Serial dissections to display the atrial myoarchitecture in a normal human heart. (a) and (b) Anterior and superior views to show the Bachmann bundle crossing the anterior interatrial groove and branching toward the left atrial appendage and note also the longitudinal fibers of the septopulmonary bundle, which arises from the interatrial groove underneath Bachmann’s bundle, fanning out to line the pulmonary veins and to pass longitudinally over the dome and in the posterior wall of the left atrium. (c) On the posterior wall, the septopulmonary bundle often crossing the circumferential myocytes coming from the lateral wall which show an abrupt change of fiber orientation in the posterior wall of the left atrium. (d) Note the highly variable anatomic orientation of the myocardial fibers making up the sleeves. The myocytes sleeves are mainly composed of circularly orientated bundles; oblique and longitudinally oriented fibers are also common. SCV = superior cava vein, ICV = inferior cava vein, RAA = right atrial appendage, LAA = left atrial appendage, LI = left inferior pulmonary vein, LLR = left lateral ridge, LS = left superior pulmonary vein, RI = right inferior pulmonary vein, RS = right superior pulmonary vein, and CS = coronary sinus.
