HOW THE BRAIN THINKS 



The matter is important, as shedding light on the 

 means by which a nerve- wave, what we call a sensa- 

 tion or an impulse, travels. An impulse going up 

 or down a single nerve follows a simple path. If, 

 for example, in your stocking feet or otherwise, you 

 impinge upon the active end of a tack, you get a 

 sharp sensation of pain, and there is an answering 

 jerk of your foot off the ground, and perhaps a 

 howl. It is comparatively easy to find the nerve 

 which carries this pain-wave, and trace it, say, as 

 far as the lower end of your spine. There it enters 

 the network of nerves which make up the spinal 

 ganglia. 



From here an automatic response is sent back 

 over another nerve, a motor-nerve, or rather a set 

 of them, which contracts the muscles of your leg 

 and foot violently and pulls it away from the tack. 

 This is what is called simple reflex action, and with 

 it your brain has nothing to do. Meanwhile, the 

 pain -wave entering the ganglion has set other 

 nerves which also enter the ganglion into action, 

 and the impulse goes travelling up to the brain, and 

 it is only when it reaches the brain-rind, or cortex, 

 that you become conscious of the hubbub down in 

 your foot. 



Put very crudely, it is as if there had been a 

 smash-up on a railroad and the nearest station 

 had been notified and had sent back what help 



261 



