chap, viii.] ARRANGEMENTS FOR ELECTROTONUS. 83 



in number, separated from one another, and each 

 having a binding screw outside. A current may 

 enter by a, reach the nerve, pass down to the next wire, 

 and off by the binding screw b. So the constant 



Fig. 44. Diagram of Arrangements for Showing Effects of Electro- 



tonus on Excitability. 



current may pass by the wires connected with a and 

 6, and the stimulating current by the wires connected 

 with c and d, or vice versa. The little box is covered 

 with a glass lid to prevent evaporation, and a piece of 

 wet blotting paper may be laid 

 in the box to keep the nerve 

 moist. At the lower left-hand 

 corner of Fig. 44 are four Grove 

 cells, of a small size, as used 

 by Du Bois-Reymond. From 

 the positive pole ( + ) a wire 

 goes to one side of the com- 

 mutator c, and from the 



negative pole a wire to the other side. This com- 

 mutator is supplied with a cross. DC is a double 

 commutator, formed of tw T o ordinary commutators, but 

 without the cross. They stand side by side, and are 

 connected together by an insulating handle, which 

 enables the bridge of both to be inclined to the same 



Fig. 45. Electrodes. 



