CHEMICAL CHANGES IN HUMAN BRAIN 461 



cells in any organ seem to grow somewhat in unison, but they are 

 influenced by cells of other organs, both through the blood and 

 through the nerves. There is no doubt that each organ directly 

 influences the growth of every other in both of these ways. 

 Certain glands secrete substances and discharge them into the cir- 

 culation that have a disproportionately large effect in influenc- 

 ing growth in most tissues. But it is not correct to assume that 

 growth is determined by these; it is only altered by them. 

 Growth seems to be a general cell process, and these substances 

 simply change the rate of this general cell development. 



The growth of the brain is probably less under the influence of 

 internal secretions than other organs. Indeed, there is strong 

 evidence that the secretions are largely under the influence of 

 the nervous system. There is no indication of internal secre- 

 tions in simple organisms, yet these organisms have a similar 

 form of growth. 



2. Relation of these data to four physiological facts 



In any interpretation of brain growth it is necessary to keep 

 in mind several important facts: 



1. A larger amount of early differentiation occurs in nervous 

 tissue than in other tissues. Though this fact is associated with 

 some definite differences that exist in the fertilized cell, the sub- 

 sequent chemical supremacy is important in evolving the marked 

 specialization. The early formation of colloidal substances such 

 as the nucleoproteins, phosphatids, and sulphatids (table 10) 

 give a peculiarity to young nerve tissue that permits it to influ- 

 ence rather markedly the development of other tissues. It is 

 more than theoretical to assume that the early start of nervous 

 tissue allows it to differentiate more than other parts of the body. 



2. The nerve cells, unlike other cells, do not regenerate. It 

 is very probable that a nerve cell if tested for regenerative power 

 early enough in its growth would regenerate just as other undif- 

 ferentiated tissues do. It very soon reaches a stage, however, 

 w^hen it is so highly specialized by the elaboration of colloidal 

 complexes (table 11) that regeneration is impossible. 



