VOLUNTARY MUSCULAR FIBRE. 



135 



fluid, transparent ground-substance. This is crossed by the 

 black stripes with shining borders, such as have been described. 

 The black stripe usually appears 

 granular, and may be divided into 

 two parallel rows of granules. Some- 

 times the two borders are equally 

 bright ; sometimes one much out- 

 shines the other. Some fibres are 

 found which, not being subjected 

 to any tension, are much more 

 drawn together. The black bands 

 are, perhaps, only half so far apart 

 as in the case just described. They 

 are never divided into two, and 

 though some appear granular, 

 others are homogeneous. The edges 

 of the fibre are no longer straight, 

 but slightly scalloped, the centre 

 of each projection being midway 

 between the black stripes. (See 

 Fig. 55.) 



When active contraction takes 

 place, the whole fibre is involved, 

 and presents nothing but a series 

 of transverse black and white bands 

 with scalloped edges. To study the 

 successive stages of contraction we 

 must wait until it is feeble and in- 

 volves but a small part of the fibre. 

 It runs like a wave from one end of 

 the fibre to the other, pauses a mo- 

 ment, and then runs back again, 

 and sometimes starts anew, but with 

 diminished force. It is hard to fol- 

 low the steps, for the elements are 

 changing their shape and position 

 at the same time ; the black stripes 

 become broader, less granular, and 

 each runs toward its neighbor in 

 the direction of the wave ; the gray band disappears, and the 

 edge of the fibre bulges. As the elements in front of the 



FiQ. 55. Semi-diagrammatic repre- 

 sentations of muscular fibres of Gyrinus. 

 Fibre supposed to be in situ and showing 

 the different appearances which the black 

 stripe may present. The wave of contrac- 

 tion is travelling toward A. At C we 

 notice that a part of the fibre, after con- 

 traction, has been subjected to stretching 

 after the passage of the wave; at B the 

 elements are in a state of active contrac- 

 tion. The longitudinal striation made 

 unavoidable in the woodcut, does not, as a 

 rule, exist in the living fibre. 



