CHAP, v.] NUTRITION. 653 



436. 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 oxidizing influences while still within the blood stream of 

 the larger channels. We have however no satisfactory evi- 

 dence of any lactic acid being oxidized 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. 



437. We may here ask 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 continu- 

 ally 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. 

 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 contractile substance 

 lodged in that living framework. These of course are at pres- 

 ent matters of speculation; but on the whole the evidence we 

 can gather tends and perhaps increasingly 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 com- 

 ing directly from the metabolism of some or other of this mate- 

 rial. 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 



