Williams that we owe the first direct experi ments in support of it. In one experiment he ascertained that the second sound was louder over the origin of the large arteries than over the surface of the ventricles, while it was the reverse with the first sound; that pressure upon the origin of the aorta and pulmonary artery suspended the second sound ; and that the second sound disappeared after the auricles bad been laid open, although the first conti nued. In a second experiment* we find the fol lowing observations stated :—" Observation 6. A common dissecting hook was passed into the pulmonary artery, and was made to draw back and thus prevent the closure of the semi lunar valves ; the second sound was evidently weakened and a hissing murmur accompanied it. A shoemaker's curved awl was then passed into the aorta so as to act in the same way on the aortic valves. The second sound now entirely ceased and was replaced by a hissing. Observation 7. The hook and the awl were withdrawn ; the second sound returned and the hissing ceased. Observation 8. The experi ment 6th was repeated with the same result, and whilst Dr. Hope listened I withdrew the awl from the aorta. He immediately said, Now I hear the second sound.' I then removed the hook from the pulmonary artery ; Dr. Dope said, Now the second sound is stronger and the murmur has ceased.'" The Dublin Committee have repeated and con firmed these experiments of Dr. Williams. In
their experiments one of the valves in each artery was transfixed and confined to the side of the vessel by a needle, and the second sound disappeared; on withdrawing the needles they re-appeared.
As the second sound thus appears to be pro duced by the shock of the blood upon the semi lunar voices, its intensity must, in a great measure, depend upon the diastole of the ven tricle drawing part of the blood back upon them, but perhaps more particularly upon the elasticity of the large arteries returning suddenly upon their contents during the diastole of the ventricles, when the distending force of the ventricles has been withdrawn. We would therefore expect that the second sound should be louder in those whose aorta retains its elas ticity, than in those (a circumstance sufficiently common in old age) in whom, from a morbid alteration of the structure of its coats, the elasticity is either lost or greatly diminished. This is an observation which, as far as I know, has not yet been verified ; but my friend Dr. W. Henderson informs me that he is positive from numerous observations that the second sound is louder in young than in older persons ; but whether this is in the exact ratio of the change upon the elasticity of the coats of the large vessels he is not at present prepared to say.