Abstract
A study of the hearts of 107 dogs revealed that the blood supply to the sinoatrial node was from three sources: right coronary artery, left coronary artery, and right internal mammary artery. A dominant pattern of supply was found in 90 hearts. Three variations are described. The importance of arterial anastomoses to the sinus node and their role in the formation of an extracardiac coronary arterial system are discussed.