56 EXPERIMENTAL PHYSIOLOGY 



making and breaking the current 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. 



FIG. 48. To test Pfluger's "Law of Contraction." 



If the nerve be very excitable l the muscle may remain in contrac- 

 tion during the whole time of the passage of a strong descending 

 current (closing tetanus), and may also remain contracted for a con- 

 siderable time after the removal of a strong ascending current (Bitter's 

 opening tetanus). If Bitter's tetanus is obtained 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 is much increased when the preparation 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. 



