EXCITATION OF NERVE AND MUSCLE BY GALVANIC CURRENT 63 



Pfliiger's law. The above (which has been misnamed " the law of con- 

 traction ") is illustrated by an experiment devised by Pfliiger. The nerve^jof a 

 nerve-muscle preparation is placed on non-polarisable electrodes, which are 

 connected with a battery of at least three cells through a commutator and 

 rheochord : a mercury key is introduced into the circuit (fig. 47). Beginning with 

 a very weak current, the rider of the rheochord being brought near to the end a 

 of the rheochord wire (see figs. 19, 20), determine the effect upon the nerve, as 

 indicated by the contraction of the muscle, of making and breaking the current 



FIG. 47. To TEST PPLUGEK'S ' ' LAW OP CONDUCTION." 



when it is (1) 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 b of the wire. Finally, the effect of a strong current is to be studied 

 by eliminating the rheochord altogether. Note down in tabular form all the 

 results obtained. (The contractions of the muscle need not be recorded 

 graphically.) 



If the nerve be very excitable 1 the muscle may remain in contraction during 

 the whole time of the passage of a strong descending current (closing tetanus), 

 and may also remain contracted for a considerable time after the removal of a 

 strong ascending current (Hitter's opening tetanus). If Ritter's tetanus is ob- 

 tained the nerve may be cut between the electrodes. The tetanus instantly 

 ceases because the point where the stimulus occurs (the original anode) is cut off. 



1 The excitability of a muscle-nerve preparation is greater when the latter is 

 made from a frog which has been in a cold place or in contact with ice, and then 

 kept for half an hour at the ordinary room-temperature before being killed. 



