INTERCALATED DISCS OF HEART MUSCLE 169 



fore, an excellent material for testing Heidenhain's interpreta- 

 tion of the intercalated discs as regions where new sarcomeres 

 ('inokommata') are being added to the growing cardiac fibers. 

 According to Heidenhain, the heart can enlarge only by inter- 

 stitial growth, i.e., by terminal additions to the cardiac elements 

 (trabeculae). In the case of the pulmonary arteries, however, 

 there appears no reason for postulating growth by this method; 

 the media here, developed from truncus arteriosus to be sure, can 

 nevertheless, undoubtedly increase in amount in the same way 

 as elsewhere in arteries. Moreover, striated muscle elsewhere does 

 not increase by means, nor show evidence of, intercalated discs. 

 But intercalated discs are present in the pulmonary media; 

 furthermore the greater abundance of the discs here coincides 

 with the time of less rapid growth, and less close developmental 

 relationship with the heart. The presence of intercalated discs 

 in the media of the cardiac end of the pulmonary arteries in the 

 mouse would seem to be correlated with the 'beat' (strain?) 

 here occurring in common with the heart. Still other facts 

 controverting Heidenhain's interpretation that the intercalated 

 discs provide for the 'interkalare Langenwachstum' of the 

 cardiac fibers are: (1) the absence of transition stages between 

 the discs and fully formed sarcomeres; (2) their absence during 

 stages of most rapid (foetal) growth ; (3) their numerical increase 

 even after the heart has attained its normal bulk; (4) their pres- 

 ence in aged and diseased hearts; and (5) their considerable struc- 

 tural variation — every type capable of resolution, however, into 

 very similar elementary units. 



Militating most strongly against Zimmermann's interpretation 

 of the discs in terms of intercellular elements, is our observation 

 of the superficial location of the complex step-like forms. The 

 more complex step-like types appear only where the entire fiber 

 is included within the plane of section. Under such circumstances 

 the successive 'steps' can be traced completely around a fiber 

 by lowering and again raising the level of focus. Many such are 

 then seen to form rings or even short spirals. The discs are of 

 course not complete in the step forms, but are interrupted, con- 



