It corresponds to the anterior and inferior interventricular grooves. Its right side is convex and bulges to the right ventricle. The septum has a posterior border which separates the right and left AV orifices and is continuous with the interatrial septum. The greater part of the septum is thick and muscular but a small area near the posterior margin is membranous. The septal cusp of the tricuspid valve is attached vertically on  this part of the septum and divides it into an anterior part  that separates the right and left ventricles and a posterior part which separates the left ventricle from the right atrium. The latter part is therefore, referred to as the atrioventricular septum.