290 



Dr. J. B. Haycraft. 

 Fio. 1. 



Bowman'* element*. 

 Qucrscheibe. 



Mittelscheibe. Hensen't line. 



Querwand. Dobie's line. 



Isotropic ! clear stripe. 



Part of a muscular fibril is represented in this figure, and it will be noticed that 

 the striping of the fibril corresponds with the position of inequalities in its 

 thickness. 



actually described the striping in the reverse focus to that generally 

 adopted, calling " clear " what we speak of as the " dim " band. 



A very simple method of determining exactly what part varicosity 

 plays in the production of the cross-striping then suggested itself 

 to my mind ; it was to immerse the fibrils in a fluid having the 

 same refractive index. Under these circumstances it is obvious 

 that these stripes, which are due to varicosity alone, will disappear, 

 bat the striping will become even more marked, if there are alter- 

 nating structures along the fibre possessed of different refractive 

 indices. At Professor P. G. Tait's suggestion, I placed the fibres in 

 a mixture of alcohol and oil of cassia, varying the proportions until 1 

 approximated to the refractive index of the fibrils. The striping 

 never entirely disappeared, bat it grew fainter and fainter, and I 

 am inclined to explain the partial failure of this experiment on the 

 grounds that, unlike a glass rod, the muscle fibres imbibed the 

 medium in which they were embedded. As a result of this slow 

 imbibition, the refractive index of the fibres would be constantly 

 altering, and it would be a matter of the greatest difficulty to make 

 it exactly the same as that of the surrounding medium. In addition 

 to this, coagulation would be almost certain to take place within the 

 fibrils, destroying their optical uniformity. 



While looking upon the results I had obtained as valuable but not 

 conclusive evidence, I sought to solve the problem in another way. 

 1 took the living muscle of a Crab or Fly, and, while examining 

 it under the microscope, I pressed down the cover-glass with 





