ORGANIC REGULATION 93 



activities of the nervous system, they are evidently 

 of such a character that the external environment, 

 is regulated just as is the internal environment. It is 

 in virtue of these nervous activities that the stream 

 of material and energy which is constantly entering 

 and leaving the body is kept so nearly constant. 

 Appetite and satiety, muscular activity and fatigue, 

 external temperature and heat loss, external light or 

 sound or other sensory stimuli and the responses to 

 them, are balanced against one another through the 

 nervous system. We cannot draw any complete line 

 of separation between the regulation of the internal 

 and that of the external environment; for evidently 

 the one is complementary to, and indispensable to, 

 the other. Regulation of the external environment 

 is in fact only the outward extension of regulation 

 of the internal environment, and the ultimate de- 

 pendence on the external environment of the organs 

 which regulate it is as evident as their more immediate 

 dependence on the internal environment. Deficiency 

 or excess in normal stimuli, normal nutrition, normal 

 temperature and respiratory exchange, are as impor- 

 tant to the nervous system as to other organs. The 

 environment determines the nervous reactions, and the 

 nervous reactions the environment, but the constancy 

 or regulation which emerges is still unexplained. The 

 conception of an organism as a mere labile structure 

 which determines, and is at the same time determined 

 by, its environment is unsatisfactory, for the reason 

 that the specific persistence of life is left unaccounted 

 for. The facts must be examined more closely. 



