THE IMPORTANCE OF PROTEID FOODS. HI 



to consist mainly of carbon, hydrogen, nitrogen, and oxy- 

 gen, and we might at first suppose that these chemical ele- 

 ments in their uncombined form would serve to nourish 

 us. Experience, however, teaches that this is not the case. 

 Four fifths of the air is nitrogen, but we cannot feed on it; 

 hydrogen gas is of no use as a food; and a lump of char- 

 coal (carbon) might fill the stomach, but would not keep a 

 man from starving. Oxygen can be utilized when taken 

 by the lungs from the air; but all other elements to be of 

 use as food must be taken, not in their separate state, 

 but in the form of complex compounds, in which they are 

 chemically combined with other things; as, for example, 

 in starch, and sugar, and fat, and oil, and albuminous sub- 

 stances. 



The Special Importance of Albuminous or Proteid Foods. 

 All the active tissues of the body are found to yield on 

 chemical analysis large quantities of proteids. (See p. 21). 

 As the tissues work this proteid is broken down, and its 

 nitrogen carried off in the form of a peculiar ammoniacal 

 substance, urea; to repair the wasted living tissue new 

 proteids must be laid down in it. So far as we know at 

 present the human body (like that of most animals) is un- 

 able to make proteids out of other things; given one vari- 



What elements do the tissues yield on analysis ? Can we feed 

 on these elements in their uncombined state 1 Name one which is 

 absorbed in a free state and used. Whence is it derived ? What 

 organs receive it? Name substances containing the necessary elements 

 in combination and used as food. 



What do we find in all the active tissues? What becomes of the 

 nitrogen of working tissues? Explain why proteids are an essential 

 article of diet. 



that under some conditions sugar and starch may be used in building tissue, 

 though they cannot do it alone; but whether they are under any circumstances 

 ever burnt before making part of a tissue is not certain. On the other hand, 

 there is some reason to suspect that albuminous substances may, when eaten 

 in excess, be oxidized in the body without ever forming part of a living cell. 



