THE PHENOMENA OF MUSCLE AND NERVE. 



73 



Tetanic Contraction. 



49. If a single induction-shock be followed at a certain interval by a 

 second shock of the same strength, the first simple contraction will be fol- 

 lowed by a second simple contraction, both contractions being separate and 

 distinct ; and if the shocks be repeated a series of rhythmically recurring 

 separate simple contractions may be obtained. If, however, the interval 

 between two shocks be made short, for instance, only just long enough to 

 allow the first contraction to have passed its maximum before the latent 

 period of the second is over, the curves of the two contractions will bear 

 some such relation to each other as that shown in Fig. 23. It will be 



FIG. 23. 



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), a second induction-shock was thrown in, at such a time that the second contraction began 

 just as the first was beginning to decline. The second curve is seen to start from the first, as 

 does the first from the base-line. 



observed that the second curve is almost in all respects like the first except 

 that it starts, so to speak, from the first curve instead of from the base-line. 



The second nervous impulse has acted on the already contracted muscle, 

 and made it contract again just as it would have done if there had been no 

 first impulse and the muscle had been at rest. The two contractions are 

 added together and the lever is raised nearly double the height it would 

 have been by either alone. If in the same way a third shock follows the 

 second at a sufficiently short interval, a third curve is piled on top of the 

 second ; the same with a fourth, and so on. A more or less similar result 

 would occur if the second contraction began at another phase of the first. 

 The combined effect is, of course, greatest when the second contraction begins 

 at the maximum of the first, being less both before and afterward. 



Hence, the result of a repetition of shocks will depend largely on the 

 rate of repetition. If, as in Fig. 24, the shocks follow each other so slowly 



FIG. 24. 



Muscle-curve. Single Induction-shock Repeated Slowly. 



that one contraction is over, or almost over, before the next begins, each 



t contraction will be distinct, or nearly distinct, and there will be little or no 

 combined effect. 

 If, however, the shocks be repeated more rapidly, as in Fig. 25, each sue- 



