338 E. R. HOSKINS 



heart, kidneys) in comparison with older rats of similar body 

 weight may be due to changes with age, independent of body 

 weight. The younger rats with more rapid metabolism may 

 have relatively larger organs. Donaldson has found a close 

 correlation between the water content of the central nervous 

 system and age in the rat. The age involution of the thymus 

 is well known, and Jackson ('13) found indications of correlation 

 with age in the weight of the eyeballs. Similar relations are 

 possible in other organs, although organ weight in general is 

 doubtless more closely correlated with body weight than with age. 



In relative (percentage) weights, the organs likewise are in 

 general correspondence with the results of Jackson ('15), the 

 differences in most cases being within the limits of normal varia- 

 bility to be expected. There was noted a marked correlation 

 between the weights of the spleen and liver in most cases. 



b. Effects of thyroid feeding. The thyroid diet apparently 

 affected the gross body weight of the rats but very slightly. 

 The 'higher dosage' animals averaged slightly heavier than the 

 controls and other groups (charts 3 and 4), but the difference 

 is perhaps too slight to be significant. If the loss in fat of the 

 thyroid-fed animals be taken into account, an increased weight 

 of the remainder of the body appears. 



The results are not incompatible with the view that by thy- 

 roid feeding with small dosage an increase in body weight- is 

 produced (as .found by Moussu '99, and Schafer '12), while with 

 higher dosage a decrease or retardation in body weight is pro- 

 duced (Magnus-Levy '95; Moussu '99, Bircher '10; Carlson, 

 Rooke and McKie '12; Gudernatsch '12, '14, '15; Hewitt' 14; 

 Cotroni '14; Romeis '15). The decrease or retardation of body 

 weight reported by most of the above mentioned investigators 

 is possibly due to the toxic effect of large doses. I observed 

 that two thyroid-fed rats apparently in good health were killed 

 by a single large dose (200 mgm.) of dried thyroid substance. 



The eviscerated body and (to a shghter extent) the integu- 

 ment of the thyroid-fed fats usually show a slight loss in rela- 

 tive weight, probably due to loss of fat. 



