LOCALISATION OF FUNCTION. 265 



sented by these groups of muscles demands many 

 associations, and it is apparently as an anatomical 

 device for furnishing such associations that their exten- 

 sive representation is significant. Experimentally it is 

 possible to partially innervate a muscle, throwing into 

 contraction only a portion of its fibres, and in making 

 delicate and finely adjusted voluntary contractions it is 

 probably necessary to innervate the different groups of 

 muscle fibres in varying proportions. The mechanism 

 for the tongue may be taken as an illustration. To the 

 tongue run many fibres from the spinal centres, and 

 since there are many they must come from different 

 portions of the motor nucleus. The tongue is moved 

 whenever the appropriate area in the cortex is stimu- 

 lated, but the contraction is slightly varied according to 

 the portion of the area aroused, and under normal con- 

 ditions the incoming impulse associated with one form 

 of movement will act through one group of efferent cells, 

 while another movement will depend on the discharge 

 of a slightly different group, thus giving an anatomical 

 basis for differences in the resultant contractions. 



It is to be noted in this connection that the move- 

 ments of the body as a whole do not belong to the group 

 requiring fine adjustment. 



As already stated, experiment shows that in man the 

 special cortical centres are somewhat separated from 

 one another, and this separation is due to masses of 

 brain substance which do not give reactions upon ordi- 

 nary electrical stimulation. Further than this, the pro- 

 portion of the brain which is thus latent, giving no 

 reaction upon stimulation, seems to be larger in man 

 than in the monkey. Certain differences between man 

 and the macaque monkey in the development of the 

 cortical centres can be explained by the fact that the 

 macaque monkey makes a greater voluntary use of its 



