ANATOMY: H. H. DONALDSON 
281 
For working purposes we assumed that 1 day in the life of the rat was equiva- 
lent to 30 days in the life of man, and that the equivalent ages in these two 
animals were represented by equal fractions of the span of life. 
One adjustment is necessary however in dealing with the data for the nervous 
system. The brain of the rat at birth is less mature than that of man at birth, 
and it is not until the rat is 5 days old that the brain is in the same phase as 
that of man at birth. 
In making any comparison therefore the data for the rat at 5 days of age are 
arranged to coincide with the data for man at birth. Using the foregoing 
methods, four comparisons have been made between the growing nervous 
system of the rat and that of man. 
The first chart is for the growth in the weight of the entire brain of the rat 
from birth to maturity, compared with that of man. When the human brain 
r _ Mt BRAIN WEIGHT ON AGE 
QMS RAT MAN GMS 
-#- 1415 
6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22 YEARS 
73 85 97 122 146 170 194 219 244 268 DAYS 
CHART 1 
Showing the increase in the brain weight of the albino rat on age (broken line and dots) 
and compared with this the increase in the brain weight of man at the equivalent ages (solid 
line) . The values for equivalent ages are on the same ordinate. 
weights are reduced, and the comparison is made in the way described, the 
two graphs run well together. Thus, at equivalent ages, the brain in these two 
forms has undergone nearly the same degree of enlargement. 
Chart 2 shows the percentage of water in the rat's brain at different ages. 
The graph indicates a rapid, followed by a slow loss of water, with advancing 
age. I have found only four corresponding records for man, namely at birth, 
2 years, 9.5 years and 25 years, and these are entered by the heavy black dots 
at the equivalent ages on the graphs for the rat. The coincidence is good. 
It has been determined that this loss of water is due to the progressive 
accumulation of myelin in the nervous system (Donaldson, '16) and the infer- 
ence is therefore justified that the formation of myelin is progressing in a like 
manner in the two forms — only it progresses 30 times as fast in the rat. 
