INTEGRATION OF SIMULTANEOUS AND SUCCESSIVE REFLEXES 949 



and less steady and the foot be less accurately directed to the spot 

 stimulated. The locus of the fatigue in the reflex arc can not be the 

 motor neuron itself, for, after this has been completely fatigued by 

 stimulation of the scratch area, the same muscles may quite readily 

 execute the flexion reflex if a painful stimulus is applied to the 

 skin of the hind leg. It must consequently lie in some part of the afferent 

 side of the reflex arc. Since we know that the nerve trunk is infatigable, 

 the natural assumption is that reflex fatigue is due to a change in con- 

 duction across the synapses of the neurons. 



Successive Combination. If antagonistic reflexes occupy a final com- 

 mon path in succession it is found that the use of it by one reflex 



Fig. 242. Successive induction illustrated by the crossed-extension reflex. The reflex, elicited 

 periodically with a stimulus of low intensity, is recorded in A and B. Between B and C a strong 

 flexion reflex was provoked and maintained for 45 seconds. C illustrates the immediately succeeding 

 extension reflex, which is greatly augmented by successive induction. The next extension (Z?) is 

 also slightly augmented, but in the third extension (E) the augmentation has passed off. The 

 signal recording the stimulation is above; time is indicated in seconds below. (From Sherrington.) 



facilitates its subsequent use in a movement antagonistic to that which 

 first occupied the motor mechanism. This phenomenon is known as 

 successive induction. 



In the spinal animal successive induction is demonstrated by us- 

 ing two reflexes that are of a more or less antagonistic character 

 for example, the flexion reflex and the knee-jerk, or better still the 

 crossed extension reflex and the flexion reflex. If we elicit the knee- 

 jerk in a spinal dog at regular intervals, with stimuli of equal intensity, 

 the extension movements (the kicks) will be approximately equal. If 

 now we apply a nocuous stimulus to the skin of the foot and so throw 

 the leg into flexion, it will be found, after the flexion movement has dis- 



