XL.] 



TWO SUCCESSIVE SHOCKS. 



219 



LESSON XL. 



TWO SUCCESSIVE SHOCKS TETANUS- 

 METRONOME. 



1. Two Successive Shocks. The primary current may be 

 broken by means of a revolving drum, i.e., using the automatic key 

 (fig. 120). Two strikers can easily be arranged on the same 

 support (IS), and their angular deviation can easily be adjusted to 

 give any required interval between the two successive shocks. 



Fig. 141 shows several tracings indicating the effect of summa- 

 tion or superposition of one contraction on another, and how the 

 result varies with the particular period or phase of the contraction 

 at which the second shock or stimulus is applied. 



FIG. 141. Effects of two Successive Shocks on a Muscle, i. Second stimulus applied 

 at x ; 2. Second stimulus applied at the second x ; 3. Second stimulus applied at x ; 

 4. Second stimulus applied at the second x . 



Make four successive experiments, using break shocks. 



(i.) Arrange the two closures for stimulation so that they are a 

 full muscle-curve apart. The second is usually slightly higher than 

 the first (fig. 141, i). 



(ii.) Arrange on a different part of the cylinder, but on the same 

 abscissa, so that the second stimulus comes in on the relaxation of 

 the foregoing contraction. As the second contraction occurs before 

 the first one has ended, it starts from a higher level (fig. 141, 2). 



(in.) If the second stimulus is so arranged as to be thrown in on 

 the ascent of the first curve, and before the apex is reached, the 



