The epicardial coronary artery system consists of the left and right coronary arteries, which normally arise from ostia located in the left and right sinuses of Valsalva, respectively. In about 50 per cent of humans a "third coronary artery" ("conus artery") arises from a separate ostium in the right sinus. The left main (LM) coronary artery ranges in length from 1 to 25mm before bifurcating into the left anterior descending (LAD) and left circumflex (LC) branches. The LAD coronary artery measures from 10 to 13 cm in length, whereas the usual nondominant LC artery measures about 6 to 8 cm in length. The dominant right coronary artery (RCA) is about 12 to 14 cm in length before giving rise to the posterior descending artery (PDA). Portions of the epicardial coronary arteries may dip into the myocardial ("mural artery" or "tunneled artery") and be covered for a variable length (1 to several millimeters) by ventricular muscle ("myocardial bridge"). The branches of the LAD artery, in their usual order of origin, are the first diagonal, the first septal perforator, other septal perforators, and other diagonal branches. Diagonal branches course laterally over the free wall of the left ventricle in the angle between the LAD and the LC. Septal branches, which leave the LAD artery at a right angle plunge deeply into the ventricular septum. The branches of the LC are variable but may include the sinus node artery (40 to 50 per cent), the left atrial circumflex branch, the anterolateral obtuse marginal, the distal circumflex, one or more obtuse marginals, and the PDA (10 to 15 per cent). The branches of the RCA include the conus artery (which may originate from a separate ostia in the right coronary sinus in 40 to 50 per cent of hearts) to the right ventricular outflow area, the artery to the sinus node (50 to 60 per cent), several anterior right ventricular branches, right atrial branches, the acute marginal branch, the artery to the AV node and proximal bundle branches, the PDA, and terminal branches to the left ventricle and left atrium.