OLFACTORY BULBS OF THE ALBINO RAT 219 
siderably greater degree than is the entire brain. When de- 
fective feeding is begun in rats about thirty days of age, the 
bulbs of rats thus experimentally stunted form about the same 
percentage of the total brain weight as do the bulbs of rats of 
the same litters killed at the beginning of the experiment. 
Whereas, under normal conditions, the bulbs of older rats (up 
to one hundred and fifty days) are considerably heavier in 
proportion than those of the young animals. With prolonged 
defective feeding the proportional weight of the bulbs tends to. 
become slightly greater. 
4. Sick animals, especially those with lung infection, show a 
marked diminution in the relative weight of the olfactory bulbs, 
accompanied by a certain amount of loss in total brain weight. 
Ill. EXERCISE EXPERIMENTS 
1. Previous experiments on the effect of exercise upon the albino rat 
Several investigators have worked upon problems connected 
with the changes in the albino rat occasioned by an increased 
amount of exercise. J. R. Slonaker in 1907 published observa- 
tions upon four rats of different ages kept in revolving cages 
for a short period. In 1912, the same author published an ac- 
count of further experiments along the same line, and although 
this time, also, the work was with a small group of rats, yet the 
experiment was continued during the natural life of the animals. 
Slonaker was working chiefly upon the problem of normal activity 
in its relation to age and sex but, incidentally, he made some few 
observations upon the comparative development of ‘exercised’ 
and normal rats. He found that “exercised rats are more active, 
more alert, and brighter in appearance than the control ones,” 
but that “the control males reach their maximum weight at 
an earlier age than exercised males, and also greatly excel them”’ 
and that “control rats live longer than exercised rats.” No 
observations were made on the effect of exercise upon any of the 
internal organs. 
Donaldson, in 1911, conducted a series of experiments to 
ascertain the effect of exercise upon the central nervous system 
