262 JAMES W. PAPEZ 
form a thin, more or less uniform stratum over the surface of the 
right atrial appendage and right surface of the base of the atrium. 
In the bovine heart they are strongly developed. In the dog 
and cat hearts they form a thin stratum more difficult to differ- 
entiate. Owing to their diffuse origin and termination, they 
cannot be accurately described. The following description is, 
therefore, an approximate one from which there are variations. 
a. The origin of those from the septal raphe is below the right 
limb of the interatrial band. It is covered by the origin of the 
band as seen on the external surface in figure 5, 3. They pass 
forward toward the atrioventricular ring and are nothing more 
than the superficial stratum of the right anterior crest. They 
are spread out and inserted into the atrioventricular ring to the 
right of the aortic orifice. 
b. The origin of those from the head of the node is hidden by 
the right limb of the interatrial band (fig. 5, 3). They pass 
forward and encircle the base of the right atrial appendage. On 
the lateral side of the atrium they divide into two limbs. One 
of these encircles the base and forms a thin stratum over the 
lower and right side of the atrium. The other sweeps upward 
around the base of the atrial appendage covering the middle 
cluster of the anterior pectinate muscles, and forms a distinct 
constriction in the lower lateral part of the base of the atrial 
appendage which is seen especially well in dog hearts, as shown 
in figure 3. 
c. Those that arise from the tail of the node form a forward 
and a lateral radiation and a posterior radiation (fig. 3, 2). The 
forward radiation is distinct only in the dog heart. T. Lewis 
has called it the ‘concentration area.’ It arises from the front 
of the tail of the node and passes along the upper margin of the 
right atrial appendage. ‘Together with the right limb of the 
interatrial band, it gives rise to the external circular bundles of 
the right atrial appendage. It covers the interlacement of the 
apical pectinate muscles, divides, and is diffused over the lateral 
surface of the right atrial appendage. In the human heart this 
radiation is generally not recognizable, but in some instances a 
small distinct superficial bundle about 1 cm. in length occurs in 
