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cular motion that that which moves returns to the place from whence it started. Hence the discovery of the course of the blood from the right ventricle, through the lungs, to the left ventricle was in no wise an anticipation of the discovery of the circulation of the blood. For the blood which traverses this part of its course no more describes a circle than the dweller in a street who goes out of his own house and enters his next-door neighbor's does so. Although there may be nothing but a party wall between him and the room he has just left, it constitutes an efficient _defense de circuler_. Thus, whatever they may have known of the so-called pulmonary circulation, to say that Servetus or Columbus or Caesalpinus deserves any share of the credit which attaches to Harvey appears to me to be to mistake the question at issue. It must further be borne in mind that the determination of the true course taken by the whole mass of the blood is only the most conspicuous of the discoveries of Harvey; and that his analysis of the mechanism by which the circulation is brought about is far in advance of anything which had previously been published. For the first time it is shown that the walls of the heart are active only during its systole or contraction, and that the dilatation of the heart, in the diastole, is purely passive. Whence it follows that the impulse by which the blood is propelled is a _vis a tergo_, and that the blood is not drawn into the heart by any such inhalent or suctorial action as not only the predecessors, but many of the successors, of Harvey imagined it to possess. Harvey is no less original in his view of the cause of the arterial pulse. In contravention of Galen and of all other anatomists up to his own time, he affirms that the stretching of the arteries which gives rise to the pulse is not due to the active dilatation of their walls, but to their passive distention by the blood which is forced into them at each beat of the heart; reversing Galen's dictum, he says that they dilate as bags and not as bellows. This point of fundamental, practical as well as theoretical, importance is most admirably demonstrated, not only by experiment, but by pathological illustrations. One of the weightiest arguments in Harvey's demonstration of the circulation is based upon the comparison of the quantity of blood driven out of the heart, at each beat, with the total quantity of blood in the body. This, so far as I
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