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Mr. Graham Brown. 



mid-brain. The extensor contraction in the contralateral reaction had 

 disappeared, but the ipsilateral reaction was untouched. The anterior 

 colliculi were then first removed. Flexor after-discharge still occurred after 

 the ipsilateral reaction. If the contralateral stimulus was applied during 

 this flexor after-discharge there occurred flexor relaxation which continued 

 after the cessation of the contralateral stimulus. When the posterior 

 colliculi were then removed the flexor after-discharge in the ipsilateral 

 reaction was not clearly marked, and the contralateral effect was not obtained 

 (the contralateral reaction in response to weak stimuli appeared to be one of 

 flexion). When the oral half of the pons Varolii was removed after a 

 transverse section the reaction which accompanied stimulation of the 

 ipsilateral area was not followed by any after-discharge. In another experi- 

 ment the flexor after-discharge had disappeared from the ipsilateral reaction 

 after removal of the cerebellum, and it remained absent after further removal 

 of the mid-brain. Here also the contralateral reaction appeared to be one of 

 flexion. 



VI. Electrical Stimulation of the Crus Cerebri. 



It may be suggested that the results above described might, in part at any 

 rate, be due to stimulation of the crura cerebri by spread of current. That 

 this is not the case is shewn by two observations. 



Of these, the first is the fact that with the strength of stimuli used the 

 ipsilateral crus cerebri is inexcitable as regards the movements of the arm of 

 its own side of the body. The contralateral crus is excitable, but the move- 

 ments of the elbow obtained from it are almost invariably flexion, whereas 

 the contralateral reactions from the area in the cross-section of the mid-brain 

 dorsal to it are those of extension. Further, there is an inexcitable field 

 between the two. 



The second observation is that the movements of flexion excited by 

 stimulation of the contralateral crus are unlike those obtained on stimulation 

 of the area in the ipsilateral side of the cross-section of the mid-brain. The 

 crus flexion is almost always a slow one and " climbs " during the course of 

 application of the stimulus, so that its maximum is almost always at the point 

 of cessation of stimulation. Cessation of stimulation is followed by a very 

 sudden relaxation, so that the arm " flops." This is extremely characteristic. 



VII. Compound Stimulation of Crus and the more Dorsal Excitable Area in the 

 Cross-section of the Mid-brain. 



In these experiments the effects of compound stimulation of various kinds 

 have been examined. Here there is not space to mention all of these, but 



