GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 51 



rius muscle, which is clamped in the middle firmly enough to prevent the con- 

 tractions of one half from moving the other, but not enough to interfere with 

 the conduction-power of the tissue. The record of the contractions is best 

 obtained by the double myograph of Hering (Fig. 15), which permits the 

 recording levers attached to the two ends of the muscle to write directly under 

 each other, so that any difference in the beginning of the contraction of the 

 two halves of the muscle is immediately recognizable from the relative posi- 

 tions of the records of their contractions. 



The current is applied to the two extremities of the muscle by non-polarizable electrodes. 

 In all experiments with the direct battery current it is essential to employ non-polarizable 

 electrodes. The form devised by Hering is very useful where the current has to be applied 

 directly to the muscle, because the two electrodes are hung from pivots in such a way that 

 they move with the movements of the muscle, and hence do not shift their position when 

 the muscle contracts. Some kind of apparatus has to be employed for quickly reversing 

 the direction of the current. A convenient in- 

 strument for this purpose is Pohl's mercury com- 

 mutator (Fig. 16). This instrument consists of 

 a block of insulating material in which are six 

 little cups containing mercury, which is in con- 

 nection with binding-posts on the sides of the 

 block. Two of the mercury cups on the opposite 



FIGS. 16, 17. Pobl's mercury commutator. 



sides of the block a and b (Fig. 17, A), are connected by wires with the battery ; two others, 

 c and d, are connected with wires which pass to the electrodes ; the remaining two on the 

 opposite side of the block, e and /, are joined by movable good conducting wires with the 

 cups c. and d in such a way that c connects with /, and d with e. Two anchor-like pieces of 

 metal are connected by an insulated handle, and are so placed that the stocks of the anchors 

 dip into the mercury cups a and b (Fig. 16). The anchors can be rocked to one side or the 

 other, so that the ends of the curved arms shall dip into the cups c and d (in which case 

 cup a will be connected with cup c, and cup b with cup d ), or so that the other ends of the 

 arms shall dip into cups e and / (in which case cup a will be connected with cup e, and by 

 means of the cross wire with cup d, and cup b will be connected with cup/, and by means 

 of the cross wire with cup c). By the arrangement shown in Fig. 17, A the current can 

 pass from the battery by way of a and c down the nerve, and by way of d and b back to 

 the battery ; or it can pass from the battery by way of a, e, d, and in the reverse direction, 

 up the nerve and back to the battery, by way of c, /, b. 



This commutator can be used in another way (see Fig. 17, B]. If the battery be con- 

 nected with it as before, and the cross wires be removed, the current can be sent at will 

 into either one of two separate circuits. For instance, if the cups c, d be connected with 

 the electrodes on one part of the nerve, and the cups e, / with the electrodes on another 



