As to the intimate nature of the mechanism which thus, by summation or by interference, gives co-ordination where neurones converge upon a common path, it is difficult to surmise. In the central nervous system of vertebrates, afferent neurones A and B in their convergence toward and impingement upon another neurone Z, towards which they conduct, do not make any lateral connection directly one with the other—at least, there seems no clear evidence that they do. It seems, then, that the only structural link between A and B is neurone Z itself. Z itself should therefore be the field of coalition of A and B if they transmit "allied" reflexes.
Whatever be the nature of the physiological process occur ring between the competing reflexes for dominance over the com mon path, the issue of their competition, namely, the deter mination of which one of the competing arcs shall for the time being reign over the common path, is largely conditioned by four factors. These are spinal induction, relative fatigue, rela tive intensity of stimulus, and the functional species of the reflex.
I. Induction occurs in two forms, one of which has been named immediate induction. The stimulus which excites a reflex tends by central spread to facilitate and lower the threshold for re flexes allied to that which it particularly excites. A constella tion of reflexes thus tends to be formed which reinforce each other, so that a reflex figure results. If the prepotent stimulus shifts, allied arcs are by the induction particularly prepared to be responsive to it or to a similar stimulus.
Immediate induction only occurs between allied reflexes. Its tendency in the competition between afferent arcs is to fortify the reflex just established, or, if transition occur, to favour transition to an allied reflex. Immediate induction seems to obtain with highest intensity at the outset of a reflex, or at least near its commencement.
The other form of spinal induction is successive induction. It is in several ways the reverse of the preceding. If the crossed-extension reflex of the limb of the "spinal" dog be elicited at regular intervals, say once a minute, by a carefully adjusted electrical stimulus of defined duration and intensity, the resulting reflex movements are repeated each time with much constancy of character, amplitude and duration. If in
one of the intervals a strong prolonged (e.g., 3o") flexion-reflex is reduced from the limb yielding the extensor-reflex movement, the latter reflex is found intensified after the intercurrent flexion reflex. The intercalated flexion-reflex lowers the threshold for the of tercoming extension-reflexes, and especially increases their after-discharge. This effect may endure, progressively diminish ing, through four or five minutes, as tested by the extensor reflexes at successive intervals. Now, as we have seen, during the flexion-reflex the extensor arcs were inhibited : after the flexion reflex these arcs are in this case evidently in a phase of exalted excitability. The phenomenon presents obvious analogy to vis ual contrast. The exaltation after-effect may ensue with such intensity that simple discontinuance of the stimulus maintain ing one reflex is immediately followed by "spontaneous" ap pearance of the antagonistic reflex.
The so-called "mark-time" reflex of the "spinal" dog is an alternating stepping movement of the hind limbs which occurs on holding the animal up so that its limbs hang pendent. It can be inhibited by stimulating the skin of the tail. On cessa tion of that stimulus the stepping movement sets in more vigor ously and at quicker rate than before. This after-increase might be explicable in either of two ways. It might be due to the mere repose of the reflex centre, the repose so recruiting the centre as to strengthen its subsequent action. But a similar period of repose obtained by simply supporting one limb—which causes cessation of the reflex in both limbs, the stimulus being stretch of the hip-flexors under gravity—is not followed by after-increase of the reflex, or the after-increase might result from the inhibition being followed by a rebound to super activity. This latter seems to be the case. The after-increase occurs even when both hind limbs are passively lifted from below during the whole duration of the inhibitory stimulus ap plied to the tail. And the reflex inhibition of the knee-extensor by stimulation of the central end of its own nerve is followed by marked rebound to superactivity of the extensor itself. Again, the knee jerk, after being inhibited by stimulation of the hamstring nerve, is more brisk than before the inhibition.