The valve “balloons” over the blood in the ventricle before the contraction of the auricle has ceased. The thin margins of its flaps come together with great rapidity. The tendinous cords holding their edges on the ventricular side, they meet, not edge to edge, but folded flap to folded flap. (3) The valve does not bulge into the auricle. On the contrary, at the height of systole it is pulled into the ventricle by the contracting musculi papillares. As the ring to which the valve is attached is diminished in size, by the contraction of the base of the heart, which continues, it will be remembered, until after the apex has begun to relax, the edges of the flaps are folded farther and still farther over by the pull of the musculi papillares, and the blood is squeezed out from between the wall of the ventricle and the indrawn valve. The “semilunar valves,” which close the apertures into the aorta and pulmonary artery, have each three flaps. The aortic semilunar valve, which has the higher pressure to bear, shows its characteristic features in a rather more marked degree than the other. Each of its three flaps is a half-cup. At the centre of the margin of the half-cup is a small fibrous nodule. The edge of the cup on either side of this is very thin. Fine elastic fibres radiate from the nodule to all parts of the flap. The wall of the aorta shows three bays, or “sinuses,” one behind each flap. Hence, when the valve is forced by the rise of pressure in the ventricle, the flap is not flattened back against the wall of the aorta. There is always a certain amount of backwash in the pocket behind it. The instant the pressure in the ventricle begins to fall, the three flaps come together with a click, so smart as to be plainly audible over most of the front of the chest. The click is the “second sound” of the heart. The auriculo-ventricular valves also make a sound when they close; but this “first sound of the heart” has a different character. It is prolonged, soft, low-pitched. It is customary to represent the sounds by the syllables “lūbb dŭp—lūbb dŭp,” the pause during diastole being of about the same length as the sounds when the heart is beating with its normal rhythm. The duration of systole is little affected by variations in the rate of beat. It is diastole that is shortened or prolonged. The second sound is due entirely to the closure of the semilunar valves. It is heard most clearly when the stethoscope is placed over the region where the aorta comes nearest to the wall of the chest—at the second rib cartilage on the right side of the breast-bone. The first sound is loudest near the apex of the heart. It is generally agreed that it is not wholly due to the closure of the auriculo-ventricular valves, but possesses a second constituent. Some persons assert that they can with the ear distinguish the clearer valvular sound at the commencement from the general rumble which overtakes it. The main part of the