314 THE DUCTLESS GLANDS 



subsequently as free iodine. The iodine content is 65 per cent. 

 It is said that a small quantity has already been synthesized. 

 Thus it appears that two definite compounds containing 

 iodine have been separated from thyroid tissue, a globulin 

 (iodo-thyro-globulin) which exists as such in the gland, and 

 a derivative of tryptophane (thyroxin) obtained as a cleavage 

 product and containing 65 per cent, of iodine. The evidence 

 before us points to thyroxin as one of the active compounds 

 of the internal secretion of the thyroid gland. But it is too 

 early to affirm that it is the only active compound. 



N. Physiological Actions of the Thyroid Secretion 



Kendall claims that thyroxin produces all the effects of 

 thyroid both physiologically and therapeutically. Thus it is 

 found as useful in myxcedema as other thyroid preparations. 



Thyroxin shows a curious delay in its action. Successive 

 daily administration may bring about death, but a single 

 injection even of enormous size produces no effect. 1 mg. of 

 thyroxin in an adult weighing 150 pounds will increase the 

 metabolic rate 2 per cent. 



Kendall suggests that in the normal animal organism 

 thyroxin is not fundamentally essential to life. This agrees 

 with the results of extirpation experiments described above, 

 and further is in accordance with the emergency theory of 

 Cannon (vide supra). It is suggested that thyroxin is a 

 catalyst, and that its function is to increase the rate of the 

 fundamental chemical reactions of the body. 



Intravenous injection of thyroid extracts or of thyroid 

 preparations produces a temporary fall of blood-pressure. 

 This effect is in all probability not a specific one. It is probably 

 the same kind of fall as is produced by the injection of extracts 

 of most organs and tissues in the body. The fall is due to 

 a dilatation of peripheral vessels throughout the body. We 

 have no reason to believe that this effect on blood-pressure 

 has any bearing on the question of the internal secretion of the 

 organ. The probable nature of the substance has been dis- 

 cussed above (p. 25). 



Gudernatsch and others have observed that feeding tad- 

 poles with thyroid substance will cause precocious differenti- 

 ation, while growth is suppressed. The tadpoles begin to 



