Nervous System. 43 



r The Gray Matter of the Cord the Center of Reflex 

 Action. In reflex action the current runs up the nerve 

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



Afferent Dorsal Root 



Sensor Fiber 



Skin 



\ 



Motor Fiber \ 

 Efferent * 



Ventral Root 



Fig. 27- Diagram of Reflex Action of the Spinal Cord. 

 (After Landois and Stirling.) 



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 irritation. 



The Parts Essential to Reflex Action of the Spinal 

 Cord: i. A sensitive surface (the skin, for instance). 

 2. Afferent nerve fibers. 3. A nerve cell, or cells, in 

 the center of the spinal cord. 4. Efferent nerve fibers. 

 5. Working organ, as muscle or gland. 



Steps in Reflex Action. In the above examples the 

 steps in order are : 



i. Stimulation of the nerve endings in the skin of the 

 foot. 2. Passage of nerve impulses up the afferent fibers 

 to the spinal cord. 3. Reception of the impulse by cells 

 of the gray matter in the cord. 4. Sending back nerve 

 impulses 5. Along efferent fibers to 6. Muscles which 

 shorten and move the foot. 



