COMPOSITION OF HUMAN BODY 



Chemically, the human body (like all other living things, for that matter) consists 

 largely of oxygen, carbon, hydrogen and nitrogen. The proportions of the other 

 elements vary somewhat with the kind of plant or animal, but there are always sev- 

 eral, and certain of these have always been found indispensable whenever we have 

 taken the trouble to experiment with them 



teins, of which there are many different kinds. They all have this in com- 

 mon, however, that they consist of the elements carbon, hydrogen, oxygen, 

 nitrogen and, in addition, either sulfur or phosphorus. Chemists have 

 shown that proteins consist of combinations of simpler nitrogenous com- 

 pounds called amino-acids. Different proteins have different combinations 

 of amino-acids. 



Proteins are thus present in almost every part of every animal or plant. 

 That is not to say that all animal and plant materials are suitable as food. 

 In many cases the proportion of protein is very low. In other cases addi- 

 tional substances present render the materials unsuitable for food, or at least 

 for human food. It means only that protein is necessary for the making of 

 more protoplasm. 



In our common foods the proteins are represented by albumen, or white- 

 of-egg; casein, the curd formed when milk sours; and gluten, the pasty 

 substance in wheat flour or bread. Similar nitrogen-containing substances 

 are present in the muscle (flesh) cells of many animals. All seeds contain 

 some proteins, some kinds in rather large proportions — as peas, beans, pea- 

 nuts, lentils and others of the bean family. 



Protoplasm Action' In active protoplasm, as we have seen, the energy 

 comes from the oxidation of "fuel". Protein itself oxidizes in living cells, 

 and yields energy. In the process it is of course destroyed, breaking up into 

 simpler nitrogen compounds, water and carbon dioxide. Other fuels, which 



iSee No. 1, p. 111. 

 97 



