CH. VI.] 



VOLUNTARY MUSCLE 



65 



FIG. 79. Transverse section through 

 muscular fibres of human tongue. 

 The nuclei are deeply stained, 

 situated at the inside of the sar- 

 colemma. Each muscle fibre 

 shows "Cohnheim's areas." 

 x 450. (Klein and Noble Smith.) 



up of what are called fibrils or sarcostyles ; and the longitudinal 

 interstitial substance with cross networks comprising the reticulun) 

 just referred to is called sarcoplasm. By 

 the use of certain reagents, such as osmic 

 acid or alcohol, the fibrils may be com- 

 pletely separated from one another. 



A transverse section of a muscular 

 fibre (fig. 79) shows the sections of these 

 fibrils; the interstitial sarcoplasm is re- 

 presented as white in the drawing. The 

 angular fields separated by sarcoplasm may 

 still be called by their old name, areas of 

 Cohnheim. 



If, instead of focussing the surface of 

 a fibre, it is observed in its depth, a fine 

 dotted line is seen bisecting each light 

 stripe; this has been variously termed 

 Dobie's line, or Krauses membrane (fig. 

 80). At one time this was believed to be 

 an actual membrane continuous with the sarcolemma. It is prob- 

 ably very largely an optical effect, caused by light being transmitted 

 between discs of different refrangibility. 



If cross membranes do exist they are not very resistant ; this was 

 well shown by an accidental observation first made by Kuhne, and 

 subsequently seen by others. A minute thread-worm, called the 

 Myorectes, was observed crawling up the interior of the contractile 

 substance of a muscular fibre; it crawled without any opposition 

 from membranes, and the track it left, closed up slowly behind it 

 without interfering with the normal cross-striations of the contractile 

 substance. This observation strikingly illustrates the fact that the 

 contractile substance in a muscular fibre is fluid, but only semi-fluid, 

 for the closing of the thread-worm's track occurred slowly as a hole 

 always closes in a viscous material. 



Another appearance which is sometimes seen is a fine clear line 

 running across the fibre in the middle of each dark band. It is 

 called Hensen's line or disc. 



A muscular fibre may not only be broken up into fibrils, but 

 under the influence of some reagents, such as dilute hydrochloric 

 acid, it can be broken up into discs, the cleavage occurring in 

 the centre of each light stripe. Bowman, the earliest to study 

 muscular fibres with profitable results, concluded that the subdivision 

 of a fibre into fibrils was a phenomenon of the same kind as the cross 

 cleavage into discs. He considered that both were artificially pro- 

 duced by a separation in one or the other direction of particles of the 

 fibre he called " sarcous elements." The cleavage into discs is how- 



