438 



PHYSIOLOGY 



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

 duration of the stimulus. X)n 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 phenomenon 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 stimu- 

 lation 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 

 intervals of -from six to ten per second, but gradually getting slower as the 

 tit dies away. The tracing of such a contraction is given in Fig. 229. 



Fio. 229. Tracing of muscular contractions during an epileptic convulsion aroused 

 by strong stimulation of the motor area. (HORSLEY and SCHAFER.) 



The main phenomena of a fit, due to irritation of any portion of the 

 motor area, were described by Hughlings Jackson in 1864, even before the 

 experimental proof of cortical localisation had been brought forward by 

 Fritsch and Hitzig. A similar condition may occur in the human subject 

 as a result of irritative lesions of this part of the cortex, such as that due to 

 the presence of a tumour or a spicule of bone pressing on the brain. Jackson 

 showed thatin this condition the convulsive movements follow a certain order 

 or ' march.' Thus if the thumb area be the seat of stimulation, the fit 

 begins by a contraction of the thumb muscles, then spreads to the muscles 

 of the hand, fore-arm and shoulder of the same side, and then to the 

 face, trunk, and leg. If it begins in the toes the order would be up the leg and 

 down the arm. The same * march ' is observed in artificial stimulation of 

 the motor area. If the convulsions are very strong they spread to the 

 leg of the opposite side and then to the whole body. The spread to the other 

 side of the body is not prevented by division of the corpus callosum, nor by 

 isolating the centres from one another, so that the sequence seems to be 

 maintained through the mediation of the sub-cortical centres. Complete 

 excision of the cortical centre for any given movement excludes this move- 

 ment from participation in the fit. In man this type of epilepsy is, in the 

 milder cases at any rate, generally unattended with loss of consciousness. 

 In animals epileptic convulsions can be excited by stimulation of any portion 

 of the cortex, though it is obtained by a weaker stimulus applied to the motor 

 cortex than to any other part. Jacksonian epilepsy is often preceded by 

 a sensation of numbness or tingling, the * aura,' in the part in which such 

 convulsions begin. In ordinary idiopathic epilepsy tactile or visual sensory 



