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 sound, if it have two constituents, or the whole sound, if there be no distinguishable valvular constituent—observers differ—is just the noise of a distant cab (_bruit du cab_) or the waves on a far-off beach; it is the sound which the ear picks up from any irregular mixture of tones which it cannot analyse. It is the resonance-tone of the ear. That the membranous valves play the leading part in producing the first sound cannot be doubted, whether by their first closure or by their subsequent vibration. We should be inclined to attribute to them the whole performance, were it not that the first sound, or at any rate a sound, is heard during the beating of a bloodless heart. If an animal be killed and the heart removed from its thorax with the utmost despatch, it will beat for about a minute while lying in the palm of one’s hand. When a stethoscope is applied to the ventricle, a “first sound” is heard. This was described as a muscular sound, owing to a misconception. It is similar to the sound which is heard when a stethoscope rests upon a contracting biceps. Until recently the voluntary contraction of a muscle was believed to be vibratory—a tetanus. The sound corresponds to a rate of about thirty-six vibrations to the second. There being reasons for thinking that muscle contracting naturally does not vibrate as fast as this, the sound was interpreted as the first overtone of the muscle-note. Muscle was said to vibrate eighteen times a second. The similarity of the first sound of the heart and the ordinary muscle-sound led physiologists to infer that the contraction of the heart also was a tetanus. But this was a mistake. Neither voluntary muscular action nor the contraction of the heart is an interrupted contraction in this sense. In the case of the musculature of the heart especially, contraction is a steady shrinking, followed by a steady relaxation. The sound produced by the bloodless heart is due to the various displacements which occur when it contracts. Its interior is very irregular, with its columns, papillary muscles, tendinous cords, valves. The displacement of these various structures is responsible for the noise.