INTERCALATED DISCS OF THE HEART OF BEEF 301 



c. The moderator band 



The moderator band is of importance in this connection because 

 of the abundant areas of transition it furnishes between branches 

 of the atrioventricular bundle and the myocardium. The transi- 

 tion area includes fibers of the nature of the subendocardial 

 Purkinje fibers. Moreover, the myocardial meshwork here 

 seems of a slightly lesser degree of modification; that is, the fibers 

 branch more regularly, and mainly dichotomously (fig. 29), at 

 very acute angles, and the discs are predominantly of the simple 

 band-type. The step-forms generally lack the deeply-staining 

 connecting membranes, which indicates, in combination with 

 other appearances, absence of a spiral twisting of the muscle 

 trabeculae. 



In transection the moderator band has an oval form (fig. 28). 

 It is enveloped by a dense areolar connective-tissue capsule. It 

 contains at opposite surfaces, just beneath the capsule and 

 within a looser areolar connective tissue, two unequal divisions 

 of the main right branch of the atrioventricular bundle. The 

 muscle tissue also is collected into two unequal bundles, sur- 

 rounded by a perimysial connective tissue layer. The larger 

 bundle contains centrally a relatively large arteriole with two 

 opposite large periarterial lymphatics, and a relatively small, 

 more peripheral venous comes. 



In the muscular portion of the moderator band contracted 

 areas alternate quite regularly with non-contracted areas (fig. 

 30). The contracted areas show relatively coarse, granular, 

 deep-staining contraction bands which alternate with wider, 

 lighter-staining discs. In the relaxed intervals the telophrag- 

 mata, Q-discs and J-discs are conspicuous. In certain deeper- 

 stained fibers additional H-discs appear (fig. 32). These latter 

 fibers are in the early phases of contraction. The contracted 

 regions stain more deeply, and have a considerably greater 

 diameter than the relaxed intervals (fig. 30). 



The muscle nuclei of the moderator jDand are located in fusi- 

 form sarcoplasmic areas. The myofibrils are arranged peripher- 

 ally, and they are relatively less abundant than in the ventricular 



