INTERCALATED DISCS OF THE HEART OF BEEF 303 



cement-substances be normally and constantly pierced by 

 nuclei. 



The peripheral position of the discs is well illustrated also in 

 the type shown in figure 33, where on the upper surface the disc 

 is in series with the telophragmata, while on the lower surface 

 it is bounded along both borders by these membranes, having 

 here suffered a slight spiral distortion. 



d. The atrioventricular connecting bundle 



Before describing the transition from Purkinje fibers to the 

 cardiac muscle, it becomes necessary briefly to describe the 

 structure of the atrioventricular bundle. This has already been 

 done more or less completely by Tawara (22), by De Witt (2), 

 by Lhamon (17) and by King (15), and we shall touch only 

 certain details which relate themselves to our investigation of the 

 intercalated discs. The atrioventricular bundle is distinctly cellu- 

 lar in structure. This conclusion is in accord with the descrip- 

 tions of all of the above-named investigators except De Witt, who 

 regards the bundle as a syncytium. Moreover, all agree that 

 certain of the myofibrils of the constituent 'cells' have an un- 

 broken course through the intercellular spaces. Agreement is 

 complete also with respect to the descriptions of the shape of 

 the cells, as somewhat modified spherical or polyhedral elements 

 with a crenated or serrated contour. The bi- tri- or quadri- 

 nucleated condition of the cells has also been noted. In general 

 our findings agree closely with those of Tawara (22) and of King 

 (15). 



In stained sections the constituent cells of the atrioventricu- 

 lar bundle are conspicuous (fig. 35). Their borders appear 

 serrated, the myofibrils pass directly without apparent modifi- 

 cation through the intercellular spaces, and the majority of the 

 cells are binucleated (figs. 35, 36, 37, 42, 43 and 50). The sev- 

 eral divisions of the branches of the atrioventricular bundle are 

 enveloped by a dense fibroelastic capsule, between which and 

 the muscular columns occurs a space (figs. 43 and 45). This 

 had been previously noted by Tawara (22), by Curran (1), by 



