12 HISTOLOGY. 



In a close examination of very thin sections of voluntary 

 muscle, especially of the red variety, the Krause's membranes 

 can often be seen passing from one fibril to another across the 

 sarcoplasm. Although it is much more difficult to make our 

 than in heart muscle, on account of the small amount of sarco- 

 plasm, it is probable that the same relation exists between fibril 

 bundles and sarcoplasm in the two kinds of muscle. If such is 

 the case, the explanation of the nature of Krause's membrane 

 is possible on the hypothesis that the contractile parts of the 

 cell form a continuous network. Concerning the nature of the 

 other striations nothing definite is known. 



During a contraction of the cell these various striations 

 undergo certain changes. They all become shorter and broader. 

 The isotropic substance becomes very thin, and the Neben- 

 scheiben approach Krause's membranes so closely that new 

 striations are formed, which are known as the contraction bands. 

 These bands usually become isotropic, while the Briicke's lines 

 (Q) acquire a doubly refractive index. In Briicke's lines the 

 distinction between the Hensen's line and the rest disappears. 

 These changes can be observed best in cells in which parts of 

 the fibre only are in contraction (so-called contraction waves). 

 In such a fibre all the transitions from one state to the other are 

 to be seen. Still plainer pictures can be obtained in fibres 

 which show the so-called lateral waves (Fig. 62). This is seen 

 in the neighborhood of a motor end plate. 



It appears that the Briicke's lines are active during contrac- 

 tion, and that the isotropic substance has only elastic prop- 

 erties and acts passively. It is certain that the striations are 

 not an essential thing for the cell contraction. Smooth muscle 

 contracts with no striated fibrils, but this contraction is much 

 slower than it is in striated muscle. Supposing that the fibrils 

 are merely a differentiated part of the primitive protoplasmic 

 framework, the contraction may be considered as a contraction 

 of this framework. The further differentiation of the fibrils 

 by the appearance of striations puts them into a physical con- 

 dition for quicker and more perfect contractions. In the 

 primitive amoeboid cells, where the network in the protoplasm 



