WATER CONTENT — MAMMALIAN NERVOUS SYSTEM 447 



amount equal to 0.31 of the protein found. The remammg 

 amount of hpoid is assumed to be that used for the sheaths. 



In table 2, the column (2) headed Corrected Protein gives the 

 observed protein (non-lipoid) plus 0.31 of itself and the column 

 (3) headed Corrected Lipoid gives the observ^ed lipoid less the 

 amount of lipoid added to the protein. 



In table 2 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 myelin formation — is for the moment 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 cxco])tion) the corrected protein 

 dhninishes and the corrected lipoid increases with advancing 

 age. Between 20 and 210 days the proportion of the lipoid 

 doubles — column (4). 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 ])rotein (neurons in the 

 strict sense = both cell bodies and axons) have 87 per cent of 

 water. From those several data we can cominite the ]iercentage 

 of water to be assigned to tlie corrected lipoid, which re])resonts 

 the myelin.^ 



The method of computation may be illustrated by the data for 

 the 20 day group. Reference to table 2 shows that at this age 

 there is 1 part of lipoid (11.12<;^o) to 8 parts of protein (SS.SS^c^. 

 This gives 9 parts, representing the entire brain and having S2.5 

 per cent of water. The product, 9 X 82.5 = 742.5. We assume 

 that the 8 parts of protein have 87 per cent 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 dif- 

 ference of these products = 742.5 - 696 = 46.5 per cent. 



'It is hardly necessary to point out that a division of the brain into mj'elin 

 on the one hand and neurons on the other fails to enumerate several structural 

 elements which are also present though representing a relatively small mass. 

 There are, in addition to the neurons, glia and ependjnnal cells; blood and lymph 

 vessels; blood and Ij-niph and a slight amount of connective tissue. Over against 

 the neurons plus this group of elements we put the myelin, but for the purposes 

 of the present discussion it is convenient to speak of the rest — the non-myelin 

 portion — of the brain as if represented by the neurons alone. 



