44 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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

 traction begins at the apex of the first. If the stimuli are closer to- 

 gether, 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 

 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- 



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 t two succeeding stimuli. 



corded curve shows no sign of relaxation between stimuli and no 

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

 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 

 form an interrupted series corresponding, 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 Phenomena 

 of Muscle and Nerve. Another proof is found in the phenome- 

 non of the muscle-tone. When a muscle is stimulated directly 

 or through its motor nerve a musical note may be heard by 

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



