190 MUSCLE AND NERVE 



pole of the galvanic circuit. In like manner the negatively 

 charged ions or anions move toward the positive. This effect 

 is brought about in any moist tissue that will conduct, and 

 the tissue is then said to be polarized. The difference of poten- 

 tial between the kations and anions sets up a current (polari- 

 zation current) in a direction opposite to that of the inducing 

 current. 



It happens thus that a polarization current produced in any 

 tissue, like nerve, may completely disguise the weaker differ- 

 ences in electrical potential due to the tissue itself. Non-polar- 

 izable electrodes consist essentially of a metal in a solution of 

 one of its own salts. The zinc-zinc sulphate combination has 

 been found most nearly perfect. One form of this electrode 

 consists of a glass tube, plugged at the end which is to come in 

 contact with the tissue with a stiff putty of kaolin made up in 

 physiological saline. Into the other end of the tube are inserted 

 a saturated solution of zinc sulphate and a strip of amalgamated 

 zinc. The liquid and metal portions of the circuit come into 

 contact at the junction of the zinc and zinc sulphate. The 

 dissociation which takes place at the kathode liberates a zinc 

 kation which deposits itself upon the zinc electrode, while the 

 sulphion at the anode unites with an atom of zinc to form zinc 

 sulphate, which remains in solution. In this way polarization 

 is largely prevented, and the tissue comes into contact only 

 with the physiological saline in the kaolin plug. 



Whenever any portion of a tissue becomes active, or in any 

 manner undergoes katabolic changes, a difference of potential 

 manifests itself in that the active portion becomes negative to 

 the rest. The difference of potential is generally small, requring 

 a sensitive galvanometer or electrometer to measure it, but in 

 some electrical fishes becomes as high as 200 volts. When a 

 muscle or nerve is intact and uninjured, it gives no evidence 

 of differences of potential, and is therefore said to be iso-electric; 

 but when the ends are cut, so as to present two sections at right 

 angles to the longitudinal surface, it will be found upon testing 

 with non-polarizable electrodes, and a suitable galvanometer, 

 that the cut and therefore dying ends are negative to the un- 

 injured longitudinal surface. . 



The current flows from the injured tissue through the muscle 

 or nerve to the electrode on the longitudinal surface, thence 



