INTERACTION AMONG REFLEXES 825 



Important among these is fatigue of the reflex arc for a particular kind 

 of stimulus. Many characteristics differentiate reflex fatigue from fatigue 

 of a nerve as observed in an isolated nerve-muscle preparation. The 

 most important of these distinguishing features are as follows: (1) 

 The fatigue comes on intermittently; thus, when the flexion reflex is 

 persistently elicited, the first sign of fatigue is an irregular decline in 

 the flexion movement followed by its entire disappearance for a short 

 time. These lapses become more and more frequent, until at last com- 

 plete fatigue sets in and no flexion occurs. (2) Reflex fatigue soon 

 passes off. (3) It appears earlier for weak than for strong stimuli. (4) 

 The movement produced by the reflex action may also change in character 

 during reflex fatigue; thus, the beat of the scratch reflex may become 

 slower and less steady and the foot be less accurately directed to the 

 spot stimulated. The locus of the fatigue in the reflex arc can not 

 be the motor neuron itself, for, after this has been completely fatigued 

 by stimulation of the scratch area, the same muscles may quite readily 

 be thrown into a perfectly normal flexion reflex by stimulation of the 

 skin of the hind leg. 



It is evident that, when two reflexes are competing with each other 

 for possession of the same final common path, the one that becomes fa- 

 tigued will be mastered by the other, especially since at the same time 

 successive induction will be well developed. Thus, ordinarily the scratch 

 reflex is much less readily elicited than the flexion reflex, and if both 

 are excited at the same time the latter will prevail; but if the flexion re- 

 flex is kept up until it shows signs of fatigue, then by simultaneous 

 excitation of both reflexes the scratch reflex will obtain the mastery. 



Another important factor is the relative strength of the competing 

 impulses. This depends partly on the nature of the reflex and partly on the 

 intensity of the stimulus. Regarding the nature of the reflex, it is important 

 to remember that crossed reflexes are usually less easily obtained than homo- 

 lateral ones, but ' of still greater importance is the species of reflex 

 that is, whether flexion, scratch, extension, etc. The reflex movements 

 produced by nocuous stimuli (nociceptive reflexes) always take precedence 

 of those produced by other kinds of stimuli; or, to put it in other 

 words, "nociceptive reflexes are prepotent in their occupancy of the 

 final common path" (Sherrington 18 ). 



The best known example of a nociceptive reflex is the flexion reflex. 

 Its movement is one produced with the intention of removing the stimu- 

 lated portion of 'the body from the source of the stimulus, all stimuli which 

 produce it being such as would elicit pain in an intact animal, or if per- 

 sisted in cause some damage to the skin. In contrast to such nociceptive 

 reflexes we may take those which are concerned in maintaining the cen- 



