94 MUSCLE UNDER POLARIZED LIGHT. [BOOK i. 



This indicates that it is the dim bands which are doubly refractive, 

 anisotropic, or are chiefly made up of anisotropic substance ; there 

 seems however to be some slight amount of anisotropic substance 

 in the bright bands though these as a whole appear single refrac- 

 tive or isotropic. The fibre accordingly appears banded or striated 

 with alternate bands of anisotropic and isotropic material. Accord- 

 ing to most authors such an alternation of anisotropic and (chiefly) 

 isotropic bands which is obvious in a dead and prepared fibre exists 

 also in the living fibre ; but some maintain that the living fibre is 

 uniformly anisotropic. 



Now when a fibre contracts, in spite of the confusion previously 

 mentioned between dim and bright bands, there is no confusion 

 between the anisotropic and isotropic material. The anisotropic, 

 doubly refractive bands, bright under crossed Nicols, occupying the 

 position of the dim band in the resting fibre, remain doubly refrac- 

 tive, bright under crossed Nicols, even at the very height of the con- 

 traction. The isotropic, singly refractive, bands, dark under crossed 

 Nicols, occupying the position of the bright bands in the fibre at 

 rest, remain isotropic and dark under crossed Nicols at the very 

 height of the contraction. All that can be seen is that the singly 

 refractive isotropic bands become very thin indeed during the 

 contraction, while the anisotropic bands, though of course becoming 

 thinner and broader in the contraction, do not become so thin as 

 do the isotropic bands ; in other words, while both bands become 

 thinner and broader, the doubly refractive anisotropic band seems 

 to increase at the expense of the singly refractive isotropic band. 



57. We call attention to these facts because they shew how 

 complex is the act of contraction. The mere broadening and 

 shortening of each section of the fibre is at bottom, a transloca- 

 tion of the molecules of the muscle substance. If we imagine 

 a company of 100 soldiers ten ranks deep, with ten men in each 

 rank, rapidly, but by a series of gradations, to extend out into a 

 double line with 50 men in each line, we shall have a rough image 

 of the movement of the molecules during a muscular contraction. 

 But from what has been said it is obvious that the movement, in 

 striated muscle at least, is a very complicated one ; in other forms 

 of contractile tissue it may be, as we shall see, more simple. Why 

 the movement is so complicated in striated muscle, what purposes 

 it serves, why the skeletal muscles are striated we do not at present 

 know. Apparently where swift and rapid contraction is required 

 the contractile tissue is striated muscle ; but how the striation 

 helps so to speak the contraction we do not know. We cannot say 

 what share in the act of contraction is to be allotted to the several 

 parts. Since during a contraction, the fibre bulges out more opposite 

 to each dim disc and is indented opposite to each bright disc, since 

 the dim disc is more largely composed of anisotropic material than 

 the rest of the fibre, and since the anisotropic material in the 

 position of the dim disc increases during a contraction, we might 



