228 E. A. Schäfer, 



opposite the constrictions which represent the junctions of the muscle 

 segments, while the intermediate parts are entirely bright. Examination 

 of the intermediate stages renders it clear how this result is produced 

 (Fig. 1, B). The dark stripes of the contracted muscle (0) are seen 

 to be brought about by the accumulation of the sarcoplasm in the 

 interstices of the muscle-columns opposite the junctions of the successive 

 segments of which the columns are made up: the sarcoplasm being 

 ostensibly forced into this position by the swelling and bulging of the 

 sarcous elements and the accompanying relative constriction of the part 

 of each segment near the membrane of Krause. I say „relative con- 

 striction" because the whole of each segment ultimately broadens as 

 the fibre contracts, but the broadening is greatest opposite the sarcous 

 element and least opposite the membrane of Krause, so that the part 

 of the segment near this membrane is relatively constricted. This 

 necessarily produces an increase in size of the interstices at the level 

 of the junctions of the successive segments, and here therefore the 

 sarcoplasm must accumulate. These accumulations of sarcoplasm oc- 

 curring at regular intervals along the fibre produce, as in the acid 

 and gold preparation (Figs. 8, 9) the effect of dark lines traversing 

 the fibre. These lines, in any optical longitudinal plane of the fibre, 

 would appear interrupted, the enlargements which form them being 

 separated from one another by the constricted part of the sareost} T les 

 traversed by thé membranes of Krause. In the optical section, how- 

 ever, it is not always possible to obtain the appearance of a pure 

 mathematical plane, for the effects of the similar enlargements in 

 planes immediately above and below the one which is exactly in focus 

 may fill up the intervals in the interrupted transverse lines and cause 

 these to appear continuous across the fibre. This appearance of con- 

 tinuity is favoured also by the effect which the constrictions of the 

 sarcostyles and the membranes of Krause produce upon the light. 

 The dark lines of the contracted muscle do not therefore represent 

 the dim bands of the resting or extended muscle, but are due to the 

 relative constriction of the sarcostyles and the accompanying accumu- 

 lations of sarcoplasm which occur at about the level of the membranes 

 of Krause, 



