CHAPTER II 

 ELEMENTS, INORGANIC MATERIALS, WATER 



Elements Found in the Body. The body is made up of a 



large number of chemical elements which are present in very 

 unequal amounts, and distributed quite unevenly in the vari- 

 ous tissues and body fluids. Certain of these elements are in 

 all living cells, others only in particular kinds of cells or in 

 particular animals. Still others are present only accidentally 

 or temporarily. The elements found most frequently are the 

 non-metals carbon, hydrogen, oxygen, nitrogen, sulphur, and 

 phosphorus ; the metals sodium, potassium, calcium, magnesium 

 and iron ; the halogens, chlorine, iodine, and, fluorine. In addi- 

 tion, there are many other elements such as silicon, copper, 

 manganese, arsenic, silver, lead, bromine, lithium, etc., found 

 only in traces or only in a few animals. 



Importance Not Determined by Amount Present. Three of 

 the elements, carbon, hydrogen and oxygen alone make up 

 over 90% of the body weight. The conclusion should not be 

 drawn, however, that these are the only important elements and 

 that those elements or compounds present in small amounts or 

 traces are relatively unimportant to the organism. Quite the 

 contrary may be the case. The body of an average-sized adult 

 man contains only about three grams of iron, and yet this is so 

 necessary to life that an animal fed for some time on a diet 

 which contains no iron will die quite as surely, though of 

 course not as quickly, as if it had been deprived of food alto- 

 gether. The principle involved may be formulated as the Law 

 of the Minimum which states that the importance of a given 

 substance to the animal organism is independent of the rela- 

 tive amount in which it is required. Striking examples of this 

 principle have developed in recent years, and we now know 



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