154 



Mr. Graham Brown. 



in the " pure " contralateral reaction. With regard to the phenomena described 

 in this paragraph there is little difference between the " de-afferented " and 

 the " normal " conditions. 



It therefore appears that the phenomena obtained when the two areas in 

 the cross-section of the mid-brain are simultaneously stimulated closely 

 resemble those obtained when the movements of two antagonists in response 

 to peripheral nerve stimulation are examined. " Algebraic summation " 

 seems to occur, and the phenomena which occur when the interrupting 

 stimulus is withdrawn and the " background " still continued nearly approxi- 

 mate to those seen under similar conditions in the peripheral reflexes where 

 the " pure " reactions are 'followed by good " after-discharge." 



4. The Compounding of Ipsilateral and Contralateral Reactions in Temporal 

 Succession. — Where the ipsilateral reaction is followed by flexor after- 

 discharge and the contralateral by extensor after-discharge the two reactions 

 m&y be compounded in such a manner that the one falls during the after- 

 discharge of the other. If this is done the contralateral reaction (extension) 

 at once produces a very sharp relaxation of the flexor after-discharge of the 

 ipsilateral reaction. On withdrawal of the contralateral stimulus an extensor 

 after-discharge is left in being, and this is at once reduced if the ipsilateral 

 stimulus is repeated, and so on (fig. 2). If the contralateral stimulus is weak 

 or of very short duration there may be incomplete relaxation of a flexor after- 

 discharge. If the ipsilateral stimulus is weak there may be a partial relaxa- 

 tion of an extensor after-discharge, and this may thereafter be reconstituted. 



5. The Geographical Position of the " Focal Point." — When stimulation 

 applied to one or other side of the cross-section of the mid-brain gives one 

 or other of these reactions it is usually found that the area from which they 

 may be obtained is comparatively large. The minimal reaction may, how- 

 ever, be localised to a comparatively small area. This area (" focal point ") 

 lies about 3 to 4 mm. ventral to the dorsal surface of the mid-brain, and 

 about 2 to 3 mm. from the mesial plane. The surrounding parts are 

 inexcitable, except, perhaps, those immediately between the focal point and 

 the mesial plane. 



In one experiment in which decerebration was comparatively high — the 

 division of the neuraxis passing just oral to the anterior colliculi — the focal 

 point was found to be much more ventral than this, about 7-8 mm. from the 

 dorsal surface. The ipsilateral reaction was of the usual type. The contra- 

 lateral reaction gave relaxation of a flexor after-discharge (if that was in 

 being) and extensor contraction, but on withdrawing the stimulus there was 

 at once sharp extensor relaxation and a marked flexor rebound contraction. 

 The mid-brain was then split in the mesial plane (the section was found to 



