TRANSACTIONS OF SECTION I. 733 
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 drawing 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. 1 B, L) (nociceptive) from the foot to the spinal segment, (ii) perhaps 
a short intraspinal neurone, and (iii) the motor neurone (fig. 1 B, rc) to the 
flexor muscle, eg., of hip. Here, therefore, we have an are 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 noci- 
ceptive 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 compromise. The scratch 
reflex is set aside by that of the nociceptive arc from the foot. ‘The stimulation 
which previously 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 movement will often suffice to completely interrupt or cut short, 
or prevent onset of, the scratch reflex. 
Suppose, again, during the scratch reflex, stimuli applied to the foot, not of the 
scratching but of the opposite side (fig. 1 B, R). Stimulation (nociceptive) of the 
foot causes 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. The antagonistic muscle is thrown out of action. If, when 
the left leg is executing the scratch reflex, the right foot is stimulated, the scratching, 
involving as it does the left leg’s flexors, is cut short concomitantly with or 
ery to the entrance into contraction of their antagonists, the left extensors. 
ig. 8 shows a record of this. This inhibition of the flexor scratching move- 
ment occurs sometimes when the contraction of the extensors is minimal or 
hardly perceptible (fig. 8). As before, the inhibition may temporarily interrupt a 
reflex or may delay its onset, or simply cut it short, the result depending on the 
time relations of the applications of the stimuli to the conflicting arcs. 
It is obvious from this that the final common path, rc, to the flexor muscle 
can be controlled by, in addition to the before-mentioned arcs, others that actuate 
the extensor muscles, for it can be thrown out of action by them. ‘The final path, 
FC, is therefore common to the reflex arcs, not only from the same-side foot 
(ie. 1 B,t) and shoulder skin (fig. 1 B, sa, 8), but also to arcs from the opposite 
oot (fig. 1 B, n), in the sense that it is in the grasp of all of them. In this last 
case we have a conflict for the mastery of a common path, not, as in the previous 
instance, between two arcs both of which use the path in a pressor manner 
although differently, but between two arcs that, though both of them control 
the path, control it differently, one in a pressor manner heightening its activity, 
the other in a depressor manner lowering or suppressing its activity. 
I said that the scratch reflex is unilateral. If the right shoulder be stimulated, 
the right hind-leg scratches; if the left shoulder be stimulated, the left hind- 
leg scratches. If both shoulders be stimulated at the same time, one or the 
