The fibrous rings of the heart (anulus fibrosus cordis) are the dense fibrous rings of connective tissue which surround the right and left atrioventricular (tricuspid and mitral) orifices and arterial (aortic and pulmonary) orifices to which are attached the atrial and ventricular muscle fibers and the aortic and pulmonary arteries. As part of the fibrous skeleton of the heart, the fibrous rings also provide origin and insertion for the myocardium. The right and left fibrous rings of heart surround the atrioventricular and arterial orifices, and are decidedly stronger upon the left than on the right side of the heart. Physiologically, this is readily appreciated in light of the pressure differential between the right and left circuits. The right fibrous ring is known as the anulus fibrosus dexter cordis, and the left is known as the anulus fibrosus sinister cordis.
The fibrous rings surrounding the arterial orifices serve for the attachment of the great vessels and semilunar valves, they are known as The aortic annulus. The attachment of the artery to its fibrous ring is strengthened by the external coat and serous membrane externally, and by the endocardium internally. The middle coat of the artery in this situation is thin, and the vessel is dilated to form the sinuses of the aorta and pulmonary artery. The left atrioventricular ring is closely connected, by its right margin, with the aortic arterial ring; between these and the right atrioventricular ring is a triangular mass of fibrous tissue, the Fibrous trigone.
Transverse Section of the Heart Showing the Fibrous Rings Surrounding the Valves