88 



MUSCULAR TISSUE. 



[CH. VI. 



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 Kraiise'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 refrangibility. 



If cross membranes do exist they are net very resistant ; this 

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



B 



Fig. 107. A. Portion of a medium-sized human muscular fibre, x 800. B. Separated 

 bundles of fibrils equally magnified ; a, a, larger, and b, b, smaller collections ; e, still 

 smaller ; d, d, the smallest which could be detached, possibly representing a single 

 series of sarcous elements. (Sharpey.) 



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

 the Myorectes, was observed crawling up the interior of the con- 

 tractile 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 



