170 H. E. JORDAN AND K. B. STEELE 



sisting of 'steps' at different anisotropic levels connected by deli- 

 cate membranes spanning the intervening 'isotropic' bands. 



Attempts to alter the number of the discs experimentally by 

 stimulation with varying strengths of an electric current have 

 proved unsuccessful. Nor are they appreciably affected in tissue 

 fixed in a state of rigor mortis. Material is now being collected 

 for a study of these discs in various pathological conditions of the 

 heart. Discs could not be demonstrated in Limulus heart. 



A comparison of the illustrations accompanying the articles 

 by Zimmermann's students, Werner and Palczewska, shows that 

 in the human heart the discs are commonly bounded on both 

 sides by the so-called ' Krause's Z-lines,' whereas in lower mammals 

 the discs are narrower than the space between two Z-lines, and 

 consequently bounded on only one side by this line. Heidenhain 

 likewise illustrates the discs in human heart muscle as bounded 

 on both sides by a Z-line. Granting that this interpretation 

 of the striped condition is correct, especially then in man do the 

 discs correspond to the Q or reputed anisotropic levels — as we 

 have urged on the basis of a different interpretation — and in so 

 far support our contention that they represent modifications of 

 the fibrillae at anisotropic levels. But all of these illustrations 

 differ from the far more widely prevalent condition of our ma- 

 terial, in that the so-called 'Z-line' or 'Krause's membrane' is 

 represented much too delicate. The darker stripe (seen both in 

 fresh and stained material) is usually stout, and frequently almost 

 half as wide as the alternate lighter segments; this is more par- 

 ticularly the case in human material. Since the sarcolemma is 

 only occasionally, and then only imperfectly, festooned between 

 these lines no definite suggestion is given of a 'Krause's mem- 

 brane.' Especially in the regions where the discs appear abun- 

 dantly are the dark stripes robust. Having naturally directed 

 our attention chiefly to these regions, we interpreted appearances 

 as indicating a condition of semi-contraction, according to the 

 illustration of Tourneux (see Traite d' Histologic; par Prenant, 

 Bouin et Maillard; tome 1, p. 442. Paris). 



It seems possible, however, in the light of this illustration and 

 the theoretical interpretation involved, that the fibers are in 



