[TA] part of the fibrous skeleton of the heart that is seen as a small, thin, round or oval nonmuscular area at the superior end of the interventricular septum; it lies just below and is continuous with the portion of the fibrous ring of the aortic valve supporting the anterior and posterior cusps and also with the right fibrous trigone; the atrioventricular bundle of conducting tissue courses along its dorsal margin and bifurcates at its inferior margin into the right and left crura. SYN: pars membranacea septi interventricularis [TA], membranous septum (2), septum membranaceum ventriculorum.