160 



H. E. JORDAN AND K. B. STEELE 



(blocks). of the discs are most likely local contractions in the fibrils, 

 the connecting deeply-staining 'membrane' being formed of the 

 lateral coalescence of less extreme locally contracted portions 

 of intervening fibrils; or it represents perhaps a thickened, dis- 

 torted membrane of Heidenhain; or it may be the product of 

 the combination of both of these elements. This is a more com- 

 mon type of disc in young hearts. 



■v' 1 ., ^ J . . 



> * a-vS l ■■'■ v}- ? 1: hl'i 



10 



11 



12 



Fig. 10 Guinea-pig heart muscle fiber, showing the super-nuclear position of 

 a three-step disc. 



Fig. 11 .Guinea-pig heart-muscle fiber, showing compact and granular varie- 

 ties of plate-like discs. The loosely granular discs are the less highly developed 

 type. 



Fig. 12 Guinea-pig cardiac fiber, with block-like discs at different aniso- 

 tropic levels and connected by delicate granular membranes giving a zig-zag 

 appearance to the complete structure. The 'blocks' are locally contracted por- 

 tions of the fibrillae, the connecting membranes represent probably 'anisotropic' 

 granules in linear arrangement, or one of the 'membranes', i. e., M or Z. 



4. Chipmunk 



Heart muscle of the chipmunk exhibits all the types of discs 

 above described. It shows also especially clearly and abundantly 

 a type illustrated in fig. 13. Here the discs appear as oval thick- 

 enings on the fibrils. In other locations three or four (still more 

 elongated) may appear at successive anisotropic levels, giving 

 the appearance of deeply-stained striped insect muscle. Fig. 14 



