CHAP. IL] THE CONTRACTILE TISSUES. 79 



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 



t 



FIG. 13. TRACING OF A DOUBLE MUSCLE-CURVE. 



While the muscle (gastrocnemius of frog) was engaged in the first contraction 

 hose complete course, had nothing intervened, is indicated by the dotted line), a 

 cond 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 INDUCTION SHOCKS REPEATED SLOWLY. 



If however the shocks be repeated more rapidly, as in Fig. 15, 

 each succeeding contraction will start from some part of the 

 preceding one, and the lever will be raised to a greater height at 

 each contraction. 



FIG. 15. MUSCLE-CURVE. SINGLE INDUCTION SHOCK REPEATED MORE RAPIDLY. 



