266 JAMES W. PAPEZ 
The electrocardiographic results of T. Lewis, Meakins, and 
White (14) show that the excitation process spreads from the 
sino-auricular node to the right atrium along the superficial 
muscle bundles and along the posterior crest. The mass con- 
traction of the atrium is evidently due to the contraction of the 
pectinate muscles and their connections with the septal raphe 
through the anterior and posterior crests. 
MUSCULATURE OF THE SINUS VENOSUS 
5. The tntercaval bundle 
The intercaval bundle (figs. 3 and 6, 5) is a large flat one that 
passes obliquely in the posterior fibrous wall of the sinus venosus. 
It arises (fig. 7, 5) in the front and left side of the orifice of the 
superior vena cava from the head of the sino-auricular node and 
from the uppermost limit of the septal raphe and in many cases 
a large slip arises in front of the orifice of the right upper pul- 
monary vein, as shown by Keith and Flack (’07, fig. 5, f). The 
upper margin of the intercaval bundle is in series with the bundles 
that encircle the orifice of the superior vena cava, the lower 
margin is in series with the right leaf of the septum secundum 
(fig. 1, 5). It curves obliquely backward and downward and 
to the right in the fibrous posterior wall of the sinus venosus. 
It forms an oblique constriction between the orifices of the venae 
cavae in the region of tubercle of Lower. Crossing the sino- 
auricular junction. and right posterior crest, it spreads out as a 
fibrous expansion over the external surface of the lower portion 
of the right atrium where it forms a superficial stratum. A 
portion of it covers the posterior and right side of the orifice 
of the inferior vena cava, as shown in figure 6, 6. Where it 
crosses the tail of the node it may receive accessions from the 
node. Its intimate attachment to the posterior crest and the 
node render its relation to these difficult to determine. Some 
of its bundles overlap the right surface of the orifice of the inferior 
vena cava and are inserted into an external fibrous sheath. The 
more proximal of these pass around the orifice to the region of 
the coronary sinus. The extent’ to which this musculature 
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