REFLEX ACTION. 



623 



But these paths have to be made passable by use ; the oftener an 

 impulse traverses a given route the more adapted such a route 

 becomes for future traffic. Thus, by practice or the habit of using 



FIG. 242. 



Diagram of the paths taken by the impulses in the brain and cord. M M, 

 motor channels; ss, sensory channels; cr., cranial nerves. 



certain sets of muscles, we constantly freshen certain channels of 

 intercommunication between the various cells of the cord, and thus 



