950 CENTRAL NERVOUS SYSTEM 



appeared, that the knee-jerk is much more pronounced than previously. 

 Similarly, if we elicit the crossed extension reflex by nocuous stimuli 

 of equal intensity applied to the opposite limb, the extension movements 

 will be approximately equal. By now throwing the limb exhibiting them 

 into the flexion reflex, the extensor movements will of course disappear, 

 but after the flexion has been discontinued, they will reappear with 

 increased intensity (Fig. 242). 



These facts show us, then, that after the final common path has been 

 occupied by a reflex of one type, it becomes more available to a reflex 

 of an opposite type. Successive induction gives rise to a series of re- 

 sponses which afford a further example of the compensatory movements 

 produced by the integrative action of the spinal cord. By facilitating 

 the occupancy of the final common path by reflexes opposite in nature 

 to those which have just occupied it, movements are brought about which 

 restore the position which was disturbed by the primary response. 

 In other words, it is evident that if the two opposite reflexes are 

 constantly competing with each other for possession of the final com- 

 mon path, they will tend alternately to occupy it, thus bringing about 

 a rhythmic movement. Such is the mechanism involved in walking; 

 the leg is lifted from the ground (flexion reflex) ; it is then brought 

 on the ground, and the mechanical push given to the plantar surface 

 of the foot brings out the extensor thrust, the appearance of which 

 is greatly facilitated by the fact that immediately before the flexion 

 reflex occupied the final common path. 



