THE CHEMICAL BASIS OF THE BODY 749 



has but little affinity for other elements. In the form of ammonia, 

 nitrous and nitric acids, it enters the plant through its roots ; as 

 protein it enters the animal, leaving it as urea, etc., which by decom- 

 position readily yields ammonia. The animal cannot utilise free 

 nitrogen any more than the plant can, though leguminous plants 

 utilise atmospheric nitrogen by symbiotic co-operation with nitrify- 

 ing bacteria. There are nitrifying and de-nitrifying bacteria. The 

 former oxidise ammonia and nitrites into nitrates, and are capable 

 of assimilating the free nitrogen of the atmosphere, which they fix 

 and supply to the plant. The latter set nitrogen free by reducing 

 nitrates to nitrites, and decomposing nitrites into nitrogen. From 

 the nitrifying bacteria in plants protein can be built up out of 

 inorganic salts in the absence of chlorophyll. Carbon is present 

 in the atmosphere in small amounts united to oxygen — i.e., in the 

 form of carbon dioxide. It is only in this form that it can be taken 

 up by plants, which in their special laboratory split off the oxygen 

 molecule and store up the carbon, returning the oxygen to the air, 

 and thus supply to the atmosphere that element of which animals 

 are continually depriving it. Under the influence of the ultra- 

 violet rays in light, the green leaves of plants are capable of manu- 

 facturing sugars and starches from carbon dioxide and water. 

 Carbon enters the animal system with the carbon of the food, and 

 leaves it either as carbon dioxide or in compounds, such as urea ; as 

 carbon dioxide it is again taken up by the plant. There is no solid 

 or fluid tissue of the body free from carbon dioxide. It is the most 

 widely distributed gas in the body. 



The Salts of the body are contained in every solid and fluid tissue, 

 though not always in the same proportion. In bone it is naturally 

 high. The age of the animal influences the amount of salts in the 

 body, young growing animals storing up material which the adult 

 rejects. The salts found are those of sodium, potassium, calcium, 

 magnesium, and iron, in the form of chlorides, sulphates, phosphates, 

 and carbonates. The nature of the diet influences the character of 

 the salts present — for example, vegetable f o d is rich in salts of 

 potassium and poor in those of sodium. The salts contribute no 

 energy to the body, but their function in nutrition is of the utmost 

 moment. They direct in some unknown way the metabolism of 

 the body. A salt-free diet produces death from what has been 

 termed salt-hunger. No matter how liberal the diet may be in organic 

 matter, if the salts be removed from it death ensues : in the case of 

 the dog in about six weeks or even less. It has been supposed 

 that life could be preserved longer under starvation, provided ample 

 water be supplied, than on an unlimited diet which is salt-free. 

 This being the case, the extraordinary importance of saline matter 

 in the food is evident. At present very little is known of the subject, 

 but it is generally believed that when the full story is known, it 

 will be found that each salt has a special function to perform in 

 nutrition. Bunge is of opinion that the salts of calcium and iron 

 which enter the body with the food are in organic combination, and 

 that they could not be replaced by inorganic salts. The great test 

 case is iron, which is largely prescribed clinically in an inorganic 

 form. Yet this is attended with success, and an increase in iron 

 can be demonstrated in the blood and tissues. Nevertheless, it 

 would appear that salts in organic combination are more readily 

 taken up than those not so combined. 



An interesting question arises in connection with the artificial 



