CHAP, v.] NUTRITION. 867 



seems as we have seen to be especially characterised by its return- 

 ing to the blood a body, viz. sugar, still containing a considerable 

 amount of energy, available for use in other parts of the body. 

 And this suggests the question whether in the normal metabolism 

 of muscular substance a similar something, still holding a con- 

 siderable quantity of energy, some proteid substance for instance, 

 may not be returned to the blood; so that the metabolism of 

 muscle is imperfectly described in saying that the results are 

 carbonic and lactic acids and an antecedent of urea. If this be so, 

 then muscles may be of other use to the body at large than as 

 mere contractile machines, just as the liver has other uses than 

 the production of bile. And the same considerations may be 

 applied to the other tissues as well. 



546. Whether the chief product of the metabolism of any 

 tissue be a proteid substance, or a fat, or a carbohydrate, proteid 

 substance is the pivot so to speak of the metabolism, and nitro- 

 genous bodies always appear as the products of metabolism. This 

 is strikingly seen in the nutrition of plants where, as far as mere 

 bulk or weight is concerned, the active metabolizing tissue is 

 insignificant compared with the mass of products of metabolism 

 heaped up in the form of starch or cellulose or some allied 

 carbohydrate. The protoplasm of a vegetable cell soon becomes 

 a mere film bearing a heavy burden of heaped up metabolic 

 products and eventually disappears ; and of that film only a part 

 corresponds to what we spoke of above as the living framework of 

 the muscle. Yet that scanty proteid-built framework is more or 

 less directly concerned in the production of the carbohydrate 

 material and the various conversions which that material under- 

 goes. Proteid, nitrogen, changes are entangled with the carbon 

 changes ; and since the products of metabolism in the plant are 

 not as in the animal cast out of the organism, but for the most 

 part heaped up within it, we find the plant storing up in parts, 

 where if they serve no useful purpose they at least do no harm, 

 nitrogenous products of metabolism, such as those known as 

 vegetable alkaloids, many of which by their amide nature betray 

 their kinship to the animal nitrogenous product urea. 



547. The rate at which in the adult, leaving aside for the 

 present the special nutrition of the young, nutrition is carried on, 

 and the characters of the nutrition, are dependent on a variety of 

 circumstances. Each tissue has of course a line of nutrition of its 

 own which circumstances may favour or hinder but cannot change 

 in nature ; the nutrition of the hepatic cell cannot be altered to 

 that of the muscular fibre. The same tissue moreover has in 

 different races and different individuals specific and individual 

 characters of nutrition ; the flesh of the dog is not the same as 

 that of a man, the muscle of one man lives differently from that 

 of another, the matabolism per unit of body weight is as we have 

 seen greater in the smaller organism, and so on. 



