334 CHESTER A. STEWART 
thymus in rats held at maintenance from the age of three to ten 
weeks. . 
Jonson (’09) likewise found in young rabbits kept at constant 
body-weight for four weeks the weight of the thymus to be re- 
duced to about one-thirtieth its initial value. The greatest loss 
is suffered by the cortex, which is reduced to one-twelfth of its 
initial weight during two weeks of maintenance. Judging from 
comparison with my own results, as well as with those of Jack- 
son, the process of hunger involution of the thymus would appear 
to be much more rapid and complete in the rabbit than in the 
rat. 
HEART 
The weight of the heart (table 2) in the rats underfed from birth 
to three weeks of age differs but little from that in the controls 
(decrease of about 10 per cent in the males; slight increase in the 
females). At six and ten weeks of age, however, there is an 
evident increase in the heart weight. In those underfed from 
birth to six weeks, the average increase is about 13 per cent. For 
the groups underfed to ten weeks, the increase from an average 
of 0.092 gram in the controls to 0.117 gram in the test rats (sexes 
combined) represents an increase of about 27 per cent (subject 
to slight correction for differences in body weight). In the rats 
underfed for very long periods, the heart weight, while variable 
in most instances, is lower than in the controls. There is a de- 
crease in the various groups, varying from about 8 to 29 per cent, 
excepting the four females at 392 days, which show an apparent 
increase of about 3 per cent (most of which may be accounted 
for as due to difference in body weight). 
, In general, therefore, the data indicate a slight increase in the 
weight of the heart in the test rats at six and ten weeks of age, 
while during the later periods the heart apparently lost weight 
in the majority of instances. However, on account of the nor- 
mal very considerable variability of the heart weight, especially 
in young rats (Jackson 713), the apparent changes in the heart 
during underfeeding are probably somewhat doubtful. 
In young rats kept at constant body weight for various periods, 
