CHEMICAL CHANGES IN HUMAN BRAIN 463 



It does not need to depend on its rate of activity for supremacy; 

 the conditions it has developed for its maintenance assure this 

 dominance though the means used for obtaining it no longer 

 exist. 



3. Concerning three periods of growth 



There are three distinct processes to be distinguished in brain 

 development. The one that takes place first is cell division. 

 This is probably almost completed at the time of the seven- 

 month-old fetus. It is worth noting that there is no evidence of 

 sheath development up to this point. There are no cerebrosides; 

 the amounts of sulphatids are increasing slowly. The phospha- 

 tids do not show any dominance. The relatively large amounts 

 of protein, and especially the nucleoproteins, suggest that chro- 

 mosome formation is very prominent. The large quantity of 

 extractives emphasizes the fact that metabolism is very rapid 

 during this period. 



From the seven-month fetus to about the time of birth, cell 

 growth is the important process. At this time the phos- 

 phatids come into prominence, while the proteins and extractives 

 retain their earlier dominance. Cholesterol, though present, is 

 not important. The same is true of sulphatids, while the cere- 

 brosides are lacking entirely or are present in but small amounts. 

 These changes arfe what one would expect in growing cells and 

 enlarging axis cylinders. How important the axis cylinders are 

 in accounting for brain growth is indicated by the fact that about 

 two-fifths of the brain consists of them. 



The third period is that of meduUation. It becomes prominent 

 soon after birth, reaches its maximum a few months after 

 birth, and slowly decreases in importance. The sheaths com- 

 prise about two-fifths of the brain substance, so it is not sur- 

 prising that cerebrosides, sulphatids, and some of the phos- 

 phatids become so prominent. The proteins and extractives are 

 skill of importance, but thoroughly masked by the new process. 

 It is probable that when the nerve cells reach the stage at which 

 conditions are proper for sheath formation, there is a release of 

 energy or an alteration in metabolism through the extension of 



