When in the closed position, the orifice is obscured by the anterior and posterior leaflets. The anterior comprises about one half to two thirds of this area, the posterior comprises about one third to one half of this area. The point of attachment of the anterior leaflet comprises one third of the circumference of the annulus (the fibrous area) and the posterior leaflets comprises two thirds of the circumference of the annulus (the muscular area). The fibrous support of the anterior leaflet is fixed, the muscular support of the posterior leaflet can enlarge and does so when.
