STEUCTURE OF MUSCLE. 



201 



constrictions, giving a scolloped border and beaded character to 

 the fibril, as in the lower portion, at c. Sometimes the striped 

 fibre cleaves into transverse portions or discs, fig. 128, b, a, b, 

 corresponding in breadth to the cross-stripes, and to the seeming- 

 segments of the fibrils. 



The following is the average diameter of the striped fibre, of 

 different classes, in fractions of an inch : — 



Fishes 

 Reptiles 

 Birds 

 Mammals 



from 



i 



>> 



75 5 

 1 



10 



1 



15 



1 

 1 1 



5 



1 



» 10 



_J 



" 35 



1 1 

 » 19 2* 



Thus, among vertebrates, fishes, and in fishes the Skates (Raia), 

 have the thickest elementary striped muscular fibre ; and its 

 elastic tunic, the ( sarcolemma,' can be best demonstrated in them. 

 When the fibre is broken across, as in fig. 129, the sarcolemma a 

 may remain, connecting the severed portions, b, b. 



The characteristic vital property of muscular fibre is to alter, 



129 



Portions of a broken elementary striped muscular fibre b, held together 

 by the untorn twisted sarcolemma a. Baia bat is. clxxxv. 



under stimulus, its relative dimensions of length and breadth. 

 When it becomes shorter and thicker it is said ' to contract ; ' and 

 by these contractions the movements of the body, and of its parts, 

 are produced. 



In the contraction of a smooth elementary muscular fibre it has 

 been seen to grow thicker at a part, and shorter, without falling 

 out of the straight line. 2 In the contraction of a striped elemen- 

 tary fibre it has been seen to grow thicker at successive parts, by 

 approximation of the cross stripes, as in fig. 130, at a, a, a, along 

 one side ; or engagi»g the whole thickness of the fibre, as at b,b,b; 

 and these successive partial thickenings, with concomitant shorten- 

 ing of the fibre, have been termed 'waves of contraction.' 3 



On the cessation of the act, the fibre may fall into zig-zag folds 



1 clxxxv. p. 510. 



xciv. Editor's note, p. 261. (1837). 



clxxxv. p. 525. 



