i ORGANISATION AND STRUCTURE OF MUSCLE 47 



Under a polarising microscope, with crossed Mcol prisms, the 

 anisotropous bands (Z ' N) and (Q), which look dark in ordinary 

 light, with a low objective, appear clear and shining, standing 

 out sharply in a dark field, while the isotropous bands (J E} and 

 (h) remain dark under the same conditions. Very beautiful 

 figures are exhibited by muscle-fibres in polarised light when the~ 

 field of vision is coloured by a mica or gypsum plate of corre- 

 sponding diameter. The anisotropous layers stand out vividly 

 in the complementary colours, according to the direction of the 

 fibres. Light falling parallel with the long axis of the fibrils is 

 singly refracted. With crossed prisms a transverse section, if 

 sufficiently vertical to the fibre axis, remains dark in all its parts 

 and at all azimuths, and does not change colour anywhere with a 

 gypsum background. 



The anisotropous portions are also uniaxial. Briicke ascer- 

 tained that they were positive by means of a movable quartz wedge ; 

 each muscle-fibre acts like the thick end of a quartz wedge when 

 lying parallel to its axis, and is therefore positive like quartz. 



Eollett has recently applied the spectrum analysis of 

 polarised light to this investigation, and has confirmed, with the 

 " spectro-polarisator," the earlier observations of Engelmann, viz. 

 that (Z) and (N) are less positively doubly-refracting than the less 

 refractile segment (Q). At the same time the remarkably clear 

 and sharp figures obtained with this method leave no doubt 

 that the accessory discs (JV) are just as much due to doubly 

 refracting segments of the fibrils as (Q) or (Z). All the inter- 

 communications of sarcoplasm, on the other hand, look quite dark 

 polarised light, so that the doubly-refracting segments of the 

 muscle-columns in the longitudinal section of the fibre " lie com- 

 pletely isolated on a dark field in regular series close to one 

 another." 



Engelmann (2) has remarked that double refractibility is a 

 widely distributed property of contractile protoplasm, appearing 

 even among protozoans. The stalk muscle of Vorticella, e.g., 

 exhibits strong double refraction, and the fibrils behave exactly 

 like the fibrils of striated muscle, i.e. are uniaxial, with the axis 

 parallel to the longitudinal direction of the fibres. In Stentor, 

 the cortical layer of plasma is usually .doubly-refracting through- 

 out its diameter, as well as the muscle-fibrils ; double refracti- 

 ility is also very conspicuous in the rays of Actinospheriuin, 



bilit; 



