56 THE CHANGE OF FORM. [BOOK T. 



length, may be observed passing along the fibres. By appropriate 

 treatment with osmic acid or other reagents, these short contraction 

 waves may be fixed, and the structure of the contracted portion 

 compared at leisure with that of the portions of the fibre at rest. 

 In Fig. 10, representing a fibre of the muscle of an insect (in which 

 these changes can be more satisfactorily studied than in vertebrate 

 muscle), the contraction wave begins near a, and has reached 

 about its maximum at b, while at c the fibre is at rest, the 

 contraction wave not having reached it (or having passed over 

 it, for the beginning and end of the wave are exactly alike). It 

 will be seen that at b, each disc of the fibre is shorter and broader 

 than at c. Further, while at c the dim band oc is conspicuous, and 

 the light band y, with its accessory markings y ', is together lighter 

 than the dim band x, at b in the fully contracted part of the fibre 

 the dim band appears light as compared with the black line y 

 occupying the middle of the previously light band. In the 

 contracted muscle then there is a reversal of the state of things 

 in the resting muscle, the light band (or part of the light band) 

 of the latter in contracting becomes dark, and the dim band of 

 the latter becomes by comparison light. Between rest and full 

 contraction there is an intermediate stage, as at d, in which the 

 distinction between rHm and bright bands seems to be largely lost. 

 The subject however is one offering peculiar difficulties in the way 

 of investigation, and while most, though not all, observers agree in 

 the broad facts which have just been stated, there is great diversity 

 of opinion concerning further details and especially as to the 

 interpretation of the various appearances observed. The accessory 

 markings in the middle of the light band have, in particular, been 

 the subject of controversies into which we cannot enter here. 



When the fibre is examined in polarised light it is seen that 

 the dim band is anisotropic, and the light band isotropic. This 

 is the case during all the phases of the contraction. At no period 

 is there any confusion between the anisotropic and isotropic material ; 

 these maintain their relative positions, both become shorter and 

 broader; but it will be observed that the isotropic substance 

 diminishes in height to a much greater extent than does the 

 anisotropic substance. The latter in fact appears to increase in 

 bulk at the expense of the former. 



Relaxation. The shortening as we have seen is followed by a 

 relaxation, the muscle returning to its original length. When an 

 appropriate weight is attached to the muscle this return is generally 

 complete, the curve speedily rejoining, as shewn in Fig. 3, the base 

 line from which it started ; but when no load is used and the 

 muscle therefore is acted upon by its own weight and that of a 

 very light lever only, the return is incomplete ; the curve, though 

 descending near to, fails to touch the base line and runs nearly 

 parallel to it for some considerable distance. The relaxation 

 is therefore obviously assisted by the extending force of the load ; 



