SKELETAL MUSCLE 



123 



and portions of four light bands. Under polarized light the dark bands 

 are doubly refractive or anisotropic, and the light ones are singly refractive 

 or isotropic. Following Rollett's suggestion, the striations are often 

 designated by letters. The dark band is called Q (an abbreviation for 

 Querscheibe, or transverse band) and the light band is called J (applied by 

 Rollett to a subdivision of the isotropic layer) . The light band is bisected 

 by the ground membrane, or Krause's membrane, which appears as a very 



FIG. 114. DIAGRAM OF MUSCLE STRIATIONS. (After Heidenhain.) 



The fibrils consist of alternating dark bands, Q, and light bands, J. J. is traversed by the ground mem- 

 brane Z, and Q by the median membrane M. In the right of the three muscle segments shown in 

 the Sgure, the bands, N, have been drawn. 



slender dark line, Z (Zwischenscheibe, or intermediate disc) . The lines 

 Z are believed to represent continuous membranes which divide the muscle 

 fiber into compartments called muscle segments, or sarcomeres. At the 

 sides of the fiber, Krause's membranes join the sarcolemma, which bulges 

 between them when the fibers are contracted (Fig. 112, A). Between 

 Z and Q, in the highly developed striated muscles of insects, a band N 

 has been described (Nebenscheibe, or accessory band). The dark band 



FIG. 115. FIBRILS OF THE WING-MUSCLES OF A WASP; THE UPPER ONE CONTRACTED; THE MIDDLE ONE 

 STRETCHED; AND THE LOWEST ONE UNCONTRACTED. X 2000. (Schafer.) 



Q becomes gradually lighter toward its central part (thus forming h or 

 Qti), and in its central part it is sometimes seen to be crossed by Hensen's 

 median membrane, M (Mittelscheibe). The latter is believed to be similar 

 to Krause's membrane, but more delicate. Like the other bands it may 

 appear dark or light according to the focus. In the muscle fibrils shown 

 in Fig. 115, the bands Q, J, and Z may be readily identified; M appears 

 as a rather broad white line which may include Qh. 



