THE REFLEX ARC 235 



by the transmitted excitation. In fact each point of the 

 path is successively receptor, conductor, and effector, 

 the effect being excitation, consisting of chemical 

 reaction and other changes, the liberation of energy, 

 and the excitation of a further point. As long as the 

 protoplasm concerned remains undifferentiated, such an 

 excitation gradient can never develop a characteristic 

 feature of the true reflex, viz., the specialized function 

 of the effector, which constitutes a part of the founda- 

 tion for the adaptive character of the reflex, but the 

 general physiological basis on which the development of 

 the reflex may occur is clearly present in such a gradient. 

 In short, according to this viewpoint the physiological 

 basis wnich underlies and initiates orderly development 

 and morphological and physiological unity and order in 

 general is the same as the basis of nervous integration 

 and of the most complicated nervous acts. The physio- 

 logical axis and the reflex arc have a common origin in 

 the protoplasmic excitation gradient. 



For the sake of convenience and clarity it is necessary 

 before proceeding farther to make certain definitions. 

 An excitation gradient is the change resulting from irrita- 

 tion or excitation at some point and its protoplasmic 

 transmission with a decrement (pp. 75-82). In such a 

 gradient each level may be successively receptor, con- 

 ductor, and effector. Through gradual changes in 

 irritability and protoplasmic conditions at different 

 levels an excitation gradient may become a more or less 

 permanent axial gradient or physiological axis of a whole 

 organism or of some part of it. As soon as local speciali- 

 zation and differentiation begin at different levels of a 

 gradient it must be distinguished from a simple gradient 



