CHAPTER XI 



EFFECT OF CHEMICAL AGENTS UPON, GROWTH 



We shall consider this subject under two heads : (1) Effect 

 upon the rate of growth, and (2) Effect upon the direction of 

 growth. 



§ 1. Effect of Chemical Agekts upon the Rate of 



Growth 



Organic growth, occurring in a material composed of water, 

 plasma, and formed substance, consists in the increment of each 

 of these components. The means and results of varying the 

 quantity of water in the organism will be discussed in the next 

 chapter ; here we are to consider the results of assimilation, 

 including the production of formed substance. The scope of 

 our work may be more precisely defined as the answer to the 

 question. What role do the various chemical substances (exclud- 

 ing water) play in the metabolic changes involved in growth ? 



There are two roles played by chemical substances in the 

 body ; and, accordingly, we may distinguish two kinds of 

 chemical agents having diverse effects upon growth. These 

 agents — foods, in the widest sense of the word — must supply 

 the material — the atoms — from which the molecules of the 

 plasma, or of its formed substance, are made up ; and, sec- 

 ondly, they must supply energy for metabolism. Foods, then, 

 yield to the organisms matter and motion ; they are hylogenic 

 (plastic) and thermogenic (respiratory). 



These two offices of chemical agents in growth are only in 

 certain cases exerted by distinct kinds of food. In the case of 

 animals, sodium chloride and iron compounds are examples of 

 wholly plastic foods, while the free oxygen taken into the body 

 is chiefly thermogenic. In the case of the free oxygen, how- 

 ever, it is quite probable that it is sometimes used in the con- 

 struction of the molecule of active albumen which, according 



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