CHAP, ii.] THE CONTRACTILE TISSUES. 91 



for the most part close under the sarcolemma are a number of 

 nuclei, oval in shape with their long axes parallel to the length of 

 the fibre. Around each nucleus is a thin layer of granular looking 

 substance, very similar in appearance to that forming the body of 

 a white blood corpuscle, and like that often spoken of as un- 

 differentiated protoplasm. A small quantity of the same granular 

 substance is prolonged for some distance, as a narrow conical 

 streak from each end of the nucleus, along the length of the fibre. 



With the exception of these nuclei with their granular looking 

 bed and the end-plate or end-plates, to be presently described, all 

 the rest of the space enclosed by the sarcolemma from one end of 

 the fibre bo the other appears to be occupied by a peculiar material, 

 striated muscle-substance. 



It is called striated because it is marked out, and that along 

 the whole length of the fibre, by transverse bands, stretching right 

 across the fibre, of substance which is very transparent, bright sub- 

 stance, alternating with similar bands of substance which has a dim 

 cloudy appearance, dim substance; that is to say the fibre is marked 

 out along its whole length by alternative bright bands and dim bands. 

 The bright bands are on an average about 1 p or 1'5/x. and the 

 dim bands about 2*5 /j, or 3 p thick. By careful focussing, both 

 bright bands and dim bands may be traced through the whole 

 thickness of the fibre, so that the whole fibre appears to be com- 

 posed of bright discs and dim discs placed alternately one upon 

 the other along the whole length of the fibre, the arrangement 

 being broken by the end-plate and here and there by the nuclei. 



When a muscular fibre is treated with dilute mineral acids 

 it is very apt to break up transversely into discs, the sarcolemma 

 being dissolved, or so altered as easily to divide into fragments 

 corresponding to the discs; and a disc may thus be obtained so 

 thin as to comprise only a single dim or bright band, or a band 

 dim or bright with a thin layer of bright or dim substance above 

 and below it, "the cleavage having taken place along the middle of 

 a band. 



When treated with certain reagents, alcohol, chromic acid, &c., 

 the fibre is very apt to split up (and the splitting up may be 

 assisted by " teasing ") longitudinally into columns of variable 

 thickness, some of which however may be exceedingly thin, and 

 are then sometimes spoken of as ' fibrillse.' Both these discs and 

 fibrillae are artificial products, the results of a transverse or 

 longitudinal cleavage of the dead, hardened or otherwise prepared 

 muscle-substance. They may moreover be obtained in almost 

 any thickness or thinness, and these discs and fibrillae do not by 

 themselves prove much beyond the fact that the fibre tends to 

 cleave in the two directions. 



The living fibre however, though at times quite glassy looking, 

 the bright bands appearing like transparent glass and the dim 

 bands like ground glass, is at other times marked with longitudinal 



