CH. XLVIII.] STIMULATION AND EXTIRPATION OP CORTEX 681 



an army. The principal centre may be compared to the commander- 

 in-chief. This highest officer gives a general order for the movement 

 of a body of troops in a certain direction ; we may compare this to 

 the principal motor-centre of the cortex sending out an impulse for 

 a certain movement in a limb. But the general does not give the 

 order himself to each individual soldier, any more than the cerebral 

 cortex does to each individual muscle ; but the order is first given 

 to subordinate officers, who arrange exactly how the movement shall 

 be executed, and their orders are in the end distributed to the 

 individual men, who must move in harmony with their fellows with 

 regard to both time and space. So the subsidiary nerve-centres or 

 positions of relay enable the impulse to be widely distributed by 

 collaterals to numerous muscles which contract in a similar orderly, 

 harmonious, and co-ordinate manner. 



There is just the same sort of thing in the reverse direction in 

 the matter of sensory impulses. Just as a private in the army, 

 when he wishes to communicate with the general, does so through 

 one or several subordinate officers, so the sensory impulse passes 

 through many cell-stations or subsidiary centres on the way to the 

 highest centre, where the mental process called sensation, that is, 

 the appreciation of the impulse, takes place. 



There are two great experimental methods used for determining 

 the function of any part of the cerebrum. The first is stimulation ; 

 the second is extirpation. These words almost explain themselves ; 

 in stimulation a weak interrupted induction current is applied by 

 means of electrodes to the convolution under investigation, and the 

 resulting movement of the muscles of the body, if any occurs, is 

 noticed. In extirpation the piece of brain is removed, and the result- 

 ing paralysis, if any, is observed. 



It is essential, when the experiment of stimulating the cortex of the brain is 

 being performed, that the animal should be anaesthetised, otherwise voluntary or 

 reflex actions will occur which mask those produced by stimulation. If, however, 

 the animal is too deeply under the influence of a narcotic the brain is inexcitable. 



On p. 379 Ehrlich's experiments with methylene blue are described. In an 

 anaesthetised animal the brain is inactive, and if the pigment is injected into the 

 blood, the brain is seen to be of a blue colour. If, however, a spot of the cerebral 

 surface is stimulated, that part of the brain is thrown into action, oxygen is used 

 up, and the methylene blue is reduced, and in consequence that area of the brain 

 loses its blue tint. If the animal is so deeply narcotised that the brain does not 

 discharge an impulse, the part stimulated remains blue. 



By such means the cortex has been mapped out into what we 

 may provisionally term motor areas, and sensory areas. 



Motor areas. These areas are also termed sensori-motor or 

 kincesthetic, for reasons which will be explained more fully later. 

 The name Rolandic area which they have also received is derived 

 from their anatomical position. 



Stimulation of them produces movement of some part of the 



