THE PHYSIOLOGY OF MUSCLE 489 



Stimulate by make-and-break shocks separated, as usual, so that the 

 curve from each is complete. Now reduce the interval between the 

 break and the make until relaxation is prevented by a second contrac- 

 tion arising from the break-shock. A compound curve is thus formed, 

 the second part much higher than the first. Compare this height with 

 that from a single break shock. 



FIG. 273 



The mechanical nature of muscular tetanus. If electrical stimuli be applied to a frog's gas- 

 trocnemius often enough, the muscle does not have time to relax before it is made to contract 

 again. Observe that the more frequent the stimulation the more slowly the muscle relaxes when 

 the stimulation stops. To be read from left to right. The time-line is in seconds. 



This superposition of contractions suggests the nature of the contracted 

 condition known as muscular tetanus (not to be confused with the disease 

 tetanus or lockjaw). As has been seen already, relaxation of even a 

 cross-striated muscle requires time (0.075 second), and if another stimu- 

 lus is imposed upon the muscle before it has fully relaxed, another con- 

 traction will at once occur. Because the latent period of contraction is 

 much shorter than the relaxation-period, the two myograms or curves 



