APPEARANCES OF MUSCLE UNDER POLARISED LIGHT. 



295 



is accumulated opposite the transverse membranes where the sarcostyles are relatively 

 contracted. 



In the living muscle also this change in the position of the sarcoplasm during 

 contraction can, with care, be observed to take place (fig. 339). In this case, also, 

 as in the wing-sarcostyles, the moniliform shape of the muscle-columns tends to 

 cause the constricted parts to appear dark, the bulged parts light in comparison, so 

 that the effect of reversal of the striae is obtained. But in the ordinary muscles 

 this effect is materially increased, and the contrast between the dark and light 

 strise of the contracted muscle is greatly enhanced by the effect of the sarcoplasmic 

 accumulations opposite the constrictions. For in the first place these themselves 

 tend to produce the appearance of dark lines or planes passing across the fibre, and, 

 besides this, the light-reflexions from their surfaces cause the muscular substance 

 between these planes to appear much brighter than would otherwise be the case. 



In alcohol-preparations (both of the wing-muscles and of the ordinary muscles) 

 in which the sarcous elements have been subsequently stained, there is no appear- 

 ance of reversal of striation ; the darkly coloured sarcous element always occupies 

 the central or bulged part of the sarcomere, and the unstained substance of the clear 

 intervals the constricted parts of the sarcostyles. 



Appearances of muscle under polarised light. It was noticed by Boeck that, like some 

 of the other tissues, muscle is doubly refracting 1 (anisotropous). Briicke however was the first 

 to point out that the fibre is not composed entirely of anisotropous substance, but that there is 

 in addition a certain amount of singly refracting or isotropous material. Since the important 

 researches of the last-named author form the basis of our knowledge of this subject, a short 

 account will be given of them here. 



In the first place Briicke distinguishes between the appearances presented by living muscle 

 examined in its own plasma and those of dead and hardened muscle examined in glycerine or 

 Canada balsam. Under the latter conditions, although a considerable variation is noticeable in 

 the relative amount of anisotropous substance, nevertheless the 

 two substances invariably take the form of alternating bands, 

 dark and light, crossing the fibre and apparently corresponding 

 in position with the light and dark stripes of the fibre as seen 

 under ordinary light. 



It is quite otherwise with living muscle. In this almost the 

 whole of the fibre may look doubly refractile, the isotropous 

 substance occurring only as fine transverse lines, or as rows of 

 rhomboidal dots which are united to one another across the aniso- 

 tropous substance by fine longitudinal lines. This account is 

 illustrated by fig. 340, which is copied from Briicke. If this 



figure be compared with fig. 331, or with the parts marked I Fig- 340. LIVING MUSCLE OF A 

 of fig. 339, which represent the living muscle of a water-beetle 

 under ordinary light, it is obvious that the rhomboid points and 

 longitudinal lines of the one correspond to the sarcoplasmic 

 lines and transverse networks of the other. The sarcoplasm there- 

 fore is singly refracting, whereas the substance of the muscle- 

 columns or sarcostyles is, in great part at least, doubly refracting.- Briicke's account of the 

 appearance of living muscle under polarised light seems to have been chiefly founded upon 

 fibres which are not extended, and in which therefore the sarcous elements occupy by far the 

 larger part of the sarcomere. In extended fibres or parts of fibres, especially those which have 

 been fixed by alcohol and mounted in Canada balsam, the fibre appears when examined between 

 crossed Nichol's prisms to be marked by alternating broad bars of light (anisotropous) and dark 

 (isotropous) substance, the former corresponding in position to the sarcous elements, the latter 

 to the clear intervals of the sarcostyles. In the wing-muscles also the sarcous elements appear 

 bright and the clear intervals, including Krause's membrane, are dark with crossed Nichols. In less 

 extended parts of the fibre, the dark or isotropous bands become relatively narrower until in the 

 contracted parts they are reduced to comparatively narrow bands, with relatively broad bright 

 (anisotropous) intervals (fig. 341). There is however no reversal of the bands, a fact of some 

 significance as indicating that the reversal which appears to occur when the fibre is examined 

 by ordinary light is really, as has been already explained, merely an optical effect, and is not 

 caused by any actual change in the relative position within the sarcomere of the substance of 

 the sarcous elements and the clear intervals (see below. Theory of Merkel). The result there- 

 fore of the examination of muscle under polarised light is confirmatory of the deductions 



J* " 



WATER-BEETLE EXAMINED IN 

 POLARIZED LIGHT WITH 

 CROSSED NICHOL'S PRISMS. 

 (Briicke. ) 



