CHAP, ir.] THE CONTRACTILE TISSUES. 95 



perhaps infer that the dim disc rather than the bright disc is the 

 essentially active part. Assuming that the fibrillar substance is 

 more abundant in the dim discs, while the interfibrillar substance 

 is more abundant in the bright discs, and that the fibrillar sub- 

 stance is anisotropic (and hence the dim discs largely anisotropic) 

 while the interfibrillar substance is isotropic, we might also be 

 inclined to infer it is the fibrillar and not the interfibrillar sub- 

 stance which really carries out the contraction; but even this much 

 is not yet definitely proved. 



One thing must be remembered. The muscle substance though 

 it possesses the complicated structure, and goes through the re- 

 markable changes which we have described, is while it is living 

 and intact in a condition which we are driven to speak of as 

 semi-fluid. The whole of it is essentially mobile. The very act of 

 contraction indeed shews this; but it is mobile in the sense that no 

 part of it, except of course the nuclei and sarcolemma, neither dim 

 nor bright substance, neither fibrillar nor interfibrillar substance 

 can be regarded as a hard and fast structure. A minute iiema- 

 toid worm has been seen wandering in the midst of the substance 

 of a living contractile fibre ; as it moved along, the muscle sub- 

 stance gave way before it, and closed up again behind it, dim bands 

 and bright bands all falling back into their proper places. We 

 may suppose that in this case the worm threaded its way in a 

 fluid interfibrillar substance between and among highly extensible 

 and elastic fibrillse. But even on such a view, and still more on 

 the view that the fibrillar substance also was broken and closed up 

 again, the maintenance of such definite histological features as 

 those which we have described in material so mobile can only be 

 effected, even in the fibre at rest, at some considerable expenditure 

 of energy ; which energy it may be expected has a chemical source. 

 During the contraction there is a still further expenditure of energy, 

 some of which, as we have seen, may leave the muscle as 'work 

 done ; ' this energy likewise may be expected to have a chemical 

 source. We must therefore now turn to the chemistry of muscle. 



The Chemistry of Muscle. 



58. 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 



