7G 



TETANUS. 



[Book i. 



contractions being separate and distinct; and, if the shocks be 

 repeated, a series of rhythmically-recurring, separate, simple con- 

 tractions may be obtained. If, however, the interval between two 

 shocks be made short, — if, for instance, it be made 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 shewn in Fig. 13. It will be 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 



Fig. 13. 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. 



short interval, a third curve is piled on the 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 afterwards. 



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

 on the rate of repetition. If, as in Fig. 14, the shocks follow each 

 other so slowly that one contraction is over, or almost over, before 

 the next begins, each contraction will be distinct, or nearly distinct, 

 and there will be little or no combined effect. 



Fig. 14. Muscle-curve. Single Imutction-shock repeated slowly. 



