The interventricular septum separates the left ventricle from the right ventricle. The interventricular septum is slanted backwards and to the right, and it also curves to the right, which completes the oval of the thick ventricle and encroaches upon the chamber of the right ventricle. The greater part of it is thick and muscular. The upper portion, which separates the aortic vestibule from the lower part of the right atrium and the upper part of the right ventricle, is much thinner and is fibrous.