The reflex from the right foot evokes at the opposite (left) knee extension ; in doing this it causes steady excitation of extensor neurones of that knee and steadily inhibits the flexor neurones.
But the scratch-reflex causes rhythmic excitation of the flexor neurones. Therefore these flexor neurones in this conflict lie as a final common path under the influence of two antagonistic reflexes, one of which would excite them to rhythmical discharge four times a second, while the other would continually repress all discharge in them.
In all these forms of interference there is a competition, as it were, between the excitatory stimulus used for the one reflex and the excitatory stimulus for the other. Both stimuli are in progress together, and the one in taking effect precludes the other's taking effect as far as the final common path is concerned.
Again, if, while stimulation of the skin of the shoulder is evoking the scratch-reflex, the skin of the hind foot of the same side is stimulated, the scratching may be 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 by steady maintained contrac tion of the flexors of the ankle, knee and hip. Here, therefore, there is an arc which embouches into a final path, common to it and to the scratch-reflex arc ; both these arcs employ the same effector organ, namely, the knee-flexor, and employ it by the common medium of the final path FC. But though the channels for both reflexes embouch upon the same final common path, the excitatory flexor effect specific to each differs strikingly in the two cases. In the scratch-reflex the flexor effect is an inter
mittent effect ; in the noci-ceptive flexion-reflex the flexor effect is steady and maintained. The scratch-reflex is set aside by that of the noci-ceptive arc from the homonymous foot.
The stimulation which previously sufficed to provoke 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 en forced inactivity, there is an interference which is tantamount to, if not the same thing as, inhibition. Though there is no cessation of activity in the motor neurone one form of activity that was being impressed upon it is cut short and another takes its place.
A stimulation of the foot too weak to cause more than a minimal reflex will often completely suffice to interrupt, or cut short, or prevent onset of, the scratch-reflex.
The kernel of the interference between the homonymous flex ion-reflex and the scratch-reflex is that both employ the same final common path FC to different effect—just as in the inter ference between the crossed extension-reflex and the scratch reflex. Evidently, the homonymous flexion-reflex and the crossed extension-reflex both use the same final common path FC. And they use it to different effect. The motor neurone to the flexor of the knee being taken as a representative of the final common path, the homonymous flexion-reflex inhibits it from discharg ing. Hence if, while the direct flexion-reflex is in progress, the crossed foot is stimulated, the reflex of the knee-flexor is in hibited. The crossed extension reflex therefore inhibits not only the scratch-reflex, but also the homonymous flexion-reflex.
Further, in all these interferences between reflexes the direc tion taken by the inhibition is reversible. Thus, the scratch-reflex is not only liable to be inhibited by, but is itself able to inhibit either the homonymous flexion-reflex or the crossed extension reflex; the homonymous flexion-reflex is not only capable of being inhibited by the crossed extension-reflex, but conversely in its turn can inhibit the crossed extension-reflex. These inter ferences are therefore reversible in direction. Certain conditions determine which reflex among two or more competing ones shall obtain mastery over the final common path and thus obtain expression.