438 PHYSIOLOGY 



opposing centres are excited simultaneously ; one reaction is prepotent, as 

 is the case with cutaneous excitation, and this reaction is attended and 

 brought about by ordered contraction of certain muscles accompanied by 

 an ordered relaxation of their antagonists. Thus the movement of opening 

 the jaw, which can be excited from a fairly large area of the cortex, involves 

 a relaxation of the normal tone of the masseter muscle. Flexion of the leg 

 demands relaxation of the extensor muscles. As in the case of the spinal 

 reflexes, this relaxation, or inhibition, can be abolished under the action of 

 strychnine, or the toxin of tetanus. After administration of either of these 

 it is impossible to evoke inhibition of any muscle. Excitation of the cortical 

 centre for the movements of the jaw causes contraction of both closers and 

 openers of the jaw, i.e. a strife in which the stronger masseter muscles 

 predominate, so that the jaw is firmly closed. 



The part played by muscular relaxation in the response to cortical stimula- 

 tion is also well seen in the case of the eye muscles. Stimulation 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 simul- 

 taneous 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, how- 

 ever, which are normally carried out by co-operation of the muscles of the 

 two sides, such as the movements of the 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 



