CHANGES IN A MUSCLE DURING CONTRACTION. 83 



know. Apparently where swift and rapid contraction is required the con- 

 tractile tissue is striated muscle ; but how the striation helps, so to speak, the 

 contraction we do not know. We cannot say what share in the act of con- 

 traction is to be allotted to the several parts. Since, during a contraction, 

 the fibre bulges out more opposite to each dim disc, and is indented opposite 

 to each bright disc, since the dim disc is more largely composed of aniso- 

 tropic material than the rest of the fibre, and since the anisotropic material 

 in the position of the dim disc increases during a contraction, we might per- 

 haps 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 intern" brillar substance is more abundant in the bright 

 discs, and that the fibrillar substance 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 substance 

 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 pos- 

 sesses the complicated structure, and goes through the remarkable 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 shows 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 nematoid worm has been 

 seen wandering in the midst of the substance of a living contractile fibre ; 

 as it moved along, the muscle substance 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 

 fibrilke. 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 con- 

 siderable 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 comparative ease. All muscles, within a certain time 

 after "general " death of the body, lose their irritability, which is succeeded 

 by an event somewhat more sudden, viz., the entrance into the condition 

 known as rigor mortis. The occurrence of rigor mortis, or cadaveric rigidity, 

 as it is sometimes called, which may be considered as a token of the death 

 of the muscle, is marked by the following features: The living muscle 

 possesses a certain translucency, the rigid muscle is distinctly more opaque. 

 The living muscle is very extensible and elastic, it stretches readily and to a 

 considerable extent when a weight is hung upon it, or when any traction is 

 applied to it, but speedily and, under normal circumstances, completely 

 returns to its original length when the weight of traction is removed ; as we 



