262 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



gave the clue to the need of the gland for iodine, and finally led to the pre- 

 vention of goiter. 



Cretinism and Myxedema. These diseases are both caused by thyroid 

 deficiency; cretinism arises before the child is born, infantile myxedema after- 

 ward. In cither one the children become dwarfs, misshapen, and underdevel- 

 oped physically and mentally, unless they are treated with thyroid hormone 

 (Fig. 15.6). Thyroid dwarfs are characteristically malformed (Fig. 15.7); 

 pituitary dwarfs are usually of normal shape but small (Fig. 15.17). Typical 

 myxedema occurs after adolescence. 



High and Low Thyroid Types. The hypothyroid type of individual has 

 a low rate of metabolism and is relatively calm and slow-moving; among dogs 



Fig. 15.6. Cretinism, a disease of the thyroid, and the importance of its treat- 

 ment. Left, A normal boy of seven years. Center, A cretin of thirteen years, 

 dwarfed physically and mentally subnormal. Right, The same boy after receiving 

 thyroid treatment for seventeen months. (Courtesy, Bronstein, Am. Jour. Med. Sc. 

 205:114, 1943.) 



it is the Saint Bernard. The hyperthyroid type, such as the Irish terrier, has a 

 high metabolism, moves rapidly, and is seldom quiet. 



Experimental Studies of the Thyroid. Removal of the thyroid glands from 

 rabbits shortly after birth produces dwarfs that are essentially like human 

 cretins. If, while still young, they are fed desiccated thyroid, they will grow 

 to normal size and maturity. 



The results of many experiments have shown that the thyroid controls the 

 change of shape that occurs as young animals become mature. This is most 

 spectacular in amphibians which go through a striking metamorphosis from 

 larvae (tadpoles) to adults. Bullfrog tadpoles (Rana catesbiana) are literally 

 rushed through metamorphosis, into dwarf frogs by feeding them desiccated 

 thyroid or implanting crystals of iodine in their bodies (Fig. 15.8). The larval 

 tail is absorbed, the legs develop, the mouth widens, and the alimentary canal 

 changes from the long watchspring shape to the more common form of the 

 adult, but the young frog does not increase in size. Merely feeding tadpoles 



