54 



ANATOMY FOR NURSES. 



[Chap. VI. 



ter, but having a length of an inch or more. Each fibre con- 

 sists of three distinct 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 con- 

 tractile substance; (3) the sarcolemma, a thin, structureless 



tube, which tightly en- 

 closes the contractile sub- 

 stance and the nuclei. 



If we examine a fresh 

 muscle fibre microscopi- 

 cally, we see that the 

 contractile substance is 

 marked with very fine in- 

 distinct longitudinal lines, 

 or stride; and in addition 



Fig. 52. -Fragments of Striped Fibres, ^q ^I^q longitudinal Stria- 

 SHOwiNG A Cleavage in Opposite Direc- . . ° 



TiONS. (Magnified 300 diameters.) A, longitu- tion it is CroSSCd by more 



dinal cleavage; c, fibrillae separated from one ^ligti^ct narrow dark and 

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



single fibrils more highly magnified, in c' the light bands or stripes,^ 

 elementary structures are square, in c" round; • •■ ,. • -i,, n ,-, 



£, transverse cleavage; a, b, partially detached ^^^^ relative WKltn Ot tllC 



disks ; b' detaclied disk, more highly magnified, gtripes varying according 

 showing the sarcous elements. ^ n^ • 



as the fibre is seen in a 

 state of contraction or relaxation. The ultimate structure of 

 muscular fibre is still by no means fully understood. This 

 much, however, is certain, that the contractile substance is a 

 complex chemical structure, and that the molecules of which it 

 is composed readily change their places under the influence of 

 certain stimuli. When a muscle contracts, the dark bands 

 swell up and shorten (the light bands are also constricted), and 

 the whole fibre broadens and shortens. This broadening and 

 shortening is brought about by the molecules of each section 

 of the fibre changing their places. We shall have a rough 

 image of the movements of the molecules during a muscular 

 contraction if we imagine a company of a hundred soldiers ten 

 ranks deep, with ten men in each rank, rapidly, but by a series 

 of gradations, extending laterally into a double line with fifty 

 men in each line. 



1 By treating a fibre with certain chemical agents, we may cause it to break 

 up longitudinally into fibrillse, and transversely into thin disks. Thus each fibre 

 is resolvable into a number of tiny structures, which elementary structures have 

 been termed sarcous elements. 



