INTERCALATED DISCS OF THE HEART OF BEEF 319 



ondary to the formation of the step-discs or a result of the dislo- 

 cation of an original straight band-disc is uncertain, and not of 

 fundamental significance. 



If the terraced forms of discs were the result of a dislocation of 

 a simple band disc, then it might seem to be required that the 

 involved telophragmata should show a distortion. Such is not 

 generally the case. Figure 13 shows an exception. But a care- 

 ful consideration of the possibilities will explain the general 

 absence of coincident telophragmatic distortion. If it be pre- 

 sumed, as seems necessary under the conditions postulated for 

 the formation of a certain type of terraced disc, that the involved 

 telophragmata are broken and the segments shifted in position, 

 they could only shift to some place between two successive telo- 

 phragmata or in series with them. The former may involve 

 fusions between portions of successive telophragmata, a phe- 

 nomenon indicated in figure 13; or a blending of telophragmata. 



The available evidence seems to force the conclusion that many 

 of the regularly terraced types of discs originate by a process of 

 secondary dislocation of band discs, and a shifting of the result- 

 ing segments to successively lower levels in a lateral direction, 

 due apparently to successively greater tensions laterally in the 

 trabeculae, the result in part of the oblique tensions caused by 

 the anastomosing branches, and in later stages in part probably 

 also to the spiral twistings of certain groups of trabeculae (e.g., 

 the bulbo-spiral band). 



With respect to the irregular types of terraced discs, in which 

 the coarser connecting membranes are invariably present, the 

 processes of formation involve the fusion of apposed portions of 

 the sarcolemmae of the adjacent trabeculae, caused to fuse by 

 reason of a mutual spiral twisting. Such fusions are common in 

 certain skeletal muscles, e.g., in the post-abdominal segments of 

 the scorpion (Jordan, (12) ) and in human cardiac muscle (Heiden- 

 hain (4) ) . In cases where peripheral discs were present in the 

 regions of the fusions of the involved fibers these would become 

 arranged in irregular step-form due to their relation to the an- 

 harmonic telophragmata of the fused fibers. Similarly in cases 

 where a single fiber is spirally twisted a portion of the sarco- 



