ANATOMY: H. H. DONALDSON 
355 
In Table 3 the data are given in five age groups for the brain and in 
one age group for the spinal cord. It is to be noted that the 10 day brain 
group — which stands just at the beginning of the myeHn formation — 
is here excluded from the discussion and we begin the comparisons which 
are to be made, with the 20 day brain group. 
In the brain series (with one exception) the corrected protein dimin- 
ishes and the corrected Hpoid increases with advancing age. Between 
20 and 210 days the proportion of the lipoid doubles. We have, in 
column (5), the observed percentage of water in the brain as a whole. 
It is assumed, as previously noted, that the corrected protein (neurons, 
in the strict sense: = both cell-bodies and axons) have 87% of 
water. From these several data we can compute the percentage of 
water to be assigned to the corrected lipoid, which represents the myelin. 
The method of computation may be illustrated by the data for the 
20 day group. Reference to Table 3 shows that, at this age, there is 
1 part of Kpoid (11.12%) to 8 parts of protein (88.88%). This gives 
9 parts, representing the entire brain and having 82.5% of water. The 
product, 9 X 82.5 = 742.5. We assume that the 8 parts of protein have 
87% of water. The product, 8 X 87 = 696. The 1 part of 
lipoid, representing the myelin, will then have a percentage of water 
equal to the difference of these products: 742.5 — 696 = 46.5%. 
The values thus obtained are entered in column (7), and, taken to- 
gether, the four entries give a mean value of 47.8% of water for the 
myelin. 
In this connection it should be noted that the spinal cord, which has 
about twice as much lipoid as the brain at the same age, gives also a 
similarly low value for the water in the myelin — 51%. 
The significance of these results lies not in the particular percentage 
of water here determined for the myelin — as that depends on the per- 
centage of water assumed for the protein — but in the similarity of the 
values found in all the five cases examined. 
We conclude from these results that there is no evidence that the 
cell bodies and their unsheathed axons suffer any significant loss of 
water between birth and maturity, and that the progressive diminution 
in the water content of the entire brain and spinal cord is mainly due 
to the accumulation of myelin — with a water content of about 50%. 
Moreover, the myehn must be regarded as a more or less extraneous 
substance, having but Httle significance for the characteristic activities 
of the neurons. As the diminution in the percentage of water in the 
central nervous system is preeminently a function of age, and as it 
appears to be due almost entirely to the formation of the myelin, it 
