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ATRIAL MUSCULATURE 
12. Septopulmonary bundles 
Septopulmonary bundles (figs. 4, 6, and 8) are the large flat 
external bundles that cover the upper and posterior surface of 
the left atrium between the pulmonary veins and encircle the 
orifices of the veins. They arise deeply from the left surface 
of the septal raphe (fig. 2, 72) behind the interatrial band and 
curve upward and to the left, chiefly in front of the orifice of 
the upper right pulmonary vein, and above the interatrial band, 
where they come to the external surface. They spread out as 
six bands, over the upper and posterior surface of the left atrium. 
The anterior septopulmonary bundle (12a) arches above the 
interatrial band, extends to the left in front of the left superior 
pulmonary vein as the superficial stratum covering the left 
posterior crest. Below the orifice of the vein it curves backward 
and superficially over the back of the atrium between the left 
inferior pulmonary vein and the coronary sinus. It covers the 
left leaf of the septum primum. 
The left intervenous septopulmonary bundle (12b) extends 
upward as a deep stratum of the foregoing and to the left between 
the left pulmonary veins and joins the anterior septopulmonary 
bundle. 
The left. posterior septopulmonary bundle (/2c) arises with 
the foregoing and spreads backward and to the left over the left 
atrium between the left and right pulmonary veins. Behind 
the left lower pulmonary vein it spreads out into the anterior 
septopulmonary bundle and curves to the left below the lower 
left pulmonary vein. Some of the fibers of the foregoing bundles 
extend onto the left pulmonary veins and give origin to the fiber 
bundles that encircle the veins in a radiate and somewhat circular 
manner. 
The right posterior septopulmonary bundle (12d), like the 
other, extends backward over the upper surface of the left atrium. 
Spreading out over the posterior surface of the atrium, it curves 
to the right below the right lower pulmonary vein. It overlies 
the left leaf of the septum secundum and septum primum. A 
portion of it extends to the back of the inferior vena cava bridging 
over the atrial junction, interlacing with the septal bundles. 
