VOLUNTARY MUSCLE 



69 



CH. VI.] 



plasm is singly refracting or isotropous; it remains dark in the 

 dark field of the polarising microscope. The fibrils or sarcostyles 

 are in great measure doubly re- 

 fracting or anisotropous, and ap- 

 pear bright in the dark field of the 

 polarising microscope. The sarco- 

 style, however, is not wholly doubly 

 refracting; the sarcous elements 



it 





Fin. 83. Wave of contraction passing over a mus- 

 cular fibre of water-beetle. R, R, Portions of 

 the fibre at rest ; c, contracted part ; i, i, inter- 

 mediate condition. (Schiifer.) 



FIG. 84. This figure (after Engelmann) illus- 

 trates the appearance of a muscular fibre 

 as examined in ordinary light (left-hand 

 side) and in polarised light (right-hand 

 side). In the upper part of the diagram 

 the fibre is not contracted, in the lower 

 part it is contracted. The dark bands 

 are seen to be bright by polarised light, 

 owing to their being doubly refracting or 

 anisotropous; during contraction, fluid 

 passes from the singly refracting or 

 isotropous light band into the doubly 

 refracting dark band, which, in conse- 

 quence, becomes widened out. 



are doubly refracting, and the clear intervals are singly refracting. On 

 contraction there is no reversal of these appearances, though of course 

 the relative thickness of the singly refracting intervals varies inversely 

 with that of the doubly refracting sarcous elements (see fig. 84). 

 The meaning and causation of the optical appearances of striated 



