268 THE GROWTH OF THE BRAIN. 



must do so, if it is to bring several motor centres under 

 its influence. The sensory impulse reaching the cortical 

 cells may thus be compared to a complex sound wave 

 striking upon resonators, each one of which picks out 

 that vibration to which it has been attuned and responds 

 to it. Moreover, to push the simile further, the pitch 

 of responsive cells may be altered by the play of other 

 impulses upon them, and thus the analysis at different 

 times is not the same. Refinement in the structure of 

 the cerebral cortex may, therefore, be developed in three 

 ways : first, by the multiplication of the pathways bear- 

 ing the incoming impulses ; second, by rendering more 

 sensitive to slight differences in the stimulation those 

 cells whose function it is to receive these impulses ; and 

 third, by increasing the number of the central cells. So 

 far as can be seen at present, the brains of the lower 

 and less intelligent mammals are inferior in all these 

 respects, but are most deficient on the side of the 

 afferent and central elements. 



Besides the prime duty of furnishing pathways for 

 the incoming impulses destined to arouse sensations, 

 the afferent elements take part in other changes 

 which are of great importance. On them primarily 

 depends the full and final development of the cortex, 

 for, to some extent, the very growth of the hemispheres 

 hangs on the manner in which they are excited, 

 healthy activity promoting fuller development. It is 

 not necessary to go into details which favour such a 

 view ; but much evidence can be obtained from cases 

 in which the course of development has been more or 

 less modified, as the result of interference with the 

 incoming impulses. 



As might be supposed, the pathways connecting the 

 efferent cells in the cortex with the lower centres in the 

 central axis, are proportionately more numerous in man 



