STRIATED MUSCLE 29.'] 



or under ordinary circumstances somewhat narrower. 

 With a high power a very thin dark granular line equi- 

 distant from each dim band is discernible in each bright 

 band, dividing the bright band into two. As these 

 apjjearances remain when the object glass is focused 

 through the whole thickness of the fibre, it follows that 

 the dim bands, the granular lines, and the clear spaces on 

 each side of each granular line, represent the edges of 

 segments of different optical characters, which regularly 

 alternate through the whole length of the fibre. Let the 

 excessively thin segments, of which the thin gi-anular 

 lines represent the edges, be called g, the thicker, pellucid 

 segments of which the bright bands on each side of a 

 granular line represent the edges, B ; and the thickest 

 slightly opaque segments of which the ground glass like 

 dim bands are the edges, D. Then the structure of the 

 fibre may be represented byZ). B. g.B.D.B. g.B. indefinitely 

 repeated, and one inch of length of fibre will contain 

 about 30,000 such segments, or alternations of structure. 



In a perfectly unaltered living fibre the striated sub- 

 stance presents hardly any sign of longitudinal striation ; 

 but near to the surface of the fibre in mammalian muscle, 

 though at various points in the depth of the fibre in the 

 muscles of the frog, faint indications are to be observed 

 of the existence of cavities each filled by a nucleus, 

 surrounded by a small amount of protoplasm (Fig. 88, 

 A, n). 



As the muscular fibre dies it undergoes a rapid altera- 

 tion : — (i) parallel longitudinal strije, often less than 2fi, 

 apart, appear, in greater or less numbers until sometimes 

 the striated substance appears broken up into a mass of 

 fine delicate fibres ; (ii) the dim bands become much more 

 opaque, and hence the transverse striation appears better 

 marked, until the dim bands may appear like sharply 

 defined discs ; (iii) the nuclei acquire sharp irregular 

 contours and become much more conspicuous, and (iv) 

 especially under certain circumstances and after particular 



