The Endocrine System 231 



the rate of metamorphosis of larvae into frogs or into salamanders as 

 the case may be. In some sections of this country (Texas, Kansas, and 

 Oklahoma) and on the Mexican plateau where the water is deficient in 

 iodine, one species of salamander fails to metamorphose into the 

 adult. These individuals known as axolotls are actually able to mature 

 sexually and to reproduce without ever acquiring their true adult 

 characteristics. If the axolotls are placed in water containing iodine, 

 they quickly lose their larval characteristics and become normal adults. 



THE PARATHYROID GLANDS 



The parathyroids are small masses of glandular tissue which has 

 been demonstrated to be present in all vertebrates but fish. In human 

 beings, there are two pairs of these small, ovate glands, one of each 

 pair being located on the posterior surface and one on the anterior 

 surface of each lobe of the thyroid gland. The number and location of 

 these glands vary in different animals, but, in general, their position is 

 near the thyroid. 



Histologically, the parathyroids are very different from the thyroids. 

 They are composed of masses of epithelial cells interspersed with capil- 

 laries. Surrounding each gland is a sheath of connective tissue which 

 sends extensions into the main body of the gland and divides it im- 

 perfectly into small lobes. The hormone of this gland, parathormone, 

 is secreted by the epithelial cells. 



Although the parathyroids had been recognized anatomically as sep- 

 arate entities for a considerable length of time, their separate function 

 was not at first realized. The first indication of their importance in the 

 functioning of the organism occurred when the thyroids were experi- 

 mentally removed from dogs and cats. These animals quickly went 

 into tetany and died. The same operation when performed on sheep 

 and goats did not produce similar results. Until one man pointed out 

 the fact that the parathyroids of these latter animals are located in such 

 a position that they are not removed when the thyroids are extirpated, 

 no one had assigned any important function to them. Thus it became 

 evident that the proper functioning of these glands was essential for 

 life. Their removal was the immediate cause of the tetany and the sub- 

 sequent death of the dogs and cats. 



The Function of the Parathyroid Glands. — Parathormone is 

 important in the metabolism of calcium and phosphorus. It should be 

 recalled that these two elements are in a dynamic state in the body. 



