REFLEXES 99 



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 

 much. more readily at some times than at others. Stimu- 

 lants like coffee and strychnin make it easier to evoke 

 reactions. Depressants like alcohol in considerable 

 doses, or the anesthetics, suppress the reflexes in a 

 definite order. We say that the stimulants lower and 

 the narcotics raise the central resistance and we have 

 reason to think that the changes are most marked at 

 the synapses. If the adjustments are to be made for 

 the best interests of the organism the average resistance 

 of the nervous system must be neither too high nor 

 too low. If it is too high the stimuli will fail of due effect, 

 while if it is too low the responses will be exaggerated 

 and disorderly. 



A person who is sensitive to coffee may "jump" at 

 the slamming of a door when he is under the influence 

 of the stimulant. This is an unprofitable reaction. 

 A man poisoned with strychnin may be thrown into 

 exhausting convulsions by very slight causes. These 

 are examples of reflex action but they no longer show 

 it as ministering to the welfare of the body. At the 

 other extreme we have the grossly intoxicated man who 



