VOLUNTARY MUSCULAR FIBRE. 



133 



band then is often replaced by two narrow black lines with a 

 light space between them. This is more frequently observed 

 when the rays strike the fibre longitudinally. 



The fibres of invertebrates, though on the same plan as 

 those of higher animals, are better fitted for study, because the 

 elements are farther apart, and because the phenomena of con- 

 traction may, in some cases, at least, be observed under the 

 microscope. 



The muscles from the thorax and legs of large flies are very 

 good. Merkel recommends that they be examined in fresh al- 

 bumen from the egg, in which they will continue to contract. 

 The fibre is crossed by narrow black stripes which, be it re- 

 membered once for all, correspond to the black stripes of ver- 

 tebrate muscle. On each side of these stripes there is a bright, 

 glittering border, which gradually shades off into a dull band, 

 midway between the two stripes. The substance between the 

 black stripes is all of one nature, the difference between its 

 middle and end portions being an 

 optical effect. The dull band corre- 

 sponds to the fine line which high 

 powers reveal in vertebrate muscle. 

 Its greater breadth is due to the 

 greater distance of the black stripes. 



Fibres from the legs and wings 

 of the large water-beetles (Hydro- 

 philus and Dytiscus) are admirable 

 objects. Schafers valuable obser- 

 vations were made on those of the 

 legs. He found the black stripe to 

 consist of a double row of highly 

 refracting granules, which were the 

 ends of dumb-bells embedded in the 

 contractile substance. These struc- 

 tures are arranged side by side, the 



adjacent ends of the dumb-bells forming the stripe, while the 

 handles constitute the slight longitudinal striations. (See Fig. 

 54.) The bright borders are due to the refraction of light from 

 the spherical heads of the dumb-bells. It is clear that they 

 must cause a greater amount of rays to pass through the sub- 

 stance directly beside them than go through the substance 

 midway between them, which latter appears dark in conse- 



FIG. 54. Muscle of large water-beetle 

 (Dytiscus) : a, dull hand ; 6, bright space 

 around ; c, the highly refractive ends of 

 the dumb-bells ; d, the handles of the dumb- 

 bells ; 6, sarcolemma. After Schafer. 



