670 FUNCTIONS OF THE CEREBRUM. [CH. XLVIII. 



movement, and the optic thalamus, in connection with sensation, 

 and especially with the sense of vision as its name indicates. 



A subsidiary centre may be compared to a subordinate official 

 in 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, har- 

 monious, and co-ordinate manner. The subject of muscular co- 

 ordination we shall consider at greater length in the next chapter, 

 on the functions of the cerebellum. 



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 stimula- 

 tion; 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 investi- 

 gation, 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 resulting 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, other- 

 wise 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. 378 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 



