VOLUNTARY MUSCULAR FIBRE. 



131 



The existence of a sarcolemma being admitted, it is clear 

 that it must be highly elastic so as to accommodate itself 

 to the changes both of length and breadth which the fibre 

 undergoes. The phenomena of contraction show, moreover, 

 that it must be attached at definite points to the muscular 

 substance. 



Fresh muscular fibre of a vertebrate animal, when teased 

 out and examined under a moderately high power, presents a 

 series of alternate black and white cross stripes, which are 

 held to be characteristic of voluntary muscle. (Fig. 53.) This 

 appearance is beautifully distinct in some 

 fibres, and very vague in others. It may 

 vary greatly in different parts of the same 

 fibre ; the stripes may run perfectly straight 

 across the fibre ; they may present a uni- 

 form curve, or they may be interrupted at 

 intervals, some parts of the line being in 

 advance of others. (See Fig. 63. ) 



As a fibre taken from an animal im- 

 mediately after death naturally draws it- 

 self together (without, however, necessarily 

 presenting the phenomena of physiologi- 

 cal contraction), it is desirable to ascertain 

 whether this modifies the appearances. To 

 do this, fibres from a recently killed animal should be ex- 

 amined in a state of extension. A cut should be made in the 

 body of a muscle, a few fibres teased out and stretched on the 

 slide under the covering glass before their attached ends are 

 divided. 



It will be seen that the light stripe is more affected by the 

 stretching than the dark one, though both are broader than in 

 the non-extended fibre ; but the most important effect is the 

 appearance, often seen with high powers, of a very narrow, in- 

 terrupted black line in the middle of the light band. 



Beside this cross striation, the fibres of vertebrates show 

 more or less plainly minute longitudinal lines. It is to be no- 

 ticed that when the cross stripes are very distinct the longitu- 

 dinal ones are very faint, or even invisible, and that when the 

 latter are well marked the former are the reverse of it. Some 

 reagents tend to divide a fibre into disks, others into fibrillae. 

 Among the former are solutions of acetic acid in water (1 in 



FIG. 53. Striped muscular 

 fibre : a, black stripe ; 6, in- 

 termediate stripe; c, white 

 stripe ; n, nucleus. After 

 Ranvier. 



