272 JAMES W. PAPEZ 
Since the left anterior crest is in large part a continuation of 
the interatrial band, it is clear, from the experiments of Bach- 
mann (716) that the excitation process reaches this region of the 
left atrium chiefly along the band. 
11. The left posterior crest 
The left posterior crest (figs. 5 and 8, //a, 11b) is a broad 
bundle, the superficial portion of which (fig. 5, 71a) is a con- 
tinuation of the upper part of the interatrial band, and the deep 
portion of which (fig. 8, 77) arises from the septum in series 
with the septoatrial bundle. It passes to the left around, the 
base of the left atrial appendage in front of the orifices of the 
left pulmonary veins. It gives origin to the small posterior set 
of pectinate muscles. Posteriorly it interdigitates with the left 
anterior crest, forming the triangular area which is attached to 
the inferior margin of the left atrioventricular ring. It les 
in front of the septopulmonary (fig. 8, 12) and septoatrial (fig. 
8, 14) bundles and forms a distinet constriction around the base 
of the left atrial appendage. The left posterior pectinate muscles 
are several in number. Their branches interlace with those of 
the anterior set. The left atrial appendage is a narrow forward 
prolongation of the left atrium. The left extremity of the inter- 
atrial band extends along the anterior margin, and midway 
between the base and apex it divides and embraces the appendage. 
This gives rise to an immediate set of pectinate muscles that 
occupy a superior position. In the human heart the posterior 
set of pectinate muscles is above the anterior set, but in the beef 
and dog hearts they are posterior. For morphological reasons 
this designation has been retained for the human heart. 
Bachmann (’16) has indicated that the spread of the excitation 
process to the left atrial appendage is along the interatrial band. 
This forms the superficial stratum of the left posterior crest. 
The deeper stratum, however, arises from the septal raphe. 
