THE CHANGE IN FORM 347 



usually greatest when the second contraction 

 starts from the summit of the first, but this rule 

 is not invariable. The summit of the summed 

 contraction does not necessarily coincide with the 

 summit of the second contraction ; the higher the 

 summed contraction, the quicker the summit is 

 reached. 



Superposition in Tetanus. Place the vibrat- 

 ing interrupter (Fig. 50) in the primary circuit. 

 Eepeat the preceding experiment, but use a series 

 of stimuli instead of only two. It will be ob- 

 served that a third contraction may be super- 

 posed on the second, a fourth on the third, and 

 so on. The shortening of muscle, however, has a 

 limit ; and when this is reached, further stimu- 

 lation merely maintains this maximum degree of 

 shortening until fatigue sets in. When the in- 

 terval between successive stimuli is very brief 

 the successive contractions appear to fuse to- 

 gether and the contraction curve becomes a con- 

 tinuous line. The more rapid the contraction, 

 the shorter must be the interval between succes- 

 sive stimuli in order to cause the disappearance 

 of the individual contractions. Thus a more 

 rapid rate of stimulation is necessary to produce 

 complete fusion in fresh, highly irritable muscles 

 than in those the irritability of which has been 

 diminished by cold or fatigue. For this reason 



