Our Egyptian and our Greek predecessors seem to have believed that they had attained to absolute and final knowledge on these subjects. While we smile at their error, let us be humble in estimating our own position and ever remember that we ourselves may be yet barely on the threshold. Our father, Harvey, has exhorted us ever to search and to study out the secrets of nature by the way of experiment. Will you pardon me if I devote the remainder of this paper to an account of a humble attempt to carry out his mandate, if I narrate briefly an experiment dealing with a yet unsolved problem in the pathology of the circulation, to which I have devoted twenty-five years of my life? I may plead the usage of speakers and writers who follow a tale or narrative by a moral or practical application, and perhaps I may also be allowed to say that the discovery that ancient Egyptian physicians advocated rest in certain forms of heart disease suggested to me the propriety of supporting this doctrine by a brief narration of my own experience in the same direction. As the Egyptians were probably ignorant as to the action of the valves of the heart, they can only have known the fact that rest was beneficial, but not the reason. Valvular defect is one of the most important and perhaps the most common of circulatory diseases. It is one which probably we shall never be able to cure, and is thus likely to remain one of the opprobia of medicine. Is it possible to treat it by prevention? This is the problem upon which I wish to speak a few words. I am the more encouraged to do this because I know that various Fellows and Members of this College hold similar views to those which I desire to unfold. Joints recover: Why does the Endocardium fail to do so?