268 JAMES W. PAPEZ 
figure 3, Ga, there is sometimes a distinct bundle that arises 
obliquely from the node and, sweeping upward over the right 
surface of the vena cava, divides to encircle the vessel. In the 
dog heart the musculature of the superior vena cava is quite 
extensive and surrounds also the orifice of the azygos vein. 
Eyster and Meek (13) believe they have shown that the ‘‘excita- 
tion usually reaches the intercaval region and the superior vena 
cava before any other regions.”’ The electrocardiographic results 
of T. Lewis, Meakins, and White (’14) indicate that there is no 
material difference of the rate of conduction from the sino- 
auricular node to the vena cava. They have shown that the 
excitation process in the dog’s heart extends radially upward. 
The variable rate which they obtained is due, as they suggest, 
probably to the obliquity and spiral arrangement of the vena 
caval musculature. 
The radial bundles (66) appear particularly in the left side, and 
in some hearts also in the posterior side, of the superior vena 
cava. They are covered by the circular set. The left ones 
arise from the uppermost limit of the septal raphe (fig. 5, 6b) 
and at the orifice of the vena cava and radiate in an upward and 
posterior direction. In the wall of the human and dog hearts 
they form an exceedingly thin stratum. The posterior radial 
set of muscle bundles is commonly absent in the human heart. 
When present, they are feeble. They arise from the tail of the 
sino-auricular node and sweep diagonally upward and to the 
left over the posterior surface of the vena cava which they tend 
to surround in a circular direction (fig. 3). They are situated 
just above the intercaval bundle, from which they are difficult 
to distinguish. The posterior surface of the superior vena cava 
is more extensive than the anterior on account of the obliquity 
with which it joins the sinus venosus. Excepting the circular 
bundles and, in some instances, the posterior radial bundles, 
it is fibrous between the vena caval musculature and the oblique 
intercaval bundle. 
The bundles around the inferior vena cava. The inferior vena 
caval bundles (figs. 4, 6, and 7) are derived from the septum 
primum (8) and from the intercaval bundle. They extend 
