232 SWALE VINCENT 



the gland. 9 This substance he calls "thyroxin." The iodin content of 

 thyroxin and the iodin content of the sulphate salt were found to be 

 65 and 60 per cent respectively. This established the molecular weight 

 of 585. Ultimate analysis and a study of the derivatives of thyroxin 

 show the structural formula to be 4, 5, 6 tri-hydro 4, 5, 6 tri-iodo, 2 

 pxy, beta,-indolepropionic acid. This substance is stated to possess the 

 active properties of thyroid gland extracts and substances. It is stated 

 that thyroxin possesses the remedial properties of thyroid gland substance 

 as tested upon patients suffering from myxedema. 



There is a delay in the action of thyroxin. Successive daily adminis- 

 tration brings about death, but a single injection even of enormous doses, 

 produces no effect. According to Plummer 1 mg. of thyroxin in an adult 

 weighing 150 pounds increases 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 extirpa- 

 tion experiments described above, and further is in accordance with the 

 emergency theory of Cannon (vide supra). Kendall is of the opinion 

 that thyroxin is a catalyst, and that its function is to increase the rate at 

 which the fundamental chemical reactions of the body are carried out. 



Intravenous injection of thyroid extracts or of thyroid preparations 

 produces a marked temporary fall of blood pressure. There is, however, 

 no reason to believe that the effect is specific. 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 direct cause of the fall is a dilatation of per- 

 ipheral vessels in different regions of the body. There is no evidence 

 that this effect on blood-pressure has any bearing on the question of the 

 internal secretion of the organ (see Vincent (b) (c), 1912, 1918). The 

 substance which produces this fall, and which can be extracted from most 

 of the organs and tissues in the body may possibly be histamine. 2 (Barger 

 and Dale.) 



In 1912 Gudernatsch made the interesting discovery that feeding tad- 

 poles with thyroid substance causes precocious differentiation of the body, 

 but suppresses further growth. The tadpoles begin to undergo meta- 

 morphosis a few days after the first application of the thyroid and weeks 

 before the control animals do so. This observation has been confirmed by 

 several observers, and the reaction is commonly supposed to be a delicate 

 test for the presence of physiologically active thyroid substance. 



But Swingle (a) (b) reports that tadpoles fed with potassium iodid, 

 flour and algsB grew more rapidly and developed limb buds more rapidly 

 than others fed with algae only. According to this observer iodin is more 

 effective than fresh thyroid tissue. Further, Allen (&) finds accelerated 



'Histamin is closely related to histidin which is a common product of protein 

 cleavage. 



