34 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



nerve cell in becoming so highly specialized in some 

 directions has lost the primitive power of reproduc- 

 tion, a power which is inherent with types of cells 

 that have become less highly specialized. Contrast 

 with these properties of the nerve cell the properties 

 of the kidney cell. Here we have a cell of epithelial 

 type, retaining the powers of regeneration which in 

 so remarkable a way pertain to epithelial cells. If 

 the secretory cells of a healthy kidney are damaged, 

 the injured elements are soon thrown off and replaced 

 by new ones. Only long-continued and deep-seated 

 injury leads to an exhaustion of this power of regen- 

 eration. But, on the other hand, the retardation 

 of this regenerative capacity is compensated by the 

 comparatively low specialization and differentiation 

 of the kidney cell. 



In the relationship between the governing parts of 

 the animal organism and the parts governed there 

 is a singularly beautiful feature of reciprocity. 

 The central nervous system, or federal government, 

 confers on the organism a highly beneficent protec- 

 tive action which is unceasing. Always alert, the 

 governing nervous centers attend not merely to the 

 needs of the various parts of the body in health, 

 but stand ready to send succor to such parts as may 

 suffer injury of one kind or another. By virtue of 

 the superior gift of consciousness the organism is 

 constantly being guided so as to secure to itself the 

 most varied advantages while escaping equally 

 varied dangers. In return for these inestimable 



