Interventricular septum: The stout wall separating the lower chambers (the ventricles) of the heart from one another. A hole in the interventricular septum is termed a ventricular septal defect (VSD).
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Note added at 5 mins (2007-03-10 14:22:26 GMT)
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http://en.wikipedia.org/wiki/Interventricular_septum
Interventricular septum (or ventricular septum, or during development septum inferius) is the stout wall separating the lower chambers (the ventricles) of the heart from one another.

The ventricular septum is directed obliquely backward and to the right, and is curved with the convexity toward the right ventricle: its margins correspond with the anterior and posterior longitudinal sulci.

[edit] Portions

* The greater portion of it is thick and muscular and constitutes the muscular ventricular septum.

* Its upper and posterior part, which separates the aortic vestibule from the lower part of the right atrium and upper part of the right ventricle, is thin and fibrous, and is termed the membranous ventricular septum (septum membranaceum).