Intervenous tubercle of Lower: true tubercle or normal interatrial fold?

Richard Lower, in 1669, first described the tubercle that now bears his name, calling it the intervenous tubercle located between the fossa ovalis and the superior vena cava. The aim of the study was to confirm the existence of the tubercle as described initially by Lower, adding details of its location, dimensions, and prevalence. We examined 100 formalin-fixed human hearts. In no heart did we find any discrete tubercle or elevation of the right atrial wall superior to the superior limbus (rim) of the fossa ovalis. In addition, we could find no morphometric differences in the thickness of the area superior to the superior limbus of the fossa. Dissections revealed that very little of the extensive musculature can be removed without opening the right atrial wall and arriving outside the heart. This is the essential criterion in distinguishing folds from "true" septal structures. When viewed in this light, it is only the flap valve of the fossa ovalis, and its immediate muscular infero-anterior rim, the so-called lower limbus, that can be removed so as to create communications between the cavities of the atrial chambers, and not exiting at the same time from the cavities of the heart. This is because the larger part of the muscular borders of the fossa ovalis is no more than an infolding of the atrial walls, which incorporates extracardiac adipose tissue within the fold. Although this process of folding unequivocally produces an intracardiac buttress, namely, the limbuses (rims) of the fossa, the buttress, being an infolding, does not constitute, according to our definition, a true septum. On this basis, we suggest that it is the superior limbus of the fossa ovalis, or the superior interatrial fold, that previously has been considered to represent the intervenous tubercle of Lower.