Interventricular septum (or ventricular septum, or during development septum inferius), abbreviated IVS, is the stout wall separating the lower chambers (the ventricle

Ventricle (heart)

In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

s) of the heart

Heart

The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 from one another.

The ventricular septum is directed obliquely backward and to the right, and is curved with the convexity toward the right ventricle

Right ventricle

The right ventricle is one of four chambers in the human heart. It receives deoxygenated blood from the right atrium via the tricuspid valve, and pumps it into the pulmonary artery via the pulmonary valve and pulmonary trunk....

: its margins correspond with the anterior and posterior longitudinal sulci.

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
  Aortic vestibule
  The portion of the left ventricle immediately inferior to the aortic orifice is termed the aortic vestibule, and possesses fibrous instead of muscular walls....
  
   from the lower part of the right atrium
  Right atrium
  The right atrium is one of four chambers in the hearts of mammals and archosaurs...
  
  and upper part of the right ventricle, is thin and fibrous, and is termed the membranous ventricular septum (septum membranaceum).

Development

• The muscular part of the interventricular septum derives from the bulboventricular flange which is developed due to differential growth of primitive ventricle and bulbous cordis. Membranous part has a neural crest origin which connects the upper free margin of the bulboventricular flange and anterior and posterior endocardial cussions of atrio ventricular valve. it also gets attached to lower border of spiral septum or the aortico pulmonary septum.