ATRIAL MUSCULATURE 263 
this region. ‘The posterior radiation from the tail spreads over 
the lateral surface of the lower portion of the atrium, a part 
above and a part below the intercaval bundle, from which it is 
often indistinguishable. In the beef heart it is quite distinct 
and has a more anterior position. 
d. The intercaval bundle (figs. 3, 5, and 6, 5) spreads over the 
lower portion of the external surface of the right atrium. It 
will be described later, (page 266). 
The spread of the excitation process through the foregoing 
external bundles has been determined experimentally by T. Lewis 
and his associates (14). They applied their electrodes to the 
epicardium covering the surface of the node and external surface 
of the right atrium. They have shown that the excitation process 
first appears at the node and spreads along these external bundles 
at a more or less uniform rate. Moreover, they have shown the 
histological connection of these bundles with the node in the 
dog heart. It appears that a similar arrangement of bundles is 
present in the human heart. It should be borne in mind that 
these superficial bundles overlie the right anterior and posterior 
crests which have a deep internal position and a more distant 
septal origin. The excitation process in the crests is, therefore, 
not readily detected by the electrocardiographic method, when the 
electrodes are applied to the epicardium. The pectinate muscles 
which arise chiefly from the crests (fig. 7,) have also a deeper 
position, but in the walls of the atrium their branches become 
exposed on the surface where they interlace with the superficial 
bundles. This is especially true in dog hearts. 
3. The right anterior crest 
The right anterior crest (figs. 7, 3, and 5, 3) is a flattened mus- 
cular band forming the anterior wall of the right atrium, above 
and to the right of the atrioventricular ring. It is best seen on 
the internal surface, although it also appears on the external 
surface (fig. 5, 3). It arises in front of the origin of the right 
posterior crest from the septum and the head of the sino-auricular 
node. It curves forward and to the right above the atrioventric- 
