The Action of Thyroi^l upon tlit- Organs of the Wliite Rat 287 



According to Donaldson's figures, the average weight of the suprarenals for 

 an animal of that body weight is 2(S mg. Among the controls of the same 

 experiment a rat of the same weight has suprarenals weighing 25 mg. 

 The increase of weight of the suprarenals is, therefore, 118 per cent, against 

 Donaldson's figure, and 140 per cent, against the control. 



In female rats the suprai'enals are normally heavier per unit body 

 weight than in males, and the percentage increase of weight caused by 

 thyroid-feeding is less. The heaviest suprarenals found in the female rats 

 weighed 74 mg. in an animal of 140 grm. body weight fed for thirty-six 

 days on 01 grm. thyroid daily. Donaldson's average for a female rat of 

 140 grm. body weight is 38 mg. In this rat the suprarenals are 94 per 

 cent, heavier than the normal averaofe. 



The examples quoted, however, are the extremes. The general averages 

 show an increase of 56 per cent, per 100 grm. body weight in the males, 

 and an increase of 41 per cent, per 100 grm. body weight in the females 

 as the result of thyroid-feeding. The increase in weight is partly of 

 medulla but chiefly of the cortex. 



In male rats the author (10) has shown that thyroid-feeding increases 

 the adrenalin content absolutely, but diminishes it relatively to the per- 

 centage weight of the suprarenals. The increase of adrenalin in the supra- 

 renals of the female rats resulting from thyroid-feeding is less constant. 

 In the female rats which thrive well under the influence of thyroid the 

 adrenalin is increased, but in those animals which show checked srrowth 

 and other toxic symptoms the amount of adrenalin is actually decreased. 

 Observations upon this subject are still being made, and will be dealt with 

 in a subsequent paper. They may have an important bearing upon the 

 explanation of the factors in thyroid-feeding which bring about the great 

 hypertrophy of some of the organs of the body. 



My results agree very closely with those obtained by Hoskins; but 

 the hj'pertrophy of the suprarenals is greater in my series than in his, 

 probably because of the larger amounts of thyroid given. Hoskins noted 

 that the reaction of the suprarenals to thyroid is relatively greater in 

 males than in females, a view which my figures confirm. 



2. The Heart. — The heart is constantly and very greatly hyper- 

 trophied under the influence of thyroid. The response is an immediate 

 one. Table VI. shows that the administration daily of 0-2 grm. fresh 

 thyroid to young male rats increases the weight of the heart by 78 per cent, 

 in eight days. Prolonged feeding with thyroid leads to a much greater 

 increase. 



The weights of the hearts of the control male rats agree very nearly 

 with the normals of Donaldson's tables; the control females have rather 

 heavier hearts than the females of corresponding body weight in the 

 Wistar Institute. The largest heart recorded in my series (No. 7, Table IV.) 

 is in a thyroid-fed male rat, and weighs 2-455 grm., the body weight of 

 the animal being 182 grm., and the body length 189 mm. For a male rat 



