ATRIAL MUSCULATURE Py i 
septum will be here called the septal raphe; it provides an ap- 
parent mechanical support for many of the larger muscular 
bundles of both atria. My preparations’ show that some of the 
external bundles commence in the sino-auricular node and that 
other deeper bundles commence in the septal raphe. 
The sino-auricular node was first described by Keith and Flack 
(07). They state that it consists of delicate, pale and pale- 
staining, primitive, undifferentiated, striated muscle fibers, plexi- 
form in arrangement with well-marked, elongated nuclei. The 
fibers resemble those of the atrio-ventricular node of Tawara. 
They are embedded in densely packed connective tissue. ‘The 
fibers lead somewhat abruptly into the surrounding atrial mus- 
culature that forms the external strata. They mention nerve 
terminals in this node. The sino-auricular node is recognizable 
as a thin stratum of fine pale musculature beneath the epicardium 
in the sulcus between the superior vena cava and right atrium. 
I have not been able to find macroscopically any nerve filament 
passing to the sino-auricular node. In contrast to this the 
abundance of nerve filaments on the septal (left) side of the 
orifice of the superior vena cava sinking into the region of the 
septal raphe is significant. Here, those authors have described 
tissue of the same character as forms the sino-auricular node. 
Moreover, they believe that the tissue of the atrio-ventricular 
node of Tawara represents a part of the same ring of undiffer- 
entiated muscle tissue that surrounds the sino-auricular junction. 
Experimental work has confirmed the view that the sino- 
auricular node is the seat of impulse formation in the normally 
beating heart. That the excitation process for at least the 
superficial bundles of the right atrium and for the interatrial 
band commences in the head of this node has been amply demon- 
strated by Wybauw (’10), T. Lewis and his associates (710-14) 
and has been confirmed by Eyster and Meek (’13-’14). 
Several external bundles commence in the sino-auricular node 
region: the external bundles that cover the upper and lateral 
surface of the right atrium (fig. 3, 2), the circular bundles of the 
superior vena cava (figs. 3 and 5, 6a), and the interatrial band 
(fig. 5, 1). The delicate fibers of the node cannot be regarded 
as giving these bundles mechanical support. 
