PFLUGER'S LAW. 233 



Find the induction shock which is just too weuk (subminimal) 

 to stimulate the nerve. 



Close the polarising circuit and stimulate the muscle will 

 contract. The influence of the kathode has raised the excita- 

 bility of the nerve, so that the previously inefficient stimulus 

 is now sufficient to excite it. 



Instead of using induction shocks, the nerve may be continu- 

 ously stimulated by applying a drop of strong salt solution 

 to the nerve in the same position, producing "salt tetanus." 

 A record can be taken, and the influence of the plus and 

 minus poles is indicated by changes in the height of the 

 contraction. 



Pfliig-ers Law. Polar excitation of nerve. 18 



Required: Rheocord, commutator, 1 key, 7 wires, and 4 or 5 cells 

 (Leclanche or dry cells). Muscle chamber on stand and drum. 



The Rheocord (Burdon Sanderson's pattern) consists of a platinum 

 iridium wire of about 20 ohms resistance, arranged for campactiiess in 

 zig-zag upon a board, and ending in screw-down terminals at each end. 

 A movable block B, with terminal, is for the purpose of establishing 

 contact at any point with the wire. The appliance is used in short 

 circuit in this experiment, and in such a manner, see Fig. 46, as to 

 vary the potential at the non-polarisable electrodes. 



Owing to the large resistance of the nerve, l cm of which is ap- 

 proximately equal to 80,000 ohms and non-polarisable electrodes 700 

 ohms each, very little actual current passes, and it becomes a question 

 of polarity. 



The copper leads are of insignificant resistance. When the rheocord 

 is out of circuit (with block off), the potential at the clay electrodes 

 is little less than that at the poles of the battery. 



As soon as the rheocord is introduced, a great drop of potential 

 occurs at the electrodes. 



Set up the connections as shown in Fig. 46. Mark the 

 position of the rocker of the commutator when the plus pole 

 is at the non-polarisable electrode nearest the spine (descending 

 current). On reversal the plus pole will be nearest the muscle 

 (ascending current). 



