ELECTRO-PHYSIOLOGY 



CHAP. 



FIG. 23. Transverse sec- 

 tion of muscle-tibrc, .Mjn 

 Squinado. S, sarcoplasm. 

 If, muscle-column. 



fibres of the Arthropoda, which, anatomically and functionally, have 



reached the highest development. In view of certain important 



differences of structure, that must correspond with no less 

 significant differences in function, two main 

 types of striated fibres may be distinguished, 

 which, although they exist only in certain of 

 the Arthropoda, are always differently localised. 

 These are what Kb'lliker has termed " typical " 

 and "a-typical" fibres; the first presenting 

 essentially the same organisation as the fibres 

 of vertebrates, while the second, which exist 

 only in the thoracic muscles of winged insects, 

 are very divergent in structure. With regard 

 to the first type, we may distinguish, as in 

 vertebrates, fibres with prismatic columns 

 (polygonal in transverse section), and fibres 



with flat bands of fibril bundles. The muscles of Crustacea 



(Astacus) are a typical example of the first, exhibiting in 



cross-section just such a mosaic of polygonal Cohnheim's areae 



as we find in most vertebrate muscle-fibres (Fig. 18, &). The 



sarcoplasm, however, is 



always more abundant ; 



it separates not merely 



single muscle-columns but 



whole groups of them, 



forming (as in the muscles 



of Amphibia) thick and 



usually nucleated lamella 



in the interior of the 



muscle : the sarcoplasm 



also forms a continuous 



sheet, greater or less in 



thickness, immediately be- 

 neath the SarCOlemma (as FIG. 24. Transverse section of muscle-fibre, HydropMlus. 



in Certain mUSCle-fibreS Of Sarcoplasm clear, muscle-columns dark. (Ro 



fishes). The muscles of Maja Squmado (Fig. 23) afford another 

 elegant illustration of this structure of fibre. In Beetles the 

 polygonal prismatic muscle-columns are very prominent ; the 

 sarcoplasm lies evenly distributed, or is unequally heaped up in 

 parts of the transverse section. 



