MINERAL METABOLISM 397 



ing is very rare except in children who may mistake boron compounds 

 for something like sugar and eat a large quantity. 



Bromine is more abundant in living materials than iodine, the 

 factor being 50 to 100 for animal tissues except the thyroid. Yet 

 there is no indication that Br~ is essential, and extensive work has 

 not produced a conclusive result. Dibromoindigo (royal purple) was 

 a dye much sought by the ancients as a badge of prestige. This 

 compound is found in small amoimts in the snail Murex brandaris. 

 Dibromotyrosine occurs in the skeletal proteins of certain other 

 snails. Otherwise natural organic compounds of bromine are un- 

 known. 



Lead occurs in traces throughout nature without known function. 

 Although the element is a cumulative poison, the quantities ingested 

 by animals are normally safe. However, the prolonged use of lead 

 sprays in orchards, for example, may increase the lead content of 

 soils and hence of crop plants to levels that can cause slowly develop- 

 ing chronic toxicity. Lead poisoning has been diagnosed in small 

 children from old dwellings painted repeatedly with lead paints. 

 Apparently the children chew painted surfaces, perhaps during 

 teething. Recent therapeutic methods are described as rapid and 

 effective. 



Nickel is another widespread element in animals without known 

 function, although in this case little serious effort has been expended 

 with modern techniques. The element is not particularly toxic, and 

 poisoning is known only among workers in ore concentrating plants, 

 where it presents a serious health problem. 



Nitrates and nitrites can be ingested in dangerously high quantities. 

 As mentioned earlier (page 325), water supplies occasionally con- 

 tain toxic amounts. In addition, certain soils in semiarid regions or 

 areas after intensive application of nitrate fertilizers may lead to high 

 NO3- levels in particular plants. Oat straw, for example, may contain 

 5 to 10 per cent of potassium nitrate and produce trembling, stagger- 

 ing, rapid respiration, prostration, and death in cattle. Other plants 

 are sometimes involved, and horses and sheep can also be affected. 

 It is believed that poisoning is caused by the nitrite ion formed 

 intestinally by reduction of the nitrate. Cases have been reported 

 primarily from Wyoming, South Dakota, and western Canada. 



Silicon as a universal component of soils is taken up by plants and 

 is ingested by animals as an incidental element so far as is known. 

 Silica is very abundant, amounting to 3 to 4 per cent of whole dried 

 plants or 75 per cent of the ash of feathers. Toxicity is not observed 



