LONDON. Royal Society, Mar. 24.—Sybil Cooper, D. E. Denny Brown, and Sir Charles S. Sherrington: Interaction between ipsilateral spinal reflexes acting on the flexor muscles of the hind-limb. The contraction evoked by reflexes exciting the same muscle when they are concurrent falls largely below the sum of the individual effects which they exert when apart. The effect of one of a pair of concurrent reflexes may default totally, i.e. be totally occluded. Such I occluding ‘interaction is quite different from the inhibitory interaction of ‘ antagonistic ’ reflexes. Its explanation seems to be that, at some structure impinged upon in common by the ‘allied ’ reflexes, (1) tetanic activation from one source precludes concurrent activation by a second, and (2) is not disturbed by the convergent activity of a second. The occlusion is a measure of the convergent overlap of ‘allied’ reflexes upon ‘motor units’ held in common. The occluded contraction emerges from occlusion without pause and step for step as the occluding activation subsides. Each individual afferent excites a reflex contraction which is of a pattern specific to that particular afferent.