An incision has been made in the epicardium (6) bordering the right atrium. The superior vena cava and upper part of the right atrium have been reflected anteriorly and to the left to display the plane of cleavage between the myocardial layers of the right and left atria. In the upper part of the area of contact of the atria there is no interconnection of the two myocardial layers. In the lower part (9), which overlies the point of entrance of the lower right pulmonary vein into the left atrium, there is considerable interweaving of muscle fibers of the two atria. The proximity of this cleavage plane to the fossa ovalis can be readily visualized, although the fossa ovalis is partially obscured in this view.