SKELETAL MUSCLE 



6l 



a distinct sheath, the sarcolemma. The sarcolemma is a transparent structure- 

 less sheath of great resistance which surrounds each fiber, figure 71. 



The substance of the fiber enclosed 

 by the sarcolemma, the contractile 

 substance, contains a number of oval 

 nuclei distributed along the length of 

 the fiber and lying just under the 

 sarcolemma or through the sarco- 

 plasm. Each nucleus is accompanied 

 by a small mass of granular proto- 

 plasm at its poles. The main mass of 

 the fiber is characterized by transverse 

 light and dark bands, figure 73, from 

 which the name striated muscle arises. 



Longitudinal striation is also ap- 

 parent under certain modes of treat- 

 ment, figure 81. The muscle fibers 

 can be split longitudinally into fibrils, 

 called sarcostyles, figures 73 and 74, 

 each of which exhibits the character- 

 istic striation of the whole fiber. 

 Under certain treatment the sarco- 

 styles break transversely into smaller discs by cleavage at the line of 

 Krause's membrane. 



The sarcostyle is, therefore, composed of a number of smaller elements 



8.E. 



FIG. 74. Diagram of Segment of Muscle 

 Fiber, showing Sarcostyle A, Sarcous 

 element ZJ, Krause's line C, Hensen's 

 line D. 



S.E. 



FIG. 75. 



FIG. 75. Sarcostyles from the Wing Muscles of a Wasp. A, A', Sarcostyles showing 

 degrees of retraction; B, a sarcostyle extended with the sarcous elements separated into two 

 parts; C, sarcostyles moderately extended (semidiagrammatic). (E. A. Schafer.) 



FIG. 76. Diagram of a Sarcomere in a Moderately Extended Condition, B. K, K, 

 Krause's membranes; H, plane of Henson; S, E, poriferous sarcous element. (E. A. 

 Schafer.) 



joined end to end. These are the sarcous elements of Bowman. The sar- 

 cous element has a highly refractive denser middle piece surrounded by a 



