86 CHEMISTRY OF MUSCLE. [BOOK i. 



formly bright ; in other .words there is a sort of reversal of the 

 situation, what was bright becoming, in its middle at least, dark, 

 and what was dim becoming relatively bright. When the fibre 

 is examined under polarized light, by which the dim bands are 

 shown to be largely composed of doubly refractive, anisotropic 

 material and the bright bands chiefly of singly refractive, isotropic 

 material, it is found that the above apparent reversal is not based 

 on any reversal of the refractive material, the anisotropic (dim) 

 band remains anisotropic, and the isotropic (bright) band remains 

 isotropic. But while both bands become broader (across the fibre) 

 and thinner (shorter along the length of the fibre), the anisotropic 

 band does not become so much thinner as does the isotropic band, 

 in other words the dim doubly refractive band or disc increases in 

 bulk at the expense of the bright singly refractive band. And 

 this accords with another feature of the fibre during contraction ; 

 namely, that the sarcolemma, which in the fibre at rest presents 

 a quite even line, is then indented at the middle of the bright 

 band at about the position of the intermediate line, and bulges 

 out opposite the dim band, that is opposite the enlarged aniso- 

 tropic disc. 



It is useless, however, to dwell on these matters until the minute 

 structure of the fibre has been more clearly and satisfactorily made 

 out than it is at present. A contraction is obviously a transloca- 

 tion of molecules of the muscle substance and may, very roughly, 

 be compared to the movement by which a company, say of one hun- 

 dred soldiers ten ranks deep, with ten men in each rank, extends 

 out into a double line of two ranks with fifty men in each rank. 

 The movement of translocation is obviously, in striated muscle, a 

 very complicated one, but how the striation helps the movement 

 we do not at present really know. All we can say is that when 

 swift and rapid contraction is needed, the contractile tissue em- 

 ployed puts on in nearly all cases the striated structure. 



The Chemistry of Muscle. 



55. We said, in the Introduction, that it was difficult to 

 make out with certainty the exact chemical differences between 

 dead and living substance. Muscle however in dying undergoes 

 a remarkable chemical change, which may be studied with com- 

 parative ease. We have already said that all muscles, within a 

 certain time after removal from the body, or, if still remaining part 

 of the body, within a certain time after ' general ' death of the 

 body, lose their irritability, and that the loss of irritability, which 

 even when rapid,, is gradual, is succeeded by an event which is 

 somewhat more sudden, viz. the entrance into the condition known 

 as rigor mortis. The occurrence of rigor mortis, or cadaveric rigid- 



