264 THE GROWTH OF THE BRAIN. 



traction of which results in co-ordinated movements, and 

 thus the nature of the reaction taking place in conse- 

 quence of the cortical discharge is modified first by 

 alterations of the initial stimulus during its passage 

 through the cortex, and second, by the conditions of the 

 lower centre as determined by impulses arriving from 

 other sources. 



Granting the relation here suggested, we are prepared 



FIG. 57. Showing the localisation of movements of different 

 characters in the arm area. (After Horsley.) 



to find that the cortical representations of different 

 muscle groups is not at all dependent on their mass, but 

 is related rather to refinement and complexity of the 

 movements which these muscles effect. Accordingly 

 the massive muscles of the trunk and leg are but poorly 

 represented, while, on the other hand, those of the face 

 and arm have much larger areas devoted to them. And 

 further, in the arm area it is to the thumb and index 

 finger, in the head area to the tongue, lips, and muscles 

 of phonation, to which the greatest extension is given. 

 The exact adjustment of the refined movements repre- 



