ATRIAL MUSCULATURE PA 
9. Musculature of the coronary sinus 
The musculature of the coronary sinus (figs. 4 and 6, C. S.) 
is for the most part circular. At the orifice, the right leaf of 
the septum primum (fig. 1) and of the septum secundum and the 
septo-coronary bundle extend into the coronary sinus. A 
pectinate ring of musculature derived from the right posterior 
crest surrounds the orifice. As the sinus is applied close to the 
posterior wall of the left atrium, it is partly buried in its mus- 
culature and receives longitudinal accessions from the left leaf 
of the septum primum and septum secundum. In the bovine 
heart the sinus is a continuation of the left common cardinal 
system and is surrounded by musculature for a distance of several 
inches. 
Keith and Flack (07, fig. 3) have shown that the bundles of 
the coronary sinus are continuous with those of the sinus veno- 
sus and the left atrium. 
MUSCULATURE OF THE LEFT ATRIUM 
10. The left anterior crest 
The left anterior crest (figs. 5, and 8, 10) is a large bundle 
that is formed in large part as a continuation of the lower portion 
of the left extremity of the interatrial band. It is seen on the 
outer surface of the atrium. The inner bundles commence in 
the septal raphe in front of the left septoatrial bundle. It 
extends to the left along the upper extent of the atrioventricular 
ring to which its thin lower margin is attached. Its upper 
margin is thick and gives origin to a feeble group of anterior 
pectinate muscles. On the left side of the atrium it interdigitates 
with the bundles of the posterior crest. Here its outer fibers 
pass backward around the base of the left atrium, intermingling 
with those of the left leaf of the septum primum. The anterior 
pectinate muscles that arise from the left anterior crest, extend 
upward as several short trunks that branch out along the left 
margin of the left atrial appendage and interlace with the inter- 
mediate pectinate muscles from the interatrial band. 
