INTEGRATION OF ACTION WITHIN THE REFLEX ARC 935 



same muscles. Nor can the seat of the refractory period be in the 

 sensory area of the skin or the afferent neuron, for if a scratch move- 

 ment is elicited by stimulation at a point A in the proper skin area, 

 the rhythm of response which it calls forth will not in any 

 way be altered by the application of a second stimulus applied at B 

 at some distance from A and having a different frequency (Fig. 236). 

 There is evidently, therefore, some part of the reflex arc that is common to 



Fig. 236. Tracing from the hind limb of a spinal dog during the scratching movements pro- 

 duced by applying stimuli at two skin points (A and B), the application of the stimuli being in- 

 dicated by the signals. Not only were the stimuli applied at different points, but at B they 

 were of much greater frequency than at A. Although there is a slight change in "local sign," it 

 will be observed that there is no alteration in rhythm, indicating that this property can not be a 

 function of the final common path. (From Sherrington.) 



impulses starting both at A and at B, for if in each of these spots a refrac- 

 tory phase occurred, then there would be interference before the two im- 

 pulses had reached the centers of the spinal cord. By exclusion, there- 

 fore, "the seat of the refractory phase seems to lie somewhere central 

 to the receptive neuron in the afferent arc" (Sherrington 18 ). 



Many other types of reflex activity illustrate rhythm due to the re- 

 fractory phase. Two laboratory examples may be given: (1) When 



