On the structure of cross-striated muscle. 221 



With regard to the changes which the cross-striation of the wing- 

 muscles undergoes during contraction, the comparison which has already 

 been made between the wing-columns in the extended and retracted 

 condition may be taken as affording an account of the differences which 

 are observed in those conditions in muscles which have been fixed by 

 alcohol. As far as concerns observations upon the living muscle, it 

 must be admitted by all who have endeavoured to follow out the 

 changes which take place in contraction, that it is a matter of the 

 greatest difficulty to determine what those changes precisely are. 

 There is, however, one point as to which most observers agree, viz: 

 that in the contracted sarcostyle there is an apparent reversal of the 

 stripes, fine dark bands being visible opposite the indentations of the 

 envelope, and alternating with bright bands, which are also narrow, 

 and which occupy the rest of the bulged and flattened muscle segment, 

 The interpretations which have been given of these altered appearances 

 are various, and I do not propose to reproduce them all here, but I 

 will refer to one only, that namely which was first advocated by Merkel 

 (loc. cit.). Merkel supposed that the reversal is due to the actual passage 

 of part of the substance of the sarcous element (Querscheibe) through 

 the clear interval to apply itself against Krause's membrane (Zwischen- 

 scheibe) whilst at the same time the substance of the clear interval 

 finds its way to the middle of the segment and applies itself to the 

 disk of Hensen (Mittelscheibe), which his theory supposes to be a 

 thin membrane or disk bisecting the segment. The two substances, 

 in fact, are supposed to execute a peculiar sort of evolution by which 

 they entirely change places, although in what way this might produce 

 or facilitate the swelling and shortening of the muscle-segment, it is 

 difficult to conceive. Apart, however, from this, it may be pointed 

 out that the appearances of the alcohol-fixed fibres which have been 

 stained by gold, do not afford the least support to the acceptance of 

 this view. In these, even in the most retracted parts of the muscle- 

 columns, the chromatic substance of the sarcous element always main- 

 tains the same relative position, i. e.. it always occupies the middle 

 or bulged part of the sarcous element. At the indented part, even in 

 the retracted fibres, there is always a clear interval although it may 



