INTEGRATION OF SIMULTANEOUS AND SUCCESSIVE REFLEXES 947 



acting alone, is applied to a point A on the skin, and immediately after a 

 second stimulus also inadequate to excite, is applied to a point B, the 

 combined effect of the two successive subliminal stimuli may give rise 

 to a reflex response. The threshold of some common part of the reflex 

 arc has been lowered by the effect of the preceding inadequate stimulus, 

 even though it was applied to a somewhat remote part of the skin. For 

 this reason a moving stimulus applied to the scratch area is far more 

 effective than a stationary stimulus applied over the same extent of area. 

 This phenomenon is called immediate induction, and it is by no means 

 confined to the spinal cord. It is well illustrated, for example, in the 

 case of vision. If a thin line drawn on a white card be looked at so that 

 it falls on the edge of the receptive field of the retina, it will not be seen 

 so well as a dot of similar width which is moved through the same 

 distance as the line. 



The Integration of Antagonistic Reflexes 



Simultaneous Combination. Two antagonistic reflexes, which use the 

 final common path to cross purposes, naturally cannot both succeed in 

 occupying it at the same time. We must examine what results when two 

 such reflexes come into competition for the control of a common motor 

 mechanism. The crossed extension reflex and the flexion reflex are 

 two responses utilizing a common group of muscles in opposite ways. 

 If we apply a stimulus to the skin of the leg of a decerebrate animal 

 while the limb is extended as the result of a stimulus applied to the 

 contralateral leg, the extension gives way completely to the flexion 

 reflex (Fig. 239). By choosing the relative strengths of the stimuli 

 properly a preexisting flexion reflex may also be interrupted by a crossed 

 extension reflex (Fig. 226). In a similar way the scratch reflex gives way 

 before a flexion reflex. The significant fact is that when antagonistic 

 reflexes are in simultaneous competition for the final common path, one of 

 them occupies the path to the exclusion of all others. Accordingly there 

 is no tendency for a fusion of their effects, and this is most fortunate, for 

 fusion would only result in confusion, since the response would be ap- 

 propriate for neither of the simultaneous stimuli. This is perhaps the 

 most important principle in the integration of spinal reflexes by the 

 final common path. 



In the competition of antagonistic reflexes for the control of the final 

 common path several principles determine the outcome. These depend on 

 (1) the nature of the reflexes, (2) the relative intensity of the stimulus, 

 and (3) fatigue. 



The nature of the reflex is dependent on the quality of the stim- 

 ulus and the purpose to which it is employed. The most prepotent re- 

 flexes are the flexions which result from the application of harmful 



