I 



i ORGANISATION AND STRUCTURE OF MUSCLE 37 



visible as a rule within the latter, even with the most powerful 

 enlargement ; the areas appear to be perfectly homogeneous. By 

 the use of proper means (alcohol, acid), which facilitate the break- 

 ing down of muscle-fibres into fibrils, or the swelling of the latter, 

 it becomes, however, possible to detect the fibrils in cross-section. 

 Cohnheim's arese then appear to be subdivided into smaller fields 

 lying in close juxtaposition (Figs. 25, b ; 26, B\ In longitudinal 

 section also, the fibrillated structure of the muscle-columns at 

 least in places becomes distinctly visible. Still it must be 

 admitted that in comparison with the certainty with which the 

 muscle-columns themselves can be demonstrated, their composition 

 as bundles of fibrils is much harder to determine. 



If the typical muscles of Arthropods already exhibit a 

 superabundance of sarcoplasm in comparison with the majority 

 of vertebrate skeletal muscles, this is to a far greater extent the 

 case with the a-typical thorax muscles of Insects. As compared 

 with the first type, these must be designated " dark " muscles, 

 which is the more legitimate, since, like the " plasmic " muscle- 

 cells and fibres of vertebrates and many invertebrates, they are 

 generally distinguished by their darker colour (reddish, or 

 brownish - yellow) from the clear, white leg -muscles. But the 

 most typical characteristic of these wing-muscles (thoracic fibrils) 

 of insects (discovered by Siebold, and first described minutely by 

 Kolliker) is their readiness to separate into very broad fibrils 

 (1 to 4 yu,), which are bedded in the cavities of the copiously- 

 developed sarcoplasm. The sarcoplasm again is richly studded 

 with " interstitial granules " which are often excessively large 

 and arranged in regular longitudinal series between the 

 fibrils. 



Further, on examining fresh preparations, the wealth of 

 tracheae is very striking. They not only wind themselves round 

 the bundles of fibrils externally, but, as appears unmistakably in 

 transverse section, penetrate inside, the individual fibres, and 

 ramify freely in the sarcoplasm. Within the meshes of the 

 network of tracheas, it is easy in cross-section to distinguish a 

 mosaic of circles, corresponding with the single fibrils, whose 

 diameter, as compared with the excessively fine, elementary fibrils 

 of the leg-muscles, is enormous. 



As a rule there is no sarcolemma in these larger bundles of 

 fibrils (corresponding to muscle- fibres) in the wing-muscles of 



