i88 AGE, GROWTH, AND DEATH 



without changing at all into nerve cells. They form 

 a distinct layer, Fig. 63, which is well known to every 

 investigator of brain structure. Soon after birth 

 these cells accomplish a second migration, but in a 

 different direction. Instead of moving in a constant 

 current over the surface of the brain, each one takes a 

 vertical pathway from the surface down towards the 

 interior of the cerebellum ; and arrived there, it changes 

 and becomes a nerve cell, or at least a part of them 

 do ; and with that the machinery of the cerebellum is 

 complete. Thus, structurally, the cerebellum at birth 

 is an uncompleted organ. Now, the cerebellum is 

 that portion of the brain which regulates the combina- 

 tion of muscular movements, which secures what the 

 physiologists term co-ordination of movements, and it 

 is not until the cerebellum has been perfected that it 

 can perform this function. Were there not some pro- 

 vision of this special sort for allowing cells to be 

 produced and added to the brain, the full complexity 

 of the brain could not be attained, because after the 

 cells have begun to change into nerve cells they lose 

 their power of multiplication, and this is a device very 

 exquisite in its working to supply to the brain the 

 number of cells needed to give it its full measure of 

 complexity. 



Another instance of the reservation of cells of a 

 simple type is afforded us by the skin, about which I 

 shall have something more to say in a few moments 

 when we speak of the process of regeneration. It is 

 not only in the period of childhood, and not only in 



