The cardiac skeleton is a collection of dense, fibrous connective tissue in the form of four rings with interconnecting areas in a plane between the atria and the ventricles. The four rings of the cardiac skeleton surround the two atrioventricular orifices, the aortic orifice and opening of the pulmonary trunks. They are the anulus fibrosus. The interconnecting areas include: the right fibrous trigone, which is a thickened area of connective tissue between the aortic ring and right atrioventricular ring; and the left fibrous trigone, which is a thickened area of connective tissue between the aortic ring and the left atrioventricular ring (Fig. 3.76).