PERCENTAGE OF WATER IN BRAIN AND CORD 83 
a. The amount of fluid in the ventricles. This is, as a rule, 
negligible in brains more than 25 days old; but in younger 
brains and especially during the first 10 days, when the ven- 
tricles are relatively large, it may be a modifying factor of 
importance. 
b. Variations in the relative weights of the several parts of the 
brain. If the brain is divided into the stem, the cerebellum, 
the cerebral hemispheres, and the olfactory bulbs, it is found that 
the most variable part of the brain is that formed by the olfac- 
tory bulbs. At times these may differ from one another by 50 
per cent, in two brains of nearly the same total weight, ranging 
therefore from 4 per cent to 2 per cent of the weight of the 
entire brain. 
The water content of the mature bulbs is high, 82 per cent. 
If, as an example, we take the water content of the entire brain 
as 78 per cent, a reduction of the relative weights of the bulbs 
from 4 per cent to 2 per cent would cause a loss of 0.1 per cent 
in the water content of the entire brain, thus reducing it to 77.9 
per cent. 
Variations in the density of the meninges or in the quantities 
of blood do not appear to have any significant influence. 
2. Variations in the water content of the brain due to histologi- 
cal differences. At the same age large rats have absolutely 
heavier, and small rats absolutely lighter brains. As there is 
reason to think that in a given mammalian species, the Norway 
rat for instance, the number of neurons composing the brain 
is approximately constant, the difference in the size of the 
entire brain must therefore mean a difference in the size of its 
constituent neurons and not a change in their number. How- 
ever, under the usual conditions of growth, shortly after birth 
myelin begins to appear on the axons. It has been shown that 
myelin is the constituent mainly responsible for the progressive 
loss of water from the brain (Donaldson, 716), and although its 
formation is closely correlated with age, it must be considered 
probable that slight fluctuations in the relative amount of 
myelin may occur. These fluctuations would produce in turn 
small changes in the percentage of water observed. 
