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A TEXTBOOK OF PHYSIOLOGY 



exert a valve-like action, so that conduction manifests itself in one direc- 

 tion only . The synapses also offer a varying degree of resistance to the 

 impulse, so that, generally, a reflex is localized ; but under the influence 

 of certain poisons, such as strychnine, a slight stimulus will evoke tonic 

 muscular spasms over the whole body. 



3. In the reflex arc there is a summation of stimuli ; a succession 

 of stimuli, each of an intensity insufficient to evoke a response when 

 applied alone, will eventually provoke a response. 



4. Excessive stimulation leads to a fatigue in the synapses, which 

 occurs principally in the connection between the nerve and the effector 

 organ. 



I'm. 406. SCKATCH REFLEX EVOKED BY A RELATIVELY FEEBLE STIMULATION AM> 



DISAPPEARING UNDER THAT STIMULATION. 



(hi increasing the intensity of the stimulus the reflex reappears. It does not ro;ippcar 

 on revetting to the original intensity of stimulation. Time in seconds. (From 

 Sherrington s " Intogrative Action of the Nervous System," by permission of 

 Yale University Press and Messrs. Constable arrl Co., Ltd.) 



5. When fatigue is not evoked by too frequent transmission of 

 an impulse, it is found that subsequent impulses call forth a reaction 

 more easily. This "facilitation," as it is termed, is really the basis 

 of habit. By facilitation, good habits, if sufficiently repeated, obtain 

 preference over bad ones, if these are not often repeated. Training 

 consists largely in the proper adjustment of the necessary reflexes, 

 and in this the law of facilitation takes a great part. The whole of 

 education consists in the obtaining of facilitation for fit paths, and in 

 the inhibition of unfit ones. 



(5. Such inhibition is a l:n\ of reflex action. Only one impulse 

 can have possession of the final common path at the same time, unless 



