286 H. K. JORDAN AND J. B, BANKS 



stresses, in essence, irreversible contraction bands (Jordan and 

 Steele (14) ), Dietrich (3) has proposed what appears to be a 

 modification of Marceau's original interpretation in terms of a 

 tendinous structure, namely, that the intercalated discs are con- 

 stant structures formed during later myocardial histogenesis to 

 provide for the functional coordination of previously incoordi- 

 nated myofibril-bundles in the branching trabeculae of the 

 muscle-plexus. 



The investigation was begun with a study of the ventricular 

 myocardium of the adult heart. The object was to discover the 

 different types of discs with respect of intrinsic structure, and 

 their varying relations to the constituent elements of the fiber: 

 the telophragmata, nuclei, sarcolemma, etc. Comparative ob- 

 servations were made between the right and left ventricles, be- 

 tween the atria and ventricles, and between the papillary muscle, 

 the columnae carneae, and the general myocardium, in order to 

 determine structural and numerical variations. Study was then 

 directed to the atrioventricular bundle, and especially to the 

 area of transition between its termination as Purkinje's fibers 

 and the myocardium, with the expectation of finding here some 

 further clue to the significance of the discs. This expectation 

 was to a considerable degree realized as will be described below. 

 The study was completed by an examination of young and 

 fetal hearts, in an attempt to discover the time and mode of 

 origin of the discs. 



The least tenable of the above-mentioned hypotheses appears 

 to be that of Heidenhain, namely, that the intercalated discs are 

 developing sarcomeres. It fails by reason of the facts, chiefly, 

 that the discs have a definite developmental history of their 

 own, that they do not occur in their definitive condition during 

 the stages of very rapid earlier fetal growth, that the sarcomeres 

 of fetal myocardium do not resemble the initial discs, and that 

 they do not disappear after the heart has attained its maximum 

 physiologic development. Heidenhain' s explanation that the 

 persistent discs in the lull-grown heart may be of the nature of 

 developmental vestiges, somewhat like the epiphyseal lines of 

 long bones, seems inapplicable. Moreover, the different types 



