CH. L.] MOTOR AND SENSORY AREAS 727 



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 coordinate 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 and absolutely uncon- 

 scious, 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. In an anaesthetised animal the brain is inactive, 

 and if methylene blue 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 term motor areas and sensory areas. 



Motor area. The name Rolandic area which this part of the 

 brain has also received is derived from its anatomical position. 



Stimulation of the motor area produces movement of some part 

 of the opposite side of the body ; excitation of the same spot is always 

 followed by the same movement in the same animal. In different 

 animals excitation of anatomically corresponding spots produces 

 similar or corresponding results. It is this which has enabled 

 one to apply the results of stimulating areas of the monkey's 



