CHAP. VI.] 



THE MUSCLES. 



53 



muscle ; the non-striated is usually withdrawn from the control 

 of the will, and is often termed involuntary muscle. 



Voluntary, striated muscle is composed of long slender fibres 

 measuring on an average about -%$-$ inch in diameter, but hav- 

 ing a length of an inch or more. Each fibre consists of three dis- 

 tinct elements : (1) contractile substance, forming the centre and 

 making up most of the bulk of the fibre ; (2) nuclei, which lie 

 scattered upon the surface of the contractile substance ; (3) the 

 sarcolemma, a thin, structureless tube, which tightly encloses 

 the contractile substance and the nuclei. 



If we examine a fresh muscle fibre microscopically, we see 

 that the contractile substance is marked with very fine indistinct 

 longitudinal lines, or strise; and if we treat the fibre with 

 certain chemical agents, 

 we find by slightly teas- 

 ing it with a needle, or 

 some other delicate instru- 

 ment, that it breaks up 

 along the longitudinal 

 lines into a number of fine 

 fibrils or fibrillse. Again, 

 if we examine the fibre 

 still further, we find that 

 in addition to the 

 tudinal striation it 

 crossed by more 

 narrow dark and light 



bands Or Stripes, the rela- transverge cleav age; , ~b, partially detached 



tive width of the Stripes disks; &' detached disk, more highly magnified, 



, . ,1 showing the sarcous elements. 



varying according as the 



fibre is seen in a state of contraction or relaxation. So that if 

 now we soak a fibre in an acid solution for twenty-four hours, 

 and then tease it, we find that instead of breaking up longitudi- 

 nally into fibrill^e, it breaks across into thin disks. We thus see 

 that by breaking up in these two directions we may conceive of 

 the fibre as being resolvable into a multitude of tiny structures, 

 which elementary structures have been called sarcous elements. 

 It is believed by many observers that these sarcous elements are 

 definite aii^ independent structures, and that they are joined 

 together side by side, and end to end, by a peculiar cementing 



s 



FIG. 52. FRAGMENTS OF STRIPED FIBRES, 

 SHOWING A CLEAVAGE IN OPPOSITE DIREC- 

 TIONS. (Magnified 300 diameters.) A, longitu- 

 dinal cleavage; c, fibrillae separated from one 

 another at the broken end of the fibre; c' c", 

 single fibrils more highly magnified, in c' the ele- 

 mentary structures are square, in c" round; B, 



