154 THE SINO-VENTRICULAR BUNDLE. 



Can this organ, which has a continuous activity, contract every minute of the 

 day and night from several months before birth until maturity and death, 

 without its constituent elements being replaced or rejuvenated? We find nothing 

 in the protoplasm of musculature that seems fundamentally different from the 

 protoplasm of any other cells, nor does chemistry reveal any reason why muscle- 

 cells should live far beyond the life period of other cells. It is true that we have not 

 observed the constant degeneration and regeneration that is so evident in many 

 other tissues, but I take the view that they are nevertheless there and can be seen 

 and their presence demonstrated if proper cytologic methods can be found. 



In many organs of lesser activity we have found evidence of such replacement, 

 and no one to-day can fail to see them in epithelial structures. In these cases we 

 have been led to the interpretation of cell-replacement by the fact that we fre- 

 quently recognize that one type resembles closely the cells that predominate in 

 the embryo. In certain instances we may see distinct evidence of the degeneration 

 of the superficial cell layer or of individual cells, and then again these may become 

 evident only in pathological conditions. In the heart we likewise have two or more 

 types of cells, one of which, the cells of the atrioventricular bundle, resembles the 

 embryonic heart-muscle cell. To argue that Purkinje fibers are already differen- 

 tiated in the embryo and do not present in the adult morphological features identical 

 with those found in the embryonic state is puerile. We have no difficulty in recog- 

 nizing as distinctive the various layers of embryonic squamous epithelium, and no 

 one would mistake them for adult; yet no one doubts that the superficial layer in 

 the adult is continually being replaced by the cells beneath which, though not 

 embryonic in appearance, yet divide and multiply as the embryonic cells. The 

 same structures which in the embryo grow and multiply by a process of karyo- 

 kinesis, in the adult absolutely fail to show such evidences of multiplication. Simi- 

 larly, as Bensley (1911) has shown in the case of the pancreas, the duct-cells repre- 

 sent the multipotential elements, which are embryonic in every sense but their 

 appearance, yet nevertheless produce both islet and acinus cells. In other words, 

 if we argue that Purkinje fibers can not be embryonic in type because they are 

 morphologically unlike the embryonic cells, we must likewise concede that because 

 the deeper layers of stratified epithelium are not embryonic in appearance they 

 can not replace the superficial, and that duct-cells can not replace islet cells and 

 acinus cells because they are unlike them. 



As regards the degeneration process, I must state that I have no evidence to 

 present. I have looked for proof of my assertion, but the same difficulty presents 

 itself that is found in studying the origin of myofibrils. After employing methods 

 such as are used for fixing and staining mitochondria, I find in the embryonic mus- 

 culature rods that have a definite color reaction. Suddenly we have fibrils that 

 have a slightly different tinge. If these fibrils are derived from these rods a certain 

 chemical or physical change must have taken place that brings about a change in 

 color reaction. Likewise we would expect a similar change of reaction when the 

 myofibrils breakdown. A single degenerated muscle-fiber is difficult to distinguish. 

 We know that in certain types of degeneration the first sign of degeneracy is the 



