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HUMAN PHYSIOLOGY 



preside over the muscles. The linked system of afferent 

 and efferent units is said to constitute a reflex arc. 

 Some investigators think that the dendrites are con- 

 tinuous with a network everywhere present in the 

 gray matter and that the fine terminal branches of 



FIG. 26. The upper figure suggests the simplest possible basis for 

 reflex action. An afferent fiber, entering through a dorsal root, comes 

 into relation with the cell-body giving rise to a motor fiber. This leaves 

 the cord by way of the ventral root. 



The lower figure shows the branching form of the afferent fiber and 

 the possibility that it may play upon a number of motor cells at differ- 

 ent levels. 



the afferent fibers also run into this network. Such a 

 view permits the belief that an impulse may wind its 

 way through the gray matter between its place of en- 

 trance and the place of its ultimate escape without any 

 interruption. 



It has been held more generally that the impulses 



