Specialized Cells: Muscles, Nerves and Sense Organs 113 



they do not occupy the whole space between the Z lines. When the 

 muscle shortens, the two kinds of filaments of actin and myosin re- 

 spectively, slide past each other. The actin filaments do not them- 

 selves shorten when the muscle contracts, but there is in the middle 

 region between two Z lines a filament which is very contractible. It 

 thus holds two rather rigid actin rods together and permits them to 

 slide through the spaces between the myosin rods. 



According to this structure, which is now fairly well supported by 



I MYOSIN ~} 



Position I 



I MYOSIN I 



TrLnnnnnJinj 



1 ACTIN I 



Position 11 



I MYOSIN ~J 



Position III 



FIG. 20. Mechanism of contraction 

 in muscle fibres 



numerous observations, the contraction of muscle is due to the sliding 

 of myosin and actin fibres alongside each other. Much less is known 

 about this aspect of the mechanism. It is certain that atp must be in- 

 volved in this process as the source of energy; but the way in which 

 it operates is uncertain, although various possible processes have 

 been suggested. It is likely that the two filaments move with respect 

 to each other, by attachments between them which are broken in one 

 way and remade in another. Thus the one filament may move along 



H 



