220 CAROLINE M. HOLT 
of the albino rat, using the same sort of apparatus—a revolving 
cage with cyclometer attachment—employed by Slonaker. He 
found that there was a slight increase in brain weight (2.4 
to 2.7 per cent) to be attributed to the effect of exercise. This 
was what was to be expected in view of the heavier brain to be 
found in the wild Norway rat. The cord showed no effect. 
The olfactory bulbs were not weighed separately. 
Hatai (15) published a series of observations based upon his 
own experiments and upon those of the present writer, showing 
the rather marked effect of the same exercise conditions upon 
the weight of the internal organs. In these experiments, the 
brains of the test animals showed an excess of 4 per cent over 
the controls with no effect upon the cord. 
2. Description of Experiments. Series D and E 
As it had thus been demonstrated that the brain of the albino 
rat could be modified by exercise in the revolving cage, it re- 
mained to determine whether, under such conditions, the ol- 
factory bulbs would show a more marked variation than the 
brain as a whole. 
For this work, also, large litters of stock albinos, were chosen. 
Each litter was weaned and divided into three groups when 
about thirty-five days old. One group constituted the ‘Initial 
Controls,’ and these were killed and examined as in the previous 
experiments. The second lot, the ‘Final Controls,’ was set aside 
in cages under the normal living conditions of the colony. The 
third group was used for the experiment. Each of these test 
animals was placed by itself in a wire revolving cage such as 
had been used by Slonaker, and later by Donaldson and Hatai. 
Each cage was 5 feet in circumference with an open nest box 
fastened to the central fixed axis. From this axis the food was 
suspended so that, theoretically, the rat must descend to the 
floor of the cage to eat. Practically, some rats soon learned 
to avoid this and so escaped a considerable amount of enforced 
exercise. 
Each cage was provided with a cyclometer. Readings were 
made and recorded six times a week. These cyclometer read- 
