ELECTRO-PHYSIOLOGY 



CHAP. 



M, 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 Kolliker has termed " tifpical " 

 and " a-tyirical " 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 



Fid. 23. Transverse sec- . 



won of muscle-fibre, ji/njrt to the first type, we may distinguish, as in 

 o. .s, sareopiasm. ver t e brates, fibres with prismatic columns 



cle-column. 



(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 Cohnheirn's arese 

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

 sareopiasm, 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 sareopiasm 

 also forms a continuous 

 sheet, greater or less in 

 thickness, immediately be- 

 neath the SarCOlemma (as Fie. 24. Transverse section of muscle-libre, ll,i.Jrnj,hilvs. 

 ill Certain mUSCle-fibreS Of Sarcoplasm clear, muscle-columns dark. (Rollett.) 



fishes). The muscles of Maja ^ai'iunlo (Fig. 23) afford another 

 elegant illustration of this structure of fibre. In Beetles the 

 polygonal prismatic muscle-columns are very prominent ; the 

 sareopiasm lies evenly distributed, or is unequally heaped up in 

 parts of the transverse section. 







