MUSCLE 



521 



myofibril is made up of solid rods of two sorts. Thick rods of the 

 protein myosin are arranged in such a way that each is surrounded 

 by six other thick rods and six thin ones (Fig. 406, C). The latter 

 are of another protein, actin, and are arranged so that each is 

 surrounded by three other thin rods and three thick ones. The 

 rods are not continuous through the fibre, but overlap in such a 



A- 

 H 



^-<r 



A. 



EH 



B. 



A. 

 H 



Myosin. 



^y 



'tMMM^U^uiiMkMML 



Actin. 



c. 



Fig. 406. — Diagrams of the 

 structure of voluntary 

 muscle, based on A. F. 

 Huxley. 



A, longitudinal section, showing A 

 and 1 bands, and H and Z lines; 



B, long'tudinal section (electron 

 microscope), showing actin and 

 myosin filaments corresponding 

 to the bands and lines in A ; 



C, transverse section (electron micro- 

 scope), showing the large filaments 

 of myosin and small ones of actin. 



way as to give the banded appearance (Fig. 406, A and B). The 

 dark or A bands are doubly-refracting the light or I bands are not. 

 In contraction the actin rods slide into the spaces between the 

 myosin rods, probably with formation of the compound acto- 

 myosin, and after that there is probably folding of the molecules. 



