448 H. H. DONALDSON 



As table 2 shows, similar computations give percentages of 

 water for the myelin in the braiii, which range from 42.7 per cent 

 to 52.4 per cent and which yield a mean value of 47.8 per cent. 

 The computation for the spinal cord gives 51 per cent. The 

 significance of these results lies not in the particular percentage 

 of water here determined for the myelin — as that depends some- 

 what on the percentage of water assumed for the protein — but 

 on the similarity of the values found in all the five cases examined. 



However, it is found on trial that one cannot depart far from 

 the value of 87 per cent for the proteins without obtaining rather 

 improbable percentages for the myelin, so that this value is 

 probably nearly right. 



DISCUSSION 



Those familiar with the pubhshed data for the percentage 

 of water in the cortex will at once perceive that the value given 

 by de Regibus (86%) seems high. The various water records 

 for the cortex run down as low as 83.5 per cent. The differences 

 between the various determinations are, how^ever, almost cer- 

 tainly due to the varying amounts of white substance included 

 in the sample, and as has already been stated the value chosen 

 is probably close to the true value. 



In connection with the computations there are, however, two 

 conditions which have been assumed to be constant but which 

 in all probability, are subject to variation. I refer to the density 

 of the myelin and to the fraction of the hpoid to be assigned to 

 the protein. As to this last condition, it would be plausible 

 to think of a larger fraction of lipoid in the axons than in the 

 cell bodies. If this were true it would be necessary to increase 

 this fraction in the case of the spinal cord or the callosum. 



It is also possible that aside from this method of distribution 

 the fraction of lipoid in the neuron may increase with age. The 

 slight loss of water during the first ten days of (rat) life is possi- 

 bl}^ due to such an increase. Finally, the density of the myelin 

 may change with age, as its chemical composition certainly does, 

 and it is conceivable that it has a higher water content when 

 first formed, as is suggested by the 63.8 per cent given for the 

 ten day record in table 2. 



