90 



HISTOLOGY. 



it cannot be made out. On treatment of the tissue with water, 

 however, the osmosis lifts the sarcolemma up so that it can be 

 plainly seen. Also in torn, twisted, or teased cells, where the 

 contents of the fibre have been extruded, it can be seen. It is 

 absent in the muscle of some lower animals. 



The striation of the muscle fibre is due to changes in the 

 physical properties of parts of the fibrils, in consequence of 

 which some portions have a different refractive index and stain 

 differently from others. The appearance changes also on high 

 and low focussing. What appears light in high focus becomes 



FIG. 60. 



JVttC/etw 



Prim itiri' fibril 



fMlij MlliYi 



[mil !&$&;;* /;y 



uiitftfiiw/ /'/ 



&ii/ ^ 



T . . . 

 Isotropic layei ?i 



] 



?itt44 u tutiiiitttl 

 \ \Y&< (\ilMfitW 

 '- '*'?!' !??.'.'.'!".'."- 

 - !?H! !5J5ff JffJJ--' 

 /-' riiiimminrinj 



// nit j/wf i f // J 



Piece of muscle fibre of the frog, broken up into fibrils, x 650. 



dark on focussing to a deeper level. The following description 

 is made from low focussing : We notice with high magnifica- 

 tion alternating light and dark bands on the fibrils. These are 

 of about equal thickness. The dark bands, the so-called 

 Briicke's lines (Q), are doubly refractive i. e., they appear 

 light in polarized light with crossed Nicol's prisms (aniso- 

 tropic). The bright bands, on the contrary, are singly refrac- 

 tive, are isotropic, and appear dark in polarized light. 



