the blood from the body and pumps it to the lungs, is the right ventricle; and that on the left side, which receives the blood from the lungs and pumps it to the body, is the left ventricle. In addition to the ventricles, which are the active pumps, each side of the heart has an additional chamber known as the _auricle_, whose purpose is to serve as a reservoir into which blood can flow during the time that the ventricles are emptying themselves. If it were not for the auricles, the movement of blood into the heart would have to stop with every beat, because while the ventricles are contracting the intake valves are closed and there would be no place to which blood could flow, but since each side has its auricle, the flow of blood goes on during the beat of the ventricles, the auricles filling up. The intake valve, in order to operate properly, should be located between the auricle and ventricle, and this is where it is. The vein opens directly into the auricle without any valve between; the auricle opens into the ventricle with the intake valve at the point of junction. The intake valves are given rather formidable names; they are sometimes spoken of as the _auriculo-ventricular valves_; that on the right side of the heart between the right auricle and the right ventricle is composed of three flaps of membrane, and has therefore been named the _tricuspid valve_. The intake valve on the left side of the heart, which is composed of but two flaps, is known as the _mitral valve_. As soon as the beat of the ventricles is over and the ventricular muscle relaxes, the blood which has accumulated in the auricles presses the intake valve open and blood begins to flow through it directly into the ventricle. Both the intake and outflow valves are composed of stout but thin sheets of membrane, so that very little pressure is required to operate them. The weight of the blood that is accumulated in the auricles during the beat of the ventricle is more than sufficient to force the valve open and allow the blood to flow on through into the ventricle. In a heart that is beating seventy-two times a minute, there cannot be much time occupied either in filling or emptying. As a matter of fact both these intervals are measured in tenths of seconds. If we take a heart that is beating at the average rate of seventy-two times a minute the whole of a single beat amounts to eight-tenths of a second. The beat of the ventricle takes about three-tenths of a second or three-eighths of the whole time; the period of relaxation of the ventricle, during which it is filling with blood through the open intake valve, is about five-tenths of a second or five-eighths of the whole time. The movement of blood is rapid enough so that this five-tenths of a second allows the ventricle to fill; in fact much less time than this is required, for in a heart that is beating at twice the average rate, the ventricle still fills with blood between beats.