40 ELECTRO-PHYSIOLOGY CHAP. 



differentiation in their fibrils. Lastly, in the Arthropoda, which 

 are, generally speaking, characterised by extreme rapidity of 

 movement, all the muscles are striated, and it is just among 

 these that we also find the most rapidly contracting fibres 

 (thoracic fibrils of insects). 



It is easy to demonstrate that the cross-striation of a niuscle- 

 cell, or fibre, depends on the cross-striation of the single fibrils. 

 Each of these appears in longitudinal section as though it were 

 regularly segmented, or, more correctly, built up of separate 

 layers, which exhibit fundamental differences in respect of optical 

 properties, and affinity for staining, as indeed of all chemical 

 and physical reactions. This construction is most evident in 

 the thick, thoracic fibrils of insects, and in the bundles of fibrils 

 known as " muscle-columns," which, in consequence of the regular 

 juxtaposition of the single often excessively fine fibrils exhibit 

 precisely the same transverse banding as that which we ascribe 

 in this instance to each elementary fibril. The optical appearance 

 of the striation is in general a regular succession of light and 

 dark bands, which lie one above the other like coins in a rouleau. 

 Such a series of strife may be very complicated in detail, since a 

 whole system of light and dark parts can be grouped together, 

 as it were, in a higher unit ; the regular, periodical alternation 

 of the individual segments is, however, a persistent character- 

 istic. The separate bands as will be shown below- -present 

 quite a different appearance in the resting and in the contracted 

 condition. The arrangement in the resting state will be first 

 considered. 



Both in Vertebrates and Invertebrates, the relaxed, striated 

 muscle-fibre, or bundle of fibrils, exhibits broad, dark, transverse 

 bands under an appropriate power of the microscope, separated again 

 by smaller, clear bands ; these last, in suitable preparations, 

 can at once be recognised as the expression of the regularly juxta- 

 posed, homogeneous segments of the fibrils, of which the muscle- 

 fibres, or muscle-columns, consist between every two transverse 

 planes of the fibre. In the simplest cases, each dark band 

 appears to be divided in the middle by a faint, clear line, each 

 light band by a dark line (Fig. 27, /, h and Z}. In many cases, 

 however, e.g. in the Arthropod muscles, the segmentation is much 

 more complex (Fig. 27, II and ///). It is convenient, with 

 Rollett, to indicate the individual segments of the fibrils, or the 



