. A text-book of physiology : for medical students and physicians . undifferentiated protoplasm, interferes withthe clearness of striation of the of this latter sort are usually of adeeper color than those in which the sarco-plasm is less abundant, and the two varie-ties have been designated as the red (moreabundant sarcoplasm) and the pale containing chiefly the less clearlystriated red fibers, for example, the dia-phragm and the heart, are characterizedphysiologically by a slower rate of contrac-tion and by a relatively small susceptibilityto fatigue. The so-call
. A text-book of physiology : for medical students and physicians . undifferentiated protoplasm, interferes withthe clearness of striation of the of this latter sort are usually of adeeper color than those in which the sarco-plasm is less abundant, and the two varie-ties have been designated as the red (moreabundant sarcoplasm) and the pale containing chiefly the less clearlystriated red fibers, for example, the dia-phragm and the heart, are characterizedphysiologically by a slower rate of contrac-tion and by a relatively small susceptibilityto fatigue. The so-called red and pale fibersmay occur in the same muscle. The sepa-rate fibrils, like the entire fiber, show twokinds of substance, the alternating dim andlight bands, and these two materials areobviously different in physical structure asseen by ordinary light. When examined bypolarized light, this difference becomes more evident, for the dimsubstance possesses the property of double refraction. When themuscle fiber is placed between crossed Nicol prisms the dim bands. 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 the left the relations ofthe dim and light bands areshown as they appear inordinary light, in the re-laxed (upper part), and thecontracted (lower part)state. On the right the re-lations of the bands areshown as they appear whenplaced between crossedNicol prisms. The whitespaces represent the dimbands. 20 THE PHYSIOLOGY OF MUSCLE AND NERVE. appear bright, while the light bands remain dark, as is shownin Fig. 3. From this standpoint the material of the lightbands in the normal fibrils is spoken of as isotropous, and that inthe dim bands as anisotropous. The anisotropic material of thedim bands consists of doubly refracting positive uniaxial particles,
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