232 NERVE. 



Examine Pohl's commutator, Fig. 23. Take care in attach- 

 ing the wires to make good contacts, also that there is 

 sufficient mercury in the cups of the commutator to cover the 

 ends of the rocker arms and those of the binding screws that 

 lead into the former. 



Prepare non-polarisable electrodes: Take a large teaspoonful 

 of China clay (Kaolin) and work it into a stiff paste with a 

 little normal saline. Perform the operation upon a clean 

 plate, and with a clean porcelain or horn spatula, avoiding all 

 contact with the fingers. Remove the glass tubes from their 

 holders in the muscle chamber, wash them clean with warm 

 water and dry them, then stop the bevelled end of each with 

 the clay for 1 cm , pressing it in compactly with the spatula. 

 Adapt the end to receive the nerve by moulding the clay into 

 a central ridge lengthways over the bevelled end. 



Replace in the holders and adjust them close to each other 

 and as near to the platinum electrodes as possible. 



Proceed as follows : 



Influence of the positive pole (anode). Turn the commutator 

 rocker so that the positive pole leads to the non-polarising 

 electrode nearest the muscle. Keep key K open. 



2. Find the minimal opening induction shock which will 

 suffice to cause a muscular contraction. 



3. Close the polarising circuit and stimulate the nerve with 

 the opening S shock there should be no contraction. The 

 production of the anelectrotonic condition (plus pole) has 

 lowered the excitability of the nerve, and the stimulus is now 

 insufficiently strong to excite it. 



Influence of the negative pole (anode). Turn the commutator 

 so as to substitute the minus for the plus pole. 

 Keep key K open. 



