NERVOUS SYSTEM. 31 



3. If the toes are again pinched, it is found that the uninjured leg 

 will draw up, but not the one whose sciatic nerve has been severed. 



4. If a wire be run down the spinal cavity, the spinal cord will- be 

 destroyed, and during the operation the uninjured leg will act spas- 

 modically, because the nerve fibers going to its muscles from the cord 

 are stimulated. 



5. Pinching the toes no longer gives response, because the cord, 

 which acted as the center of this reflex action, is destroyed. 



The Gray Matter of the Cord the Center of Reflex 

 Action. In simple sensation of touch, pressure on the 



Afferent Dorsal Root 



Sensor Fiber 



Skin 



Motor Fiber v v 



Efferent Ventrg| Root 



Fig. 1 3. Diagram of Reflex Action of the Spinal Cord. 

 (After Landois and Stirling.) 



toes starts a nerve current or nerve impulse which runs up 

 to the brain. The sensation is in the brain, but is referred 

 to the foot. Hence we should be careful not to speak of 

 a sensation being carried. In voluntary muscular action 

 the impulse starts from the brain, goes to the muscles, and 

 makes them shorten or relax. 



But in reflex action the current runs up the nerve to the 

 spinal cord. The gray matter of the central part of the 

 cord receives the message, and sends back a nerve impulse 

 to the muscles to make them shorten and pull the foot 

 away from the source of injury. 



