ATRIAL MUSCULATURE 265 
derived directly from the sino-auricular node enter into its 
composition. The internal bundles, however, form a large part 
of the crest. They arise (fig. 1, R, 4) in the anterior part of the 
atrial septum from the septal raphe. They curve upward and 
to the right in front of the orifice of the superior vena cava and 
then downward on the right of both venae cavae. They are 
overlaid by the sino-auricular node. Along its course the crest 
gives rise to about ten posterior pectinate muscles (fig. 7), and 
as these are given off the crest diminishes in size. One or 
two upper pectinate muscles are large and extend into the atrial 
appendage. The succeeding ones are progressively smaller. In 
the wall of the atrium the pectinate muscles divide into numerous 
branches that interlace with those of the anterior pectinate 
muscles. The lower pectinate muscles are small and extend to 
the lower right part of the right atrioventricular ring. The 
lowermost one encircles the orifice of the Gorenary sinus and is 
joined by the right leaf of the septum primum (fig. 7, 8) which 
extends through the (right) valve of the inferior vena cava. 
The external bundles have already been described under the 
external bundles of the right atrium. The uppermost ones run 
parallel to the sino-auricular node and commence in it as attenu- 
ated fibers. They spread out with the pectinate muscles which 
they cover externally. Below, they are reinforced by the inter- 
caval bundle which spreads downward to the lower part of the 
right atrioventricular ring. 
Keith and Flack (’06—’07) considered that the right posterior 
crest “probably belongs to the sinus venosus, since it extends 
beneath the endocardium of the atrium from the position of one 
venous valve to the other.’”’ In the same article, however, they 
state “that the auricles (atria) are outgrowths from the dorsal 
wall of the auricular (atrial) canal.’’? The latter seems to be the 
best view. Hence, the anterior and posterior crests may be 
regarded as derived from the annular musculature of the atrial 
canal. As the crests become separated the intervening mus- 
culature becomes arranged in the form of an anterior and poste- 
rior set of pectinate muscles. 
