The left atrioventricular orifice (left atrioventricular opening or mitral orifice) is placed below and to the left of the aortic orifice. It is a little smaller than the corresponding aperture of the opposite side. It is surrounded by a dense fibrous ring, covered by the lining membrane of the heart, and is reguarded as the bicuspid or mitral valve. References External links Cardiac anatomy