A text-book of physiology, for medical students and physicians . g dim and light bands, andthese two materials are obviously differentin physical structure as seen by ordinarylight. When examined by polarized lightthis difference becomes more evident, forthe dim substance possesses the propertyof double refraction. When the musclefiber is placed between crossed Nicol prismsthe dim bands appear bright, while thelight bands remain dark, as is shown inFig. 3. From this standpoint the materialof the light bands in the normal fibrils isspoken of as isotropous, and that in the dimbands as anisotropo
A text-book of physiology, for medical students and physicians . g dim and light bands, andthese two materials are obviously differentin physical structure as seen by ordinarylight. When examined by polarized lightthis difference becomes more evident, forthe dim substance possesses the propertyof double refraction. When the musclefiber is placed between crossed Nicol prismsthe dim bands appear bright, while thelight bands remain dark, as is shown inFig. 3. From this standpoint the materialof the light bands in the normal fibrils isspoken of as isotropous, and that in the dimbands as anisotropous. The anisotropicmaterial of the dim bands contains doublyrefracting positive uniaxial particles, and Engelmann has shown that such particles may be discoveredin all contractile tissues. The inference made by him is thatthis anisotropic substance is the contractile material in theprotoplasm, the machinery, so to speak, through which itsshortening is accomplished. In the striated fiber this conclu-sion is supported by the fact that during contraction the size. Fig. 3.—To show theappearance of the dim(anisotropic) and light(isotropic) bands at restand in contraction, as seenby ordinary and by polar-ized light. The figure rep-resents a muscle fibril(beetle) in which the lowerportion has been fixed in acondition of contraction.—{Engelmann.) 20 THE PHYSIOLOGY OF MUSCLE AND NERVE. of the dim bands increases at the expense of the material in thelight bands.* As shown in the schema given in Fig. 3, the bandexhibiting the property of double refraction seems, during thephase of contraction, to absorb or imbibe material from theisotropic substance constituting the so-called light band. Thisobservation, if beyond doubt, would have the greatest signifi-cance in revealing the location and mode of action of the realcontractile material of the fibers. It has, however, been calledinto question by recent work (see p. 75). The Extensibility and Elasticity of Muscular Tissue.—Musculartissu
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