MUSCLE 543 



saturated zinc sulphate 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 made. Before the tube is 

 filled with the zinc sulphate solution, the lower part of it is plugged 

 with china clay made up with normal saline. The clay just projects 

 through the opening, and thus comes in contact with the tissue. 

 When these electrodes are properly set up, there is very little polariza- 

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

 with saturated zinc sulphate solution, are better. The amalgamated 

 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. 157) 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 FIG. 157. POHL'S COM- 

 by the wire connected with i, and back MUTATOR. 



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 convenient 

 form is shown in Fig. 158. 



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

 time relations of many physiological phenomena which are graphically 

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

 beat of a heart. For this purpose a tracing showing the speed of 

 the travelling-surface in a given time is often taken simultaneously 

 with the record of the movement under investigation. For a slowly- 

 moving surface it is sufficient to mark intervals of one or two seconds, 

 and this is very readily done by connecting an electro-magnetic 

 marker (such as the electric signal of Deprez) with a circuit which is 

 closed and broken by the seconds pendulum of an ordinary clock 

 (Fig. 159) or a metronome (Fig. 60, p. 170). For shorter intervals 

 a tuning-fork is used, which makes and breaks a circuit including an 

 electro-magnetic marker, or writes on the drum directly by means of 

 a writing-point attached to one of the prongs. 



