said that almost all the blood passed from the right to the left side of the heart across the septum of the heart, but Harvey, maintaining that the septum was not porous, proved that all the blood, not merely some of it, went round by the lungs.
Of course, this pulmonary or "lesser" circulation was taught before his time, notably by Servetus and M. K. Columbus; but it was Harvey who first showed that the valves of the heart and the valves of the veins absolutely prevent any other direction of flow except from the veins to the right heart and thence via the lungs to the left heart. This doctrine of the porosity of the septum died hard, for we find Harvey, towards the close of his life, attempting to convince an objector—Professor Riolanus—that if only he will pour water into the right heart and tie all vessels to and from the lungs, not one drop will get into the left ventricle.
No one before Harvey had fully understood the venous valves. His own professor at Padua—Fabricius—had talked a great deal of nonsense about them in a treatise entirely devoted to them. Harvey said that they were not primarily for supporting columns of blood, but for preventing any back-flow towards the periphery, seeing that they were present in the veins of the head in which the blood (under gravity) flowed past them with the greatest ease: here, since they opened towards the heart, they could not support any column of blood.
In Chapter II. of his great book—the "De Motu"—his chief point is, "the charge of blood is expelled by force," that is, the heart is dynamic for the circulation, a point by no means admitted before his time, for M. R. Columbus denied the heart to be even muscular. Harvey is absolutely clear on this point; he writes:
It is in virtue of one and the same cause that all arteries of the body pulsate, namely, the contraction of the left ventricle.
Again in Chapter V. he writes:
The one action of the heart is the transmission of blood and its distribution by means of the arteries to the very extremities of the body.
In Chapter VI. he gives a remarkably good account of the circulation through the fetal heart, that is, before lungs are developed; it is surprisingly accurate to have been done three hundred years ago. In Chapter VII. he returns to the circulation through the lungs and clearly arrives by induction at the existence of capillaries; the word of course he does not use, he calls them "porosities of the flesh," but he understands perfectly that arteries do not become veins without undergoing some complete change in structure and nature. He says if arteries became veins, there would be a pulse in the veins, marvellously good physiology for 1628!
Owing to his having no microscope sufficiently powerful, Harvey could not see the capillaries even in those transparent animals which he
