Innumerable hypotheses have been proposed to explain the mechanism of an attack of angina pectoris. Huchard1collected about eighty such hypotheses in his monograph. It is not within the scope of this paper to enumerate or to discuss the great majority of these. However, the two that seem to be commanding most attention at the present time merit a brief outline. 1. The "coronary hypothesis," stated first by Parry,2holds that the attack of angina pectoris is dependent on a temporary interference with the blood supply of a part of the heart muscle. This hypothesis is based on the following points: (a) the frequency with which advanced coronary disease is found post mortem in patients who have died from angina pectoris; (b) the similarity of the syndromes of angina pectoris and coronary occlusion in regard to the character and location of the pain; (c) the adequate explanation of the mechanism