180 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



the interspace between eveiy two septal lines depends 

 upon the reagent employed. With dilute acids and 

 strong solutions of salt, the iuter-scptal 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- 

 trarj', renders the inter-septal substance more opaque, 

 and the septal zones consequently appear very well 

 defined. 



In living and recentl}' 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, which 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 contrar}^, produces no such effect, 

 and consequently remains dark ; while the septal lines 

 again have the same property as the inter-sei)tal 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 



