8 Life and Health 



animal economy. They are combined in various propor- 

 tions, the compounds differing widely from the elements 

 of which they consist. Thus, oxygen and hydrogen unite 

 to form water, and water forms more than two-thirds of 

 the weight of the whole body. 



10. Inorganic Salts. A large number of the elements 

 of the body unite one with another by chemical affinity and 

 form inorganic salts. Thus, sodium and chlorine unite and 

 form chloride of sodium, or common salt. This is found 

 in all the tissues and fluids, and is one of the most impor- 

 tant inorganic salts the body contains. It is absolutely 

 necessary for continued existence. 



By a combination of phosphorus and oxygen with sodium, 

 potassium, calcium, or magnesium, the various phosphates 

 are formed. The phosphates of lime and soda are the most 

 abundant of the salts of the body. For instance, they form 

 more than half the material of the bones. 



The special place of iron is in the coloring matter of the 

 blood. Its various salts are also found in small quantities 

 in the ash of bones, in muscles, and in many other tissues. 



11. Organic Compounds. Besides the inorganic materials, 

 there exists in the human body a series of compound sub- 

 stances formed by the union of the elements just described, 

 which always 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. Human beings and the lower animals 

 take such of these organized materials as they require, and 

 build them up into the materials of their own bodies, often 

 forming still more highly organized forms in the process. 



Whatever other elements they may contain the organic 

 compounds of the body are invariably composed chiefly of 

 carbon, hydrogen, and oxygen. 



