1) A small nodular mass of specialized muscle fibers located in the interatrial septum near the opening of the coronary sinus. It gives rise to the atrioventricular bundle of the conduction system of the heart.

2) A small mass of tissue that is situated in the wall of the right atrium adjacent to the septum between the atria, passes impulses received from the sinoatrial node to the ventricles by way of the bundle of His, and in some pathological states replaces the sinoatrial node as pacemaker of the heart.