24 APPLIED PHYSIOLOGY 



be more correct, however, to speak of such waste pro- 

 ducts as internal excretions, and to reserve the term 

 internal secretion for the products of glandular organs 

 which are not provided with ducts opening on to a free 

 surface. In any case, the fact that removal of an organ 

 is followed by certain derangements of metabolism is 

 no proof that the organ in question produces even an 

 internal excretion, for the changes observed might quite 

 as well be due to the blood being no longer deprived of 

 some of those constituents which should be taken out of 

 it by the organ removed. There is, therefore, a great 

 deal of loose thinking on the whole subject, and in 

 considering the influence of internal secretions on meta- 

 bolism it will be well to restrict ourselves to the more 

 exact connotation of the term indicated above. 



Now, of internal secretions in the strict sense we 

 know for a certainty of one only namely, the secretion 

 of the thyroid gland, the active constituent of which is 

 the iodine-containing compound known as ' iodothyrin.' 

 This is not the place in which to speak of the exact 

 chemical nature of this compound, and, indeed, but 

 little is known about it, but that it exerts a profound 

 influence upon metabolism there can be no question. 

 That influence may be described as one of stimulation 

 resulting in a great increase in the rate of oxidation 

 in the tissues. The increased elimination not only of 

 carbonic acid gas, but also of urea, which follows thyroid 

 feeding shows that both the fatty and nitrogenous 

 tissues are involved in the increase of katabolism which 

 it brings about. The pronounced effect upon the nitro- 

 genous tissues presumably the muscles is important 



