98 HUMAN PHYSIOLOGY 



the gray matter. The manner of their ending is some- 

 what in doubt but in all probability they lead to 

 the dendrites of various cells. Impulses led into the 

 nervous system by afferent fibers can thus be carried 

 to the receiving processes of those nerve cells which 

 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 

 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 

 which come out to call the muscles into play are not 

 the same impulses that went in a moment before. They 

 are supposed to have had their origin in particular acts 

 or " discharges" of the motor cells under the influence 

 of stimuli applied to the dendrites. Whichever concep- 

 tion finally prevails we must believe that there are paths 

 of easy transmission between certain afferent and certain 

 efferent channels. It is these paths which make the 

 reflexes under most conditions so advantageous. 



Synapses. The term synapse is used to denote the 

 junction of two units in the nervous system. A synapse 

 is said to exist where the fine terminations of an efferent 

 fiber are joined to the dendrifes of a motor cell. It is 

 not certain how far this meeting of the two structures 

 is a literal joining but it is a junction from the stand- 

 point of functional capacity. The nervous system com- 

 prises an innumerable host of elements each influenced 

 by a number of others and influencing still different 

 ones in its turn. We say that the communication is 

 through synapses and we picture them as in the dia- 

 grams that accompany this chapter. We have at least 



