AbstractOne of the most common congenital abnormalities of the human heart is a defect in the development of the membranous part of the ventricular septum which, in this study, is designated the MVS. The WS is a thin fibrous membrane, about 1 cm long, which extends upward and to the right from the muscular ventricular septum to the adjacent part of the aortic fibrous annulus that also gives attachment to the right posterior (noncoronary) and anterior (right coronary) aortic valve cusps. It is of considerable clinical importance that there lies between the muscular ventricular septum and the MVS the atrioventricular (AV) bundle of the cardiac conduction system. The MVS has an irregular quadrangular form and has right and left surfaces. This study is based on macroscopic and histological sections of more than 30 normal and abnormal hearts. © 1994 Wiley‐Liss, Inc.