Those who to-day are so familiar with the course of the circulation of the blood through the arteries and veins find it difficult to understand how the recognition of this phenomenon could have been so long delayed; it seems so simple, yet to the ancients it was perfectly incompre-hensible! Although every one had recognized that blood would flow from an incision, few stopped to reason thereupon. From time immemorial it had been supposed that the veins had their origin in the liver, and were the only vessels which contained blood, since the arteries were always found empty after death; the latter were held to contain only air or spirit. The circulation was supposed to leave and return to the liver through the venous canals by undulating movements similar to those of the waves of the ocean; and this was the doctrine of the Asclepiadæ,{155} and probably of Erasistratus. Galen modified this view by showing that the arteries contained blood; he knew it was poured into the right cavities of the heart by the great veins, but he believed that only a small quantity passed from the right ventricle into the lungs, and that the major portion reached the left ventricle by passing through pores in the inner ventricular septum. This opinion was uncontested until the middle of the sixteenth century. Then the theologian, Michael Servetus, who, in 1553, perished as the victim of Calvin's jealousy, denied the passage of the blood through this septum, contending that it was returned from the lungs to the left side of the heart by the pulmonary veins. This was a happy thought, and a great step toward the truth. Soon after Columbus demonstrated anatomically that the conjecture of Servetus was plausible, by showing the function and real use of the valves of the heart. Cesalpinus came still nearer to the truth, and explained, as did Columbus, the course of the circulation through the lungs, but he opined that blood and vital spirits passed from the arteries into the veins during sleep, because at that time there was swelling of the latter and diminution of the pulse. Valves in the veins were known, and it had been shown that ligature of an artery in the living animal stopped the flow below it, while if a vein were tied there was shrinkage above the ligature, and swelling below it. Such was the state of science at the beginning of the seventeenth century; there remained, practically, but one step to take,--to find the true course of the blood. [Illustration: 0176]