102 THE ORIGIN OF THE NERVOUS SYSTEM 
it is not necessary, for the persistence of the relation, 
that the original gradient shall persist unchanged. 
After definite anatomical relations of parts and definite 
conduction paths have developed, the dominant region 
may still maintain its dominance, even though the 
metabolism or other physiological change in it is less in 
amount than in the parts which it dominates. The 
initiation of a nervous impulse in a receptor of the head 
and its conduction through the nervous system to an 
effector certainly does not involve a very large amount 
of metabolic, electrical, or other physiological change, 
but by means of such impulses the nervous system 
maintains its dominance, even though the physiological 
change brought about in the effector by the impulses 
may be far greater in amount than that in the nervous 
system. The physiological gradient represents merely 
the first step in the determination and establishment of 
the relation of dominance and subordination. 
PHYSIOLOGICAL ISOLATION 
Since this relation of dominance and subordination is 
a feature of the physiological gradient and since the 
length of the gradient is limited, the range within which 
such dominance is effective must be limited, though 
variable, according to physiological condition. In 
terms of Lillie's hypothesis the range of dominance must 
be determined primarily by the range of effectiveness of 
the electrical gradient which is a feature of the physio- 
logical gradient. This limitation of the range of 
physiological dominance makes possible the occurrence 
of physiological isolation, i.e., of the more or less com- 
plete isolation of a part of the body of the organism 
