INTERCALATED DISCS OF THE HEART OF BEEF 307 



Figure 45 shows a transection of one of the terminal branches 

 of the left atrioventricular bundle where it passes under the 

 endocardium to unite with the ventricular myocardium. The 

 fibers are still enveloped by a connective tissue sheath enclosing 

 a subjacent lymphoid space. The nuclei lie in a finely-granular, 

 delicately-reticular, central, sarcoplasmic area of fusiform shape 

 (figs. 46 to 48). The myofibrils are peripherally arranged, in 

 general in delicate radial lamellae (fig. 47). Simple discs occur 

 sparsely, in close connection with the telophragmata (figs. 46 

 and 48). Some are located at nuclear levels. Deeply-staining 

 connecting-membranes may occur in the step-discs (fig. 48). 

 They probably represent the fused sarcolemmae along the line 

 of union of two cells which have fused in the formation of a 

 fiber. 



Figure 46 would at first consideration seem to furnish incon- 

 trovertible proof of the inadequacy of the interpretation of in- 

 tercalated discs as intercellular cement substances, for here we 

 have a single elongated cell upon which appear several inter- 

 calated discs, at least one of which is supernuclear in position. 

 But conditions like that illustrated in figure 51 rob this illus- 

 tration of its apparent finality in this connection, since it indi- 

 cates that these discs may actually be related to lateral sur- 

 faces of fusion. This possibility, moreover, in part at least 

 explains the supernuclear position of intercalated discs. An 

 attempt will be made below to harmonize the apparent dis- 

 crepancies here suggested. 



When we pass now again to the moderator band, we find the 

 same series of events. The cells of the atrioventricular bundle 

 (fig. 50) pass more or less abruptly into Purkinje fibers (fig. 49, 

 above) , and the latter by more gradual stages pass into the myo- 

 cardial meshwork (fig. 49, below) where simple band-forms of 

 discs appear. 



The difference in shape of the nuclei in these several regions is 

 also noteworthy. In the cells of the atrioventricular bundle the 

 nuclei are generally spherical or stoutly oval, and paired; in the 

 Purkinje fibers they are still substantially of the same shape, 



