510 NUTRITION. 



the more distinctly structural work. Thus the rnucin of the salivary mucous 

 cell finds its analogue either in the contractile substance itself, or more prob- 

 ably in some early nitrogenous product of the explosion of the contractile 

 substance, such as may correspond to the myosin of rigid muscle. The 

 metabolism of the hepatic cell seems, as we have seen, to be especially cha- 

 racterized by its returning 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 mus- 

 cular substance a similar something, still holding a considerable 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 produc- 

 tion of bile. And the same considerations may be applied to the other 

 tissues as well. 



459. 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 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 undergoes. Proteid, nitro- 

 gen, 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, nitro- 

 genous 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. 



460. 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 favor 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 a 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 nutri- 

 tion. 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 ( 437), 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 ali- 



