Left panel. Lateral view of the atria showing the location of the sinus node (fusiform green colour), and the arrangement in the left atrium of the 4 pulmonary veins.

Middle panel. Anterior view a normal heart photographed in attitudinally appropriate position. As can be seen, the right atrium lies anterior to its alleged left-sided counterpart. Note the location of the transverse sinus (white broken line).

Right panel. External appearance of the right and left atriums viewed from superior view. Note the relationship to the aorta and pulmonary trunk (PT). Ao= Aorta; LAA = left atrial appendage; LSPV = left superior pulmonaiy vein; PT= Pulmonary trunk;  RAA= right atrial appendage; RIPV = right inferior pulmonary vein; RSPV = right superior pulmonary vein; SCV = superior caval vein

Left panel. Specimen view from the tilted right superior perspective, to show the course of the terminal sulcus or groove between the superior vena cava and the right atrium and appendage. Note the close relationship of the right pulmonary veins to the posterior interatrial sulcus.

Middle panel. Epicardial visualization of the left posterolateral wall after removing the epicardium. This dissection showing the subepicardial muscle fibers of the septopulmonary bundle in the roof and posterior wall of the left atrium and the vein of Marshall ending  in the coronary sinus.

Right panel. Specimen seen in a left lateral view to  show the relationship of the left atrial appendage, which in this case has the shape of a chicken wing, with the left pulmonary artery, and the superior cava Vein with the right pulmonary artery. Note how the left atrium is the most  posterior of the cardiac chambers. CS= coronary sinus; GCV= great cardiac vein; ICV= inferior cava vein; LA= left atrium; LAA = left atrial appendage; LPA= left pulmonary artery; LPVs= left pulmonary veins; LSPV = left superior pulmonaiy vein; LV= left ventricle; RAA= right atrial appendage; RIPV = right inferior pulmonary vein; RPA= right pulmonary artery; RSPV = right superior pulmonary vein; SCV = superior caval vein

Left panel. Right lateral view of the heart to show the course of the terminal sulcus (sulcus terminalis), usually, a fat-filled groove, between the cava veins and the right atrium. The dominant feature on the right side is a large triangular-shaped atrial appendage can be seen along the lateral wall The terminal sulcus corresponding to crista terminalis internally.

Middle panel. Is a view of the right lateral wall with transillumination. Note that the sulcus has the same disposition and location as the terminal crest (crista terminalis), but the crest is endocardial and from which pectinate muscles originate. Right panel. Endocardial view of the right atrium with transillumination to see the origin of the pectinate muscles from the crista terminalis.ICV= inferior cava vein; RAA= right atrial appendage; RIPV = right inferior pulmonary vein; RSPV = right superior pulmonary vein; SCV= superior caval vein

Left panel. Histological section of the sinus node body (Masson trichrome stain) within a dense matrix of connective tissue (green color) and showing a central position of the nodal artery. Note its subepicardial location and irregular contour intermingled with the neighboring myocardium without a discrete fibrous border.

Middle panel. Lateral epicardial views of the heart.The sinus node is located in the terminal sulcus, close to the cavoatrial junction. Note by transillumination in this figure that the superior cava vein showing myocardial extensions from the right atrium which can reach more than 10 mm in length inside the superior cava vein.Right panel. The sinoatrial node artery (SNa) arising from the right coronary artery (in most cases)  and courses along the anterior. interatrial groove toward the superior cavoatrial junction (precaval location).Ao= Aorta; PT= pulmonary trunk; RAA= right atrial appendage; RCA= right coronary artery; RI = right inferior pulmonary vein; RS = right superior pulmonary vein; RV= right ventricle;  RVOT= right ventricular outflow tract; SCV= superior caval vein.

Left and right panels. Right and left superior views of the atria are shown. Between the atria and the arterial trunks is the transverse sinus of Theile. The oblique sinus is located posterior to the left atrium and pulmonary veins.The right superior pulmonary vein  passes posterior to the superior cava vein and the right atrium. Note the anatomic variants of the pulmonary veins. The left panel show a left common pulmonary vein, a common variant seen in up to 25% of cases. The right panel show a normal arrangement of the four pulmonary veins entering the left atrium. In most cases the superior pulmonary veins have a larger ostium and a longer trunk than do the inferior pulmonary veins.LAA = left atrial appendage; LIPV= left inferior pulmonaiy vein; LSPV = left superior pulmonaiy vein; RAA= right atrial appendage; RIPV = right inferior pulmonary vein; RSPV = right superior pulmonary vein; SCV = superior caval vein

Left and right panels. Dissections to display the atrial myoarchitecture in anormal human heart. The left panel is a view of the posterior and inferior walls of the atria  shows abrupt changes in orientation of the myocardial strands, and this view of the left side shows myocardial strands in the region between the left superior and left inferior  pulmonary veins crossing the posterior interatrial groove into the right atrium. The right panel shows the called  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. In the right atrium, the myocardial fibers made up of myocytes run parallel to the cava veins and the pectinate muscles but perpendicular to the crista terminalis.LIPV= left inferior pulmonaiy vein; LSPV = left superior pulmonaiy vein; RIPV = right inferior pulmonary vein; RSPV = right superior pulmonary vein;

Left and right panels. Dissections to display the atrial myoarchitecture in a normal human hearts after removing the epicardium. Anterior and inferior views to show the Bachmann bundle crossing the anterior interatrial groove.The anterior wall of the left atrium behind the transverse sinus and below the Bachmann bundle can be thin (<2mm).

Left and right panels. Dissections to display the atrial myoarchitecture in a normal human hearts after removing the epicardium. Arrangement of the subepicardial fibers seen from the anterior aspect. Bachmann ‘s bundle crosses the septal raphe (anterior interatrial groove) and runs parallel to the circumferential fibers of the anterior wall of the left atrium and branching toward the left atrial appendage. Oblique fibers arising from underneath the circumferential fibers of the Bachmann´s bundle become nearly longitudinal as they cross the dome or roof between the left and right pulmonary veins and directed towards the posterior wall of the left atrium. These fibers encircle or loop around the insertions of the pulmonary veins.LAA = left atrial appendage; LSPV = left superior pulmonaiy vein; PA= Pulmonary artery RAA= right atrial appendage; RIPV = right inferior pulmonary vein; RSPV = right superior pulmonary vein; SCV = superior caval vein

Left panel. Cross-histological section (Masson’s trichrome stain) shows Bachmann’s bundle and its rightward extension (black arrows) toward the sinus node. Right panel. Dissections to display the atrial myoarchitecture.Anterior-superior view to show the Bachmann´s bundle crossing the anterior interatrial groove and branching toward the left atrial appendage and note also the septopulmonary bundle, fanning out to line the pulmonary veins and to pass longitudinally over the dome and in the posterior wall of the left atrium.  LAA = left atrial appendage; LSPV = left superior pulmonaiy vein; PA= Pulmonary artery RAA= right atrial appendage; RIPV = right inferior pulmonary vein; RSPV = right superior pulmonary vein; SCV = superior caval vein