WHERE SELENIUM POISONING 



In certain portions of the 

 North Central great plains, 

 plants absorb enough sele- 

 nium from the soil to injure 

 animals that feed upon them. 

 The poisoning may result in 

 a slow disease known as 

 "blind staggers"or as"alkali 

 disease", or it may be quickly 

 fatal. As a result of the 

 selenium, the joints of the 

 leg-bones become badly 

 eroded. The hoofs develop 

 abnormalities or drop off. 

 Locomotion is impaired. The 

 effect of the selenium per- 

 sists, for the animals do not 

 usually recover even if re- 

 moved from such a region 

 and fed a good ration 



OCCURS 



iron is an essential constituent of the hemoglobin (see page 205). Copper 

 compounds are generally poisonous to most kinds of protoplasm; yet for 

 some species copper is necessary in small amounts. Copper is an essential 

 element in the bluish oxygen-carrier hemocyanin of the king crab and the 

 lobster. 



In some of the Western states the soil contains the element selenium. 

 This element is present also in plants growing in such soil, although it does 

 not appear to affect them in any way. But animals that feed upon such plants 

 are often seriously poisoned (see illustration opposite). In other regions 

 variation in the amount of fluorine in the soil may be important to us. 



The element fluorine, which is very widely but unevenly distributed, 

 seems to play a role in the assimilation of calcium and phosphorus, and so 

 affects the formation of the teeth. A study of 7000 girls and boys of high- 

 school age in various middle and southwestern states brought out the fact 

 that there was much more tooth decay, or caries^ in communities whose water 

 supplies were free of fluorine than in communities using water with 0.5 or 

 more parts fluorine per million parts water. Thus, the population of a certain 

 part of Texas, Deaf Smith County, was found to have an exceptionally low 

 number of decayed teeth ; and this relative freedom from dental caries is asso- 

 ciated with more than usual amounts of fluorine in the local waters. 



In other regions unusual amounts of fluorine in the soil and soil waters 

 apparently bring about the development of "mottled teeth" among the 

 children living there. Nobody wants blotchy teeth, but nobody wants caries 



102 



