ELEMENTARY VITAL PHENOMENA 



243 



of insects is a long thin cylindrical cell, consisting of sarcoplasm, 

 which is bounded externally by a somewhat denser layer, the 

 sarcolemma, and contains numerous nuclei elongated longi- 

 tudinally (Fig. 103). Embedded in this sarcoplasm, and extending 

 parallel from one end of the fibre to the other, lie the regularly 

 segmented muscle-fibrillse (Fig. 104, A). If the muscle-seg- 

 ments of a fibrilla be examined with very high powers, it is 

 found that they all possess the same structure, the same arrange- 

 ment of their constituents being repeated in every segment. 

 Each segment is separated from the two adjacent segments by the 

 so-called Dohie's line [Zivischenscheibe] (Fig. 104, s), and contains 



VlG. 103. — Cross-striated muscle-fibres. A^ Two excised pieces of muscle-fibre (at the left above, 

 the end of a fibre) ; the cross-striation is clearly to be seen, likewise many spindle-shaped 

 - muscle-nuclei. (After Schiefferdecker.) S, Two uninucleated cross-striated muscle-cells from 

 the heart, at the left from man, at the right from the frog. (After Disse.) C, Cross- 

 section of a muscle-fibre of an insect ; three nuclei are to he seen and, embedded in the 

 sarcoplasm, the cross-sections of innumerable fibrillEe. (After RoUett.) 



two different substances, of which the one is doubly light-refract- 

 ing, or anisotropic, and lies in the middle of the segment (Fig. 104, 

 q, or q + m + q), while the other is singly refracting or isotropic, 

 <ind in two portions borders the anisotropic substance (Fig. 104, *). 

 In the middle of the anisotropic layer there appears more or less 

 distinctly a clearer zone, which is termed Eensen's disc or 

 Hensen's line [Mittelscheiie] (Fig. 104, 7n). Finally, there occur 

 in many muscle-fibres, but not as a constant constituent of all, 

 one or two accessory discs [Nebenscheibe] (Fig. 104, n) lodged in 

 the isotropic substance. The general constituents of the 

 muscle-segment are the anisotropic layer and the two isotropic 

 layers bordering it ; of these the anisotropic substance is darker, 



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