74 THE CONTRACTILE TISSUES. 



ceeding contraction will start from some part of the preceding one, and the 

 lever will be raised to a greater height at each contraction. 



FIG. 25. 



Muscle-curve. Single Induction-shock Repeated More Rapidly. 



If the frequency of the shocks be still further increased, as in Fig. 26, 

 the rise due to the combination of contraction will be still more rapid, and 

 a smaller part of each contraction will be visible on the curve. 



In each of these three curves it will be noticed that the character of the 

 curve changes somewhat during its development. The change is the result 



FIG. 26. 



Muscle-curve. Single Induction-shock Still More Rapidly. 



of commencing fatigue, caused by the repetition of the contractions, the 

 fatigue manifesting itself by an increasing prolongation of each contrac- 

 tion, shown especially in a delay of relaxation, and by an increasing dimi- 

 nution in the height of the contraction. Thus, in Fig. 24, the contractions, 

 quite distinct at first, become fused later ; the fifth contraction, for instance, 

 is prolonged so that the sixth begins before the lever has reached the base 

 line ; yet the summit of the sixth is hardly higher than the summit of the 

 fifth, since the sixth, though starting at a higher level, is a somewhat 

 weaker contraction. See also, in Fig. 25, the lever rises rapidly at first but 

 more slowly afterward, owing to an increasing diminution in the height of 

 the single contractions. In Fig. 26 the increment of rise of the curve due 

 to each contraction diminishes very rapidly, and though the lever does con- 

 tinue to rise during the whole series, the ascent after about the sixth con- 

 traction is very gradual indeed, and the indications of the individual con- 

 tractions are much less marked than at first. 



Hence, when shocks are repeated with sufficient rapidity, it results that 

 after a certain number of shocks, the succeeding impulses do not cause any 

 further shortening of the muscle, any further raising of the lever, but 

 merely keep up the contraction already existing. The curve thus reaches 

 a maximum, which it maintains, subject to the depressing effects of exhaus- 

 tion, so long as the shocks are repeated. When these cease to be given, the 

 muscle returns to its natural length. 



