66 



THE ROYAL SOCIETY OF CANADA 



Table II, with one or two exceptions, does not show such hyper- 

 trophies. Rat 4, experiment 2, and the rats of experiment 6 will 

 be considered later. The differences between experimental and con- 

 trol animals of the same series are usually only a few per cent., and 

 in either direction, and can be considered as within the possible 

 individual variations. An analysis of Table II is given in Table III, 

 expressing the percentage differences (extremes and means) between 

 experimental and control animals. The spleen and lymph glands 

 show greatest variations. Lymphatic tissue seems normally subject 

 to unusually great variations. The adrenals of female rats seem still 

 to be definitely hypertrophied, while thyroid tissue averages dis- 

 tinctly less than normal (but its macroscopic appearance showed 

 that in all cases it was functioning normally). The remaining organs 

 are practically normal. 



TABLE III 



Liver 



Kidneys. . 

 Heart. . . . 

 Spleen. . . . 

 Lymph 



Glands. 

 Adrenals.. 

 Thyroid. . 

 Muscle. . . 

 Testes. . . . 



Average 



% 



+ 4 



+ 6 



+ 7 



+ 7 



- 9 

 + 15 

 -10 



- 2 



Exceptions 



Rat 4, experiment 2, and rats 1 (control) and 7 and 9 (thyroid- 

 fed), experiment 6, show markedly enlarged thyroids, and at the same 

 time markedly hypertrophied livers, kidneys, hearts, and adrenals, 

 with (except the last rat) distinctly diminished muscle tissue. The 

 results suggest an actual hyperthyroidism in these rats, and the 

 results for total growth are in agreement. Unfortunately before these 

 results had become apparent the thyroid tissue had been discarded. 



Neglecting these obviously abnormal cases it would seem that 

 in a period of from 4 io 7 weeks after cessation of thyroid-feeding the 

 distinct hypertrophy which is produced in 18 days has almost, if not 

 quite, disappeared. 



We may suppose, therefore, that during thyroid-feeding with 

 such doses as have been employed in these experiments an undue 

 amount of thyroxin is present in the circulating blood. This leads 



