INTERACTION AMONG REFLEXES 823 



flexion movement. Nevertheless, the more closely allied the receptors 

 are to one another, the more easily does summation occur. 



The mutual reinforcement of allied reflexes lasts for a short time after 

 the stimulation has been removed, the phenomenon being now known as 

 successive integration of allied reflexes. It can be illustrated also in the 

 case of the scratch reflex. If point A on the skin area is excited with a 

 stimulus that in itself would be inadequate, immediately after an effec- 

 tive stimulus has been discontinued at point B, then the scratch move- 

 ment will be kept up smoothly although it will of course become modi- 

 fied in local sign. For the same reason, a moving stimulus applied 

 to the scratch area is far more effective than a stationary stimulus ap- 

 plied over the same extent of area. In such a case the stimulus that 

 excites a reflex tends by its occupancy of the nervous pathway to faeili- 



Fig. 217. Showing region of body of dog from which the scratch reflex can be elicited. (From 



Sherrington.) 



tate the spread along the same pathway of succeeding allied stimuli; 

 towards such it lowers the threshold of excitability of the reflex arc. 



This phenomenon is also often called immediate induction, and it is 

 by no means confined to the spinal cord. It is well illustrated, for ex- 

 ample, 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. 



From these facts we see, therefore, that, when two allied impulses are 

 being transmitted to the final common path, the one is likely to reinforce 

 the other, and that this tendency to reinforce the allied impulse is main- 

 tained for a brief period of time after the impulse has been removed. We 

 may now proceed to consider the factors which will become operative 

 in determining to which of two competing or antagonistic reflexes the 

 final common path will become available. 



