ATRIAL MUSCULATURE 267 
extends onto the inferior vena cava varies in different hearts, 
most often it is absent. The entire intercaval bundle is often 
absent or so feebly developed that it cannot be defined with 
certainty. 
In reference to cardiographic results on this region, T. Lewis 
says: ‘These leads cover the root of the cava and overlie strong 
bands of septal musculature which pass across the line of the 
lead. It is a region of the dog’s heart from which we almost 
invariably obtain prominent extrinsic effects, and we attribute 
them to the radiation of currents from muscle lying deeply be- 
neath the contact area.” 
Keith and Flack (07, fig. 5, f) have clearly illustrated this 
bundle in the human heart. It is to be noted that the apparent 
origin of the upper portion of this bundle in front of the orifice 
of the upper right pulmonary vein seems anomalous. However, 
a T-stem connects this bundle with the septum; and it seems 
likely that this portion of the bundle is a deploying in both direc- 
tions of its narrow septal stem. 
6. The superior vena cava bundles 
The superior vena cava bundles (figs. 3, 5, and 6, 6a, 6b) sur- 
round the orifice of the vena cava for a distance of 2 cm. or more. 
They are variably developed in different hearts. All muscula- 
ture above the sino-auricular node is considered as belonging 
to the superior vena cava. These bundles consist of a circular 
and a radial set. 
The circular bundles (6a) are superficial to the radial ones and 
in the human and dog hearts they predominate. On the right 
and chiefly on the front of the orifice they arise from the node in 
an oblique manner and spread out over the front of the vena 
cava which they tend to encircle spirally. Others arise from the 
head of the node and beneath the origin of the interatrial band 
and from the uppermost limits of the septal raphe in the left wall 
of the superior vena cava. These spread out over the left and 
posterior surface of the vena caval orifice where they are in series 
with the intercaval bundle. In the dog heart, as is shown in 
