264 JAMES W. PAPEZ 
ular ring, giving rise to the right anterior pectinate muscles. 
Its anterior or lower margin is attached to the thin fibrous atrio- 
ventricular ring. Between the origins of the right anterior and 
posterior crests there is formed a thin spot. The right anterior 
crest encircles the base of the right atrium and near the mouth 
of the coronary sinus interlaces with the terminal pectinate 
columns of the right posterior crest. From here they are con- 
tinued as a thin stratum to the atrioventricular node. The right 
anterior pectinate muscles (fig. 7) number nine to fourteen. 
The ones in the apex of the right atrial appendage are large. 
The others.are smaller. They arise from the anterior crest 
singly or in three radiating clusters. The trunks of the pectinate 
muscles branch out and the branches interlace with those of the 
posterior pectinate columns. The upper pectinate cluster arises 
as a strong column from the anterior portion of the atrial septum 
in front of the origin of the posterior crest. Its pectinate muscles 
extend toward the apex of the atrial appendage. The middle 
cluster is usually definite. It forms an external constriction 
below the base of the atrial appendage. Keith and Flack (’07) 
designated the anterior crest as the ‘annular fibers of the auricle’ 
and MeMurrich in Piersol’s Anatomy speaks of them as ansiform 
fascicull. 
From the experiments of T. Lewis, Meakins, and White, it 
would appear that the excitation process is conducted by the 
external bundles of the atrium from the sino-auricular node into 
the anterior pectinate muscles and thence into the anterior crest. 
However, the fat and vessels in the coronary sulcus raise the 
epicardium from the anterior crest to such an extent that direct 
application of electrodes cannot be made. 
4. The right posterior crest 
The right posterior crest (figs. 7, 4, and 1, 4) is a strong mus- 
cular column, called by His the terminal crest, because it forms 
the boundary between the sinus venosus and the right atrium. 
It is seen on the internal surface of the atrium and gives rise to 
the posterior set of right pectinate muscles. External bundles 
