MOVEMENT 49 



The muscle substance is essentially protoplasm endued 

 with the special power of contracting. Each fibre is bounded 

 by a delicate structureless sheath, the sarcolemma (Fig. 16), 

 inside which nuclei can be distinguished ; the fibre has 

 therefore been formed by the coalescence of a large number 

 of cells whose boundaries can no longer be detected. The 

 striation of the fibres is comparatively coarse in the muscles 

 of many insects and the structure has been minutely studied 

 by various observers. According to the researches of 

 C. Janet (1895) and E. Schafer (i89i)on the wing-muscles 

 of ants, wasps, and beetles, each light stripe is traversed by 

 a fine line (Dobie's line or Krause's ** membrane " (Fig. 

 16, Id)) which sometimes appears broken (a " dotted line "). 

 The portion of a fibril between two such lines is a sarcomere, 

 and the central dark region of the sarcomere appears as 

 a band or disc ; the dark substance or sarcous element 

 has a striped aspect because it is traversed by two sets of 

 longitudinal pores which are open towards Dobie's line in 

 either direction but closed towards the median plane of the 

 sarcomere. The substance of the sarcous element, as well 

 as the pores, becomes shorter and broader when the fibril 

 contracts, longer and narrower when it relaxes ; in the latter 

 condition a clear transverse line (Hensen's line) may be 

 distinguished in the median plane of the sarcomere. The 

 paler and presumably more fluid constituent (hyaloplasm) 

 of the sarcomere is largely absorbed in the pores during con- 

 traction and squeezed out of them during relaxation. An in- 

 terstitial network with radially arranged filaments (Fig. 16, c) 

 has been described in the neighbourhood of Dobie's line. 

 The arrangement of the various constituents is such that 

 contraction can take effect only in the longitudinal direction 

 of the fibre. All the fibrils contract together and the amount 

 of contraction of each fibril may be regarded as the sum of 

 the contractions of the sarcomeres that compose it. Not 

 that all the sarcomeres in a fibril contract simultaneously ; 

 a wave of contraction whose velocity can be measured passes 

 along each fibre, and therefore along the whole muscle from 

 end to end. The muscles of insects are, as we have already 



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