A text-book of physiology . tinct; and, if the shocks berepeated, a series of rhythmically-recurring, separate, simple con-tractions may be obtained. If, however, the interval between twoshocks be made short, — if, for instance, it be made only just longenough to allow the first contraction to have passed its maximumbefore the latent period of the second is over, — the curves of thetwo contractions will bear some such relation to each other asthat shewn in Fig. 14. It will be observed that the second curveis almost in all respects like the first, except that it starts, so tospeak, from the fir
A text-book of physiology . tinct; and, if the shocks berepeated, a series of rhythmically-recurring, separate, simple con-tractions may be obtained. If, however, the interval between twoshocks be made short, — if, for instance, it be made only just longenough to allow the first contraction to have passed its maximumbefore the latent period of the second is over, — the curves of thetwo contractions will bear some such relation to each other asthat shewn in Fig. 14. It will be observed that the second curveis almost in all respects like the first, except that it starts, so tospeak, from the first curve instead of from the base-line. Thesecond nervous impulse has acted on the already contractedmuscle, and made it contract again just as it would have done ifthere had been no first impulse, and the muscle had been at two contractions are added together, and the lever is raisednearly double the height it would have been by either alone. Ifin the same way a third shock follows the second at a sufficiently. Fig. 14. Tracing of a Double Muscle-curve. While the muscle (gastrocnemius of frog) was engaged in the first contraction(whose complete course, had nothing intervened, is indicated by the dotted line), asecond induction-shock was thrown in, at such a time that the secoud contractionbegan just as the first was beginning to decline. The second curve is seen to startfrom the first, as does the first from the base-line. short interval, a third curve is piled on the top of the second ; thesame with a fourth, and so on. A more or less similar resultwould occur if the second contraction began at another phaseof the first. The combined effect is, of course, greatest whenthe second contraction begins at the maximum of the first, beingless both before and afterwards. Hence, the result of a repetition of shocks will depend largelyon the rate of repetition. If, as in Fig. 15, the shocks follow eachother so slowly that one contraction is over, or almost over, beforethe
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