448 MUSCLE PHENOMENA. 



consists of a glass tube, at one end of which is an opening ; 

 the lower part of this tube is filled with China clay, mixed with 

 normal saline solution, which projects a little through the open- 

 ing so that it may come into contact with the tissue to be in- 

 vestigated. The portion of the tube above the clay is filled with 

 a saturated solution of zinc sulphate, into which dips an amalga- 

 mated zinc wire. When the experiment is of long duration the 

 form represented by the middle figure in the illustration is best 

 adapted; the U-shaped tube is filled with the zinc sulphate solu- 



FIG. 254. Method of recording muscular contraction (Lombard). 



tion, and into one end dips the wire, and into the other a glass 

 tube filled with clay, on which the tissue rests. 



PohVs Commutator (Fig. 252). In order to reverse the direction 

 of the current, PohPs mercury commutator is used. This consists 

 of a block of some insulating material, as paraffin or wood, with 

 six cups containing mercury, each of which is connected to a 

 binding-post. In Fig. 253, A, it will be seen that a and b are con- 

 nected with the battery, c and d with the electrodes, and e and / 

 with movable wires in such manner that c connects with / and 

 d with e. Above the block is a bridge of some insulating mate- 

 rial, glass or vulcanite, to which two semicircles of wire are 

 attached, one of which is connected with a wire dipping into the 

 cup a, and the other with a wire dipping into the cup b. This 

 bridge can be rocked back and forth, so that if the ends dip into 

 the cups c and d, a will be connected with c, and b with d ; while 



