Pectinate muscles

Pectinate muscles of the left atrium are muscular ridges found primarily within the left auricle (left atrial appendage), forming a complex network that gives the left auricle its characteristic trabeculated, or rough, endocardial surface. In contrast, the main body of the left atrium is largely smooth-walled, with pectinate muscles confined to the appendage and not extending into the atrial body proper.