The Action of Thyroid upon the Organs of the White Rat 233 



ment, brain, and hinfjs, and noted a small increase in the weight of most 

 of these in the thyroid-fed rats. His animals were observed over long 

 periods, but were given very little thj'roid. The gross body-weights of 

 the thyroid-fed animals averaged very little more than those of the control 

 animals, so little, indeed, that Hoskins doubts if the increase of weight is 

 of any special significance. The thyroid-fed animals lose an amount of 

 fat which nearly counterlialances the increase of weight of other structures. 

 Hoskins points out, that, taking this into consideration, his results are not 

 incompatible with the view expressed by Moussu (18) and Schiifer (19), 

 that small doses of th3-roid produce an increase in body weight. On the 

 other hand, Hoskins agrees with a large numljer of observers cited in his 

 paper that larger does of thyroid bring about a decrease or retardation of 

 body weight. 



My own results embodied in this paper are in general agreement with 

 those of Hoskins. I have employed .somewhat larger doses, or doses of 

 similar size more fre(|uently administered, and it is interesting to note that 

 the degree of hypertrophy obtained in several organs is much greater than 

 that obtained by Hoskins. In other organs my figures more nearly 

 approach those of Hoskins, and may be of service in confirming and 

 emphasising his results. Hoskins ascribes the hypertrophy to the 

 increased rate of metabolism promoted by thyroid-feeding, and compares 

 his results to a somewhat siinilar condition described by Hatai (7) in 

 which hypertrophy of organs is induced by increased exercise. The degree 

 of hypertrophy reached by some of the organs in my series seems to me to 

 point to something further than this, and suggests a specific influence of 

 the thyroid upon certain structures of the body. 



II. Methods and Material ExMployed. 



The experiments have been carried out on white rats exclusively. The 

 male animals in the first series were young adults born in the spring of 

 1916, and were mostly from stock reared for several generations in this 

 laboratory. A few were from an outside stock, but the controls and thyroid- 

 fed animals were in this case paired against one another. No essential 

 diflerences were noticed in the control animals of the two stocks. The 

 female rats were all from the laboratory stock, and each thyroid-fed 

 animal and its control were taken from the same litter. These animals 

 were born in the autumn of 1916. According to Jackson (14) and King 

 (15) the variability in body weight of albino rats of the same litter is only 

 about half as crreat as that due to general racial variation. Even in the 

 same litter, however, the individuals of the same sex often show consider- 

 able diflferences in weight and disposition, and for that reason animals as 

 like as possible in the same litter were selected. 



The rats were isolated from one another during the period of experi- 

 ment, and kept each in a separate cage. The isolation in some cases 



