

CHAPTER XXj. 

 FOODS. 



Foods as Tissue-formers. Hitherto we have considered 

 foods merely as source of energy, but they are also required 

 to build up the substance of the Body. From birth to man- 

 hood we increase in bulk and weight, and that not merely by 

 accumulating water and such substances, but by forming more 

 bone, more muscle, more brain, and so on, from materials 

 which are not necessarily bone or muscle or nerve-tissue. 

 Alongside of the processes by which complex substances are 

 broken down and oxidized and energy liberated, constructive 

 processes take place by which new complex bodies are formed 

 from simpler substances taken as food. A great part of the 

 energy liberated in the Body is in fact utilized first for this 

 purpose, since to construct complex unstable molecules, like 

 those of protoplasm, from the simpler compounds taken into 

 the Body, needs an expenditure of kinetic energy. Even 

 after full growth, when the Body ceases to gain weight, the 

 same synthetic processes go on; the living tissues are steadily 

 broken down and constantly reconstructed, as we see illus- 

 trated by the condition of a man who has been starved for 

 some time, and who loses not only his power of doing work 

 and of maintaining his bodily temperature but also a great 

 part of his living tissues. If again fed properly he soon 

 makes new fat and new muscle and regains his original mass. 

 Another illustration of the continuance of constructive 

 powers during the whole of life is afforded by the growth of 

 the muscles when exercised properly. 



Since the tissues, on ultimate analysis, yield mainly car- 

 bon, hydrogen, nitrogen and oxygen, it might be supposed 

 a priori that a supply of these elements in the uncombined 

 state would serve as material for the constructive forces of 

 the Body to work with. Experience, however, teaches us 

 that this is not the case, but that the animal body requires, 

 for the most part, highly complex compounds for the con- 



