ELEMENTARY VITAL PHENOMENA 245 



fibre. If the contraction proceeds very rapidly, as in the fibrillse 

 of infusorian cells and cross-striated muscle-fibres, the fibre gives 

 a very quick twitch, the single events of which cannot be followed 

 by the eye. Thus, the stalk of Vorticella contracts suddenly, 

 assuming a screw-shape as a result of the spiral winding of the 

 muscle-fibre and drawing the head of the animal tightly down 

 to the foot of the stalk (Fig. 101, &). The smooth muscle-fibres 

 of the tissues, on the contrary, generally contract extremely slowly 

 and never show sudden twitches like the infusorian myoids and 

 cross-striated muscle-fibres. But, while in the smooth muscle- 

 fibrilla no further events, apart from the change of form, are to be 

 noted microscopically, the cross-striated fibrilla, in correspondence 

 with its complex structure, shows in the phase of contraction 

 highly characteristic changes of its striation (Fig. 105). 

 During the contraction of a single muscle-segment the following 

 phenomena, which Engelmann (73, 75, 78) first carefully 

 analysed, may be observed. The segment becomes shorter and 



--i 



FIG. 105. Single muscle-segment ; /, extended ; //, contracted ; 1, in ordinary light ; 2, in 

 polarised light ; a, the anisotropic disc ; i, i, the isotropic discs. 



thicker, which is to be expected from the shortening and 

 thickening of the whole fibrilla. Meanwhile, remarkable changes 

 occur in the relation of the isotropic to the anisotropic substance. 

 The latter increases in volume and the former decreases, while the 

 volume of the whole segment remains unchanged. At the same 

 time the anisotropic substance, which before was denser and darker, 

 becomes less dense and lighter, i.e., less refracting, while the iso- 

 tropic substance undergoes the reverse changes, becoming denser 

 and darker, i.e., more refracting than it was before. These changes 

 are extremely important, for they show that contraction consists in 

 a passage of substance from the isotropic discs into the anisotropic, 

 and, moreover, of substance that is of less consistency than that 

 of the anisotropic disc. Recently, by means of photography Schafer 

 ('91, 2, 3) has studied more carefully the microscopic changes 

 in this process, and has discovered the interesting fact that in the 

 anisotropic disc extremely fine tubes run parallel to one another 

 and in correspondence with the direction of the fibres almost 

 up to Hensen's disc (Fig. 106); in contraction the isotropic 



