342 



ELEMENTARY PHYSIOLOGY 



LESS. 



change thus set up is j)ropagated as a molecular disturb- 

 ance (or impulse) along the sensory (afferent) nerve, a.f. 

 to c, a part of the central nervous system (the spinal 

 cord). The changes which then take place in c result in 

 the setting up of a molecular disturbance in the motor 

 (efferent nerve), e./. , which is conveyed outwards along 

 that nerve to the muscle iH^ usually on the same side of 

 the body. Sometimes the impulse is sent out along a 

 motor nerve to some muscle MP- on the opposite side of 

 the body. 



-- Sp.C. 



Fia. 108.— Diagram to illustrate the Paths of Reflex Action. 



Sp.C, spinal cord. S, some sensory surface ; a./, afferent or sensory 

 nerve ; c, central connection in nervous system ; «./, e.f, efferent or motor 

 fibres ; 3/1, 3/-, muscles. Tlie arrows sliow the directions in which the 

 impulses travel. 



2. Sensations and Consciousness. — A reflex action 

 may take place without our knowing anything about it, 

 and hundreds of such actions are continually going on in 

 our bodies without our being aware of them. But it very 

 frequently happens that we learn that something is going 

 on, when a stimulus affects our afferent nerves, by having 

 what we call a feeling or sensation. We class sensa- 

 tions along with emotions and volitions, and 



