PRACTICAL PHYSIOLOGY 



49 



with the omission of the induction coil and its 

 accessories), test the so-called law of contraction, or, 

 in other words, the conditions of excitation of a 

 motor nerve by the making or breaking of a con- 

 stant current. The rheochord is employed as 

 before to vary the strength of the current, the com- 

 mutator to vary its direction up or down the ijerve. 

 It will have been observed in the last experiment 

 that the closing or opening of the polarising cur- 

 rent itself acts as a stimulus to the nerve, the result 



FIG. 32. To TEST PFLUGER'S " LAW OF CONTRACTION." 



varying with the direction and strength of the cur- 

 rent. The conditions of this excitation, so far as 

 regards strength and direction of current, are now 

 to be worked out. Beginning with a very weak 

 current, the rider of the rheochord being brought 

 near to the end b of the rheochord wire (see Fig. 

 15), determine the effect upon the nerve, as indi- 

 cated by the contraction of the muscle, of making 

 and breaking the current when it is (i) ascending 

 and (2) descending. Repeat the experiment, using 

 a moderate strength of polarising current i.e., with 

 the rider of the rheochord near the end a of the 



