OLFACTORY BULBS OF THE ALBINO RAT 233 
we see that the average for 12 controls (90 to 160 days old) was 
4.26 per cent while only two test animals fell as low as this , 
(one of these was of abnormally light body and brain), and the 
averages were 4.41 per cent and 4.74 per cent for four and one- 
half months and eight months respectively. 
5. Summary 
1. The results of the present experiments agree with those of 
previous investigators in that they show no marked effect of 
exercise either upon body length or body weight in the albino rat. 
2. The female albino becomes very active earlier than does 
the male but the activity of the male later increases to such an 
extent that the total activity for the two sexes for long periods 
is probably about equal. 
3. These experiments suggest that there is an increase in 
fertility correlated with increase in the size of the reproductive 
organs. 
4. The brain weight is slightly increased by exercise. 
5. The weight of the olfactory bulbs of albino rats exercised 
in revolving-cages for periods of from thirty to one hundred days, 
is considerably increased. The bulbs of such rats form from 
4.41 to 4.74 per cent of the total brain weight as compared with 
4.20 to 4.32 per cent in rats reared under normal colony conditions. 
These bulbs show an increase of 5 to 11 per cent over and above 
the increase in weight manifested by the entire brain. 
1V. CONCLUSIONS 
From the preceding observations we may conclude that we are 
able to modify the olfactory bulbs of the rat by changing the 
conditions under which it lives and to modify them to a con- 
siderably greater degree than we can change the rest of the brain. 
In eases of stunting, the bulbs tend to overcome the effect, to 
a certain extent, as time goes on. With exercise the effect seems 
to increase with age. Yet the bulbs respond more markedly 
to the stunting effect of defective feeding or sickness than to 
the stimulating effect of exercise. 
A histological study of these modified bulbs will be presented 
in the second part of this paper. 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 27, NO. 2 
