influences growth (see illustration, p. 310). If the pituitary becomes over- 

 active after the long bones have reached their normal full growth, the 



^f 



I 



Thyroid I Thyroid ^1 Thyroid 



Plant-fed Muscle-fed Adrenal-fed 



control control control 



After Gudernatsch 



INFLUENCE OF THYROID ON DEVELOPMENT 



In three series of experiments, young tadpoles fed on thyroid tissue developed toward 

 adult stage so rapidly that they hardly had time to grow. These are full size 



bones of the face, jaw, hands and feet may now continue to grow. Such 

 disproportionate enlargements of the face, the nose, the lips and the hands 

 are often very distressing. 



Hormones and Development As an organism increases in size, it 

 normally changes also in proportions. Everybody recognizes that the body 

 of a mature person is in many ways different from that of a very large 

 baby (see illustration, p. 347). In bodies like our own the maturing appears 

 to be related to the rate of metabolism. But the rate of metabolism is in 

 turn very much influenced by the hormones, especially by thyroxin, the 

 thyroid hormone (see illustration above). 



Where the thyroid is deficient at birth or in early infancy, the child 

 remains sluggish, sometimes to the point of being idiotic. This is not the 

 same as being born with a defective brain. It means that although all the 

 parts of the organism are present, they are not operating effectively. Chil- 

 dren in this condition, called cretins, have appeared in some regions in much 

 larger proportions than in others. In fact, it was long generally believed that 

 the population of certain parts of Switzerland and of other mountainous 

 regions were degenerate because there were so many cretins among them. 

 It was assumed that this condition was inherited and represented a defec- 

 tive stock. 



In recent times, however, we have learned to distinguish idiocy, due to 

 defective brain development, from cretinism, due to a thyroid deficiency. 

 Moreover, we have learned to cure and prevent cretinism (see illustration, 

 p. 311). The hormone produced in the thyroid has been chemically identi- 

 fied and is today produced synthetically. It is distinguished from most other 

 organic compounds by the presence of iodine. Where the soil — and the food 



309 



