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APPLIED PHYSIOLOGY 



223. How the spinal cord acts. The mind in the cells 

 of the spinal cord is like a telegraph operator, who does 

 not send messages except as others tell him to. In the 

 first place, the mind in the brain sends all its orders 

 through its servants, the cells of the spinal cord. When 

 it wishes to move the hand, it sends word to the cells of the 

 spinal cord, and they send word to the muscles of the arm 

 so quickly that we do not know that they had anything to 

 do with moving the arm. 



224. Reflex acts. In the second place, the spinal cord 

 sends orders when asked to do so by the cells of the body. 



Diagram of reflex action. 



a tack pricking the hand. b sensory nerve. c nerve cell in the spinal cord. 

 d motor nerve. e muscle moving the hand away from the tack. 



The nerves which carry the messages from the cells of the 

 body go to the cells of the spinal cord, and also to the 

 brain. The cells of the spinal cord are much nearer 

 the cells of the body than the brain is, and when they get 

 word that anything has harmed a part of the body, they 

 have the power to order the muscles to snatch that part 

 away without waiting for word from the brain. Thus, when 

 the cord receives word that the hand touches a hot stove, 

 it at once sends an order for the muscles to snatch the 

 hand away, and it is done by the time the brain feels 

 the burn. When anything suddenly hurts you, you cannot 



