HUMAN PHYSIOLOGY: INTRODUCTION 291 



The cells of the body are made up of certain chemical 

 substances, compounds of the simple elements, sulphur, 

 phosphorus, carbon, oxygen, hydrogen and nitrogen and a 

 few other substances (potassium, chlorine, calcium and 

 magnesium). Comparatively few elements, therefore, are 

 found in the animal tissues. These are, however, united in 

 many ways to form many different compounds. 



Chemical compounds of the body. — The chemical 

 compounds found in the body are proteids, carbo-hydrates, 

 fats, acids, and salts. 



Proteids contain carbon, oxygen, hydrogen, and nitrogen. 

 These are called the nitrogenous compounds. 



Carbo-hydrates contain carbon, hydrogen and oxygen, 

 the former predominating. 



Fats contain also chiefly oxygen, carbon and hydrogen, 

 the latter predominating. The carbo-hydrates and fats 

 are known as non-nitrogenous substances. 



Since these are the chief substances that the body is built 

 up of, they must also be the chief substances in the foods we 

 eat. 



OBSERVATION OF A FEW SIMPLE CHEMICAL 

 ELEMENTS 



Oxygen, the properties of. — The most necessary ele- 

 ment in all the world is oxygen. Neither plant nor animal 

 can live without it. Fire cannot burn without it. It forms 

 about one-fifth of the atmosphere. It is a colorless, odor- 

 less and tasteless gas. Most of the other simple elements 

 will combine with it, especially at a high temperature. 



Oxidation and combustion. — Oxidation is the union of 

 oxygen with any other substance. We say that a substance 

 is oxidized if it has taken up oxygen. Thus carbon, when 

 it takes up oxygen, is oxidized and becomes carbon-dioxide. 

 When oxidation is rapid or accompanied by light or great 



