MUSCLE. 



93 



is simple and uniform, the contraction takes place slowly and 

 in no definite direction. When a similar network, however, 

 has been differentiated into longitudinal thickenings (fibrils) 

 and these fibrils further changed physically, the contraction is 

 quick, and takes place in the direction of the stronger strands 

 of protoplasm i. e., the fibrils. 



By the action of weak acids, Briicke's lines swell up and 

 Krause's membranes are unchanged, so that the fibrils have the 



FIG. 62. 



Rarcolemnia 



Lateral contraction wave of Cassida equiestru. (After Kollet.) The formation of the 

 contraction band is well seen, at the left, as thick black lines. 



form of a row of beads. With stronger acids, Briicke's lines 

 split, and disks are formed which contain Krause's membrane 

 in their centre. The so-called Bowman's disks are formed by the 

 action of 93 per cent, alcohol. Here the splitting is at Krause's 

 membrane and Briicke's lines are left intact. The Krause's 

 membranes are the most resistant of all the lines, and seem to 

 be closely related both to the sarcoplasm and the sarcolemma. 



