INTERCALATED DISCS OF THE HEART OF BEEF 293 



ranged with respect to the telophragniata so that the opposite 

 telophragniata joined opposite (upper and lower) borders of the 

 disc-sections. In anticipation of the ensuing discussion it may be 

 stated here that the fundamental causal factor in the formation 

 of this terraced disc is believed to be the unusual stresses im- 

 posed upon the peripheral myofibrils in the region of the area 

 of fusion, incidental to the functional re coordination required 

 of the fibrils. The location of discs generally near the levels 

 where branches arise also becomes comprehensible under this 

 hypothesis. 



The band-forms in figure 9 are located at telophragniata levels. 

 In the upper portion of the field one lies superjacent to a nu- 

 cleus. This same disc extends for some distance into the adjacent 

 fiber. Such disposition of the broader discs, that is, a location 

 across several fibers, is a common feature. It occvirs extensively 

 even in the Limulus heart, w-here the discs are numerically rare 

 and of the simplest 'comb' type (fig. 41). The condition indi- 

 cates a local functional alteration influencing several adjacent 

 fibers in a transverse plane. Such discs can be plausibly inter- 

 preted on no hypothesis involving growth phenomena, inter- 

 cellular cement, or tendinous structures. They appear to sig- 

 nif}^ identical modifications resulting from identical functional 

 phases at the same transverse level of the heart musculature. 



Fusion of two adjacent trabeculae is further illustrated in 

 figure 10. Here two groups of narrow band-discs occur, con- 

 nected by the fused sarcolemmae. The formation of discs is ey\- 

 dently closely associated with the processes of fusion among 

 fibers. But the location of the discs with respect to the surface 

 of fusion is a matter of fundamental significance. The point is 

 wtII illustrated in both of the figures 9 and 10. The discs do not 

 lie in the line of fusion but at riglit (or oblique) angles to it, and 

 in ahnement with the telophragmata. It is readily conceivable 

 that the fusion of the fibers involved a functional recoordination 

 of gi'oups of peripheral myofibrils. This produced unusual strains 

 at certain levels. Such levels offer, theoretically, favorable 

 sites for the formation of discs by process of modification of con- 

 traction bands (essentially an irreversibility) according to the 

 hj^^othesis here adopted and discussed below. 



