STRIATED OR VOLUNTARY MUSCLE. 



463 



lines, or intercalated discs, has been interpreted as expressing a later differentiation 

 into fibre-cells, the cross-lines being regarded as indicating the limits of the compo- 

 nent fibres. According to Jordan, ^ how- 

 FiG. 4S2. ever, the intercalated discs are neither cell 



boundaries ( Zimmermann) nor growth- 

 zones (Heidenhain), but must be inter- 

 preted in terms of the ultimate fibrillae, 

 not of the whole fibre, and are due to 



Fig. 483. 



Muscle-fibres of human heart. X 375. 



f 



J 



I ^h-lt « 



ma* 



•J, 





I* 









Diagram showing the form and arrangement of the 

 intercalated discs. {M. Heidenhain.) 



accumulations of anisotropic substance, associated in some way with contraction. 

 The heart muscle possesses a large amount of sarcoplasm, as evidenced by the con- 



FiG. 484.  



Capillary blood-vessel 











i*^-% 





Undifferentiated sarcoplasm 





Nucleus 



■m 



Fibres of cardiac muscle in transverse section. X 375. 



siderable accumulation surrounding the nucleus, as well as the thicker strata separat- 

 ing the muscle-columns. 



^ Anatomical Record, vol. v, No. ii, 1911, 



