GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 



53 



The muscle-fiber also presents a longitudinal striation which indicates 

 that it is composed of finer elements placed side by side, termed fibrillae. 

 The fibrillae extend throughout the entire length of the fiber, though they 

 are not of uniform thickness. That portion of the fibrilla correspond- 

 ing in position to the dim band is thick, prismatic, or rod-like in shape, and 

 termed a sarcostyle; that portion corresponding in position to the bright 

 band is extremely thin and narrow and presents at its middle a slight en- 

 largement or nodule. The fibrillae are embedded in a clear transparent fluid 

 which, from its supposed nutritive character, is termed sarcoplasm, or 

 interfibrillar substance. The diminution in caliber of the fibrillae at different 

 levels would permit of the accumulation and storage of a larger amount of 

 this nutritive material than could otherwise be the case. It is for this 

 reason that the fiber at these points presents a brighter appearance. 



When the muscle-fiber is examined 

 by polarized light, the dim band ap- 

 pears bright and the bright band appears 

 dim against a dark background, indicat- 

 ing that the former is doubly refracting 

 or anisotropic, the latter singly refracting 

 or isotropic. 



FIG. 16. A. Diagram of 

 arrangement of the contrac- 

 tile substance according to 

 the view of Rollett; the 

 granular figures represent 

 the contractile elements, the 

 intervening light areas the 

 sarcoplasm. B. Small 

 muscle-fiber of man, the 

 corresponding parts in the 

 two figures are indicated; 

 t, i, I, respectively the trans- 

 verse, the intermediate, and 

 lateral discs, n. Muscle 

 nuclei. (Pier sol.} 



FIG. 15. MUSCLE-FIBER 

 OF A RABBIT. a. Dark 

 band. b. Light band. c. In- 

 termediate line. n. Nucleus. 

 (Landois and, Stirling) 



This interpretation of the structure of the muscle-fiber has been subjected 

 to criticism in recent years by Heidenhain. This observer regards the trans- 

 verse, line in the bright band as did its discoverer Krause as a true membrane 

 which is attached laterally to the sides of the sarcolemma. The fibrillae he 

 also regards as continuous but of uniform thickness, and passing directly 

 through the transverse membrane by which they are supported and main- 

 tained in their normal relation. In this view the fibrilla consists of alternate 

 regions of a doubly refracting and a singly refracting material. The sarco- 

 plasm is, therefore, confined to the interfibrillar spaces. Fig. 17. 



The fiber of the pale muscle is similar histologically to the fiber of the 

 red muscle. It, however, does not contain so much granular protoplasm as 

 does the fiber of the red muscle and hence does not intercept the light to the 

 same extent. The greater the quantity of granular protoplasm the darker 

 the muscle. 



