270 JAMES W. PAPEZ 
8. The right leaf of the septum primum 
The septum primum (figs. 2, 8, and 7, 8) is a large bundle of 
muscle tissue that forms the lower boundary of the oval fossa. 
It is joined by fibrous tissue to the ventricular septum. It is 
divisible into two distinct portions, the right leaf for the sinus 
venosus and the large left leaf for the base of the left atrium. 
The two leaves have independent origins and are almost entirely 
separate.. Since the left leaf belongs to the left atrium, only 
the right leaf will be described here. 
The right leaf of the septum primum (fig. 7, 8) arises from 
the septal raphe on the right side in front of the oval fossa, and 
passes downward and to the right in the (right) valve of the 
inferior vena cava, that extends along the front of the orifice 
of the vessel, and intermingles with the terminal expansions 
of the right crests and intercaval bundle. This leaf passes, 
therefore, between the orifices of the inferior vena cava and 
coronary sinus, to both of which it contributes musculature. 
Some of its bundles sweep upward and form the posterior border 
of the oval fossa. 
The lower portion of this bundle, that may be called the 
septocoronary bundle, arises from the lowermost portion of the 
septal raphe and spreads out in a fan-shaped manner in front of 
the coronary sinus covering the right side of the atrioventricular 
node (fig. 1, A.V. N.). The bundles that form its right surface 
appear to be attached to the fibrous upper extent of the inter- 
ventricular septum, but its deeper bundles stand in intimate 
relation to the atrioventricular node. T. Lewis has shown that 
muscular contraction in this region appears early, as is indicated 
by an early extrinsic deflection on the cardiogram. Eyster and 
Meek have shown that this region receives the excitation process 
early before it has spread fully into the right atrium. 
Keith and Flack (’07) have shown that the musculature of 
the venous valve, which is here called the right leaf of the septum 
primum, is formed by an infolding of the atrial wall at the sino- 
auricular junction. ‘To the right this musculature spreads out 
and joins freely with that of the right atrium. 
