234 THE ORIGIN OF THE NERVOUS SYSTEM 



considerable degree of physiological integration, since 

 it correlates two more or less different organs, the 

 receptor and effector, or, more strictly speaking, subordi- 

 nates one to the other. Even in the primitive receptor- 

 conductor-effector system of Vorticella within the limits 

 of a single cell and without any visible nervous structure 

 there is a considerable degree of integration and biologi- 

 cal purpose. 



Every neurologist and physiologist recognizes that 

 between the highly adaptive reflex arcs of the vertebrate 

 with their highly specialized receptors, conductors, and 

 effectors, and the sensitivity and conductivity of living 

 protoplasm in general some sort of relation must exist. 

 The problem of the physiological origin of this adapted 

 unit reaction, the reflex, is a part of the problem of the 

 origin and evolution of the nervous system in its physio- 

 logical aspects and its consideration is the first step 

 toward any adequate conception of the nervous system 

 as a functional system. 



Viewed from its physiological side the reflex consists 

 in initiation, transmission, and end-effect of excitation 

 induced by some external agent. From this point of 

 view the relationship is evident between the reflex arc 

 and the protoplasmic excitation gradient, which is the 

 first step in axiate integration. In the primitive proto- 

 plasmic gradient, receptor, conductor, and effector are 

 not differentiated, but any part of the protoplasm or at 

 least of the protoplasmic surface may perform any of 

 these functions. The receptor is merely the point first 

 or most intensely excited by an external stimulus, the 

 conductor is any protoplasm which conducts or trans- 

 mits the excitation, and the effector is any point excited 



