The left atrium is a thin walled cavity. Most of the wall is smooth. Musculi pectinati are present only in the auricle of the atrium. The interatrial septum separates the cavity of the right atrium from the left atrium. There may be a depression corresponding to the fossa ovalis in the septum. The four pulmonary veins, two rights and two left from the lungs open into the upper lateral part of the left atrium. The left atrioventricular orifice is situated anteroinferiorly and is guarded by the mitral orifice.