CHAPTER XX. 



FOODS. 



Foods as Tissue Formers. Hitherto we have considered 

 foods merely as sources of energy, but they are also re- 

 quired to build up the substance of the Body. From birth 

 to manhood 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 com- 

 plex 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 com- 

 plex unstable molecules, like those of protoplasm, from the 

 simpler compounds taken into the Body, needs an expendi- 

 ture 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 con- 

 stantly reconstructed, as we see illustrated 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 a^ain 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 



