The phenomena of Magnus and De Kleijn1 occur with striking regularity and in a pronounced degree in the so-called decerebrate state of both animal and man; that is, in the animal after ablation of both hemispheres of the cerebrum and in man when disease of the cerebrum is of such extent as to eliminate more or less completely its influence on the periphery. In this state the limbs, always the anterior, less constantly the posterior, change posture as a result of passive displacement of the head. These reactions vary with the type of head displacement. They may be bilaterally identical, the limbs on both sides undergoing flexion or extension, or the limbs on one side undergo flexion, while those on the other side undergo extension. The first type of reaction may be one of two: the anterior limbs may flex and the posterior extend or vice versa; or all