INTERCALATED DISCS OF HEART MUSCLE 171 



condition of full, or nearly full, contraction. During contrac- 

 tion the substances of the anisotropic and isotropic bands are 

 supposed to intermingle and ultimately change their relative 

 locations. Such a transition condition may account for the 

 indistinct, or absence of, stratification of the anisotropic and iso- 

 tropic substances under the micropolariscope. Moreover, in the 

 contracted condition (according to Tourneux' diagram; see also 

 M. Heidenhain, 'Plasma und Zelle,' '11, p. 677) the darker stripe 

 is at the level of the Z-line, itself supposed to consist of aniso- 

 tropic substance. The Z-line seems to have thickened by reason 

 of the accumulation of 'anisotropic' substance about it, forming 

 the 'contraction band' of Rollet. Thus the darker stripe may 

 indeed represent the Z-line, plus considerable additional aniso- 

 tropic substance. The 'Z-lines' of Heidenhain and Zimmermann 

 correspond apparently to the darker lines in our specimens, repre- 

 senting more likely a 'contraction band.' The illustrations of 

 these investigators are faulty in that they show the darker stripes 

 too delicate, always single, continuous, and too uniform. If the 

 darker stripes are indeed the Z-lines, now grown robust in con- 

 traction, the regions of the fibers containing the intercalated 

 discs are in a state of more pronounced contraction, according to 

 the theory of Rollet and Tourneux. This deduction, then, is in 

 complete accord with our position that the discs are somehow 

 a concomitant of contraction; and further that they represent 

 modifications (irreversible contractions?) of the fibrils at the dark 

 (anisotropic) levels, the anisotropic substance having shifted in 

 contraction to the Z-line. It seems clear that a complete eluci- 

 dation of the question of the structure and function of the inter- 

 calated discs awaits fuller knowledge of the physical and chemical 

 changes undergone by the cardiac myofibrillae during contraction. 

 Our interpretation of the robust, sometimes double, dark stripes 

 (the only stripes visible in non-human material) as the aniso- 

 tropic bands seems in closer accord with our knowledge of skeletal 

 and striped muscle generally. 



