First sound. The chief cause of the first sound of the heart appears to be the vibration of the auriculo-ventricular valve, and also, but to a less extent, of the ventricular walls, and the coats of the aorta and pulmonary artery, all of which parts are suddenly put into a state of tension at the moment of ventricular contraction. The second sound is more complete than that of the first. It is probably due entirely to the sudden closure and consequent vibration of the semilunar valves when they are pressed down across the orifice of the aorta and pulmonary artery. Pulse. The heart of a healthy adult man in the middle period of life acts from seventy to seventy-five times per minute. The frequency of the heart's action gradually diminishes from the commencement to near the end of life. In persons of sanguine temperament, the heart acts somewhat more frequently than in those of the phlegmatic; and in the female sex more frequently than in the male; in children, more frequently still. Capacity. The capacity of the two ventricles is probably exactly the same. From the mean of various estimates taken, it may be inferred that each ventricle is able to contain on an average about three ounces of blood, the whole of which is impelled into the respective arteries at each contraction. Every time the ventricles contract three ounces of blood are pumped out of the heart into the lungs and heart respectively. Calculating seventy pulses per minute, the quantity of blood passing through the heart would be about 211 ounces, or 14 1/4 pints per minute; or 895 pints per hour, or 21,480 pints in 24 hours. Velocity. The velocity of the stream of blood is greater in the arteries than in any other part of the circulatory system, and in them it is greatest in the neighborhood of the heart and during the ventricular systole; the rate of movement diminishes during the diastole of the ventricles, and in the parts of the arterial system most distant from the heart. The rate is calculated to be about from 10 to 12 inches per second in the large arteries near the heart. THE BLOOD.