iw) 
ATRIAL MUSCULATURE 275 
the left leaf of the septum secundum, as shown in the figures. 
It sweeps upward and to the left into the anterior wall of the 
atrium and backward into the superior and posterior walls 
between the right and left pulmonary veins. Some of its bundles 
pass around the base of the atrial appendage with the left poste- 
rior crest (fig. 8) and in front of the orifices of the left pulmonary 
veins for which they provide an inner circular stratum. Posteri- 
orly, the septoatrial bundle spreads out and intermingles with 
the left leaf of the septum primum. So complete is the fusion 
of these bundles that it is difficult to determine that the latter is 
not a continuation of the septcatrial bundle. 
Externally the left septoatrial bundle is covered by the ante- 
rior and posterior crests and by the septopulmonary bundles. 
On the posterior surface, however, it appears as a triangular 
area between the diverging limbs of the left posterior septo- 
pulmonary bundles (fig. 6, 14). 
The bundles that surround the orifices of the pulmonary 
veins are derived from the external septopulmonary stratum of 
bundles (fig. 6, 12, 6, c, d, e), and from the internal or septo- 
atrial stratum of bundles (fig. 8, 74). Those for the right veins 
are chiefly from the former, those of the left vems are largely 
from both strata. The bundles encircle the orifices, although 
sparse radiate bundles may occur. On the inner surface below 
the lower right pulmonary veins and behind the oval foramen 
the left leaf of the septum primum gives off a concentration 
area which gives rise to fiber bundles that encircle the lower 
right pulmonary vein on its inner surface (fig. 2, 13). 
15. The left leaf of the septum primum 
The left leaf of the septum primum (figs. 2 and 8, 15) is a 
large flat bundle most distinctly seen on the interior of the left 
atrium. It arises from the lowermost portion of the septal 
raphe on the left side of the atrioventricular node region below 
the oval fossa (fig. 2, 15). It sweeps to the left along the poste- 
rior portion of the left atrioventricular ring as far as the base of 
the left appendage where it meets and intermingles with the left 
