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, 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 contrary, 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 of 
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 
