A REVISION OF THE PERCENTAGE OF WATER IN 
THE BRAIN AND IN THE SPINAL CORD 
OF THE ALBINO RAT 
HENRY H. DONALDSON 
From the Wistar Institute of Anatomy and Biology 
ONE CHART |: 
In four studies previously published (Donaldson ’10, 11, 11a 
and ’11 b), the percentage of water in the central nervous system 
of the albino rat has’ been considered. A revision of this topic 
was thought necessary, however, for several reasons. In the 
first place, I wished to record the observed values on which had 
been based an extensive table giving the percentage of water in 
the brain and spinal cord according to age, each entry in this 
table giving also the weight of the brain, or spinal cord to be ex- 
pected. This table 74, from The Rat (Donaldson ’15)—is given 
in full as an appendix to the present paper. Further, I wished 
to state the conditions to be observed in the use of ‘his table. 
In the second place, a statement previously made, required cor- 
rection :—It has been stated (Donaldson ’10) that at a given age 
the percentage of water in the brain or spinal cord is not sig- 
nificantly modified by the absolute weight of the organ. This 
conclusion is incorrect, for more careful study shows that weight 
is a factor in modifying the percentage of water. The evidence 
for such a conclusion will appear in the course of this paper. 
It is the main purpose of the present study, then, to show how 
the percentages of water, which appear in the appended table 74, 
have been obtained, and, incidentally, to show how new data 
must be treated when they are to be corrected for weight. In 
considering the main points, the brain and the spinal cord will 
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