INTERCALATED DISCS OF THE HEART OF BEEF 325 



3. In the adult heart the discs are still for the most part periph- 

 eral, as revealed both in transverse sections and in teased prepa- 

 rations. They never extend completely through a fiber. They 

 are always intimately associated with telophragmata. The 

 teloplu*agmata are in close union with the sarcolemma, the 

 nuclear wall and the myofibrils. In their simplest form, the 

 discs shade laterally into a telophragma, the latter apparently 

 bisecting the disc. In the more highly differentiated types 

 (mechanically modified discs) telophragmata frequently bound 

 one and occasionally both surfaces of the disc. 



4. The unit of structure of the simple band-disc is a modified 

 bacillary portion of a myofibril at a telophragma level. Such 

 units are grouped into bands of various widths (longitudinally) 

 and breadths (transversely) to form the initial discs. 



5. The more complex terraced, serrated and irregular types of 

 disc are derived from the simple band-forms through the opera- 

 tion of secondary extensive mechanical and possibly also chemi- 

 cal factors. The fundamental mechanical factors are irregular 

 tensions operating in opposed or oblique directions upon cer- 

 tain regions during the development and functional activity of 

 the heart. The irregular direction of the stresses are determined 

 by the syncytial (meshwork) character of the myocardium. The 

 primary results of such stresses are further modified during 

 development by spiral twistings of single fibers involving occa- 

 sionally an inturning of portions of the sarcolemma, and by 

 similar mutual twistings of two adjacent fibers resulting in 

 lateral fusions. 



6. Terraced or step-like discs result in part from a segmenta- 

 tion of the original band-discs and a secondary dislocation of the 

 resultant segments consequent to dissimilar tractions upon 

 successively more lateral segments; in part they arise also along 

 (approximately at right angles to) the oblique surfaces of fusion 

 of adjacent fibers (cells), presumably as strain effects (modified 

 irreversible? contraction bands) resulting from a re coordination 

 of the peripheral myofibrils of the fusing fibers. Terraced discs 

 may be formed also as secondary modifications of original band 

 discs by spiral twistings of single fibers or of two adjacent fibers. 



