The conceptions of the structures of the heart and vessels, of their actions, and of the motion of the blood in them, which Galen entertained, are not stated in a complete shape in any one of his numerous works. But a careful collation of the various passages in which these conceptions are expressed leaves no doubt upon my mind that Galen's views respecting the structure of the organs concerned were, for the most part, as accurate as the means of anatomical analysis at his command permitted; and that he had exact and consistent, though by no means equally just, notions of the actions of these organs and of the movements of the blood. Starting from the fundamental facts established by Erasistratus respecting the structure of the heart and the working of its valves, Galen's great service was the proof, by the only evidence which could possess demonstrative value; namely, by that derived from experiments upon living animals, that the arteries are as much full of blood during life as the veins are, and that the left cavity of the heart, like the right, is also filled with blood. Galen, moreover, correctly asserted--though the means of investigation at his disposition did not allow him to prove the fact--that the ramifications of the vena arteriosa in the substance of the lungs communicate with those of the arteria venosa, by direct, though invisible, passages, which he terms anastomoses; and that, by means of these communications, a certain portion of the blood of the right ventricle of the heart passes through the lungs into the left ventricle. In fact, Galen is quite clear as to the existence of a current of blood through the lungs, though not of such a current as we now know traverses them. For, while he believed that a part of the blood of the right ventricle passes through the lungs, and even, as I shall show, described at length the mechanical arrangements by which he supposes this passage to be effected, he considered that the greater part of the blood in the right ventricle passes directly, through certain pores in the septum, into the left ventricle. And this was where Galen got upon his wrong track, without which divergence a man of his scientific insight must infallibly have discovered the true character of the pulmonary current, and not improbably have been led to anticipate Harvey. The best evidence of the state of knowledge respecting the motions of the heart and blood in Harvey's time is afforded by those works of his contemporaries which immediately preceded the publication of the _Exercitatio Anatomica_, in 1628. And none can be more fitly cited for this purpose than the _de Humani Corporis Fabrica_, Book X, of Adrian van den Spieghel, who, like Harvey, was a pupil of Fabricius of Aquapendente, and was of such distinguished ability and learning that he succeeded his master in the chair of anatomy of Padua.