CHAP, v.] NUTRITION. 825 



some we probably ought to regard as actually entering into the 

 processes themselves. Of these matters however we know very 

 little. 



543. The end-products of muscular metabolism are as we 

 have seen carbonic acid, lactic acid, and kreatin or some other 

 nitrogenous bodies, and we have already (87) said all we have to 

 say concerning the formation of these products. We may how- 

 ever briefly consider here the question, what is the relation of 

 these various metabolic processes to the structural elements of the 

 tissue ? When we say that the muscular fibre is continually 

 undergoing metabolism do we mean that every jot and tittle of the 

 fibre is undergoing change and that at the same rate ? We can 

 hardly suppose this. It seems unlikely, for instance, that the 

 metabolism of the fibrillar substance is identical with that of the 

 interfibrillar substance, whatever be the view we take as to the 

 properties or meaning of the two substances. Further, if we 

 accept the suggestions made in 87 as to a contractile substance, 

 which, though having peculiar qualities, being peculiarly related 

 to and having peculiar connections with the rest of the fibre, may 

 in a broad way be compared with the glycogen of a hepatic cell, 

 we can conceive that this contractile substance may be manufac- 

 tured without the whole of it at least having been at any time an 

 integral part of what we may in a stricter sense call the real living 

 substance of the fibre. We should thus be led to regard the 

 metabolic events occurring in muscle as falling into two classes at 

 least; those taking place in the living more permanent framework, 

 and those bearing on the formation and destruction of the con- 

 tractile substance lodged in that living framework. Further, if 

 we suppose that the metabolism by which the muscles supply 

 so much of the heat of the body, and which as we have seen 

 may and does go on independently of contractions, is not a 

 metabolism of the same contractile substance difFering from the 

 metabolism of a contraction in being so ordered that all the 

 energy goes out as heat, none being employed to effect a change 

 of form, but is a metabolism of some other ' thermogenic ' sub- 

 stance, we should have to add a third class to the other two. 

 These of course are at present matters of speculation ; but on 

 the whole the evidence we can gather tends and perhaps in- 

 creasingly tends to shew that in muscle there does exist such 

 a framework of what we may call more distinctly living sub- 

 stance which rules the histological features of the fibre, and 

 whose metabolism though high in quality does not give rise to 

 massive discharges of energy, and that the interstices so to speak 

 of this framework are occupied by various kinds of material related 

 in different degrees to the framework and therefore deserving to 

 be spoken of as more or less living, the chief part of the energy 

 set free by muscle coming directly from the metabolism of some 

 or other of this material. And the same view may be extended 



