572 A MANUAL OF PHYSIOLOGY 



a greater interval than the latent period, so that the second 

 falls into the muscle during the contraction produced by the 

 first. The result here is very different : traces of two con- 

 tractions appear upon the muscle-curve, the second curve 

 being that which the second stimulus would have caused 

 alone, but rising from the point which the first had reached 

 at the moment of the second shock (Fig. 179). Although 

 the first curve is cut short in this manner, the total height 

 of the contraction is greater than it would have been had 

 only the first stimulus acted ; and this is true even when 

 both stimuli are maximal. Under favourable circumstances, 

 when the second curve rises from the apex of the first, the 



FIG. 180. TETANUS. 



f, 5 stimuli per second ; 2, 15 per second ; 3, 15 per second, when muscle was more 

 exhausted than in 2. 



total height may be twice as great as that of the contraction 

 which one stimulus would have caused (p. 623). 



Not only may we have superposition or fusion of two 

 contractions, but of an indefinite number; and a series 

 of rapidly following stimuli causes complete tetanus of the 

 muscle, which remains contracted during the stimulation, or 

 till it is exhausted (Fig. 180). 



The meaning of a complete tetanus is readily grasped if, 

 beginning with a series of shocks of such rapidity that the 

 muscle can just completely relax in the intervals between 

 successive stimuli, we gradually increase the frequency 

 (p. 624). As this is done, the ripples on the curve become 

 smaller and smaller, and at last fade out altogether. The 



