244 



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 fibrillse 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 amoeboid movement 



H 



FIG. 104. A, Two isolated muscle-fibrillge ; z, Dobie's line ; i, isotropic substance ; q, anisotropie 

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

 substance ; q, anisotropie 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 fibrillse. 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 fibrillae in either a smooth or a cross-striated muscle- 

 cell evidently causes a shortening and thickening of the whole 



