STRIATED OR VOLUNTARY MUSCLE. 



459 



possessing a fine grain the secondary bundles correspond with the fasciculi, but in 

 muscles of coarse texture each fasciculus includes a number of secondary bundles 

 between which the ramifications of the epimysium extend. The characteristic picture 

 presented in transverse sections of muscles (Fig. 474) illustrates the relation of the 

 fibres to the larger groupings of the muscular elements. 



Structure of the Muscle-Fibre. — Each fibre corresponds to a greatly 

 elongated multinucleated muscle-cell, and consists of a sheath, or sarcolemtna, and 

 the contained sarcous substmice. 



The sarcolemma forms a complete investment of the fibre and alone comes 

 into contact with the surrounding connective tissue by which the muscle-fibres are 

 attached either to one another or to the tendinous structures upon which they exert 

 their pull. The sarcolemma is a transparent, homogeneous, elastic membrane which 

 so closely invests the contained sarcous substance as to be almost invisible under 

 ordinary conditions. Being of greater toughness than the muscle-substance, it often 

 withstands mechanical disturbance, as teasing, while the latter becomes broken ; 

 where such breaks occur the sarcous substance sometimes contracts within the sarco- 

 lemma, which at the points of fracture then becomes visible as a delicate tubular 

 sheath stretching across the space separating the broken ends of the more friable 



w^ 



-M 



Fig. 476. 



fii 



In ^m ^^ 



' ■QES PKS Hi'-J' 



Diagrams illustrating; structure of striated muscle-fibre. A, usual view; 5, correct view, showing sustentacuiar 

 septa continued across fibre from sarcolemma; C, septa shown after vanadiuni-hcematoxyliu staining. 2^, interme- 

 diate disk {Zwischenscheibe) ; J. light band; Q^ transverse disk (_Querscheibe)\ M, median disk {Mittelsckeibe) ; 

 .S, sarcolemma. {After M. Heidenliain.) 



J 



'WN*^ 



sarcous substance (Fig. 475). In teased preparations the sarcolemma is sometimes 

 also seen projecting beyond the sarcous substance, as a coat sleeve covers the stump 

 of an arm. 



The sarcous or muscular substance within the sarcolemma in turn consists 

 of two parts, the less differentiated passive sarcoplasm and the highly specialized 

 contractile Jibrillce in which the active changes take place resulting in the contraction 

 of the muscle-fibre. 



Since the highly characteristic appearance of cross-striation which distinguishes 

 the fibres of voluntary muscle, as well as supplies the reason for its designation as 

 striped or striated, depends upon the arrangement of the contractile fibrillae, the 

 details of the latter first claim attention. 



The cross-striation consists of alternate dark and light bands which extend the 

 entire width of the fibre and depend upon the differentiation of the contractile fibrillae 

 into segments of greater or less density. Close lateral approximation of the more 

 dense and deeply staining segments in the fibrillae, lying side by side within the sarco- 

 lemma, produces the dark band ; the similar relation of the less dense and non-staining 

 segments produces the impression of the light band. If it were possible to isolate the 

 individual contractile fibrillae, each would present the details shoavn in the accompany- 

 ing diagram (Fig, 476). The dark, broad transverse disk {Q) of doubly refracting, 



