REFLEX ACTION 585 



applying the electrodes lightly to its surface or by pinching the skin 

 with a pair of forceps. If more convenient, the foot may be immersed 

 in a weak solution of acetic acid. In either case, the stimulus pres- 

 ently gives rise to a contraction of the muscles of the corresponding 

 leg which results in its withdrawal from the seat of the stimulation. 

 If electrical stimuli are employed, the student should sharply dif- 

 ferentiate between the direct effect of the current as evinced by a 

 twisting of the toes, and the reflex effect, consisting in a more general 

 muscular action and the actual withdrawal of the leg. It should also 

 be observed that the stimulus is applied in this case to the tactile 

 receptors of the skin and that the response consists in a seemingly 

 purposeful movement. This reaction is similar to the one occurring 

 in us whenever our integument is suddenly stimulated, say, in a me- 

 chanical way. The subsequent contraction of the musculature 

 necessary to perform the protective movements corresponding to 

 this stimulus, is non-volitional, i.e., the response is had without that 

 its character can be changed by the will. In many cases, of course, 

 we obtain a perfect sensory concept of this act, but the sensorium is 

 activated in this instance after the completion of the primary act 

 and cannot, therefore, influence the latter in any way. But if this 

 cutaneous stimulus is first received in consciousness and is there 

 subjected to volition, the resulting response ceases to be a reflex and 

 becomes a complex reaction. 



Reflex Time. Reflex Fatigue. — The time elapsing between 

 the moment of the application of the stimulus and the beginning of the 

 response, is known as the reflex time. As is easily observed in the re- 

 flex frog, this factor varies with the strength of the stimulus and 

 •the irritability of the nervous system. It has been stated above that 

 a series of slight stimuli are more effective than one strong one, and 

 that the best results are obtained if the receptor itself is stimulated 

 and not the afferent path leading from it. Thus, if a tetanic current 

 of very moderate strength is applied to the sole of the foot of the reflex 

 frog, a perfectly definite muscular response is evoked, consisting in a 

 seemingly purposive removal of the foot from the seat of stimulation. 

 There is, of course, no intent present, because this result "is wholly 

 dependent upon the general structural arrangement of the leg. If 

 the intensity of the stimulus is now increased, the response follows with 

 the same mechanical precision, but at a somewhat earlier moment. 

 In other words, the reflex time is inversely proportional to the strength 

 of the stimulus. It is also possible to vary the reflex time by altering 

 the receptive power or irritability of the nervous system. Depressive 

 agents, such as the narcotics, lengthen it, while stimulants, such as 

 strychnin, oxygen, warmth, etc., shorten it. 



We are thus justified in applying to reflex action such characteriza- 

 tions as ''subminimal reflex stimulus," meaning thereby the stimulus 

 which just fails to elicit a reflex response, or ''reflex threshold," indi- 

 cating thereby the stimulus which is just becoming effective. It 



