9 



NORMAL HISTOLOGY AND ORGANOGRAPHY. 



nating light and dark bands. The dark bands are 

 doubly refractive to light, or anisotropic, while the 

 light bands are singly refractive, or isotropic. The 

 dark bands represent a predominance of the sarco- 

 style substance, and the light bands a predominance 

 of the sarcoplasm. In the middle of the light band 

 a dark line can be seen, known as Krause's membrane. 

 In the middle of the dark band a light-colored, line is 

 present, known as Hens en's median disc. The latter 

 disappears when a fiber contracts. 



H-H-t 



Sarcomere. 



Hensen's me- 

 dian disc. 



Fig. 57. Diagram of voluntary muscle fiber; A, Fiber relaxed; B, 

 fiber contracted. 



These transverse markings are all due to the dis- 

 tribution of the sarcoplasm and the regular con- 

 strictions of the sarcostyles. The sarcostyles are 

 not of uniform dimensions, but at regular intervals 

 show dilatations alternating with constrictions. The 

 dilatations appear at regular intervals and in the 

 same transverse plane of the muscle fiber, thus 

 giving rise to the dark band. Krause's membrane 

 is not a membrane, but represents minute nodal 

 points of the sarcostyles, placed in the same trans- 



