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. 
