CHAPTER XIII 



THE REFLEX ARC IN RELATION TO THE GRADIENT 



THE PHYSIOLOGICAL CHARACTERISTICS OF REFLEX ARC 

 AND EXCITATION GRADIENT 



Students of the nervous system are agreed that the 

 reflex is the unit reaction (Sherrington) or the physio- 

 logical unit (Parker) upon which nervous function and 

 integration are built up, but they are not in complete 

 agreement as to what constitutes a reflex unit's simplest 

 terms. According to Sherrington (1906), for example, 

 a reflex arc consists fundamentally of receptor, conduc- 

 tor, and effector, while Herrick's conception (Herrick, 

 1918) and Parker's (1919) also involves the presence of 

 a central organ, the adjuster, and so of a central nervous 

 system. 



But wherever we may determine to draw the line 

 between the true reflex and the simpler response it is 

 evident that we have in the reflex arc, first, the initiation 

 of excitation in the receptor by some external agent, 

 second, the transmission of excitation over some path, 

 the conductor, from the point of excitation, usually 

 with more or less central complication, and third, the 

 production by the transmitted excitation of some effect 

 through the agency of a more or less specialized organ, 

 the effector. In the higher animals even the simpler 

 reflexes are rather highly adapted reactions in which 

 the biologically purposive character is more or less 

 evident, and the reflex arc in such cases represents a 



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