THE PHENOMENON OF CONTRACTION. 41 



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 

 to an increasing extent as the second contraction falls later and 

 later in the period of relaxation after the first contraction.* If 

 instead of two we use three successive stimuli, falling into the muscle 



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. 



at proper intervals, a still further summation occurs. In this way 

 the total extent of shortening in a muscle completely tetanized may 

 be several times as great as that of a single maximal contraction. 



The Discontinuous Character of the Tetanic Contraction 

 The Muscle-tone. In complete tetanus the muscle seems to 

 be in a condition of continuous uniform contraction; the re- 

 corded curve shows no sign of relaxation between stimuli and no 

 external indication, in fact, that the separate stimuli do more than 

 to maintain a state of uniform contraction. It can be shown, how- 

 ever, that in reality each stimulus has its own effect, and that the 

 chemical changes underlying the phenomenon of contraction are 

 probably not continuous, but form an interrupted series correspond- 

 ing, within limits, to the series of stimuli sent in. The clearest 

 proof for this belief is found in the electrical changes that result 

 from each stimulus, and the facts relating to this side of the question 

 will be stated subsequently in the chapter on The Electrical 

 * Von Kries, "Archiv fur Physiologie," 1888, p. 537. 



