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.