MUSCLE TISSUE 



119 



sar CO plasm (interfibrillar substance) and shows 

 (Fig. 6i), longitudinal striations and cross 

 striations. 'V\\v longitudinal striations are 

 due to i^arallel running ulliniatc t'lbrillic (Fig. 

 Oi, a), which lie in and are more or less sep- 

 arated from one another by the sarcoplasm. 

 Each iibrilla when examined, unstained, by 

 reflected light is seen to be comj)osed of alter- 

 nating light and dark segments. As like 

 segments lie in the same transverse plane, 

 the whole muscle fibre appears composed of 

 alternate light and dark bands (Figs. 6i 

 and 62), and this distinction is maintained 

 even in stained specimens, as the light bands 

 stain little if any, with most staining agents, 

 while the dark bands react strongly to stains. 

 The light band is composed of a singly re- 

 fracting (isotropic) substance, the dark band 

 of a doubly refracting (anisotropic) sub- 

 stance. Through the middle of the light 

 band runs a tine line {Krauses line, Fig. 61, r, 

 62, r), while an even finer line (Hensen's line, 

 Fig. 61, (7, 62, a) can sometimes be seen run- 

 ning through the middle lighter portion of 

 the dark band. Both Krause's and Hensen's 

 lines cross the intervening sarcoplasm as well 

 as the fibrillae and extend to the sarcolemma, 

 thus completely crossing the fibre. As all of 

 these structures run through the entire thick- 

 ness of the fibre, they in reality constitute 

 discs of muscle substance (Fig. 62). By 

 means of certain chemicals these discs may be 

 separated, the separation taking place along 

 the lines of Krause. Each "muscle disc" 

 thus consists of that portion of a fibre in- 

 cluded between two adjacent lines of Krause 

 and is composed of a central dark disc, and on 

 either side one-half of each adjacent light disc. 

 A muscle fibre is thus seen to be divisible 

 longitudinally into ull Fig. 6 1 , a) , 



two sets of striations 



m 

 Fig. 60. — Semidiagram- 

 matic Drawing of Parts of 

 two Muscle Fibres which 

 have been broken, showing 

 the relations between Mus- 

 cle Substance Proper and 

 Sarcolemma (Ranvier.) 

 m, a, Retracted ends of 

 muscle substance, between 

 which is seen the sarcolem- 

 ma with several adherent 

 muscle nuclei; B, thin layer 

 of muscle substance which 

 has adhered to the sarco- 

 lemma; n, muscle nucleus; 

 s, sarcolemma; p, space 

 between sarcolemma and 

 muscle substance. 



transversely into mus- 



