92 THE NATURE OF LIFE 



makes a great improvement. Feeding 

 thyroid gland also helps but is less effec- 

 tive than transplantation. 



Lack of thyroid activity lowers the 

 whole metabolism, excessive activity of 

 the thyroid has the opposite effect. The 

 mind instead of being sluggish is over- 

 active, often excitable and troubled by 

 anxious fear, the eyes may protrude, the 

 body tends to be taller than normal. In 

 short the symptoms are quite the oppo- 

 site of those found when the thyroid is 

 subnormal. 



To what extent the effect of the thy- 

 roid depends upon its content of iodine 

 is not yet wholly clear. We know that 

 both thyroid tissue and iodine have a re- 

 markable effect on development in cer- 

 tain animals. Feeding thyroid to tad- 

 poles causes them to develop rapidly into 

 frogs; on the other hand feeding thymus 

 delays metamorphosis and the tadpoles 

 become gigantic in size. Feeding iodine in 

 the form of inorganic salts or free iodine 

 has less effect than feeding thyroid. 



