ON NUTRITION IN GENERAL. 509 



framework will consist of substances which have not yet undergone metabo- 

 lism, 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 metabo- 

 lism, or are substances whose presence is in some way necessary for the car- 

 rying 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 the interstices. Hence such phrases as 

 " tissue proteid " and " floating proteid " ( 436), are undesirable 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. 



457. The products of muscular metabolism pass into the lymph bath- 

 ing 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, sub- 

 ject only to the influence of the vascular walls, we might fairly expect that 

 the lactic acid discharge from the muscles would be subjected to oxidizing 

 influences while still within the blood stream of the larger channels. We 

 have, however, no satisfactory evidence 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 dis- 

 posed of by the liver ; and that if the liver fail 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 evidence is 

 all against this view. In the absence of actual knowledge we infer that it 

 is in the liver oxidized into carbonic acid and water, thus adding its con- 

 tribution to the supply of heat, or prepared in some way for oxidation else- 

 where. Probably such a change is not confined to the liver, but takes place 

 in other organs, such as the spleen. Thus the kind of action on which we 

 dwelt in treating of urea, namely, that the products of the metabolism of 

 one organ are carried to other organs for further elaboration and possible 

 utilization, applies to the non-nitrogenous as well as to the nitrogenous pro- 

 ducts of muscular metabolism ; and if a muscle gives rise to other non-nitro- 

 genous products than carbonic and lactic acid these are probably disposed of 

 in some such way as the lactic acid. In speaking of glycogen in the winter 

 frog ( 380) we said that possibly the glycogen so stored up might arise 

 from sugar brought to the liver from other tissues. If that be so, we should 

 further expect that some at least of that sugar, either as such or as some 

 allied substance, would come from the skeletal muscles which form so large 

 a part of the body of the frog ; and if so, we must conclude that under the 

 special circumstances obtaining in the winter frog the muscles discharge into 

 the blood a non-nitrogenous product not in the form either of carbonic or 

 lactic acid. It is perhaps, however, more probable that the sugar in ques- 

 tion comes from a metabolism of the fat stored up in the " fatty bodies " and 

 elsewhere. 



458. As far as we can see at present the plan of nutrition thus briefly 

 sketched out for muscle holds good for the other tissues as well, the chief or 

 at least the most conspicuous differences bearing on the nature and proper- 

 ties of and the changes undergone by the material formed by and held by 



