38 REFLEX ACTION. 



We can now understand, that when anything touches our 

 toes it starts what is called a Nerve Current or Nerve Impulse, 

 which runs up the nerve to the spinal cord, and through this 

 to the brain. If the brain is entire and awake, we have what 

 we call a sensation of touch. 



The feeling seems to be in the foot ; but there is no feeling 

 in the foot. Without the brain there is no sensation. If the 

 nerve has been cut no sensation is produced, because the nerve 

 current fails to reach the brain. It cannot pass the break in 

 the nerve any more than an electric current can pass if the 

 wire be cut. 



Nerve-Cell 



Afferent Fiber / \ . Efferent Fiber 



Muscle 

 Fig. 14. Scheme of Reflex Arc. (After Landois & Stirling.) 



But our frog has lost his brain ; he neither feels nor wills 

 to do anything. How is it that he draws up his leg when his 

 toes are pinched ? 



4. Run a wire down the cavity of the spinal column. This 

 destroys the spinal cord. During this operation it will be seen 

 that the muscles of nearly the whole body are set into violent 

 action. This is because nerve impulses are produced by this 

 irritation of the motor nerve-cells of the cord which pass out 

 to the muscles, causing them to shorten. 



5. Pinch the toes again. The foot is no longer drawn up. 

 The muscles and nerves are still able to act, as is seen on 



pinching the end of the nerve still connected with the foot. 



