826 METABOLISM AND STRUCTURE. [BOOK n. 



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 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 more temporary nutrition as bearing on the accumula- 

 tion 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 con- 

 tinuity 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 

 acid, sarcolactic acid. Did any considerable amount of oxidation 

 take place in the blood stream while the blood is flowing along 

 the larger channels, subject only to the influence of the vascular 

 walls, we might fairly expect that the lactic acid discharged from 

 the muscles would be subjected to oxidising influences while still 

 within the blood stream of the larger channels. We have 

 however no satisfactory evidence of any lactic acid being oxidised 

 in this way. On the contrary, there is a certain amount of 

 experimental and other evidence that lactic acid present in the 

 blood is somehow or other disposed of by the liver; and that if the 

 liver fails to do its duty lactic acid may appear in the urine. 

 It is tempting to suppose that it might there by a synthetic 

 effort be converted into glycogen, the liver thus utilizing some of 

 the muscular waste product, but the experimental and other 



