CHAP, ii.] 



THE CONTRACTILE TISSUES. 



49 



contraction or spasm will be followed by a second spasm, the two 

 bearing some such relation to each other as that shewn by the 

 curve in Fig. 6, where the interval between the two shocks was 

 just long enough to allow the first spasm to have passed its maxi- 

 mum before the latent period of the second was over. 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 raised nearly double the height it would have been 

 by either alone. A more or less similar result would occur if the 

 second contraction began at any other 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. If in the same way a third shock follows the second 

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

 the second. The same with a fourth, and so on. 



FIG. 7. MUSCLE THROWN INTO TETANUS, WHEN THE PRIMARY CURRENT or AN IN- 

 DUCTION-MACHINE IS EEPEATEDLT BROKEN AT INTERVALS OF SIXTEEN IN A SECOND. 



To be read from left to right. 



The upper Hue is that described by the muscle. The lower marks time, the 

 intervals between the elevations indicating seconds. The intermediate line shews 

 when the shocks were sent in, each mark on it corresponding to a shock. The lever, 

 which describes a straight line before the shocks are allowed to fall into the nerve, 

 rises almost vertically (the recording surface travelling in this case slowly) as soon 



the first shock enters the nerve at a. Having risen to a certain height, it begins 

 to fall again, but in its fall is raised once more by the second shock, and that to a 

 greater height than before. The third and succeeding shocks have similar effects, 

 the muscle continuing to become shorter, though the shortening at each shock is 

 less. ^ After a while the increase in the total shortening of the muscle, though the 

 individual contractions are still visible, almost ceases. At b, the shocks cease to be 

 sent into the nerve ; the contractions almost immediately disappear, and the lever 

 forthwith commences to descend. The muscle being lightly loaded, the descent is 

 very gradual ; the muscle had not regained its natural length when the tracing was 

 stopped. 



F. -i 



