314 Basic Structure of Vertebrates 



which is probably a nutrient medium rather than ordinary cytoplasm. 

 The wall of the fiber, much more prominent than an ordinary cell-wall, 

 is called the sarcolemma. 



The alternate dark and light bands on the individual fibril are due 

 to physical differences such that, in polarized light, the dark bands 



Fig. 255. Motor nerve-ending; part of a longitudinal section of muscle-fiber. 

 (After Boeke. Courtesy, Bremer- Weatherford: "Text-Book of Histology," Phila- 

 delphia, The Blakiston Company.) 



are doubly refractive (anisotropic) while the lighter bands are singly 

 refractive (isotropic). Both the dark and the light bands are traversed 

 by finer markings seen only under high magnification. 



The relation of a nonstriated fiber to its nerve is apparently of the 

 simplest sort. A terminal twig of nerve merely attaches to the surface 



