Membranous part

The membranous part of the interventricular septum is a small, thin, fibrous area located at the dorsal aspect of the interventricular septum, immediately ventral to the aortic valve and adjacent to the septal leaflet of the tricuspid valve. It is distinguished from the muscular septum by its lack of myocardial tissue and is composed primarily of dense connective tissue, including type I collagen fibers arranged in transverse bundles, with elastic fibers present at the transition to muscle tissue.

The membranous septum is subdivided into two components: the atrioventricular membranous septum, which lies between the right atrium and left ventricle situated dorsal to the base of Cuspis septalis of Valva atrioventricularis dextra, and the interventricular membranous septum, which separates the right and left ventricles.

The membranous part of the interventricular septum is typically very small, with a mean length of less than 1 mm in human adult hearts, well visible in cats and may be absent in some species like adult ox.

In the dog the membranous part can be seen by transmitted light under the septal cusp of the right atrio- ventricular valve adjacent to the origin of the aorta. When the foramen fails to close, a subaortic defect or interventricular foramen is left.