INTRODUCTION 5 



We must keep in mind that, with slight exceptions, none 

 of these elements exist in the body in their elementary 

 form. They are combined in various proportions to make 

 compounds which, as a rule, do not resemble the elements 

 of which they are made up. Thus, oxygen and hydrogen 

 unite to form water, and water forms more than two thirds 

 of the weight of the whole body. 



9. Organic Compounds. There is in the human body 

 a series of compound substances which require the agency 

 of living structures for their formation. They are built up 

 from the elements or from simple mineral compounds by 

 plants, and are called organic compounds. 



Animals take as many of these organic substances as 

 they require and build them up into the materials of their 

 own. bodies, which process often results in the formation of 

 still more highly organized forms. 



10. The Three Great Classes of Organic Compounds. The 

 principal organic compounds found in the body or in our 

 food are usually divided into three great classes : (i) proteids 

 (albuminous substances), (2) carbohydrates (starches, sugars, 

 and gums), (3) fats. 



The proteids, the type of which is egg albumin, or the 

 white of egg, are found in muscle and nerve, in glands, in 

 blood, and in nearly all the fluids of the body. 



NOTE. The proteids, because they contain the element nitrogen 

 while the others mentioned above do not, are frequently called 

 nitrogenous, and the other two are known as non-nitrogenous 

 substances. 



The extent to which these three great classes of organic materials 

 of the body exist in the animal and vegetable kingdoms, and are util- 

 ized for the food of man, will be described in the chapter on food 

 (Chapter IV). 



