42 ESSENTIALS OF REFLEX ACTION. 



1. 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 (usually many). 



5. Working organ, as muscle or gland. 



In the experiment performed the steps in order were : 



1. Stimulation of the nerve endings in the skin of the toe. 



2. Passage of a nerve impulse up the afferent fibers to 

 the spinal cord. 



3. Reception of the impulse by a cell, or cells, of the gray 

 matter in the cord. 



4. Sending back nerve impulses 



5. Along efferent fibers of several nerves, to 



6. Muscles which shorten and move the foot. 



It is important that we understand the nature of reflex 

 action, for very many of the processes of the body are regu- 

 lated by it. Not only the more manifest motions, such as 

 winking when anything comes quickly toward the eye, dodg- 

 ing, jumping when suddenly touched by anything hot or 

 when pricked by a pin, when startled by a sudden loud noise, 

 making a quick effort to regain one's balance when stumbling 

 or slipping ; when asleep, brushing away a fly or drawing up 

 the foot when it is exposed to the cold not only are all 

 these reflex actions, but it is through reflex action that there 

 are brought about the necessary continual adjustments of the 

 essential processes of life, circulation, respiration, diges- 

 tion. 



Cramp is a spasmodic shortening of the muscles, attended 

 with pain. 



Tetanus (a word also used for lock-jaw) is a spasmodic 

 and continuous shortening of the muscles, causing rigidity of 

 the parts they supply. It is due to the disordered and exces- 

 sive stimulation of the muscles through the nerves. 



