When the blood supplied for the various processes which are carried on in the living body has undergone a certain degree of change, it requires to be purified by the act of respiration. For this purpose it is urged onwards by fresh blood from behind into the veins; and returning in them from all parts of the body, enters a cavity of the heart called the _right auricle_. At the same time the purified blood returning from the lungs by the pulmonary veins, passes into the _left auricle_. When these two cavities, which are distinct from each other, are sufficiently dilated, they contract, and force the blood which they contain into two other much more muscular cavities called respectively the right and left _ventricle_, all retrogression into the auricles being prevented by valves, which admit of a passage in one direction only. The ventricles then contract in their turn with great force, and at the same instant; and propel their blood, the right, by the pulmonary artery into the lungs; the left, which is much the stronger of the two, into all parts of the body, by the great artery called the _aörta_, and its branches; all return being prevented as before by valves situated at the orifices of those vessels, which are closed most accurately when the ventricles relax, by the backward pressure of the blood arising from the elasticity of the arteries. Thus the purified blood passes from the lungs by the pulmonary veins through the left auricle into the ventricle of the same side, by which it is distributed into all parts of the body, driving the vitiated blood before it; and the vitiated blood is pushed into and along the veins to the right auricle, and thence is sent into the right ventricle, which propels it by the pulmonary artery through the lungs. In this manner a double circulation is kept up by the sole agency of the heart, through the lungs, and through the body; the contractions of the auricles and ventricles taking place alternately. To prevent any backward motion of the blood in the superficial veins, which might happen from their liability to external pressure, they are also provided with simple and very complete valves which admit of a passage only towards the heart. They were first remarked by Fabricius ab Aquapendente, and exhibited in his lectures to Harvey among the rest of his pupils; but their function remained a mystery till it was explained by the discovery of the circulation. It is related by Boyle, upon Harvey’s own authority, that the first idea of this comprehensive principle suggested itself to him when considering the structure of these valves.