352 
ANATOMY: H. H. DONALDSON 
TABLE 1 
Percentages of water in the human brain 
Koch and Mann, (K); Weisbach, (W) 
HUMAN 
AGE, YEARS 
OBSERVER 
PERCENTAGE 
OF WATER 
EQUIVALENT 
RAT AGE DAYS 
25.0 
Birth 
2.0 
9.5 
(W) 
(K) 
(W) 
(W) 
(K) 
88.3 
81.1 
79.2 
(77.0) 
(77.8) 
Birth 
26 
115 
290 
We find that the span of life for man is about thirty times as long as 
that for the rat and hence by reducing any observed human age to one- 
thirtieth of its value we obtain the corresponding rat-age to which it is 
equivalent. These equivalent ages are given in Table 1. As is seen 
from the chart, when the water determinations for man are entered in 
accordance with this age relation they agree well with those for the rat. 
We conclude, therefore, that essentially the same process is occurring 
in the brains of both man and the rat — only in the rat it is proceeding 
thirty times faster than in man. These relations have been pointed 
out here in order to show that the conclusions reached at the end of 
this paper will probably apply to man, although directly based on the 
rat. 
The fact that there is a progressive loss of water in the brain and in 
the spinal cord with advancing age having been established, it is desir- 
able to consider the organs themselves and to find how this loss is dis- 
tributed within them. The following analysis applies both to the brain 
and to the spinal cord — but, for convenience, it will be given as if for 
the brain alone. The brain is very largely composed of nerve cells or 
neurons. When reduced to its simplest terms a neuron consists of a 
cell body — with a principal outgrowth — the axon. All axons are free 
from visible myelin sheaths when young — but as they mature many, 
but not all, acquire rather thick sheaths of myelin. MyeKn is a mor- 
phological term used to designate these sheaths, which are composed 
mainly of lipoids and which give the nerve fibers a white color. Broadly 
speaking, there is little or no visible myeHn present in the brain at birth — 
while with advancing age it increases rapidly, and at maturity forms a 
considerable portion of the entire brain. 
It is my purpose to determine by this study whether the loss of water, 
which occurs in the brain between birth and maturity, takes place 
equally in the cell body and its axon, on the one hand, and in the myelin 
sheaths, on the other, or whether this loss is unequally distributed. 
