutest subdivisions, and terminates in capillaries; these once more converge into venous trunks, and carry the altered blood from the liver. Here is a complete circle, analogous to that larger circle which traverses the whole body. It is, indeed, very like that found in the fish. The fish has a heart, it is true, but that heart only drives the blood into the gills, not into the rest of the body-it is a pulmonary, not a systemic heart. The blood, aerated in the capillaries of the gills, descends through them

into the aorta, not into the heart : and from the aorta it passes to the rest of the body, from whence it returns through veins to the heart. Those who have never watched with attention the circulation in the capillaries, may perhaps imagine that the mere force of the heart which propels the blood into the gills, will suffice to propel it also through the general circuit. They will see the heart of the fish beating vigorously, and will imagine these pulsations suffice. But this will no longer seem so plausible, if we place the tail or gills of a tadpole, or the foot of a frog, under the microscope, and delight ourselves with the wondrous spectacle. We shall immediately perceive that the blood flows with far greater rapidity in the arteries than in the capillaries; we shall note that, although the heart continues its vigorous pulsations, and the blood in the arteries advances in regular leaps, the currents in the capillaries are very irregular, sometimes momently arrested, and even reversed. Instead of the leaping rush, we perceive an unsteady flow, which is never at any time equal in rapidity to the flow in the arteries. This capillary retardation is a very important point in the present discussion. Although precise estimates are excessively difficult in such cases, we need accept the estimates yet made as only approximative, and our argument is equally fortified by them, for the difference in the rate is enormous in the arteries the blood rushes at the rate of a foot per second; in the capillaries only an inch per minute. What does that indicate? It indicates that, when the heart has driven the blood along the arteries at rapid rate, there is a considerable retardation occurring in the capillaries, which must call for some new force to restore the rate. When we find the blood of the fish driven into the capillaries of the lungs by the heart, and, in spite of the retardation there experienced, driven from the lungs to all the rest

of the body with a newly-acquired velocity, we ask, Whence comes this new velocity? Not from the heart, since that force has been retarded. Clearly then from some other cause.

Nor do the difficulties end here. The heart may be removed in coldblooded animals, and the capillary circulation will continue for some time, in spite of that removal. I had observed this some time ago; but when preparing this paper for publication, I determined on critically investigating the point; and for this purpose removed the heart of a Triton, with as much care as possible, and found the circulation going on in the tail for some minutes afterwards, nor did it entirely cease on separating the tail from the body.* While the fact was thus indubitable, I had many doubts as to the cause. But the fact is enough for our present purpose; and that it also occurs in warm-blooded animals may be inferred, since after death various processes of secretion, and some even of growth (as of hair, beard, &c.), are known to take place; and this surely implies capillary circulation. "After most kinds of death," says Dr Carpenter, "the arterial system is found, subsequently to the lapse of a few hours, almost or completely emptied of blood; this is partly, no doubt, the effect of the tonic contraction of the tubes themselves; but the emptying is commonly more complete than could thus be accounted for, and must therefore be partly due to the continuance of the capillary circulation. It has been observed by Dr Bennet Dowler, that in the bodies of individuals who have died from yellow fever, the external veins frequently become so distended with

blood, within a few minutes after the cessation of the heart's action, that when they are opened the blood flows in a good stream, being sometimes projected to the distance of a foot or more. It is not conceivable that the slowly-acting tonicity of the arteries could have produced such a result as this; which can scarcely therefore be attributed to anything else than the sustenance of the capillary circulation by forces generated within itself."

Not further to multiply examples, we may take our stand on these, and pronounce the Harveyan doctrine to be incomplete, since it fails altogether to account for many important phenomena. Whatever influence the action of the heart may exert, it is not the sole cause of the circulation, but only one of the causes. But before attempting to assign the other causes, let us see the part actually assignable to the heart.

The motions of the heart consist in the alternate contractions and relaxations of its muscular walls. The process is this: The two antechambers (auricles) suddenly contract; immediately afterwards, but while the auricles are still contracted, the two chambers (ventricles) also contract, having been powerfully expanded by the rush of blood from the auricles. This contraction is named the systole of the heart. It continues for a moment, and is followed by a relaxation of the two auricles, which is immediately succeeded by the relaxation of the two ventricles. This relaxation is named the diastole. During each beat, two sounds may be heard; one dull, which may be imitated by pronouncing the word lubb; the other, quick

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