REFLEX TIMES IN THE SOUTH AFRICAN CLAWED FROG. THIRD NOTE

The reflex in the hind limbs of the spinal Xenopus, in response to a single break induction shock applied to the web of one foot, has been recorded by two string galvanometers recording simultaneously on one photographic plate. The reflex consists of homonymous flexion and heteronymous extension. The electrical responses consist of polyphasic or oscillatory variations, of which there are two types: I uncomplicated, due to a single activation of the muscle through its motor nerve; and II complicated, due to successive activations through the nerve and showing evidence of interference. Evidence is not found in this reflex of reciprocal innervation. Both extensor and flexor muscles yield electrical responses in both limbs. In the same‐side limb reflex activity starts in the knee‐flexor muscle —gracilis—but the extensor muscle—cruralis—becomes active less than a thousandth of a second later. The gastrocnemius follows the thigh muscles after an interval corresponding to the delay of a set of synapses in the spinal centre plus an interval due to its longer nerve path. In the limb opposite to the stimulated foot activity appears simultaneously in the extensor and flexor of the knee, and this follows the onset of activity in the same‐side limb by an interval—the cross‐over time—corresponding to the delay of a set of synapses. The outflow of excitation to the gastrocnemius acting heteronymously is simultaneous with that to the thigh muscles, so that the crossed gastrocnemius follows the crossed cruralis and gracilis by only a short interval due to its longer nerve path. The outflow of excitation to the tibialis anticus, homonymous, and heteronymous is simultaneous with that to the thigh muscles. The delay of a synapse‐set at 14°C. is about 6 σ (thousandths of a second). It is suggested that this is not the delay at a single synapse. Analysis of the reflex delays of the thigh muscles shows that one synapse‐set is involved in the intracentral conduction of the homonymous reflex, and two synapse‐sets are involved in the heteronymous reflex. While it is the general rule that the effective excitation for the crossed reflex passes through the centre by a path containing two synapse‐sets, the presence of a preliminary deflection indicates that a slight excitation is transmitted by a shorter path from one side to the other. Certain variations from the usual time relations are found to occur.