96 MUSCLE UNDER POLARIZED LIGHT. [BOOK i. 



stage below the object, and the other in the eye-piece, the fibres 

 stand out as bright objects on the dark ground of the field when 

 the axes of the prisms are crossed. On closer examination it is 

 seen that the parts which are bright are chiefly the dim bands. 

 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 singly 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 

 refractive, bright under crossed Nicols, even at the very height of 

 the contraction. 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 translocation 

 of the molecules of the muscle-substance. If we imagine a com- 

 pany of 100 soldiers ten ranks deep, with ten men in each rank, 

 rapidly, and yet 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 



