654 INFLUENCES DETERMINING NUTRITION. [BOOK n. 



substance, but one more directly than the other, and this is what 

 was meant by the terms ' directly ' and ' indirectly,' used in 434. 



438. 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 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 arid 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 frame- 

 work is more or less directly concerned in the production of the 

 carbohydrate material and the various conversions which that 

 material undergoes. 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. 



439. In the preceding chapters of this work we have had 

 abundant evidence that the metabolism of the tissues is subject 

 to the government of the central nervous system ; the contrac- 

 tion of a muscle, the secretory activity of a gland, the increased 

 or diminished production of heat all afford instances of nervous 

 impulses affecting metabolism. In most of these instances the 

 changes induced fall within the downward, katabolic, phase 

 and have a downward character ; thus when a muscle contracts, 

 the result is a conversion of more complex bodies into simpler 

 bodies ; and the same as far as we can see is true of most other 

 cases. But it is open for us to suppose that nervous impulses 

 might affect the upward, anabolic, phase and have a constructive 

 influence. 



At all events we are not justified in assuming that a nervous 

 impulse can only produce disruptive katabolic changes such as 

 are seen in muscular contraction or in secretion. The effects 

 of stimulating a nerve going to a muscle or a salivary gland 

 are striking and obvious and the behaviour of a muscle or a 

 gland as far as contraction and secretion are concerned is, within 

 certain limits, under experimental control. But there are cer- 

 tain phenomena, seen chiefly in the course of disease, and lying, 

 to a very small extent only, within the control of experiment, 



