If instead of a single stimulation a number of stimuli rapidly succeeding each other are applied either directly to the muscle itself or to its motor nerve, we get, in place of a single contraction, a continued contraction or tetanus. As this is due to a fresh contraction of the muscle occurring before the previous one has had time to relax, it is evident that the number of stimuli requisite to produce this will vary with the length of each single contraction in a muscle. Thus in the muscles of the tortoise, which contract and relax very slowly, tetanus may be produced by 3 stimuli per second, while in the white muscles of rabbits 20 may be necessary, and in some muscles of birds 70 stimuli per second are insufficient. It has been said that with as rapid stimuli as 250 per second the tetanus ceases, and after a single initial contraction a muscle goes to rest just as if a constant instead of an interrupted current had been used. Kro-necker and Stirling have shown that, with no less than 22,000 interruptions per second, tetanus is still obtained; but when such extremely rapid stimuli are applied, the muscle still contracts about the ordinary rate of 20 per second; and this is also the case when chemical stimuli are applied to the nerve, or when the muscle is irritated by the nerve-centres, either voluntarily or by artificial stimuli applied to them. It seems therefore probable that the number of contractions of the muscles in tetanus are not due to the number of stimuli sent down from the nerve centres, but that the rate is determined either by the ends of the nerve in the muscle or by the muscle itself.1

The form of a tetanus curve may be modified very considerably by the action of drugs : thus substances which diminish the contractile power of muscle cause the tetanus curve to fall very rapidly notwithstanding the continued application of stimuli either to the muscle itself or to its nerve (vide Ammonia).