264 Prof. C. S. Sherrington. Reciprocal [Mar. 25, 



that it is less by an amount represented by the difference between the height 

 of contraction in Obs. II and Obs. Ill respectively. Comparison of Obs. II 

 with Obs. Ill furnishes, therefore, one proof that under the concurrent stimu- 

 lation the contraction of the extensor like that of the flexor shows algebraic 

 summation of neural states of unlike sign. And comparison of Obs. II with 

 Obs. IV (fig. 9b) furnishes further proof of the same thing. Obs. IV is a 

 repetition of Obs. II, the intensities of the stimuli being the same in Obs. II, 

 but the sequence of the stimuli in Obs. IV is in reverse order to their 

 sequence in Obs. II. Obs. IV opens with stimulation of the contralateral 

 afferent; this stimulation produces contraction of the extensor with no 

 obvious effect on the flexor, the preparation not being a tonus preparation as 

 regards the flexors. After the contralateral stimulus has been in operation 

 for rather more than a second the ipselateral afferent is stimulated, the 

 stimulation of the contralateral continuing as before. The effect of the 

 concurrent stimulation of the two nerves is that the contraction of the 

 extensor is considerably reduced in height, and that a contraction of the 

 flexor occurs, but this contraction of the flexor is much less than that given 

 by the ipselateral stimulus acting alone as in Obs. I and Obs. II. After 

 about 2 seconds' duration the ipselateral stimulus is withdrawn, there then 

 ensues immediately a disappearance of the flexor's contraction and a resump- 

 tion by the extensor of the full height of contraction it showed originally 

 under contralateral stimulus alone. Finally the contralateral stimulus is 

 withdrawn and the contraction of extensor soon begins to decline and 

 somewhat gradually passes off. The difference between the abrupt reduction 

 of the extensor's contraction directly the ipselateral stimulus (inhibitory) 

 joined the contralateral and the delayed and gradual dying out of the 

 contraction on mere withdrawal of the exciting stimulus (contralateral) is 

 characteristic of the difference between onset of inhibition and mere 

 cessation of excitation in a tonus preparation. 



Fig. 9b, Obs. V, taken a minute later than Obs. IV, supplies a repetition 

 of the latter with increased strength of the ipselateral stimulus. The result 

 then given during the concurrent stimulation of the opposed afferents is 

 different. The simple change in the intensity of stimulation of one of the 

 afferents removes the result from the type 3 to type 1. Under the con- 

 current stimulation in this observation the flexor contracts while simul- 

 taneously the contraction of the extensor is apparently completely 

 suppressed, whereas under the concurrent stimulation in Obs. II and IV 

 both the flexor and extensor are synchronously in active contraction. 



In this third type (fig. 9, Obs. II, Obs. IV, and fig. 10) of result, 

 therefore, concurrent stimulation of the two antagonistic afferents produces 



