50 HUMAN PHYSIOLOGY. 



Histology of the Skeletal Muscle-fiber. A muscle-fiber consists 

 of a transparent elastic membrane, the sarcolemma, in which is 

 contained the true muscle element. Examined microscopically, the 

 fiber presents a series of alternate dim and bright bands, giving to 

 it a striated appearance. 



When the bright band is examined with high magnifying powers, 

 a fine, dark line is seen crossing it transversely. It was supposed b% 

 Krause to be the optic expression of a membrane which divides the 

 cavity of the sarcolemma into a series of compartments, each of 

 which contains a dim band of sarcous or muscle substance, bounded 

 at either extremity with the half of a bright band. This membrane 

 has since been resolved into a row of granules. 



The muscle-fiber also exhibits a longitudinal striation, indicating 

 that it is composed of fibrillae, placed side by side and embedded in 

 some interfibrillar substance, to which the name sarcoplasm has been 

 given. The fibrillae, which are arranged longitudinally to the long 

 axis of the fiber, are grouped by the intervening material into 

 bundles of varying size, the muscle columns. The fibrillae which ex- 

 tend throughout the length of the fiber are not of uniform thickness, 

 but present at regular intervals well-marked constrictions. 



In the region of the dim band the fibrilla presents itself in the form 

 of a homogeneous prismatic rod, termed sarcostyle, separated from 

 neighboring rods by a slight amount of sarcoplasm. Between two 

 successive rods is found a dark granule, united by a thin band of 

 similar material to the ends of the rods. The transverse row of 

 granules corresponds to Krause's membrane., 



In the region of the granules there is a diminution of the sarcous 

 substance, but an increase in the amount of sarcoplasm, and as the 

 latter is more transparent than the former, the fiber presents at this 

 point a conspicuous bright band. Rollet considers the sarcostyles 

 to be preexistent, not the result of post-mortem or chemic changes, 

 and the seat of the contractile elements. The sarcoplasm is a passive 

 material similar in its properties to protoplasm. 



Briicke has shown that when the muscle-fiber is examined under 

 crossed Nicol prisms the dim band appears bright and the bright 

 band appears dim against a dark background, indicating that the 

 former is doubly refractile, or anisotropic, the latter singly refractile, 

 or isotropic. The fiber, therefore, appears to be composed of alternate 

 discs of anisotropic and isotropic substance. 



