256 JAMES W. PAPEZ 
commences in this node. They have also suggested that the 
excitation process spreads from the node through both right 
and left atria along several muscular paths. The right posterior 
crest (taenia terminalis) has been considered the principal pri- 
mary pathway for the excitation process in the right atrium. 
The interatrial band has been considered the primary pathway 
for the excitation to the left atrium (G. Bachmann, 716). It is 
the purpose of this paper to show the gross arrangement of the 
muscular bundles of the atria and to indicate how this arrange- 
ment may serve to spread the excitation process through them. 
MATERIAL AND METHOD 
For this work the method of cleaning and stripping the mus- 
culature has been found most useful. Human, beef, and dog 
hearts in abundance were used. 
The cardiac orifices were tied and the hearts were distended 
with a 10 per cent solution of formalin under a pressure of about 
30 em. of the fluid. The fluid was allowed to act for a day or 
two, depending upon the size of the heart. The hearts were then 
washed in running water for a day and transferred to alcohol. 
To preserve the distended shape and facilitate external dis- 
sections, the atria were then stuffed with absorbent cotton. To 
display the bundles the epicardium and endocardium were 
removed. 
SINO-AURICULAR NODE AND SEPTAL RAPHE 
From the preparations at hand it is clear that all of the princi- 
pal muscular bundles of the atria radiate from one central area 
which surrounds the orifice of the superior vena cava. This 
area is for the most part buried in the anterior part of the atrial 
septum, but in front and to the right of the orifice of the vena 
cava it comes to the external surface. The portion that appears 
in the groove between the vena cava and the right atrium has 
been designated the sino-auricular node by Keith and Flack 
(07); it is the seat of impulse formation for the atria in the 
normally beating heart. The portion that is buried in the atrial 
