ELEMENTS, INORGANIC MATERIALS, WATER 25 



preserving the neutrality of the blood as above noted. Sodium 

 chloride is the only inorganic substance which is contained in 

 a mixed diet in amounts insufficient for the body's needs. Ac- 

 cordingly our food must be salted. Herbivorous animals, living 

 on plants in which potassium is in excess of sodium, crave salt, 

 and it is well known that cattle must be "salted" to keep them 

 in good health. Sodium and potassium also affect the irritabil- 

 ity of muscle and nerve tissue so that their role in the organism 

 may perhaps include little understood regulatory functions. 

 These metals may be detected by the flame test or by precipita- 

 tion as sodium pyroantimonate or potassium cobaltinitrite. 



Calcium and magnesium, while found in all cells in the body, 

 are present in largest amount in the bones and teeth, which 

 contain about 99% of all the calcium and 70% of all the mag- 

 nesium in the body. Their phosphates and carbonates make up 

 about 98.5% of all the inorganic material in bone. That these 

 metals have other roles to play in the body, however, is demon- 

 strated by the failure of blood and milk to clot in the absence 

 of calcium. Calcium is detected by precipitation as calcium 

 oxalate. Magnesium is present in much smaller amounts than 

 calcium, and less is known of its physiological activities. It is 

 interesting in this connection that magnesium sulphate acts as 

 an anaesthetic on mammals and that it paralyzes the endings of 

 the motor nerves in the muscles. Magnesium is detected by 

 precitation as magnesium ammonium phosphate. 



Iron, though present only to the amount of a few grams in 

 the body of an adult man, still is distributed very widely. Its 

 most conspicuous role is in connection with the hemoglobin of 

 the blood, of which it is a constituent. This substance carries 

 oxygen from the lungs to the tissues. Iron probably is pres- 

 ent also in traces of inorganic iron compounds. The spleen 

 contains relatively much, so that it has been suggested that the 

 spleen has some role to play in connection with the iron com- 

 pounds of the blood. Also the liver is concerned with the fate 

 of hemoglobin, and serves as a clearing house through which 

 hemoglobin from worn out corpuscles is broken down and ex- 



