Activities of the Mid-Brain. 



159 



the phenomena when crus is pitted against the excitable area which lies 

 more dor sally in the cross-section of the mid-brain are of great interest. 



1. Immediate Compounding of Crus against Contralateral Reaction 

 {Extension). — The crus reaction used was the typical contralateral flexion. 

 In one experiment (" de-afferented "), where the " background " of the 

 compound reaction was that of the contralateral reaction (extension with 

 extensor after-discharge) the interrupting crus reaction caused extensor 

 relaxation accompanied by flexor contraction of the typical crus type. 

 Withdrawal of the crus stimulus gave flexor relaxation and reconstitution 

 of extensor contraction. Withdrawal of the " background " contralateral 

 area stimulus was followed by the usual extensor after-discharge. When 

 the contralateral reaction is made to interrupt the crus reaction a relaxation 

 of the flexor contraction has been observed during double stimulation. 



2. Immediate Compounding of Crus against Ipsilateral Reaction {Flexion). — 

 Here when the ipsilateral reaction serves as the " background " there occurs 

 flexor augmentation of contraction when the interrupting crus stimulus is 

 applied. On withdrawal of that stimulus again, the ipsilateral reaction still 

 continuing, the flexor contraction remains augmented. The subsequent after- 

 discharge of the flexor on withdrawal of the " background " ipsilateral reaction 

 may remain at this level. 



3. Compounding in Temporal Succession. — When the ipsilateral reaction 

 from the more dorsal excitable area in the cross-section of the mid-brain 

 gives a marked flexor after-discharge and the contralateral crus stimulus is 

 then applied, augmentation of flexor contraction may occur. But, on 

 withdrawal of the crus stimulus, there is immediate and very sudden flexor 

 relaxation — exactly similar to that which follows a " pure " contralateral 

 crus stimulus. This phenomenon is the more remarkable in that it may 

 occur where the flexor after-discharge after the ipsilateral reaction is of 

 great extent and very great duration. It seems to indicate that the flexor 

 relaxation after crus stimulation is, as it were, a positive phenomenon 

 (fig. 6). This phenomenon has been observed in the "de-afferented" 

 condition. It has also been observed after removal of the cerebellum. 



VIII. Stimulation of other Points in the Mid-brain and Hind-brain. 



In passing, I should like to note some effects of the stimulation of other 

 points in the hind-brain and mid-brain. 



In one experiment, where the level of cross-section in decerebration was 

 just above the anterior colliculi, a curious bilateral kick-like movement of 

 the lower limbs was obtained on stimulation of a small area on either side of 

 the cross-section. This was to the side and dorsal in position, and it is 



