CHAP. IT.] 



THE CONTRACTILE TISSUES. 



77 



If, however, the shocks be repeated more rapidly, as in Fig. 15, 

 each succeeding contraction will start from some part of the 

 preceding one, and the lever will be raised to a greater height at 

 each contraction. 





FIG. 15. MUSCLE-CURVE. SINGLE INDUCTION-SHOCK REPEATED MORE RAPIDLY. 



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

 Fig. 16, 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. 



FIG. 16. MUSCLE-CURVE. SINGLE INDUCTION-SHOCK REPEATED STILL MORE RAPIDLY. 



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 of commencing fatigue, caused by the 

 repetition of the contractions, the fatigue manifesting itself by an 

 increasing prolongation of each contraction, shewn especially in a 

 delay of relaxation, and by an increasing diminution in the height 

 of the contraction. Thus in Fig. 14 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. So, also, in Fig. 15, the 

 lever rises rapidly at first, but more slowly afterwards, owing to an 

 increasing diminution in the height of the single contractions. In 

 Fig. 16 the increment of rise of the curve due to each contraction 

 diminishes very rapidly, and though the lever does continue to 



