S'J4 THE CENTRAL NERVOUS SYSTEM 



Integration of Antagonistic Reflexes. — Although the phenomenon of 

 immediate induction encourages integration of allied reflexes, yet it is 

 frequently succeeded by one of successive induction, in which just the 

 opposite conditions occur; the resistance in the reflex pathway becomes 

 lowered for a type of movement antagonistic to that which first occu- 

 pied the reflex. To understand clearly what relationship this bears to 

 immediate induction, it may be well to take the instances in which these 

 phenomena apply in the case of vision. When the eye, after darkness, 

 is suddenly directed to a light and then closed, there remains a bright 

 image (positive after-effect) of the light; but if the light is looked at 

 for some time, then on closing the eyes it will be seen as a dark pat- 

 tern (negative after-effect). In the former instance we have an exam- 

 ple of immediate induction, in the latter, one of successive induction. 



In the spinal animal successive induction is demonstrated with equal 

 ease by using two reflexes that are of a more or less antagonistic charac- 

 ter — 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 Ave apply a nocuous stimulus to the skin of the foot and so throw 

 the leg into flexion, it v\ ill be found, after the flexion movement has dis- 

 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 

 marked intensity. 



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. In other words, it is evident that if the two op- 

 posite reflexes are constantly competing with each other for possession 

 of the final common 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. 



Other Factors Which Determine the Occupancy of the Final Common 

 Path. — Besides immediate and successive induction, several other fac- 

 tors affect the relative availability of the reflexes to afferent stimulation. 



