152 METABOLISM 



The salts are of no importance from an energy point of 

 view. They maintain the normal composition and osmotic 

 pressure in the liquids and tissues of the body. Furthermore, 

 they are in some way bound up in the structure of the living 

 material, so that they are necessary to its normal reactions. 

 They are even found in proteins, and their removal changes 

 the properties of proteins, so that ash-free native proteins 

 are not coagulable by heat and globulins are precipitated. Cal- 

 cium plays a special part in the coagulation of blood and milk 

 and in the contractions of cardiac tissue. Iron salts are 

 necessary for the production of hemoglobin. Sodium chloride 

 is the only salt that is consciously added to the diet. The 

 average man ingests from ten to twenty grams per diem. The 

 need of sodium chloride is felt by those animals that live on 

 vegetable food. The explanation given for this is as follows: 

 Most vegetables contain a large amount of potassium salts. 

 When these enter the body they react with sodium chloride, 

 forming by their reaction salts foreign to the blood, which 

 are promptly excreted by the kidneys. As a result, the normal 

 percentage of sodium chloride is lessened. 



During starvation the body clings to its salts; the amount 

 of sodium chloride excreted soon falls to a low figure (0.6 gram). 

 If ingested in superabundance it is promptly excreted. Calcium 

 salts play an important part in the growth of the skeleton. 

 When dogs are given a calcium-free diet they fall into a con- 

 dition resembling rickets in children. While most of the cal- 

 ciilm passes through the body, undergoing no change, a portion 

 must be involved in the tissue metabolism. Calcium is con- 

 stantly being eliminated from the inner surface of the intestine 

 in small amount, perhaps 0.15 to 0.16 gram per day. The 

 deposition of calcium increases with age in the arteries, so 

 that they lose their elasticity. Under pathological conditions 

 it may give rise to arterial sclerosis and senile cataract. Milk 

 is a food which is rich in calcium and phosphorus, evidently 

 related to the needs of the developing child. The calcium 

 is in the form of an organic combination, united with proteins 

 as in caseinogen. It is more easily assimilated in the body 

 in this form. The same is true of iron. In ordinary dietaries 

 the iron amounts to only 8 to 10 milligrams per day. A daily 

 excretion takes place through the walls of the intestine. No 



