ATRIAL MUSCULATURE 261 
It arises by its deep surface from.the head of the sino-auricular 
node in front of the orifice of the superior vena cava. This 
origin extends along a diagonal line, the left end of which termi- 
nates in the atrial septum. The bulk of the fibers arises near 
the septum below the band. Some of the bundles arise above 
the head of the node on the front of the vena cava and are dis- 
tinctly traceable into the fiber bundles of the node. These 
upper fiber bundles, from the upper part of the band, extend to 
the left atrium and form a small part of the left posterior crest 
(fig. 4, 77) encircling the base of the left atrial appendage. Be- 
hind them is the large left septopulmonary bundle (fig. 5, 12). 
The larger portion of the fiber bundles that arise from the head 
of the sino-auricular node along the diagonal line extends to the 
front of the left atrial appendage and divides to encircle the 
appendage midway between the tip and the base. Here it 
produces a sharp constriction, causing the left atrial appendage 
to be bent upward. The right end of the band extends into the 
front of the right atrial appendage and divides to encircle it near 
its base in a manner similar to that on the left side, but the 
constriction is slight and may be wanting. In the left atrium 
it gives origin to the middle or apical group of pectinate muscles. 
Both ends of the band give origin to a thin superficial stratum 
of fiber bundles that encircle the atrial appendages. On the 
right side this stratum is well defined, on the left side it is sparse 
and often absent so that the deeper pectinate stratum is distinctly 
seen on the surface. This is the case in the dog heart on both 
sides so that the deeper pectinate stratum is distinctly superficial. 
The encircling fibers which arise in the ‘concentration area’ 
(fig. 3, 2) of T. Lewis in the dog heart must not be confused with 
superficial circular stratum found in the human and _ bovine 
hearts. The superficial fibers are seen well in beef hearts. 
2. The external bundles of the right atrium 
The external bundles (figs. 3, 2, and 5, 3) that encircle the right 
atrium are derived from the septal raphe, head and tail of the 
node, and from the intercaval bundle. In the human heart they 
