48 ELECTRO-PHYSIOLOGY CHAP. 



where each plasma ray acts like a doubly-refracting fibre with 

 one optical axis, which is parallel to the longitudinal direction 

 of the fibre, and therefore, generally speaking, with the direction of 

 contraction in the plasma. The same properties also characterise 

 the fibrils of the epithelial muscles of Hydra. The behaviour 

 of the bi-obliquely striated muscle-cells of many vertebrates in 

 polarised light is also remarkable. According to Engelmann, 

 e.g., " the optical axis of the fibrils does not coincide, as might be 

 expected from analogy, with their longitudinal direction, but in- 

 variably with the long axis of the muscle-fibres " : the latter, no 

 matter what angle the fibrils form with the fibre-axis, are always 

 at maximum clearness, provided the axis lies at an angle of 45 

 to the plane of polarisation between the crossed Nicol prisms. 



Double refractibility therefore appears as a characteristic 

 property wherever the contractile particles of plasma lie perma- 

 nently in a definite direction, and moreover seems invariably to 

 denote uniaxial particles, whose optic axis coincides with the direct 

 tion of contraction. The striated fibrils must, with Engelmann, be 

 regarded as consisting mainly of an isotropous basal substance, 

 running longitudinally, in which the doubly-refracting particles 

 (which must be regarded as the seat of the contracting forces) 

 are arranged in regular striae, corresponding with the metabolous 

 segments. 



It remains to give a brief account of the changes in cross- 

 striation which take place during the contraction of the muscle- 

 fibre, since they are of almost equal morphological and physio- 

 logical interest. As we learn by observation, the changes of form 

 in fibre, or fibril, of the muscle are a shortening and a thickening ; 

 and this of course not merely in the entire fibril, but in each 

 individual tract of the same, and each individual segment. 



If the attention is fixed on a contracted spot in a living 

 fibre, e.g. in insect muscle, which frequently exhibits short waves 

 of contraction, with relatively low velocity, long after preparation, 

 it is easy to distinguish two kinds of transverse bands within the 

 wave ; one small and invariably dark, the other clear and some- 

 what broader. The contracted fibres therefore present on the 

 whole an aspect similar to the resting fibres, i.e. a regular 

 alternation of dark and light cross -bands, only the single stria; 

 are much closer together, the dark bands much smaller than in 

 the relaxed fibre. 



