264 THE BODY AT WORK 



fibre is illuminated with polarized light. The dim disc is 

 then found to be doubly refracting ; the bright disc is not. 

 When the prism in the tube of the microscope is placed with 

 its axis at right angles to the axis of the prism which inter- 

 venes between the source of light and the stage of the micro- 

 scope, a succession of bright bands is seen corresponding to 

 the dim bands seen with unpolarized light. The rest of the 

 fibre is invisible, because it has not the property of twisting 

 the undulations of light which the lower prism has set all in 

 the same plane. Various hypotheses as to the cause of con- 

 traction, or, to speak more correctly, as to what happens 

 during contraction, have been based upon the thinning of the 

 bright discs. It is assumed that the dim discs have a definite- 

 ness of structure which the bright discs do not possess. They 

 are thought of as being traversed by pores, or as consisting of 

 short rods. Microscopists who take the latter view believe 

 that during contraction the more fluid substance, sarcoplasm, 

 which occupies the bright bands is drawn into the dim bands 

 between the short rods, or sarcostyles, which are consequently 

 separated more widely. 



No tissue could be more unsuitable than muscle for micro- 

 scopic examination ; for none other offers the same optical 

 difficulties. This will be evident to anyone who considers 

 the description already given of the markings which it ex- 

 hibits. Whatever may be the true interpretation of these 

 markings, it is clear that they point to an almost infinite 

 multiplication of minute elements adjusted with absolute 

 accuracy side by side and end to end. A cylinder filled with 

 these transparent objects has to be viewed by transmitted 

 light. The elements, whatever may be their nature, refract 

 light in different degrees. It is impossible to eliminate the 

 effects of internal reflection, refraction, and interference of 

 waves of light. The most alluring hypothesis must be accepted 

 with a considerable amount of reserve. Any fact which seems 

 to militate against it must be taken into consideration. The 

 view set forth above, in general terms, is very attractive to 

 everyone who wishes to bring muscle within the category of 

 machines. Suppose we accept the hypothesis that the dim 

 band is a plate made of sarcostyles surrounded by sarcoplasm 

 then the impulse which reaches a fibre causes an alteration in 



