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 trigonum fibrosum, which represents the os cordis seen in the heart of some of the larger animals, as the ox and elephant.
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.
Each ring receives, by its ventricular margin, the attachment of some of the muscular fibers of the ventricles; its opposite margin presents three deep semicircular notches, to which the middle coat of the artery is firmly fixed.
The attachment of the artery to its fibrous ring is strengthened by the external coat and serous membrane externally, and by the endocardium internally.
From the margins of the semicircular notches the fibrous structure of the ring is continued into the segments of the valves.
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.