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other; three shocks, closely following, cause three contractions, still further blending, and so on, until the shocks come so fast that the individual contractions are all fused together to form a curve. Tetanus, therefore, is not one contraction, but a state brought about by the Fig. 35.—Curve showing the production of tetanus. a to b, individual contractions; b to c, muscle now tetanic. The slope of line from b to c shows that muscle is becoming fatigued; e, indicates moment when induction shocks stopped; c d, slow relaxation. fusion or summation of many contractions. With the frog's muscle, about fifteen shocks per second are sufficient to cause tetanus; the muscles of a tortoise require only two to three; the muscles of a rabbit from ten to twenty; the muscles of birds about seventy; and the muscles of insects over three hundred per second. If the number of shocks is very much increased, even to as many as twenty-four thousand per second, tetanus is still produced.