REFLEXES 99 



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 afferent 

 fiber are joined to the dendrites 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 

 the same right to do this that the organic chemist has 

 to his molecular formulae. 



There are two or three properties which we associate 

 with synapses and we cannot reason clearly about the 

 working of the nervous system excepting as we keep 

 our faith in these characters. First of all, we believe 

 that synapses transmit effects in only one direction. 

 They have been described as having a "valve action." 

 A nerve cell, therefore, cannot affect the cells which 

 affect it but always plays upon others and these in turn 

 must send impulses over new courses. The student 

 is likely to think that it is the nerve fibers which have 

 this significant polarity but it is not; the fibers can con- 

 duct in both directions. It is the synapses which refuse 

 to reverse the transmission. 



Another property of the synapse is that of variable 

 resistance. It is clear that the nervous system reacts 



