REFLEX ACTION. 



39 



Such actions as the struggling of the chicken whose head 

 has been cut off, and of the brainless frog in drawing up the 

 foot when the toes are pinched, are examples of Reflex Action 

 of the Spinal Cord. 



When the toes are pinched, the nerve current, or nerve 

 impulse, runs up the nerve to the spinal cord. The gray mat- 

 ter in the central part of the spinal cord receives the message, 

 and sends back a nerve impulse to the muscles to make them 

 shorten, and pull the foot away. 



Afferent Sensor Fiber 

 \ 



Dorsal Root 



Muscle 



Motor Fiber \ 



Ventral Root. 

 Fig. 15. Diagram of Reflex Action of the Spinal Cord. (After Landois & Stirling.) 



The sciatic nerve is composed of many fibers. If this nerve- 

 is traced outward, it is found to be continually subdividing, 

 and sending small branches to the muscles, and finally in the 

 muscles one fine nerve fiber goes to each muscle fiber. Many 

 fibers go on past the muscles to the skin. We can feel in any 

 part of the skin, and we can tell just where we are touched. 

 These fibers from the skin, then, carry nerve impulses inward, 

 as those going to the muscles carry impulses outward. 



In skinning an animal we see many bands of white or 

 transparent tissues connecting the skin and the sheaths of 



