If you will follow the course of the arrows in Fig. 4 you will see at once that—in accordance with the views of Columbus—the blood passes from the right side of the heart, through the lungs, to the left side. Then, adds Harvey, with abundant proof, it passes through the arteries to all parts of the body; and then, at the extremities of their branches in the different parts of the body, it passes (in what way he could not tell, for his means of investigation did not allow him to say) into the roots of the veins—then from the roots of the veins it goes into the trunk veins—then to the right side of the heart—and then to the lungs, and so on. That, you will observe, makes a complete circuit; and it was precisely here that the originality of Harvey lay. There never yet has been produced, and I do not believe there can be produced, a tittle of evidence to show that, before his time, any one had the slightest suspicion that a single drop of blood, starting in the left ventricle of the heart, passes through the whole arterial system, comes back through the venous system, goes through the lungs, and comes back to the place whence it started. But that is the circulation of the blood, and it was exactly this which Harvey was the first man to suspect, to discover, and to demonstrate. But this was by no means the only thing Harvey did. He was the first who discovered and who demonstrated the true mechanism of the heart's action. No one, before his time, conceived that the movement of the blood was entirely due to the mechanical action of the heart as a pump. There were all sorts of speculations about the matter, but nobody had formed this conception, and nobody understood that the so-called systole of the heart is a state of active contraction, and the so-called diastole is a mere passive dilatation. Even within our own age that matter had been discussed. Harvey is as clear as possible about it. He says the movement of the blood is entirely due to the contractions of the walls of the heart—that it is the propelling apparatus—and all recent investigation tends to show that he was perfectly right. And from this followed the true theory of the pulse. Galen said, as I pointed out just now, that the arteries dilate as bellows, which have an active power of dilatation and contraction, and not as bags which are blown out and collapse. Harvey said it was exactly the contrary—the arteries dilate as bags simply because the stroke of the heart propels the blood into them; and, when they relax again, they relax as bags which are no longer stretched, simply because the force of the blow of the heart is spent. Harvey has been demonstrated to be absolutely right in this statement of his; and yet, so