Final answer:
The membranous material between the two lateral ventricles is called the interventricular septum, a thick structure distinguishing the two heart chambers.
Step-by-step explanation:
The membranous material located between the two lateral ventricles is known as the interventricular septum. Unlike the interatrial septum, the interventricular septum is normally intact after its formation during fetal development and is considerably thicker. This is because the ventricles produce much greater pressure when contracting. The interventricular septum does not consist of serous membrane like those lining the body's cavities but is instead a thick muscular and membranous wall separating the two ventricles of the heart.