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-nbrillse (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 Dobies line [Zivischenscheibe] (Fig. 104, 2), arid contains 



FIG. 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.) B, 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 be seen and, embedded in the 

 sarcoplasm, the cross-sections of innumerable fibrillse. (After Rollett.) 



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 iso tropic, 

 and in two portions borders the anisotropic substance (Fig. 104, i). 

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

 distinctly a clearer zone, which is termed Hensens disc or 

 Hensen's line [Mittelscheibe] (Fig. 104, m). 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 iso tropic 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, 



R 2 



