The right auricle communicates on the one hand with the veins of the general system and on the other with the right ventricle. The valvular curtain between the right auricle and the right ventricle is named the tricuspid; by it the auricle is guarded from the ventricle. The ventricle leads directly into the pulmonary artery and this in turn into the lungs. The pulmonary artery is guarded by three semilunar valves. The left auricle again communicates on the one hand with the pulmonary vein and on the other with the left ventricle, which is guarded by the mitral or bicuspid valve. The left ventricle leads directly into the aorta, which is also guarded by three semilunar valves. The aorta is a large artery which conveys the blood to the general system. The arrangement of the heart's valves is such that the blood can pass only in one definite direction, and this is--from the right auricle the blood passes into the right ventricle, and thence into the pulmonary artery, by which it is conveyed to the capillaries of the lungs. From the lungs, the blood, which is now purified and altered in color, is gathered by the pulmonary veins and taken to the left auricle. From the left auricle it passes into the left ventricle, and thence into the aorta, by which it is distributed to the capillaries in every portion of the body. The Heart's action. The heart's action in propelling the blood consists in the successive alternate contractions and dilatations of the muscular walls of the two auricles and ventricles. The auricles contract simultaneously; so do the ventricles; their dilatations also are severally simultaneous; and the contractions of the one pair of cavities are synchronous with the dilatations of the other. Valves--Bi and Tricuspid. During auricular contraction the force of the blood propelled into the ventricle is transmitted in all directions, but being insufficient to raise the semilunar valves, it is expended in distending the ventricle and in raising and gradually closing the auriculo-ventricular valves (tricuspid and bicuspid valves). These when the ventricle is full form a complete septum (partition) between it and the auricle. The arterial or semilunar valves are brought into action by the pressure of the arterial blood forced back towards the ventricles, when the elastic walls of the arteries recoil after being dilated by the blood propelled into them in the previous contraction of the ventricle. The sounds. When the ear is placed over the region of the heart two sounds may be heard at every beat of the heart, which follow in quick succession, and are succeeded by a pause or a period of silence. The first sound is dull and prolonged; its commencement coincides with the impulse of the heart and just precedes the pulse at the wrist. The second is a shorter and sharper sound, with a somewhat flapping character, and follows close after the arterial pulse.