32 PHYSIOLOGY. 



The Parts Essential to Reflex Action of the Spinal 

 Cord : - 



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. 



5. Working organ, as muscle or gland. 



Phases of Reflex Action. In the above experiment 

 on the frog 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. 



- Nerve-Cell 



Afferent Fiber x Efferent Fiber 



Sk -- ^-Muscle 



Fig. 14. Scheme of Reflex Arc. 



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

 gray matter in the cord. 



4. Sending back a nerve impulse 



5. Along an efferent fiber, or fibers, to 



6. Muscles which shorten and move the foot. 



Importance of Reflex Action. It is important that 

 we understand the nature of reflex action, for very many 

 of the processes of the body are regulated by it. Not 

 only the more manifest motions, such as winking when 



