Examine the interventricular septum [cross-section]. It is composed of two regions. The majority of the septum is made up of a thick muscular portion [at the level located between the fingers]. However, just inferior to the cusps of the aortic valve [at the level located between the fingers] [region located at the tip of the middle probe], the interventricular septum consists of a thin smooth membranous portion. Failure of the membranous portion of the septum to close leads to a common congenital cardiac malformation called ventricular septal defect (VSD).
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