82 THE CONTRACTILE TISSUES. 



Hence when one Nicol is placed beneath the stage of a microscope so that the 

 light from the mirror is sent through it, and another Nicol is placed in the eye- 

 piece, the field of the microscope will appear dark when the eye-piece Nicol is 

 rotated so that its optic axis is at right angles to the optic axis of the lower Nicol, 

 and consequently the light passing through the lower Nicol is stopped by it. If, 

 however, the optic axis of the eye-piece Nicol is parallel to that of the lower Nicol, 

 the light from the latter will pass through the former and the field will be bright ; 

 and as the eye-piece is gradually rotated from one position to the other the bright- 

 ness of the field will diminish or increase. 



Both the Nicols are composed of doubly refractive material. If now a third 

 doubly refractive material be placed on the stage, and therefore between the two 

 Nicols, the light passing through the lower Nicol will (in a certain position of the 

 doubly refractive material on the stage, that is to say, when its optic axes have a 

 certain position) pass through it. and also through the crossed Nicol in the eye- 

 piece. Hence the doubly refractive material on the stage (or such parts of it as 

 are in the proper position in respect to their optic axes) will, when the eye-piece 

 Nicol is crossed, appear illuminated and bright on a dark field. In this way the 

 existence of doubly refractive material in a preparation may be detected. 



When muscle prepared arid mounted in Canada balsam is examined in 

 the microscope between Nicol prisms, one on the 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 refractive or isotropic. The 

 fibre accordingly appears banded or striated with alternate bands of aniso- 

 tropic and isotropic material. According to most authors such an .alterna- 

 tion 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 men- 

 tioned 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 bands 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 

 anistropic 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 aniso- 

 tropic band seems to increase at the expense of the singly refractive isotropic 

 band. 



57. 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 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 com- 

 plicated 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 



