INTERCALATED DISCS OF HEART MUSCLE 



161 



shows a step-like disc with six 'steps' and five 'risers.' The 

 'steps' consist of deep-staining granules; the 'risers' are more 

 delicate granular structures. Several of the discs shade off later- 

 ally be}^ond the 'risers,' into more compact granular anisotropic 

 bands. It would be impossible, we believe, to interpret this 

 structure in terms of a cement line or cell boundary. The 'steps' 





1 



■■'■*■**-- 



14 



15 



13 



Fig. 13 Branching cardiac fiber of chipmunk, with numerous isolated larger 

 and smaller oval discs, for the most part at the darker levels of the fibrillae. 



Fig. 14 Cardiac fiber of chipmunk with step-like discs, composed of coarse 

 granular thick 'steps' and delicate granular 'risers.' 



Fig. 15 Fiber of humming-bird heart, showing the short, compact deeply- 

 staining discs characteristic of this muscle. 



of the disc here are clearly at the levels, and displace portions, of 

 the darker bands, the entire structure lying superficially. 



