43O Chemical Differentiation of the Brain 



ever, all the errors of analysis, as well as of such calculations as are based 

 on assumed factors, enter into this fraction. After accounting for. the 

 cholesterol in the brain of the 50 and 100 mm. pig fetus, 13 in which this 

 was estimated directly by Mendel, there remained undetermined 2 to 3 

 per cent of the total solids. Considering the number of groups estimated, 

 this is not a very discouraging result. (In other tissues, which contain 

 little cholesterol, the undetermined is recorded as neutral fat.) 



Anatomical distribution. Cholesterol is principally of interest as a con- 

 stituent of the medullary sheath to which it adds a sort of mechanical 

 stability. But it is present in the cell bodies also, possibly contributing 

 to the cell membranes. According to Lorrain Smith 14 it is one of the sub- 

 stances responsible for the color which the medullary sheath gives with 

 Weigert's stain. 



Total sulphur and total phosphorus. 



It may not be out of place to state briefly the reasons for selecting these 

 two elements for special determination in preference to others. As far 

 as the phosphorus is concerned, the importance of the nucleins to all liv- 

 ing cells and the phosphatides to the nervous system in particular, amply 

 justify its selection. The reason for selecting sulphur in preference to 

 the much more generally studied nitrogen, may,- however need a word of 

 explanation. 



Nitrogen is studied for two reasons: because it is an important element 

 in the building up of the proteins, and because it is easy of estimation. 



Sulphur is just as characteristic of proteins, in fact more so, as it does 

 not enter into the non-protein groups such as the nucleic acids. Among 

 the lipoids, too, sulphur enters into only one group, the sulphatides, while 

 nitrogen occurs in all except cholesterol. 



In other words, to estimate sulphur in the protein fraction is to esti- 

 mate an element essentially characteristic of the more truly protein part. 

 To estimate it in the lipoid fraction, enables one to distinguish one par- 

 ticurar, and, as growth curves show, a very interesting group of lipoids. 

 Besides, as has already been pointed out in a previous paper 15 sulphur 

 occurs in the tissues in several states of oxidation and thus gives us some 

 indication of the intensity of reactions of oxidation which are so impor- 

 tant to growing tissues, and about which we know so little. It seems wise 

 therefore to estimate the sulphur, and in case there are any special reasons 

 to study nitrogen, to study it rather in the form of one of its definite 

 groups of compounds such as the purine bases or the amino-acids. 



13 Koch, Mathilde L. : this Journal, xiv, pp. 267-279, 1913. 



14 Smith, Lorrain: Journ. of Path, and Bact., xv, pp. 179-181, 1911. 



15 Koch, W. and Upson, F. W. : Proc. Soc. for Exp. Biol. and Med., vii, 

 pp. 5-6, 1909. 



