CHAP, v.] NUTRITION 475 



We have seen that the oxidations of the body are, as in the 

 case of muscle, of a peculiar character, and carried on by the tissues 

 themselves. While at present we should be hardly justified in 

 denying that any oxidations at all take place in the blood-plasma, 

 such as do occur must be slight in amonnt as compared with those 

 going on in the tissues. We might also say that one body only, 

 viz. lactic acid, presents itself as a substance likely to be directly 

 oxidised in the blood itself; and even with regard to this the evi- 

 dence is as much against as for any such direct oxidation taking 

 place. The great mass of the oxidation of the body is of an 

 indirect kind, determined by the activity of the several tissues. 

 The blood serves as an oxygen carrier for the tissues ; and it is not 

 itself the large combustion agent it was once thought to be. The 

 tendency of all recent inquiries is to shew that the body cannot be 

 compared, either as a whole, or in its parts, to a furnace for the 

 direct combustion of combustible food. On the contrary, we are 

 driven nearer and nearer to the conclusion that all food which has 

 become absorbed into the blood must become tissue before it be- 

 comes waste product, and only becomes waste product through a 

 metabolism of the tissue. When we say " become tissue" we must 

 leave it at present wholly undecided how far the constant meta- 

 bolism which this view demands affects the so-called structural 

 elements of the more highly organized tissues ; it is quite open 

 however, as we have already suggested, for us to imagine that in 

 muscle, for instance, there is a framework of more stable material, 

 giving to the muscular fibre its histological features, and under- 

 going a comparatively slight and slow metabolism, while the 

 energy given out by muscle is supplied at the expense of more 

 fluctuating molecules which fill up so to speak the interstices 

 of the more durable frame-work, and the metabolism of which 

 alone is large and rapid. 



The characteristic feature of proteid food is that it increases the 

 oxidative, metabolic activity of the tissues, leading to a rapid con- 

 sumption, not only of itself, but of non-nitrogenous food as well. 

 Where therefore a rapid renewal of the tissues is sought for, an 

 excess of proteid food may be desirable. But it must be borne in 

 mind that by the very nature of its rapid metabolism, proteid food 

 must tend to load the body with the so-called extractives, i.e. with 

 nitrogenous crystalline bodies. How far these are of use to the 

 body, and what part they play, is at present unknown to us. That 

 they are of some use is suggested by the beneficial effects of the 

 extractum carnis when taken as food in conjunction with non-nitro- 

 genous material, though it is possible that the dietetic value of 

 this preparation may be due to the small amount of non-crystalline 

 extractives which it contains. That when in excess these nitro- 

 genous products may be highly injurious is indicated by the little 

 we know of the connection between the symptoms of gout and the 



