28 



ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



conductors, such as are presented by animal tissue, invariably leads 

 to a polarization of the current. The two metals of the battery, copper 

 and zinc, are surrounded by electrolytes, the tendency of which is to 

 pass toward the opposite pole. Thus, the positive ions, Cu and H, 

 progress toward the cathode, whereas the OH and SO4 pass toward 

 the anode, which inside the cell is the zinc. Presently the copper plate 

 becomes covered with bubbles of H, which place a high resistance in 

 the path of the current and eventually neutralize it. This phenomenon 

 is known as polarization. 



A similar action takes place at the points of contact between the 

 electrodes and the tissue. It may be prevented by the use of the so- 

 called non-polarizable electrodes. Those devised by DuBois-Reymond 

 consist of zinc terminals immersed in a solution of zinc sulphate. A 



FIG. 6. STIMULATING ELECTRODES. 



A, B, and C, Boot electrodes; D and E, clinical electrodes; F, hand electrodes. (Harvard 



Apparatus Co.) 



very simple form may be constructed by taking two pieces of glass tub- 

 ing, measuring 4 mm. in diameter and 6 mm. in length. One end of 

 each tube is filled with modeling clay moistened with normal saline 

 solution. Above the plug the tube is filled with a saturated solution 

 of sulphate of zinc into which is placed a short rod of amalgamated zinc 

 carrying the ends of the copper wires. The points of contact with the 

 tissue are wrapped in cotton moistened with saline solution. These 

 electrodes must be immersed in saline solution for some hours before 

 they are used so as to render the clay permeable (Fig. 7) . 



4. The Different Types of Electric Currents. If the two poles of a 

 battery are connected with one another by wires and a simple key, the 

 current begins to flow as soon as the bridge has been closed, and ceases 

 to flow after the latter has been opened. The strength of this current 



