96 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



toward the cathode; at the cathode the free sodium ion acts upon the water, 

 HHO, forming NaOH and liberating hydrogen, which gives its charge to the 

 cathode and accumulates upon it in the form of gas. The anions, Cl, 

 with their negative charges move toward the anode; there the chlorin acts 

 upon the water, forming HC1 and liberating oxygen. It is obvious that in 

 quantitative studies of the electrical currents of animal tissues polarization 

 will destroy the accuracy of the results ; the demarcation current will show 

 a diminution due not to changes in the nerve, but to physicochemical changes 

 at the leading off electrodes. To prevent polarization du Bois-Reymond devised 

 the non-polarizable electrodes consisting of zinc terminals immersed in zinc 

 sulphate. Theoretically any metal in a solution of one of its salts may be used, 

 but experience shows that the zinc-zinc sulphate electrode is most nearly 

 perfect. Each electrode where it comes into contact with the tissue is made 

 of one of these combinations. Various devices have been used. For instance, 

 the electrode may be constructed as shown in the diagram (Fig. 41). A 

 short glass tube of a bore of about 4 mms. is well cleaned one end, 

 which is to come into contact with the nerve, is filled, as shown, by a plug of 

 kaolin made into a stiff putty with physiological saline (solution of NaCl, 0.7 per 

 cent.). The kaolin should have a neutral reaction and unless good kaolin is 

 obtainable it is better to use a plug made of clean filter paper macerated in 

 physiological saline and packed tightly into the end of the tube. Above this 

 plug the tube is filled in for a part of its length with a saturated solution, of 

 zinc sulphate into which is immersed a bar of amalgamated zinc with a copper 

 wire soldered to its end. With a pair of such electrodes the conduction of 

 the current through the nerve or muscle to the metallic part of the circuit 

 may be represented as follows: 



Zn on en rn m m en en Zn 



The liquid part of the circuit comes into contact with the metallic part 

 at the junction of Zn and ZnSO 4 . At the cathode it may be supposed that 

 the Zn cation instead of acting upon the water and liberating hydrogen, 

 deposits itself upon the zinc electrode; at the anode the sulphion (SO 4 ) 

 attacks the zinc instead of the water, forming ZnSO 4 . In this way polarization 

 is prevented, and by the construction of the electrode the living tissue is 

 brought into contact only with the plug of kaolin moistened with physio- 

 logical saline. Such electrodes are indispensable in studying the electrical phe- 

 nomena of living tissues, and also in all investigations bearing upon the polar 

 effects during the passage of an electrical current from a battery. Ordinarily, 

 however, when it is only desired to stimulate a nerve or muscle, metal (plat- 

 inum) electrodes are employed. 



The Action Current or Negative Variation. Du Bois-Rey- 

 mond proved that when the excised muscle or nerve is stimulated 

 its demarcation current suffers a diminution or negative variation. 

 If, for instance, the excised nerve gives a demarcation current suf- 

 ficient to cause a deflection in the galvanometer of 50 mms., then 

 if the nerve is stimulated by a series of induction shocks the galva- 

 nometer will show a lessened deflection, say, one of 40 mms. The 

 negative variation in this case is equal to 10 mms., on the scale of 

 the galvanometer used. It has been shown that this negative varia- 

 tion is due to a current in the opposite direction whose strength, in 



