ATRIAL MUSCULATURE 269 
downward over the vena cava, but do not form circular bundles 
around it. The right side of the orifice is limited by the right 
posterior crest. The lower portion of the intercaval bundle (5) 
spreads downward over the posterior and right surface of the 
inferior vena cava and terminates in a fibrous expansion. The 
upper portion of the right leaf of the septum primum (fig. 7, 8) 
spreads over the left and posterior sides of the inferior vena 
cava and terminates in a fibrous expansion. The posterior 
surface of the inferior vena cava between the right leaf of the 
septum primum and the intercaval bundle is usually fibrous. 
The lower portion of the right leaf of the septum primum forms 
a strong band in front of the orifice of the inferior vena cava 
along the base of the (right) valve of the inferior vena cava. 
This frequently forms a sharp fold between the orifice of the 
inferior vena cava and that of the coronary sinus. 
Keith and Flack (’07, fig. 3) have illustrated the bundles on 
the dorsal surface of the inferior vena cava. 
7. The right leaf of the septum secundum 
The septum secundum (figs. 1, 7, 2, 13, and 7, 7) or limbus of 
the oval fossa is a strong muscular bundle which arises in the 
septal raphe (fig. 2, R), chiefly on the right side, and arches 
backward over the oval fossa. It is divisible into a right and 
a left leaf, but the separation of the leaves is to some extent an 
arbitrary matter. 
The right leaf of the septum secundum (figs. 1, 7, and 7, 7) 
belongs to the sinus venosus. It arches over the left side of the 
orifice of the inferior vena cava, above and posterior to the orifice 
of the coronary sinus. There it is inserted into a fibrous expan- 
sion over the inferior vena cava. Above, it is in series with the 
intercaval bundle, but it is situated deeply in the atrial junction. 
A portion of this bundle turns to the left onto the orifice of the 
coronary sinus. 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 27, NO. 3 
