On the structure of cross-striated muscle. 211 



The different appearances of the wing-muscles, according' to their 

 degree of stretching, which occurs even in the fresh muscle, but 

 which is most evident in the alcohol-gold muscle, has been described 

 by several of those authors who have given special attention to the 

 subject of these muscles, but has never received a satisfactory expla- 

 nation. That it is susceptible of some simple explanation there can 

 be little doubt, and it behoves us to consider the conditions of the 

 retracted and extended muscle with the view, if possible, of elucidating 

 the question. 



Fig. 28 shows part of a muscle column, one end of which is 

 retracted while the other end is somewhat extended. In the latter the 

 membranes of Krause are visible: they are not seen in the former. The 

 probable reason for this has been already alluded to (p. 207) and 

 need not here again be touched upon: we may confine ourselves to 

 the conditions of the sarcous elements and of the clear intervals in 

 the two parts. In the retracted part the clear intervals are very 

 short, the sarcous elements are almost in contact with one another or 

 at least with the membrane of Krause which lies invisibly between 

 them: in the extended part, on the other hand, the clear intervals are 

 longer in proportion: the sarcous elements are now well separated 

 from one another, there is no marked difference in their length, but 

 they are less in diameter than in the retracted part. The most ob- 

 vious difference in appearance, therefore, is the increase in length of 

 the clear interval. One interpretation of this increase in length is to 

 suppose it to be composed of an extensible and highly elastic substance 

 which has become stretched by the extending force and which, with 

 removal of that force, would when free to move again, produce a 

 retraction by virtue of its elasticity. This interpretation, however, 

 presupposes solidity in the substance occupying the clear interval, it 

 would be impossible to apply it to liquid. But as we have seen 

 (p. 209), there is every reason to believe that the material in the 

 clear interval is fluid or semi-fluid, and if so, the interpretation falls to 

 the ground. The only alternative (if we except the sudden passage 

 of fluid from without into the interior of the muscle segment, which 

 the presence of the lateral membrane of the segment, the con- 



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