496 PHYSIOLOGY 



The part played by muscular relaxation in the response to cortical 

 stimulation is also well seen in the case of the eye muscles. Stimula- 

 tion of the centre for eye movements on the convex surface of the 

 frontal lobes on the right side causes ' conjugate deviation ' of both 

 eyes to the left. This movement involves contraction of the right 

 internal rectus and left external rectus, and a simultaneous inhibition 

 of the tone of the right external rectus and left internal rectus. If all 

 the muscles of the right eye be divided except the external rectus, this 

 eye looks permanently towards the right side, i.e. a right external 

 strabismus or squint is produced. On now exciting the right cortex 

 both eyes move to the left, although the right internal rectus is divided. 

 The movement of the right eye stops at the middle line, and is brought 

 about simply by a relaxation of the tone of the right external rectus 

 muscle (Sherrington). 



This movement of both eyes on stimulation of one side of the brain 

 shows that the function of each hemisphere is not entirely unilateral 

 with regard to the muscles of the body. As a rule the response to 

 excitation of the motor area for limbs is strictly unilateral. In the 

 case of those movements, however, which are normallv carried out bv 



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co-operation of the muscles of the two sides, such as the movements 

 of ths trunk, neck, and eyes, stimulation of the motor area in one 

 hemisphere evokes a movement involving the muscles of both sides 

 of the body, i.e. the cortical representation is one of movement rather 

 than one of muscles. Where an action is carried out by similar con- 

 tractions of corresponding muscles on the two sides, the movement 

 itself is bilaterally represented in the cortex. Types of such reactions 

 are found in closure of the mouth, contraction of the abdominal 

 muscles, erection or flexion of the trunk. It seems that under such 

 circumstances there is a free communication between the lower motor 

 centres of the two sides, since the bilaterality of the response is not 

 altered by extirpation of the cortex of the opposite hemispheres to 

 that which is being stimulated. 



CORTICAL EPILEPSY. When electrical excitation, of any strength 

 over the minimum effective stimulation, is applied to the motor area 

 of the cortex, the movements evoked tend to persist for a short time 

 beyond the duration of the stimulus. On still further increasing the 

 strength of the current, the contraction spreads to adjoining muscles, 

 and finally may affect all parts of the body, giving rise to the pheno- 

 menon known as an epileptic convulsion. The same effect may often 

 be caused by weak stimuli, if the irritability of the cortex be raised in 

 consequence of repeated previous stimulation. A typical fit consists 

 of two parts. The first effect of the stimulation is a strong tonic 

 contraction ; this outlasts the stimulus for some time, and then 

 gives way to a series of clonic contractions, repeated at first at 



