MUSCLE. 



91 



It is to be observed also that there are other lines dividing 

 these bands, so that each is bisected. The light band (isotropic) 

 possesses in its centre a thin, very dark, doubly refractive line, 

 the so-called Krause's membrane (Z), which was described first 

 by Amici. In the dark band we find usually a light line i. e., 

 it refracts light less strongly that the rest of the auisotropic sub- 

 stance. This is the so-called Henseris line (h). 



FIG. 61. 



Diagram of cross-striations of a beetle's muscle. (After Rollet.) 7. with higher; //, 

 with lower focussing of the objective ; Q, Briicke's liue ; h, Hensen's line,; J, isotropic sub- 

 stance ; N, accessory line (Nebenscheibe) ; E, isotropic substance (Endscheibe) ; Z, Krause's 

 membrane. 



In some arthropods we meet with a still greater differentia- 

 tion. There is a band present dividing into two equal parts the 

 isotropic substance between Krause's membrane and Briicke's 

 line i. e., between Zand Q. This is called the accessory line 

 (Nebenscheibe) (N). This quite inconstant line usually refracts 

 light more weakly than the Briicke's membrane, but is doubly 

 refractive (Fig. 61). 



