Diagram 23.—Contractions with Two Stimuli at Different Intervals of Time.
Muscle is thrown into a state of contraction by an impulse reaching it from a nerve, but it contracts quite as readily if excited directly by a mechanical or electrical shock. A second shock causes a second contraction, or, if the muscle is still in a state of contraction owing to the first, causes it to contract still more. ([See Diagram 23.]) If a number of stimuli are applied to a muscle in such rapid succession that the effect of the preceding one has not passed off by the time the next arrives, it will contract as far as possible, and remain contracted—a state known as tetanus. ([See Diagram 24.]) A muscle is therefore kept in a state of contraction by a continuous nervous effort, not arranged and then left contracted.
Diagram 24.—Tetanus.
Diagram 25.—Fatigue Curves.
Fast drum: a, point of stimulation. Every tenth contraction recorded.
Diagram 26.—Effect of Fatigue on Muscular Contraction.