PHYSIOLOGICAL GRADIENTS 97 



may become almost inconceivably complex. In the 

 mammalian nervous system, for example, the polarities 

 of the different neurons form all possible angles with the 

 primary body axes, yet it seems evident that these 

 polarities arise in response to local conditions. 



PHYSIOLOGICAL DOMINANCE AND SUBORDINATION IN 

 RELATION TO THE AXIAL GRADIENTS 



Turning to another aspect of physiological integra- 

 tion, it is evident that a relation of dominance and 

 subordination is a characteristic feature of the axial 

 gradient. The region of most intense activity, the high 

 end of the gradient, must influence the physiological 

 condition at other levels to a greater degree than these 

 other levels can influence it. It is the region of highest 

 electronegativity and the primary factor in determining 

 the electrical gradient, and therefore must dominate 

 or control other levels to a greater or less extent. When 

 a new excitation occurs at any level of an already 

 established physiological gradient it must, if the electric 

 current is the transmitting factor, as Lillie maintains, 

 be transmitted to a greater distance down the gradient 

 than up, because as it passes to higher levels its own 

 electric current will be more and more completely com- 

 pensated by the current arising at the higher levels. 

 When an axial gradient is already present we may expect 

 to find, at least in the simpler organisms, a difference of 

 this kind in transmission with respect to the two direc- 

 tions, upward and downward. Figure 7 is an attempt 

 to show in a purely diagrammatic way the reason for 

 the disappearance within a comparatively short distance 

 of an excitation transmitted up a physiological gradient. 



