Cæsalpinus, consequently, tried by a very moderately searching criticism, presents himself to us as but very little farther advanced than the ancients in his ideas on the motion of the blood.--The interpretation which successive generations of men give to a passage in a writer, some century or two old, is very apt to be in consonance with the state of knowledge at the time, in harmony with the prevailing ideas of the day, and, doubtless, often differs signally from the meaning that was in the mind of the man who composed it. The world saw nothing of the circulation of the blood in Servetus, Columbus, Cæsalpinus, or--Shakespeare, until after William Harvey had taught and written. The truth is, that some of the foremost grounds of Harvey’s claims to rank as a discoverer are very commonly overlooked. We always associate his name and fame with the development of the ultimate fact of the circulation of the blood. But Harvey, as a step to this conclusion, first demonstrated the heart as the means by which the circulation was effected; and he farther showed that there was but _one_ kind of blood, common to both the arteries and the veins. Up to his time the heart was regarded as the passive cistern of the blood, and the elaboratory of the vital spirits; it was not known as the moving instrument in any efflux or reflux of the blood, or even of any lesser circulation that had been previously asserted or conjectured. The moving power was still the respiratory act. Harvey may be said to have first broached, as he also essentially completed the physiology of the heart’s actions. The circular motion of the blood followed as a necessary corollary from these. The “motion of the heart” has even precedence in the title of his immortal work; the chapter in which he first enters properly on his subject (Chap. 2), is devoted to its consideration. And then, no physiologist up to Harvey’s time had questioned the existence of two kinds of blood, one appropriate to each order of vessels, and answering different ends in the economy.