294 ELEMENTS OF PHYSIOLOGY 



and they, being aroused, in turn send motor impulses to 

 the muscles. When a person wills to move his hand, an 

 impulse goes to the spinal cells, and they in turn send im- 

 pulses that cause a contraction of the muscles in the arm. 

 The brain sends about ten impulses per second to keep the 

 muscle in action. Thus, before it can relax, another im- 

 pulse reaches it. Each muscle has its own set of spinal 

 cells in the cord which act as a relay station between the 

 muscle and the brain. 



542. The Spinal Cord and Reflex Action. The cells of 

 the spinal cord have another very important duty ; they 

 largely control reflex action. At times it would take too 

 long for the brain to act, so the body needs a quicker 

 governing power to supply its needs or protect it from 

 injury. If the finger touches a hot object, the brain be- 

 comes conscious of the burn and of the movement of the 

 finger at about the same time. There are also many acts 

 that are performed so often that the spinal cord acquires 

 the habit of sending back the appropriate impulses. Thus 

 muscles and other organs may be controlled and the brain 

 be relieved to attend to other duties. 



543. The same motor cells of the spinal cord that pro- 

 duce motion in a part, also exercise an oversight that con- 

 trols the growth and nutrition of the cells in that part. 

 The impulses from the spinal cells furnish a constant 

 stimulus to growth and repair. 



544. Examples of Reflex Action. A crumb entering the 

 larynx brings on a fit of coughing. A dash of cold water 

 makes us hold our breath. A pinch of snuff or pepper 

 causes us to sneeze. If the foot of a sleeping person is 

 tickled, he will kick ; if a fly settles on his face, he will 

 brush it off. Sometimes a person in an unconscious con- 

 dition will drink a cup of water if it is placed to the lips. 



545. If the back is broken, reflex actions persist although 

 all sensation and volition concerning the parts below the 



