MUSCLE 



625 



And as any slight chemical difference, or even perhaps a difference 

 of physical state, between the two electrodes will cause them and the 

 tissues to form a battery evolving a continuous current, it is often 

 desirable to use unpolanzable electrodes. 



Unpolarizable Electrodes. Some convenient forms of these are 

 represented in Fig. 213, A piece of amalgamated zine wire dips into 

 saturated zinc sulpnute solution contained in the upper part of a 

 glass tube. The lower end of the tube may be straight, but drawn 

 out so as to terminate 

 in a not very large 

 opening, or it may be 

 bent into a hook, in 

 the bend of which a 

 hole is mr,de. Before 

 the tube is filled with 

 the zinc sulphite solu- 

 tion, the lower p^rt 

 of it is plugged with 

 china clay made uo 

 with physiological s?,lt 

 solution. The clay just FlG - 213- UNPOLARIZABLE ELECTRODES. 



projects through the A, hook-shaped; B. U -tubes ; C, straight. D, clay 

 opening, and thus in contact with tissue: S, saturated zinc sulphate 

 comes in contact with solution ; Z, amalgamated zinc wire, 

 the tissue. When these 



electrodes are properly set up, there is very little polarization for 

 several hours, but for long experiments, U-shaped tubes, filled 

 with saturated zinc sulphate solution, are better. The amalga- 

 mated zinc dips into one limb, and a small glass tube filled with clay, 

 on which the tissue is laid, into the other. 



Pohl's Commutator (Fig. 214) consists of a block of paraffin or 

 wood with six mercury cups, each in connection with a binding-screw 

 (not shown in the figure). Cups i and 6 and 2 and 5 are connected 

 by copper wires, which cross each other 

 without touching. The bridge consists 

 of a glass or vulcanite cross-piece a, to 

 which are attached two wires bent 

 into semicircles, each connected with a 

 straight wire dipping into the cups 3 

 and 4 respectively. With the bridge in 

 the position shown in the figure, a 

 current coming in at 4 would pass out 

 by the wire connected with i , and back 

 again by that connected with 2, in the 

 direction shown by the arrows. When 

 the bridge is rocked to the other side 

 so that the bent wires dip into 5 and 6, 

 the direction of the current is reversed. The cross-wires may be 

 taken out altogether, and the commutator used to send a current 

 at will through either of two circuits, one connected with i and 2, 

 and the other with 5 and 6. 



Du Bois-Reymond's Short-circuiting Key. A cheap and con- 

 venient form is shown in Fig. 215. 



Time-markers Electric Signal. It is of importance to know the 

 time relations of many physiological phenomena which are graphi- 

 cally recorded ; for example, the contraction of a skeletal muscle or 



40 



FIG. 214. POHL'S COM- 

 MUTATOR. 



