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 



