186 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



the interspace between every two septal lines depends 

 upon the reagent employed. With dilute acids and 

 strong solutions of salt, the inter- septal substance swells 

 up and becomes transparent, so that it ceases to be dis- 

 tinguishable from the septal zone. At the same time a 

 distinct but faint transverse line may appear in the 

 middle of its length. Strong nitric acid, on the con- 

 trary, renders the inter-septal substance more opaque, 

 and the septal zones consequently appear very well 

 defined. 



In living and recently dead muscle, as well as in 

 muscles which have been preserved in spirit or hardened 

 with nitric acid, the inter-septal zones polarize light; and 

 hence, in the dark field of the polarizing microscope, the 

 fibre appears crossed by bright bands, w^hich correspond 

 with the inter-septal zones, or at any rate, with the 

 middle parts of them. The substance which forms the 

 septal zones, on the contrarj^, produces no such effect, 

 and consequently remains dark ; while the septal lines 

 again have the same property as the inter-septal sub- 

 stance, though in a less degree. 



In fibres which have been acted upon by solution o 

 salt, or dilute acids, the inter-septal zones have lost 

 their polarizing property. As we know that the reagents 

 in question dissolve the peculiar constituent of muscle, 

 myosin, it is to be concluded that the inter-septal sub- 

 stance is chiefly composed of myosin. 



Thus a fibril may be considered to be made up of 



