96 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



4- 

 Na 



Cl 



between the poles A and C through a solution of sodium chlorid. During 

 the passage of the current the cations, Na, with their positive charges 

 move 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. In conse- 

 quences of these chemical actions at the poles an electromotive force is de- 

 veloped at the cathode which diminishes the current passing from A to C. 

 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 physi- 

 cochemical 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 amal- 

 gamated 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 



S0 4 S0 4 Cl Cl Cl S0 4 S0 4 



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 



