NERVE 193 



fibres with the visuo-sensory area, and fibres pass from 

 it down the cerebro-pontine tracts to reach the cerebellum 

 (fig. 58). 



The discharging part of the cortex may be con- 

 sidered as a map of the various muscular combinations 

 throughout the body, the map being mounted so that the 

 lower part represents the face, the middle part the arm. and 

 the upper part the leg, probably corresponding closely to the 

 map of cutaneous and muscle-joint sensibility, although this 

 may lie rather more posteriorly. Each large division is filled 

 in so that all the various combinations of muscular movement 

 are represented (fig. 95). It must be remembered that 

 these centres do not send nerves to sinsjle muscles, but that 

 they play upon the spinal centres to produce combined move- 

 ments of sets of muscles. 



These movements involve inhibition as well as excita- 

 tion, just as the spinal reflexes do. 



This is very clearly shown as regards the eye movements. 

 In the monkey, the resting position of the eyes is straight 

 forward with the optic axes parallel. If all the nerves to the 

 ocular muscles be cut, this position is assumed, and, if the 

 position of the eye be passively altered, upon removing the 

 displacing force, it springs back to this position. If the III. 

 and IV. nerves of the left side be cut (p. 161), so that the 

 external rectus alone is unparalysed, then, exciting a part of the 

 cortex which causes movements of the two eyes to the right, 

 produces not only a movement of the right eye in that 

 direction, but a movement of the left eye to the right as far 

 as the middle line — the position of rest — showing that the 

 VI. nerve has been inhibited. 



Stimulation of the cortex causes flexion more readily 

 than extension, apparently because the inhibitory mechanism 

 for the extensors is better developed than that for the flexors. 

 Sherrington finds that this condition is reversed under the 

 influence of strychnine or of tetanus toxin, and that stimuli, 

 which in normal conditions will cause flexion, now cause 

 powerful extension, and hence co-ordinated movement is 

 impossible. 



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