THE SINO- VENTRICULAR BUNDLE. 151 



using an acid fixative in order to decalcify and soften the tissue prohibits the use of 

 finer cytologic stains. It is therefore very fortunate that in the pig this node is not 

 restricted to this limited area but streams down into the ventricles, accompanying 

 and gradually merging into the third part. 



In figure 1 this area is shown. The section is taken parallel to the ventricular 

 septum and lies a little below the atrial junction. At a is shown the ventricular 

 musculature with many nuclei in the same fiber; between a and b is loose connective 

 tissue which forms Curran's (1910) bursa and which can readily be injected; b 

 shows the third part and c the second part. It will be noted that it consists of an 

 intricate interlacement of fibrils that show but little cross striation. These fibrils 

 constitute a syncytium that incloses clear areas containing nuclei. It will be noted 

 that there are generally two nuclei in the same area. In several places, where but 

 one nucleus is drawn, another one can be found in a lower level of the section. 

 Again the membrane of one nucleus is directly continuous with that of another. 

 Unquestionably we are dealing here with amitotic nuclear division. 



The transition from the second to the third part of the system is greater than 

 between any other two parts. The third part or ventricular stem (6) shows an 

 enormous increase in the number of fibrils. It is difficult to determine whether this 

 increase has been brought about by a splitting of the fibrils or whether it has 

 developed from a differentiation of the cytoplasm. Each nucleus is now surrounded 

 by its own perinuclear space. The bundle is here divided up into a number of 

 strands by the invasion of delicate collagenic fibers and presumably nerve-fibers. 



It seems that we are dealing here also with a syncytium, as far as the fibrils 

 are concerned. Nevertheless, there is a definite condensation of fibrils midway 

 between the nuclei. This gives it an appearance that would suggest cellular delim- 

 itations. The question whether the fibrils would then be considered as exoplasm 

 or as extracellular deposits lies without the scope of this paper. 



In the fourth or preterminal part (see fig. 2) we find the cell area increased in 

 size and two nuclei are found to be situated in the same perinuclear space, which 

 again indicates a multiplication of nuclei, as it does in the second part. The fibrils 

 are less numerous in a given area than they are in the second part of the bundle. 

 We find a considerable increase in the amount of collagenic fibers, which we recog- 

 nize in the gross as the sheath of the bundle. 



In the fifth or terminal part (see fig. 3), which constitutes the Purkinje 

 fibers proper, we find that the nuclei have moved to the border of the cell area. 

 Each one is surrounded by its own perinuclear space. In the pig we find as a rule 

 four nuclei for one cell area. The striation of the fibrils is a little more marked than 

 it is in the other parts of the bundle, but not as much as in cardiac muscle proper. 

 The collagenic fibers are restricted to the border. A few delicate strands can be 

 seen which delimit the cell areas. In various parts (shown at d) of this Purkinje 

 system we see transition to the cardiac musculature; this transition consists in an 

 increase and a more longitudinal arrangement of the fibrils and a further multipli- 

 cation of the number of nuclei. In the pig this nuclear multiplication brings about 

 rows of nuclei, as many as eight, that are situated in the central core of the fiber. 



