PERIPHERAL emboli usually arise in the heart as a result of underlying cardiac pathology. They occur in rheumatic heart disease (rheumatic mitral stenosis almost exclusively) following coronary occlusion, in arteriosclerotic heart disease with auricular fibrillation, and as a result of bacterial endocarditis. Rarely, a paradoxical embolus of venous origin may pass an abnormal major venous-arterial shunt and lodge peripherally. The advent of the operation of mitral commissurotomy for mitral stenosis, with its attendant intracardiac manipulation, has introduced another major cause for arterial embolization. In many cases of mitral stenosis there are thrombi filling the left auricular appendage through which the finger is placed to correct the valvular lesion. These thrombi may be old, firm, fibrotic, and adherent, in which case the danger of embolization is minimal, or they may be fresh, soft, and poorly attached with a marked tendency to break off and lodge somewhere in the peripheral arterial tree.