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



3. The number of nerve cells remains constant. Rather early 

 in growth, probably as early as the seventh fetal month (table 10) 

 the number of nerve cells is largely determined. No amount of 

 functioning produces an increase. This would indicate that the 

 chemical changes in brain growth are fixed within rather close 

 limits. It also suggests that development in the brain is essen- 

 tially different than elsewhere. Probably the main processes are 

 determined at the time of the formation of the cells. The or- 

 ganization is such, however, that smaller but no less important 

 (speaking physiologically) change occurs during later activity. 



4. Nervous tissues remain constant in composition under con- 

 ditions that markedly alter many other tissues. The chemical 

 composition of the nervous system must be related to this suprem- 

 acy. The large amounts of several of the lipins (table 10) seem 

 to be of importance. Though the large amount of colloidal mate- 

 rial in the form of lipins and proteins is often supposed to be indi- 

 cative of slower metabolism and a lack of dominance (Child, 

 '11), the chemical condition in the brain would suggest that col- 

 loidal structure is equally important wdth the rate of metabolism 

 in maintaining dominance. It te conceivable that in the case of 

 the brain its early importance, due to the high rate of metabol- 

 ism, should be maintained through specialized activity, even when 

 this rate is no longer greater than the rate in other tissues. The 

 highly specialized nerve fiber and cell are made of many com- 

 pounds that are but slightly available to other tissues, because 

 such substances are in almost irreversible equilibrium with 

 metabolizing substances elsewhere. 



Another factor that is undoubtedly involved is the selective 

 nature of the membranes surrounding the cells of the nervous sys- 

 tem. Very probably such membranes or surfaces are much more 

 common than is supposed, thus providing means of keeping the 

 various tissues in equilibrium with each other. If the membranes 

 in the nervous system are more nearly irreversible than in other 

 parts, the condition exists that is favorable for maintenance un- 

 der circumstances that use up other tissues. 



No other tissue has a chance to supplant the nervous system 

 with its highly specialized pathways to all parts of the body. 



