INTl.KM PION AMONG KEFLEX] 825 



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

 of stimulus. Many characteristics differentiate reflex fatigue from fat:. 

 of a nerve as observed in an isolated nerve-muscle preparation, 'i 



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 dgn 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 Btrong stimuli. 4 

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

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

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

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

 be the motor neuron itself, for, after this has been completely fat i lt 

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

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

 shin 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 <>ne that becomes fa- 

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

 successive induction will be well developed. Tims, ordinarily tl itch 



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 tht competing 

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

 intensity of the stimulus. Regarding the nature of tJir refii X, 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 refl- 



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



produced by nocuous stimuli [nociceptive refl< xes always tal ence 



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

 words, "nociceptive reflexes are prepotent in their occupancy 

 final common path" (Sherrington 1 *). 



The best known example of a »• is the flexion reflex. 



Its movement is one produced with the intention of removi] mu- 



tated portion of the body from the source of the stimulus, all stimuli which 



produce it being such as would elicit pain in an intacl animal, or if | 

 sisted in cause some damage to the skin it- Buch nociceptive 



reflexes we may take those which an ncerned in maintaining the cen- 



