THE NERVOUS SYSTEM 185 



causing them to act without the previous action of the other 

 cerebral mechanisms. This is seen in Jacksonian epilepsy, 

 where, as the result of it may be 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 Rolandic convolutions, in front of the fissure, 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. 98 and 99.) It must be remembered 

 that these centres do not send nerves to single muscles, but 

 act upon groups to produce definite movements, through the 

 lower spinal centres, and their action may involve not merely 

 stimulation of some muscles but inhibition of others. (See 

 p. 60.) 



In these areas the lesion must be extensive to cause com- 

 plete paralysis of any group of muscles. A limited lesion 

 may simply cause a loss of the finer movements. Thus, a 

 monkey with part of the middle portion of the Rolandic areas 

 removed may be able to move its arm and hand about, but 

 may be quite unable to pick up objects from the floor of 

 its cage. 



As in the case of the receiving, so in that of the discharg- 

 ing mechanism, it is the most recently acquired functions 

 which are most easily lost. This is well illustrated by the 

 results of lesions of the left inferior frontal convolution 

 the area which presides over movements of the lips and 

 tongue. This is the centre which has to be specially educated 

 in the use of spoken language ; and, when this region is only 

 slightly injured, the power of using language may be lost 



