STIMULATING THE MOTOR CORTEX IN ANIMALS. 737 



a very slight shifting of the electrodes to one or other side of the 

 boundary will cause the movement to be limited to one part only, 

 provided the excitation be of minimal intensity. If, however, the 

 stimulus be strong or be too prolonged, a spread of effect is at once 

 produced. 



Within the limits of the several areas above enumerated, varieties of 

 movement are obtained which indicate a still further differentiation, or 

 in other words an intra-areal localisation. 1 It is obvious that these 

 intra-areal localisations are likely to be most marked within the larger 

 areas, and we should further expect the best differentiation in connection 

 with those parts which are concerned with the more complex and 

 precise movements directed by the will. These in the monkey are 

 chiefly the movements of the upper limb, and especially the hand, and 

 movements concerned with facial expression. Further, it is found that 

 for many movements, if not for all, there is in each case a sort of focal 

 point within the area, from which on the average of a large number of 

 experiments the movement is more readily or more frequently obtained 

 than elsewhere. This 

 is illustrated in the 

 accompanying dia- 

 gram from Beevor 

 and Horsley,in which 

 each dot represents 

 an instance in which 

 a movement of the 

 hallux was primarily 

 obtained in a large 

 number of experi- 

 ments, and in which 

 therefore the point 



about which the FIG. 337. Diagram from Beevor and Horsley, showing by relative 

 shading the effect, in a number of individuals of Macacus 



greatest number OI sinicus, of stimulation within the area where movements of 

 dots are accumulated the opposite hallux are elicited. 



has been termed the 



focal point for that movement. It is possible that its focal point 

 does actually represent an absolutely localised centre of representa- 

 tion of each movement, but the fact that the movement is in many 

 instances also got with extreme readiness in a circumjacent area of 

 greater or less size, and that in many individuals it may be produced 

 even more readily from other parts of such circumjacent area than 

 the average focal point, seems to point to the conclusion, which is that 

 usually accepted, that localisation of particular movements is rather 

 connected with small areas of cortex than with mere points on the 

 cerebral surface ; in a few cases only do these areas seem to be so 

 extremely limited in size as to merit being spoken of as points. It 

 is further rare to find that the movement which is provoked is simple 

 and uncomplicated by other movements, although occasionally this is so. 

 Thus it is often possible by stimulation at the upper end of the Eolandic 

 fissure to obtain a simple flexion or extension of the hallux, or by 

 stimulation of the middle of the ascending parietal convolution to obtain 

 simple flexion of the fingers, or at a little lower level simple opposition of 



1 The intra-areal localisations have been largely worked out by Beevor and Horsley, 

 Phil. Trans., London, 1887 and 1888, B. 

 VOL. ii. 47 



