828 INFLUENCES DETERMINING NUTRITION. [BOOK n. 



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 

 nitrogenous bodies alway 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 not 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 metabolism per unit of body weight is as we have 

 seen greater in the smaller organism, and so on. 



Within the limits and subject to the conditions however 

 thus fixed by race and personality, general influences produce 

 general variations in nutrition. The rate of nutrition of a tissue 

 for instance is dependent on the food, on the amount and nature 

 of the food material brought to the tissue by the blood. We 

 have seen that proteid food, in contrast to carbon food markedly 

 increases the metabolism of the body. Since this increase tells 

 not only on the nitrogenous but also on the carbon metabolism 

 ( 523), it ^ cannot be the result of a mere luxus consumption 

 of the proteid food itself; and unless we suppose that the presence 

 of the excess ^of proteid material either in the alimentary canal, 

 or while passing through the capillaries of some organ such as 

 the liver, acts as a stimulus to some reflex nervous machinery 



