22 PRINCIPLES OF PHYSIOLOGY 



fluorine, iodine, bromine, manganese, and 

 copper, but the presence of some of these sub- 

 stances may be accidental. It should be noted 

 that carbon and nitrogen, along with hydrogen 

 and oxygen, are essential to life. Oxygen and 

 hydrogen are often in the proportions that form 

 water 2 of hydrogen to 1 of oxygen. From 

 these elements complex chemical substances 

 are built up. Thus the living matter in the 

 cells of a plant, mainly under the stimulus 

 of light, so combines carbon, hydrogen and 

 oxygen as to form starches, sugars, and fats ; 

 and it may also form still more complex 

 chemical substances, known as proteins, 

 which contain carbon, hydrogen, and oxygen, 

 and the all important element nitrogen. 

 Such bodies, often termed proximate prin- 

 ciples, thus formed in a plant, may become 

 the food of an animal and be built into its 

 tissues, or be directly used up in various 

 transformations connected with vital activity. 

 The term proximate principle was given by 

 the earlier physiological chemists, because they 

 thought that certain substances, such as the 

 proteins (albumen, etc.) existed in the tissues 



