Left fibrous ring

The left fibrous ring (annulus fibrosus sinister) of the canine heart is the dense connective tissue structure that surrounds the left atrioventricular (mitral) valve orifice. It forms part of the fibrous skeleton of the heart and provides firm support for the attachment of the mitral valve leaflets, helping maintain the shape and competency of the valve during the cardiac cycle. This ring also serves as an anchoring site for adjacent myocardial fibers and contributes to the mechanical stability of the left side of the heart, while participating in the electrical insulation between atrial and ventricular myocardium.