The musculi pectinati of the atria are internal muscular ridges generally on the anterolateral surface of the chambers. They are only present in the area derived from the embryological true atrium.
In the right atrium, this is the area lateral to the crista terminalis. They run roughly perpendicular to the crista terminalis in an anteroinferior direction and converge on the superior margin of the atrioventricular orifice. The musculi pectinati extend into the right atrial auricle.
In the left atrium, the musculi pectinati are less pronounced than in the right atrium. They are mainly sited on the inside of the auricle.