Studies in Animal Locomotion

1. If the actively contracting muscles at the posterior end of an intact, spinal or decapitated eel are subjected to external restraint, a posterior resistance reflex is elicited. This reflex involves very powerful contraction in the muscles lying anteriorly and ipsilaterally to the point of restraint and thereby moves the fish backwards. 2. If the actively contracting muscles at the anterior end of a spinal or decapitated eel are subjected to external restraint (and a persistent stimulus applied simultaneously to the tail), an anterior resistance reflex is elicited. This reflex involves very powerful contraction in the muscles lying posteriorly and ipsilaterally to the point of restraint and thereby moves the fish forwards. 3. The muscular pattern of a posterior resistance reflex is incompatible with that of a forward locomotory rhythm. Sufficiently strong external restraint inhibits a locomotory rhythm; sufficiently strong tactile stimuli (capable, when acting alone, of eliciting a forward locomotory rhythm) rapidly and completely inhibit a posterior resistance reflex. An anterior resistance reflex is slowly and incompletely inhibited by stimuli which evoke a backward locomotory rhythm. 4. If the muscular pattern of a resistance reflex is compatible with that of a locomotory rhythm (e.g. an anterior resistance reflex and a forward locomotory rhythm), the two reflexes are not mutually inhibitory but reinforce each other. 5. The resistance reflexes are used by the intact eel when escaping from a confined environment. It is suggested that they may, at other times, adjust and modify the centrally controlled locomotory rhythm which has been shown to involve only a small proportion of the total musculature.