452 C. G. MACARTHUR AND E. A. DOISY 



this the rate decreases to adult age. During adult life they prob- 

 ably increase very slowly and are one of the colloidal factors to 

 be considered in retarding metabolism in old age. 



We have some reasons for beUeving that lecithin is the phos- 

 phatid largely found in the nerve cells (Cowdry, '14). It looks 

 as though it were rather closely associated with the nucleo- 

 proteins in carrying on vital activities. Cephalin is probably 

 present in both the cells and axis cylinders, though more largely 

 in the latter. Very little is known about sphingomyelin, but it 

 it is probably largely to be found in the sheaths. It would be 

 very interesting to study the increase in each of these phos- 

 phatids during growth. 



Lecithin, and especially cephalin, because of their auto-oxi- 

 dation characteristics, are believed to be closely related to nerve- 

 tissue oxidation (Signorelli, '10; MacArthur and Jones, '17). 



Cerebrosides 



Phrenosin (Levine and Jacobs, '12) and kerasin (Rosenheim, 

 '13), the two brain cerebrosides, may be parts of an unstable 

 complex made up of sulphatid, phosphatid, and cerebroside. 

 If this is true, the data in this paper would indicate that with 

 growth this complex increases in complexity (fig. 1), because the 

 cerebrosides as analyzed do not appear until birth (tables 2, 5, 

 and 8), when myelination becomes the dominant brain activity. 

 This may mean that they are in some way dependent on the 

 presence of other constituents for their production. They are 

 peculiar in the rapidity with which they assume such a prominent 

 place in developing nerve tissue. 



The cerebrosides are probably more directly related to sheath 

 formation than any other constituent (Smith and Mair, '12- '13). 

 Their maximum rate of formation does not occur as early as in 

 the case of the other brain constituents (at four months instead 

 of at birth) (table 11). Then about 0.025 gram are added each 

 day. 



