448 Chemical Differentiation of the Brain 



SUMMARY. 



The principal results of this study may be summarized as follows : 

 Well-marked and characteristic chemical changes occur in the rat- 

 brain during its growth and these changes are obviously correlated 

 with its anatomical differentiation. 



The principal chemical changes noted are: 



1. A general decrease in the per cent of water which is not due 

 entirely to medullation since the decrease begins before medulla- 

 tion (Donaldson '10). 



2. A diminution in the relative per cent of protein in the total 

 solids due to the formation of a large amount of lipoid matter. 



3. The lipoids which appear coincident with medullation and of 

 which the development is pari passu with medullation are the cere- 

 brosides and sulphatides. These, therefore, are chiefly found in 

 the medullary sheaths. 



4. There is a great outburst of phosphatide formation at the 

 very beginning of medullation, but the phosphatides are present 

 also in large amounts before medullation. The phosphatides are 

 present, therefore, in the cells as well as the sheaths. 



5. The extractives are present in largest amounts during fetal 

 and early life when growth and metabolism are at a maximum. 

 Particularly the water-soluble, organic sulphur compounds (neutral 

 sulphur) diminish relatively with age, while the colloidal sulphur 

 increases. The relations of the neutral sulphur may be interpreted, 

 therefore, as indicating the intensity of metabolic activity. 



6. The great increase of colloidal matter with age clearly indi- 

 cates that this, in the form of supporting structures, constitutes a 

 relatively inactive material which presumably serves to localize 

 chemical processes. The accumulation of this material is probably 

 one factor producing the general slowing of metabolism character- 

 istic of senescence. This would thus become one cause of senes- 

 cence as Child has suggested. 



