CHAP, v.] NUTRITION. 865 



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 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, and indeed is supported by what we know of the 

 changes in secreting cells. 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 541. Such a mode of expression 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 distinguish between growth as bearing on the frame- 

 work, and mere temporary nutrition as bearing on the accumu- 

 lation and expenditure of the lodged material. We may add 

 that since some of the material so lodged in the framework will 

 consist of substances which have not yet undergone metabolism, 

 but are either about to be worked up into the framework itself, 

 or are about to be transformed in a more direct way into some 

 product of metabolism, or are substances whose presence is in 

 some way necessary for the carrying on of metabolic processes in 

 which they themselves take no bodily part, we must recognize a 

 continuity without any sharp break between this material which 

 we regard as part of the tissue, and the lymph which simply 

 bathes the tissue and flows through its interstices. Hence such 

 phrases as ' tissue proteid ' and ' floating proteid,' 522, are unde- 

 sirable if they are understood to imply a sharp line of demarcation 

 between the " tissue " and the blood or lymph, though useful as 

 indicating two different lines or degrees of metabolism. 



544. The products of muscular metabolism pass into the 

 lymph bathing the fibre and so, either by a direct path into the 

 capillaries or by a more circuitous course through the general 

 lymphatic system, into the blood. The fate of the carbonic acid 

 we have fully treated of in dealing with respiration ; the little we 

 know concerning' the nitrogenous product or products has been 

 stated in dealing with urea ; the third recognized product is lactic 



