INTERCALATED DISCS OF HEART MUSCLE 155 



rows of granules as areas of local contraction in otherwise relaxed 

 and distinct fibrils, the inter-granular membrane possibly repre- 

 senting a condensed membrane of Heidenhain (M line; meso- 

 phragma, Heidenhain) . 



The discs are almost invariably at the level of, and, where 

 present, displace the dark (anisotropic) bands. 5 However, they 

 are frequently wider than these bands. Occasionally, when ro- 

 bust, they may extend approximately halfway or even entirely, 

 through the lighter band (fig. 6). But they are certainly not 

 generally bounded on both sides by Krause's membrane (telo- 

 phragma, Heidenhain), an observation urged in favor of an inter- 

 cellular interpretation, as has been stated by some investigators, 

 e.g., Heidenhain. 



In fig. 2 all of the discs appear as wider or narrower more or 

 less compact granular bands. The two separate collections are 

 localized at the points of stress. One disc appears to span the 

 line of junction. Several lie partially over the nucleus. All are 

 superficial and of comparatively little depth. 6 These structural 

 variations, superficial and occasional super-nuclear position, ren- 

 der untenable interpretation as cement lines. Fig. 3 illustrates 

 four successive superficial discs unconnected by 'risers' to form 

 'steps.' Moreover, they gradually shade laterally into the ani- 

 sotropic bands which they in part displace. In fig. 4, three discs 

 are shown overlying the deeper nucleus. In fig. 5 are illustrated 

 two wide discs of finely granular character. Such discs are fairly 

 common. In this particular fiber the two discs lie at slightly dif- 

 ferent depths. The important point is that in passing from a 



8 The term anisotropic is not here employed in a definitely physical sense. It is 

 used simply to denote the darker-staining substance in the Q and Z band of striped 

 muscle, in accordance with common custom. Careful study of cardiac muscle 

 with the micropolariscope, under the same physical conditions that very strik- 

 ingly revealed anisotropic vegetable fibers and inorganic crystals, failed, however, 

 to disclose any definitely anisotropic substance in the myofibrillae. The point 

 urged is the similarity between the darker transverse and the intercalated discs. 

 It would seem, moreover, that not all striped muscle can be resolved into sharply 

 defined isotropic and anisotropic discs. On the other hand, as far as the micro- 

 polariscope gives evidence, 'stripes' appear in the absence of anisotropic granules. 



6 Hence 'discs' strictly defined is a misnomer; they are more properly desig- 

 nated 'bands.' The Q and J 'bands,' moreover, are more correctly denominated 

 'discs.' 



