44 2 Chemical Differentiation of the Brain 



day period, the total sulphatides formed surpass the cerebrosides, 

 nearly equal the phosphatides, and are more than one-third the 

 proteins. The constant production of sulphatides is, therefore, 

 a marked feature of late medullation, just as that of the phos- 

 phatides is of the early medullation. The sulphatides diminish in 

 their rate of formation far less than any other constituents. 



Finally we have the period from 210 days on: the period of 

 stationary or adult life. We have no definite chemical data as 

 to any changes occurring during this period, but from such data 

 at hand, as the periods just studied, we can assume that the 

 growth processes during adult life are practically stationary except 

 perhaps a very gradual increase in the per cent of solids. 



The enlargement of the brain may, therefore, in great part 

 be accounted for chemically by the formation of the medullary 

 sheath. Donaldson 31 has found that some 88 per cent of the 

 volume of the adult brain is composed of the axons and their 

 sheaths, while the cell bodies with their dendrites and the sup- 

 porting tissues together only make up the remaining 12 per cent. 

 The axones, therefore, medullated or non-medullated, are mainly 

 responsible for the increase of the size of the brain and for the 

 changes which it undergoes during post natal growth. To bring 

 more vividly before the eye the relative rate of growth of the 

 various constituents, we have prepared Charts 1 and 2 from 

 Tables III and IV. These charts are self explanatory. We have 

 also prepared Charts 3 and 4, an explanation of which is given 

 below. 



Chart 3 shows the relation of lipoid 32 to protein. In this the 

 weights of the several constituents are represented for one brain 

 at each age. This chart shows that while both the proteins and 

 the lipoids are increasing in absolute weight, the proportion of 

 lipoid to protein is becoming greater and greater as the tissue 

 grows older. This indicates that the rate of increase for the 

 lipoids is greater than for the proteins (also brought out in Table 

 VI). At 120 and at 210 days we find that the lipoids and pro- 



31 Donaldson, H. H. : Journ. of Nervous and Mental Disease, xxxviii, p. 

 260, 1911. 



32 Lipoids here include the phosphatides, cerebrosides, and sulphatides; 

 cholesterol, which is classed as lipoid, is here recorded in the "undeter- 

 mined." 



