part of heart annulus fibrosus annulus fibrosus cordis BTO:0003628 BrendaTissueOBO Lower's ring One of four fibrous rings that surround atrioventricular and arterial orifices of the heart, providing attachment for the valve leaflets and maintaining patency of the orifice. As part of the fibrous skeleton of the heart, the fibrous rings also provide origin and insertion for the myocardium. coronary tendon fibrous ring of heart