416 



ORGAN SYSTEMS OF MAN 



dorsal fissure 

 M t 

 vwhite matter 



moscl& 

 or qiand 



Sense 

 orqon 



Fig. 16-16. A schematic view of the spinal cord in cross-section. The arrangement of the nerve cells is indicated on 



the left and the simple reflex pathway on the right. 



the peripheral and central nervous system 

 is called a reflex. 



The cord and reflex action. Every person 

 has experienced the operation of a simple 

 reflex in his own body, as, for example, the 

 quick withdrawal of his bare foot when it 

 encounters a sharp object. It is difficult 



to determine whether the foot was removed 

 after or just prior to the sensation of pain. 

 A simple experiment with a frog can show 

 tliat the reaction could have taken place 

 without the sensation of pain. If the brain is 

 removed and the skin irritated with acetic 

 acid, the frog will withdraw its foot by ap- 



