STRUCTURE OF MUSCLE. 



163 



each composed of a pair of cells Fig- 80 



containing the densest form of myo- 

 line, and are hence highly refrac- 

 tive while the transparent spaces 

 are constituted by a pair of cells con- 

 taining a more fluid myoline. When 

 the fibrils are collected together so as 

 to form an ultimate fibre or fasciculus, 

 the appearance of the cell is altered; 

 those which look dark in the single 

 fibril, that is, the most refractive, being 

 ranged side by side, constitute the 

 bright band ; while the transparent 

 cells of the single fibril are the shaded 

 stria of the fibre. 



When the ultimate fibril is very 

 much stretched, the two highly re- 

 fractive cells appear each to be double ; 

 while the transparent space is evidently composed of four cells. 



The ultimate fibre of organic life [fig. 81, D, E] is a simple homo- 

 geneous filament, much smaller than the fibre of animal life, flat, and 

 without transverse markings. Besides these characters there may 

 generally be seen a dark line or several dark points in its interior, and 

 not unfrequently the entire fibre appears enlarged at irregular dis- 

 tances. These appearances are due to the presence of the unobli- 

 terated nuclei of cells from which the fibre was originally developed. 

 The fibres of organic life are collected into fasciculi of various size and 

 are held together by dark nuclear fibres similar to those which bind 

 the fasciculi of fibrous tissue (p. 114). 



The development of muscular fibre is effected by means of the 

 formation of nucleated cells out of an original blastema, and the con- 

 version of those cells, by a process already described (p. 5) into the 

 tubuli of ultimate fibres, while their contents, by a subsequent de- 

 velopmental action are transformed into ultimate fibrils. According 



* Structure of the ultimate muscular fibril and fibre of animal life. 



A. An ultimate muscular fibril in the state of partial contraction. 



B. A similar fibril in the state of ordinary relaxation. This fibril measured 

 __i__ of an inch in diameter. 



c. A similar fibril put upon the stretch, and measuring i of an inch in 

 diameter. 



D. Plan of a portion of an ultimate fibre, shewing the manner in which the 

 transverse striae are produced by the collocation of the fibrils. 



Nos. 1,1. The pair of highly-refractive cells; they form the dark parts of the 

 single fibrils, but the bright parts of the fibre D. In the stretched fibril c, each 

 cell has the appearance of being double. 2, 2. The pair of less refractive cells, 

 light in the single fibrils, but forming the shaded stria in D. The transverse 

 septum between these cells is very conspicuous ; and in c two other septa are 

 seen to exist, making the number of transparent cells four. In D, the tier of 

 cells immediately above the dark tier is partially illumined from the obliquity of 

 the light. 



