82 MUSCULAR TISSUE [CH. VI. 



depth, a fine dotted line is seen bisecting each light stripe ; this has 

 been variously termed Dobie's line, or Krauze's membrane (fig. 107). 

 At one time this was believed to be an actual membrane continuous 

 with the sarcolemma. It is probably very largely an optical effect, 

 caused by light being transmitted between discs of different refrangi- 

 bility. 



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

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

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

 Myorectes, was observed crawling up the interior of the contractile 



Fio. 107. A. Portion of a human muscular fibre, x 800. B. Separated bundles of fibrils equally 

 magnified; a, a, larger, and b, b, smaller collections; c, still smaller; d, d, the smallest which 

 could be detached, possibly representing a single series of sarcous elements. (Sharpey.) 



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 d'isc. 



4- muscular fibre may not only be broken up into fibrils or muscle- 



