Activities of the Mid-Brain. 



149 



This is accompanied by reciprocal flexor relaxation if there is flexor tonus at 

 the commencement of stimulation. The extensor contraction is a more slow 

 movement than the flexor contraction in the ipsilateral reaction. Having 

 attained a maximum this persists throughout the period of stimulation and 

 is continued after termination of stimulation as extensor postural after- 

 discharge. This is often as well maintained as the flexor after-discharge in 

 the ipsilateral reaction, but sometimes it dies away more rapidly. Occasion- 

 ally augmented extensor contraction may be seen, and it sometimes happens 

 that the terminal phenomena consist of extensor relaxation and flexor 

 rebound contraction. This is rare and has occurred when there was con- 

 siderable flexor tonus in being at the time of application of the ipsilateral 

 stimulus — although even in these circumstances extensor after-discharge is 

 the more common. The flexor rebound has been observed to change to 

 extensor after-discharge after mesial longitudinal section of the mid-brain. 

 Good after-discharge may be seen in the " de-afferented " condition. 



3. The Synchronous Compounding of Ipsilateral and Contralateral Reactions. — 

 The two reactions may obviously be synchronously compounded in such a 

 manner that the ipsilateral interrupts a contralateral " background " or the 

 contralateral an ipsilateral " background." 



When compounded against an ipsilateral " background " (flexion) the effect 

 of stimulation of the contralateral area (extension) is to produce relaxation 

 of the " background " flexor contraction. This may be complete or it may be 

 incomplete. When the relaxation is not complete it is found that stronger 

 contralateral stimulation produces greater flexor relaxation during double 

 stimulation. The flexor relaxation may be accompanied by reciprocal 

 extensor contraction — which is, however, not so great in extent as that 

 in the "pure" contralateral reaction (fig. 1). On the other hand there may 

 appear no extensor contraction during double stimulation — even when that 

 is present in the " pure " contralateral reaction. Although the extensor 

 contraction is a slow one the flexor relaxation is a very rapid movement, but 

 the latency of flexor relaxation is usually great. When the interrupting 

 contralateral stimulus is withdrawn and the ipsilateral stimulus is continued 

 there occurs a restitution of flexor contraction. This is usually a rapid 

 movement even where there is a good extensor after-discharge in the contra- 

 lateral reaction. The restituted flexor contraction may attain a level as 

 great as that at the corresponding point in a " pure " ipsilateral reaction 

 (fig 3, reaction " a "). Withdrawal of the ipsilateral " background " stimulus 

 is followed by a flexor after-discharge just as in the "pure" reaction. In 

 one instance an extensor terminal contraction and flexor terminal relaxation 

 were seen. With the exception of the last phenomenon and of extensor 



