Ratio per 1000 examined 

 0.25.1.00 

 1.01-3.99 



^M 4.00 -10.00 

 ^H 10.01 - 27.00 



GOITER DISTRIBUTION IN THE UNITED STATES 



In the First World War, drafted men from the Great Lakes region and from the Pa- 

 cific Northwest had more goiters per thousand than other groups. The soils in these 

 goiter areas contain relatively little iodine, which the body uses in making thyroxin, 

 the thyroid hormone. It is as if the gland enlarged to keep up production from a 

 diet deficient in iodine 



bonate. The exoskeletons of crustaceans (lobsters, crabs, and so on) also 

 contain large quantities of calcium carbonate. 



Iodine, which exists in relatively large amounts in sea water, has been 

 found to be essential in the life of land mammals and birds and other 

 classes of animals. We should not have suspected that from the very small 

 amounts actually present in our tissues — about forty parts per million. 

 Iodine appears to be an essential constituent of thyroxin, which is secreted by 

 the thyroid gland, located in front of the neck (see illustration opposite). 

 In some mountainous regions, and in upland areas having moderate or high 

 rainfall, the iodine has been leached out of the soil. The plants seem to 

 thrive about as well here as elsewhere. But the animals that feed on these 

 plants indicate the lack of iodine in their development and in their activities 

 (see page 311). 



Iron is another element present In relatively small amounts yet absolutely 

 necessary in the metabolism of many species. In animals having red blood, 



101 



