THE PHENOMENON OF CONTRACTION. 41 



tractions, on the one hand, to a perfect fusion of the contractions, 

 a complete tetanus, on the other. Tetanic contractions present 

 two peculiarities in addition to the mere matter of duration, 

 which is governed, of course, by the duration of the stimu- 

 lation: First, the more or less complete fusion of the contrac- 

 tions due to the separate stimuli. This, as stated above, is the 

 distinctive sign of a tetanus. Second, the phenomenon of sum- 

 mation in consequence of which the total shortening of the muscle 

 in tetanus may be considerably greater than that caused by a maxi- 

 mal simple contraction. 



Summation. The facts of summation may be shown most read- 

 ily by employing a device to send into the muscle two successive 

 stimuli at varying intervals. If the second stimulus falls into the 

 muscle at the apex of the contraction caused by the first stimulus, 

 then, even if the first contraction is maximal, the muscle will shorten 

 still farther; the first and second contractions are summated, giv- 

 ing a total shortening greater than can be obtained by a single stim- 

 ulus (see Fig. 20). The extent of the summation in such cases 

 varies with a number of conditions, such as the intervals between the 



Fig. 20. Summation of two successive contractions. Curve 1 shows a simple con- 

 traction due to a single stimulus, the latent period being indicated at the beginning of the 

 contraction. Curve 2 shows the summation due to two succeeding stimuli. 



stimuli, the relative strengths of the stimuli, the load carried by the 

 muscle, etc. Taking the simplest conditions of a moderately loaded 

 muscle and two maximal stimuli, it is found that the greatest sum- 

 mation occurs when the stimuli are so spaced that the second contrac- 

 tion begins at the apex of the first. If the stimuli are closer together, 

 so that, for instance, the second contraction follows shortly after 

 the first has begun, the total shortening is less, and the same is true 



