Activities of the Mid-Brain. 



155 



have passed out of the mesial plane into the left half of the neuraxis at 

 the level of the posterior collieuli). Immediately thereafter the focal point 

 was found to be in the usual more dorsal position. Ipsilateral stimulation 

 gave the usual reaction, and contralateral stimulation gave the same reaction 

 as before, save that there was marked extensor after discharge and no flexor 

 rebound contraction. 



V. The Effect of Various Lesions. 



1. Mesial Section between the Bight and Left Halves of the Mid-brain. — 

 When the reactions are obtained from the dorsal focal point there may be no 

 change in them after this lesion. The phenomena during the immediate and 

 successive compounding of the two reactions may be the same as before 

 (figs. 1, 4). The excitability may be depressed slightly, or it may remain 

 unchanged, or it may even appear to be raised. The effects of mesial section 

 in a case where the lower focal point was effective have been described in 

 the previous section. 



2. Right Semi-section of the Mid-brain between Anterior and, Posterior 

 Collieuli. — After this lesion it is found that the ipsilateral reaction (from the 

 left side of the mid-brain) is unimpaired, but the contralateral reaction (from 

 the right side of the mid-brain above the level of the semi-section) is 

 abolished. The contralateral reaction may at once be obtained by stimulation 

 of the caudal surface of the cut in the mid-brain. 



3. Division of the Bight Superior Cerebellar Peduncle. — This was found to 

 have no appreciable effect upon the reactions and their compound effects. 

 The experiments upon the cerebellar peduncles, in view of the effects of 

 removal of the whole cerebellum, were not continued. 



4. Complete Bemoval of the Cerebellum. — In several experiments the 

 cerebellum has been completely removed. After this lesion there is at first 

 no change in the two reactions. Flexor after-discharge follows the ipsilateral 

 reaction and extensor after-discharge the contralateral, and the effects of 

 compounding the two in temporal succession is the usual one (fig. 5). This 

 may last for 30 minutes or more. Thereafter the flexor after-discharge 

 disappears, the withdrawal of the ipsilateral stimulus being followed by sharp 

 flexor relaxation. In one experiment the extensor after-discharge still 

 persisted. No change in the excitability of the reactions may occur. In 

 other cases the flexor after-discharge may disappear from the moment of 

 removal of the cerebellum (the reactions have usually been tested within one 

 minute of that removal). In one case the contralateral reaction reversed to 

 flexion with the same strength of stimulus which before the removal gave 

 extension. The procedure of removal of the cerebellum has been observed 



M 2 



