ATRIAL MUSCULATURE 259 
It is interesting to note that a large terminal of the right 
vagosympathetic nerve ends in the upper portion of the septum 
behind the interatrial band by dividing into numerous filaments 
that sink into the raphe and terminate in the various bundles. 
That is, the nerve enters the triangular interval, bounded in 
front by the interatrial band, to the right by the vena cava and 
to the left by the upper right pulmonary vein. A small filament 
passes behind the orifice of the superior vena cava in the inter- 
caval bundle. The others sink into bundles that radiate from 
the septal raphe. In the beef heart a fine terminal is traceable 
to the base of the right appendage, where it terminates just 
below the head of the sino-auricular node. No terminal filaments 
were traceable into the sino-auricular node. 
Extension of nodal tissue into the region of the septal raphe 
has not been demonstrated. One of the figures of Keith and 
Flack (07) shows a portion of the sino-auricular node in the 
left side of the orifice of the superior vena cava above the region 
of the septum. They suggested that a ring of such tissue may 
extend into the septum and include the atrioventricular node. 
Histological studies of Keith and Flack (’07), T. Lewis (1014), 
and others have served to define the head and tail of the node 
in the groove between the superior vena cava and the right 
atrium. . 
Extension of the pace-making function into the region of the 
septal raphe has not been demonstrated. Since the septal raphe 
is buried behind the interatrial band to the left of the superior 
vena cava it is not approachable externally for electrocardio- 
graphic work. It is approached with difficulty from the inner 
endocardial surface. Here the raphe is covered by muscular 
bands, and is, therefore, not immediately subendocardial. 
Ganter and Zahn (’12—’13) cooled the septum from the right side 
and found that atrial rhythm is influenced only by cooling of 
the region of the atrioventricular node. Primary negativity 
also appears only in this region of the septum. It is a generally 
accepted view that the atrioventricular node is the principal 
seat of impulse formation in the atrial septum and normally 
conducts the excitation process into the ventricles. Only when 
