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200 VETERINARY PHYSIOLOGY 



causing them to act without the previous action of the other 

 cerebral mechanisms. This is seen in Jacksonian epilepsy, 

 where, as the result of a spicule of bone or a thickened 

 bit of membrane, one part of the cortex is from time to 

 time excited, and by its action produces movement of certain 

 groups of muscles. 



Experimental observations have fully confirmed and extended 

 the conclusions arrived at from such pathological evidence. 



If parts of these convolutions be excised in the monkey, 

 the animal loses the power of voluntary movement of certain 

 groups of muscles, while if they are stimulated by electricity 

 these groups of muscles respond. 



These convolutions, just in front of the fissure of Rolando, may 

 be considered 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. Each large division is filled in so that all the 

 various combinations of muscular movement are represented 

 (figs. 99 and 101). It must be remembered that these centres 

 do not send nerves to single muscles, but act upon groups to 

 produce definite combined movements, through the lower spinal 

 centres. 



(6) The action of these centres involves not only stimulation of 

 certain muscles, but the inhibition of others. 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. or IV. nerve of the left side be cut so 

 that the external rectus alone is unparalysed, then, exciting a 

 part of the cortex which causes movements of the two eyes to 

 the opposite side, 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 under the influence of strychnine or 



