- oughly by the experimental method. Furthermore, they
were often disconnected, and this lack of continuity obliged him to supply missing links at several points; in other words, nobody had as yet demonstrated the important fact that the blood travels regularly in an unbroken circuit, and it was to this great task that Harvey devoted himself at the period which we are now considering. He carried out all these investigations with the most painstaking care and made public announcement of his discoveries only after the lapse of an extraordinary length of time; his chief object being that ample opportunity might thereby be afforded for complete verification. The following are among the more important questions which he investigated and to which he furnished satisfactory solutions. He learned, for example, that the auricle and ventricle of each side of the heart do not contract simultaneously but in succession. When the right auricle contracts the blood which it then contains passes into the right ventricle; and when the right ventricle contracts the blood is driven into the pulmonary artery. From this vessel it passes ultimately into the pulmonary vein, and from the latter into the left auricle, which then contracts and drives the blood into the left ventricle. The latter chamber next contracts and forces the blood into the aorta, whence it is carried into all the arteries of the body. From these, in turn, it passes into the veins and thence back to the right auricle of the heart—the point from which it started. He corroborated the finding—by other anatomists who had preceded him—of membranous valves at the spots where the blood passes from one chamber to the other; and he compared these valves to little doors which open to permit the passage of the blood in one direction, but which close when there is any tendency for it to pass in the opposite direction. The valves of the right auricle, for example, allow the blood to pass into the right ventricle, but prevent it from returning into the auricle. Then, further, the valves of the right ventricle permit the blood to pass into the pulmonary artery, but prevent it from returning into the ventricle. The valves of the left auricle permit the blood
