100 HUMAN PHYSIOLOGY 



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 

 cannot be roused to meet an emergency. Central re- 

 sistance must be of a medium grade if all is to go well 

 with the individual. 



A subject who is described as " nervous" is one whose 

 resistance is more or less lowered from the normal. He 

 will be unreasonably disturbed by external conditions 

 which a more robust person readily ignores and he will 

 tire himself by countless reactions which were better 

 not carried out. Low resistance is seen to be opposed 

 to conservation of one's resources. It is undoubtedly 

 more common as a constitutional fault than a high 

 resistance which must be indicated by stolidity of 

 temperament. 



Resistance and Coordination. Reflex acts are, in 

 most cases, clearly coordinated. That is to say, a 

 number of muscles are used in their execution and their 

 effects are combined for a common end. Some muscles 



