Ou the structure of cross-striated muscle. 203 



It has been already noted that the sarcostyles of the wing- 

 muscles are enclosed within a relatively large amount of sarcoplasm, 

 embedded in which are peculiar highly refracting granules. Sarco- 

 styles and sarcoplasm are bound up together into comparatively large 

 bundles or fibres, 1. by the ramifications of fair-sized tracheae, 2. by a 

 delicate sarcolemma. It would appear that a sarcolemma is not 

 always present (Kölliker, op. cit. p. 700). By some authors its occur- 

 rence has been altogether denied, but it is certainly found in those 

 wing-muscles which have come under my own observation. 



The tracheae send fine branches into the interior of the bundle 

 or fibre which ramify throughout the sarcoplasm. 1 ) 



The sarcostyles — the so-called „wing-fibrils" of authors — are 

 easily isolated, either in the fresh and living condition or in alcohol- 

 hardened muscle. In the former case their contractions can actually 

 be observed. If these isolated sarcostyles are looked at in the fresh 

 condition, the most striking feature about them is the existence at 

 regular intervals, which appear to vary with the degree to which the 

 column is stretched, of fine transverse lines which are, in fact, the 

 optical expression of disks or membranes which serve to separate the 

 muscle-column into segments (intersegmental membranes, membranes 

 of Krause, Zivischenscheiben). On either side of these fine transverse 

 lines the muscle -substance is rather brighter and clearer, the rest of 

 the segment looks relatively dim, hence a certain degree of cross- 

 striation which is, however, in the fresh tissue comparatively indistinct, 

 (Fig. 25). I have failed to detect longitudinal fibrillation in the 

 fresh condition. 



If acid be now added, while the sarcoplasm is unaltered or becomes 

 more distinct, the muscle-columns swell up and become indistinct and 

 the delicate cross-striation just alluded to after a time almost entirely 

 vanishes. The swelling is at first less marked opposite the transverse 

 membranes, so that the columns tend to become moniliform (they are 



*) This is sometimes mentioned "as peculiar to the wing-muscles, but I have 

 seen fine tracheae ramifying also in the interior of the leg-fibres of some insects 

 (Bombus, Locusta). A similar fact in mentioned by Kölliker. 



