THE REFLEX ARC 



239 



to that in the coelenterates, while the central organ, the 

 adjuster, is of still later origin. With the exception of 

 Kleinenberg's conception of the neuro-muscle cell, these 

 hypotheses and suggestions are concerned more with 

 the origin of the nervous system as a morphological 

 structure than as a physiological entity. A brief con- 

 sideration will show that from the physiological point of 

 view these different views appear rather as special cases 

 of a general physiological process than as mutually 

 exclusive conceptions. 



FIG. 69. Diagram of simple multicellular arcs: s, sensory cell 

 (receptor); n, nerve cell (conductor); m, m, muscle cells (effectors). 



If the reflex arc develops from the protoplasmic 

 excitation gradient, the development and evolution of 

 the nervous system as a morphological structure must 

 follow in general the lines of the chief or the most 

 persistent excitation gradients established in the pro- 

 toplasm of a particular species. According to the con- 

 ception of physiological axiation developed in the 

 earlier chapters, an excitation gradient may arise in 

 primitive protoplasm at any point of the surface where 

 excitation occurs, and any particular gradient may 

 persist only momentarily or for a short time, as is 

 apparently the case in Amoeba (Hyman, 1917). Under 



