hemoglobin content. Rats fed on milk from the cows in such regions show 

 nutritional deficiencies and die in large proportions unless minerals are 

 added to their diet. 



It is possible that some of the elements or compounds which we find in 

 various plant and animal bodies are residues of material taken in but no 

 longer used by the protoplasm; that is, they are waste products. In some 

 species, for example, the roots or the underground stems contain crystals of 

 a calcium compound, calcium oxalate, which we can recognize by the acrid 

 taste, as in jack-in-the-pulpit. These crystals may represent wastes resulting 

 from metabolism or leftovers from processes in which the plant has more 

 calcium than the living protoplasm can use (see illustration, p. 215). 



From actual experience and special experiments, we know that some of 

 the elements found in protoplasm are indispensable — sodium, potassium, 

 calcium, phosphorus, magnesium, iron and chlorine, for example. Where 

 farming goes on year after year, some of the minerals from the soil are 

 carried off with each crop. In time the soil can no longer maintain the 

 plants. In this country, farming has in the past consisted largely of work- 

 ing fields until they could yield no more, and then moving on. For this 

 reason many of the abandoned farms are quite worthless. 



Special Elements Some dozen elements take part in the growth and 

 activity of most kinds of protoplasm. In addition, many species use certain 

 minerals in special ways. The bones and teeth of vertebrates, for example, 

 are typically hard and rigid. We find that they contain very large propor- 

 tions of calcium phosphate, which consists of calcium, phosphorus and 

 oxygen. Again, the shells of moUusks consist of almost pure calcium car- 



Larynx 



Parathyroid 



glands behind 



thyroid 



Trachea- 



Thyroid 

 gland 



The food and water which the 

 organism takes in contain a very 

 small proportion of iodine. The 

 product of the thyroid gland, 

 however, the thyroxin, contains 

 65 per cent of iodine by weight. 

 Apparently this gland absorbs 

 all the iodine that the body 

 receives and concentrates it in 

 the thyroxin, which is essential 

 to "the normal growth and devel- 

 opment of the organism. Unless 

 there is sufficient iodine in the 

 diet, the thyroid cannot make 

 enough thyroxin. The parathy- 

 roid glands influence calcium 

 concentration in the body fluids 



IODINE AND THE THYROID 

 100 



