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GENERAL PHYSIOLOGY 



denser and more strongly refracting, while the isotropic substance 

 appears richer in water, brighter, less dense, and less refracting. 

 In every muscle-fibre similar discs of the individual fibrillae lie in 

 the same transverse plane, so that the whole fibre appears regularly 

 banded or cross-striated (Fig. 103, A). The cross-striated muscle- 

 fibres of vertebrates often reach a very considerable length, al- 

 though they represent only a single, multinucleate cell — e.g., the 

 fibres from the long skeletal muscles of man are more than a 

 decimetre in length, and each fibrilla in them extends from one 

 end to the other. 



In the movement of both smooth and cross -striated muscle- 

 fibres, two phases can be distinguished, as in amcsboid movement — 



A B 



Fig. 104. — A, Two isolated muscle-fibrillag ; z, Dobie's line ; i, isotropic substance ; q, anisotropic 

 substance. (After Ranvier.) B, Two single muscle-segments ; z, Dobie's line ; i, isotropic 

 substance ; g, anisotropic substance containing Hensen's disc, m. The segment at the right 

 possesses an accessory disc, n, in the isotropic substance. 



that of contraction and that of expansion. Contraction consists of 

 a shortening and thickening of the fibrillae. This process passes 

 from the place of its origin in the form of a contraction-wave 

 over the whole fibrilla. The particles, therefore, shift themselves 

 in the longitudinal direction in such a manner that they come to 

 lie beside one another in a larger cross-section. In this way 

 the whole surface of the fibrilla becomes diminished, although 

 not to its minimum, the spherical form, as is the case in 

 naked protoplasmic masses. The simultaneous contraction of 

 the single fibrillse in either a smooth or a cross-striated muscle- 

 cell evidently causes a shortening and thickening of the whole 



