306 H. E. JORDAN AND J. B. BANKS 



muscular in nature. They however lack intercalated discs; the 

 intercellular spaces and cement substance haVe no resemblance 

 to intercalated discs. The atrioventricular bundle is originally 

 and definitively cellular. The myocardium is both originally 

 and definitively syncytial, and the intercalated discs arise as 

 secondary modifications at certain levels of the trabeculae 

 (transiently fusiform elements in the early fetal heart) in rela- 

 tion to telophragmata. The closest points of resemblance be- 

 tween the cells of the atrioventricular bundle and the trabeculae 

 of the myocardium are the presence of myofibrils, and their con- 

 tinuity through intercellular spaces and intercalated discs 

 respectively. 



e. The fibers of Purkinje 



We may now more profitably return to the description of the 

 transition area between the atrioventricular bundle and the 

 ventricular myocardium. Here we encounter the fibers of 

 Purkinje. Tawara was the first to describe the continuity of the 

 atrioventricular bundle with the Purkinje fibers. This obser- 

 vation has been repeatedly confirmed by other investigators; 

 vide, e.g., Retzer (21). The Purkinje fibers (cells) are essen- 

 tially identical with the cells of the atrioventricular bundle, 

 only somewhat modified by elongation, fusions into fibers, and a 

 higher degree of differentiation. The latter consists in a rela- 

 tively greater abundance of myofibrils, which are more regu- 

 larly disposed and less distinctly aggregated into smaller bundles, 

 coarser and more conspicuous telophragmata, and the presence 

 of simple band- and step-forms of intercalated discs (figs. 46 to 

 48 and 51). 



The Purkinje-fiber transition-area is definitively a syncytium. 

 This conclusion agrees with De Witt's (2) description of these 

 fibers as forming a syncytium in man, dog, cat, sheep and calf. 

 In the dog embryo De Witt describes the Purkinje fibers as com- 

 posed of 'single short clear cells.' The definitive condition in 

 the beef heart still gives evidence of the originally cellular 

 structure of these Purkinje fibers (fig. 51). 



