508 NUTRITION. 



the part only of securing by their presence favorable conditions for the due 

 metabolic processes, somewhat after the way in which the presence of calcic 

 phosphate determines the curdling of milk ; but some we probably ought to 

 regard as actually entering into the processes themselves. Of these matters,, 

 however, we know very little. 



456. 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 ( 85) said all we have to say concerning the formation of 

 these products. We may, however, 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 under- 

 going metabolism do we mean that very jot and tittle of the fibre is under- 

 going 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 suggestion made in 85 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 com- 

 pared with the glycogen of an hepatic cell, we can conceive that this contrac- 

 tile substance may be manufactured 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 bear- 

 ing on the formation and destruction of the contractile 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 me- 

 tabolism 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 " substance, 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 increasingly tends, to 

 show that in muscle there does exist such a framework of what we may call 

 more distinctly living substance 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 to other tissues. Both the framework and the intercalated 

 material undergo metabolism, and have, in different degrees, their anabolic 

 and katabolic changes ; both are concerned in the life of the living substance 

 but one more directly than the other, and this is what was meant by the 

 terms " directly " and " indirectly," used in 454. Such a mode of expres- 

 sion seems preferable to the more common one, based on the analogy of a 

 firearm, of the muscle fibre firing off the contractile material ; in the firearm 

 there are no such connections between the machine and the charges as obtain 

 in the living mechanism. We may perhaps further be led by this to dis- 

 tinguish between growth as bearing on the framework, and mere temporary 

 nutrition as bearing on the accumulation and expenditure of the lodged 

 material. We mav add that since some of the material so lodged in the 



