VOLUNTARY MUSCLE. 59 



structure is very distinct, and the following description from 

 Schafer is very instructive. He says : 



" The wing-muscles of insects are easily broken up into sarco- 

 styles (fibrils), which also show alternate dark and light striae. 



" The sarcostyles are subdivided at regular intervals by thin 

 transverse disks (membranes of Krause) into successive portions, 

 which may be termed sarcomeres. Each sarcomere is occupied by 

 a portion of the dark stria of the whole fiber (sarcous element) : 

 the sarcous element is really double, and in the stretched fiber 

 separates into two at the line of Hensen. At either end of the 

 sarcous element is a clear interval separating it from the mem- 

 brane of Krause ; this clear interval is more evident the more the 

 sarcostyle is extended, but diminishes to complete disappearance 

 in the contracted muscle. The cause of this is to be found in the 



Nucleus. 



Muscle- 

 substance. 

 Sarcolemma. 



FIG. 52. Cross-section of striated muscle-fibers : 1, of man ; 2, of the frog ; the 

 relations of the nuclei to the muscle-substance and sarcolemma are clearly visible ; 

 X 670 (Bohm and Davidoff ). 



structure of the sarcous element. Each sarcous element is per- 

 vaded with longitudinal canals or pores, which are open in the 

 direction of Krause' s membranes, but closed at the middle of the 

 sarcous element. In the contracted or retracted muscle the clear 

 part of the muscle-substance has passed into these pores, and has 

 therefore disappeared from view, but swells up the sarcous element 

 and shortens the sarcomere in the extended muscle ; on the other 

 hand, the clear part has passed out from the pores of the sarcous 

 element, and now lies between this and the membrane of Krause, 

 the sarcomere being thereby lengthened and narrowed. The sar- 

 cous element does not lie free in the middle of the sarcomere, but 

 is attached laterally to a fine enclosing envelope, and at either end 

 to Krause's membrane by very fine lines, which may represent 

 fine septa running through the clear substance." 



Schafer regards the sarcomere as similar to the protoplasm of 

 an ameboid cell, the substance of the sarcous element being repre- 



