Junction of left atrium and pulmonary veins

The common pattern of the superficial myocardial fibers of the left atrium (posterior aspect). A main circular fascicle (a, a', a", and a"') runs peripherally around the area of the openings of the pulmonary veins. An interatrial fascicle (b) runs between the right (RA) and the left (LA) atrium. Some fibers (c) descend from the left atrium into the left part (a') of the main circular fascicle. Circular fibers leaving the main fascicle turn around the openings of the pulmonary veins, forming sphincter-like structures; other fibers extend over the veins as myocardial sleeves. Loops of fibers coming from the atrium are seen over the right superior pulmonary vein (RSPV) and returning to the atrium. Oblique, vertical (e), and transverse (f, f') fascicles of fibers are also seen on the posterior atrial surface.