340 PHYSIOLOGY 



first one reflex and then another, and the activities so excited would pro- 

 duce confusion in the conduct of the animal, if there were not some means by 

 which at any one time only one reaction should be in the act of being carried 

 out. The imperative stimulus should dominate the actions of the body as a 



FIG. 169, A and B. (SHERRINGTON.) 



The flexion reflex observed as reflex contraction (excitation) of the flexor muscles 

 of the knee (A), and as reflex relaxation (inhibition) of the extensor muscle (B). 

 The stimulus was a series of weak break induction shocks applied to a twig of the 

 internal saphenous nerve below the knee. Observation B was made four minutes 

 after A . Note the summation of stimuli, in each case six stimuli being required before 

 the reaction was evoked. 



whole. Just as, in the mental world, attention must be undivided if we 

 are to avoid confusion of judgment, so in the lower nervous activities there 

 must always be concentration on one act or another. There may be a 

 struggle of different stimuli, but one must finally be prepotent and annul 

 altogether the influence of the others. The study of the spinal animal 

 shows that this concentration of energy is obtained by the process of inhibi- 



