September 8, 1904] 



NA TURE 



463 



flexion of its own leg and extension of the opposite. In 

 numerous instances reflex contraction of one set of muscles 

 is accompanied by reflex relaxation of their antagonists. 



counted. We thus arrive at the following reflex chain for 

 the scratch reflex : (i.) The receptive neurone (Fig. i B, sa), 

 from the skin to the spinal grey matter of ihp corresponding 

 spinal segment in the shoulder. iliis 

 is the exclusive or private path of the 

 arc. (ii.) The long descending proprio- 

 spinal neurone (Fig. i B, pa), from the 

 shoulder segment to the grey matter of 

 leg segments, (iii.) The motor neurone 

 (Fig. I B, Fc), from the spinal segment 

 of the leg to the flexor muscles. This 

 last is the final common path. The 

 chain thus consists of three neurones. 

 It enters the grey matter twice, that 

 is, it has two neuronic junctions, 

 two synapses. It is a disynaptic 

 arc. 



Now if, while stimulation of the skin 

 of the shoulder is evoking the scratch 

 reflex, the skin of the hind foot is 

 stimulated (Fig. 2), the scratching is 

 arrested. Stimulation of the skin of 

 the hind foot by any of various stimuli 

 that have the character of threatening 

 the part with damage causes the leg to 

 be flexed, drawing the foot up. This 

 reflex response to noxious stimuli of the 

 foot is one of great potency. The draw- 

 ing up of the foot is effected by strong 

 tonic contraction of the flexors of ankle, 

 knee, and hip. In this reaction the 

 reflex arc is (i.) the receptive neurone 

 (Fig. I B, l) (nociceptive) from the foot 

 to the spinal segment, (ii.) perhaps a 

 short intraspinal neurone, and (iii.) the 

 motor neurone (Fig. i B, fc) to the 

 flexor muscle, e.g. of hip. Here, there- 

 fore, we have an arc which embouches 

 into the same final common path as sa. 

 The motor neurone Fc is a path common 

 to it and to the scratch reflex arcs ; both 

 arcs employ the same effector organ, a 

 hip flexor. And, as you see, a condition 

 for one reflex is the absence of the 

 other. 



The channels for both reflexes finally 

 embouch upon the same common path. 

 The flexor effect specific to each differs 

 strikingly in the two cases. In the 

 scratch reflex the flexor effect is an 

 intermittent contraction of the muscle, 

 in the nociceptive reflex it is steady and 

 maintained. The accompanying tracing 

 (Fig. 2) shows the result of conflict 

 between the two reflexes. The one 

 reflex displaces the other from the 

 common path. There is no com- 

 promise. The scratch reflex is set 

 aside by that of the nociceptive arc from 

 the foot. The stimulation which pre- 

 viously sufficed to evoke the scratch 

 reflex is no longer effective, though it 

 is continued all the time. But when the 

 stimulation of the foot is discontinued 

 the scratch reflex returns. In that 

 respect, although there is no enforced 

 inactivity, there is inhibition. There is 

 interference between the two reflexes, 

 and the one is inhibited by the other. 

 Though there is no cessation of activity 

 in the motor neurone, one form of 

 activity that was being impressed upon 

 it is cut out and another takes its place. 

 A stimulation of the foot too weak to 

 cause more than a minimal reflex move- 

 ment will often suffice to completely 

 interrupt or cut short, or prevent onset 

 of, the scratch reflex. 



Suppose, again, during the scratch reflex, stimuli applied I The antagonistic muscle is thrown out of action. If, when 

 to the foot, not of the scratching but of the opposite side the left leg is executing the scratch reflex, the right foot 

 (Fig. I B, r). Stimulation (nociceptive) of the foot causes | is stimulated, the scratching, involving as it does the left 



NO. 18 I 9, VOL. 70] 



, ;;. — Interference of the reflex from the skin of the opposite foot with I 

 FC, the flexor inuscle of the left hip (Fig. i B, Fc) u, the signal line the notch in which 

 marks the beginning, continuance, and conclusion of a skin stimulation of the rigbtfoot 

 (Fig. B, r). s, signal line similarly marking the period of stimulation of the skin of 

 the left shoulder (Fig. i B, sa). The ability of stimulus s to produce the scratch reflex 

 takes effect only on concluding, stimulus r; that is, s obtains connection with the ^«a/ 

 common path (the motor neurone of the flexor muscle) only on r's relinquishing it. Stimulus 

 R, while excluding s from FC, causes slight contraction of FC's antagonist, and coincident 

 slight relaxation of fc itself. Time in fifths of seconds. Read from left to right. 



