Activities of the Mid-Brain. 



151 



division of the mid-brain. The letters G-H (ordinates g, g'-h, h!) denote the period 

 of stimulation of the ipsilateral " dorsal focal point " (posterior longitudinal 

 bundle ?) in the cross-section of the mid-brain at the anterior colliculi. The letters 

 K-L (k, k'-l, V) in a similar manner denote contralateral stimulation. The upper 

 record registers contraction (up) and relaxation (down) of the elbow flexor — 

 supinator longus. The lower record registers similar movements of the elbow 

 extensor — humeral head of triceps. Below these are the signal lines and a time 

 tracing which registers seconds. A millimetre scale is reproduced, having been 

 drawn upon the record before varnishing. 



The first reaction is an ipsilateral one. On withdrawal of the stimulus the flexor 

 after-discharge is extremely poor. 



The second reaction is a contralateral one. Extensor contraction occurs. This is 

 here accompanied by abnormal flexor contraction — not usually seen. There is an 

 extensor after-discharge which is not well marked, but a sudden relaxation of this 

 is seen at the commencement of the third reaction, which opens with the ipsilateral 

 reaction. 



In the third reaction the two stimuli are compounded synchronously with an 

 ipsilateral "background." In the phase of double stimulation (k, k'-l, V) there is 

 extensor contraction and a partial and slight flexor relaxation. In this phase small 

 rhythmic irregularities are evident in the two records. These are related to a 

 slowing and deepening of respiration which then occurred. It will be observed that 

 the extensor contraction is less in extent than that of the " pure " contralateral 

 reaction (the second reaction of the record). On withdrawal of the contralateral 

 stimulus there occurs flexor restitution and extensor relaxation. Flexor after- 

 discharge occurs on withdrawal of the ipsilateral " background " stimulus, and is 

 much better sustained than that in the preceding " pure " ipsilateral reaction. 



In the fourth reaction of the record the " background " is contralateral (extension). 

 The contralateral stimulus is applied in the flexor after-discharge of the preceding 

 reaction and causes flexor relaxation (at k, k'). During double stimulation (g, g'- 

 h, h') there occurs a partial and slight extensor relaxation accompanied by reciprocal 

 flexor contraction. "Withdrawal of the ipsilateral stimulus is followed by flexor 

 relaxation and extensor restitution of contraction. Withdrawal of the " back- 

 ground " contralateral stimulus is followed by extensor after-discharge. 



This figure demonstrates the presence of flexor after-discharge in the ipsilateral 

 reaction and extensor after-discharge in the contralateral ; of the effects of com- 

 pounding the two in temporal succession ; and of the effects of synchronous 

 compounding with ipsilateral and contralateral "backgrounds," all after mesial 

 longitudinal division of the mid-brain. 



contraction during double stimulation all these points have been observed in 

 the " de-afferented " condition as well as in the " normal." 



When compounded against a contralateral " background " (extension) an 

 interrupting ipsilateral stimulus (flexion) evokes extensor relaxation and 

 flexor contraction. Where the ipsilateral stimulus is comparatively weak 

 the extensor relaxation may be incomplete (fig. 2). Where stronger it may 

 be complete. Withdrawal of the interrupting ipsilateral stimulus is accom- 

 panied by a sharp relaxation of the flexor contraction. This may occur even 

 when there is a flexor after -discharge in the ipsilateral reaction and when 

 the contralateral " background " stimulus is ineffective. The flexor relaxation 



