84 PHYSIOLOGICAL PHYSICS. [Chap. vm. 



side at the same time and by the same movement. 

 K is a simple mercury key interposed in the circuit of 

 the four Groves. At the right side of the figure is E, 

 a Daniell's element, connected, for the production of 

 tetanus, with the primary coil of the induction 

 machine, a key x being interposed. The secondary 

 coil ii is arranged in short circuit with the key k'. 

 To return to the single commutator c. Wires from it 

 pass to the rheocord R, arranged in short circuit. 

 The long circuit from R goes by the mercury key K, 

 to the left side of the double commutator DC, the wire 

 from d of the rheocord going through K to d' of the 

 double commutator, and that from c of the rheocord 

 going to c' of the double commutator. If now the 

 bridge of the commutator c be inclined in the direc- 

 tion of the continuous arrow, the wire to c of the 

 rheocord is + the wire c' is then -J-, and suppose DC 

 inclined in the same direction, the wire from cup 4 

 is -}-. So that a current going by that wire would 

 reach the nerve by number 4 wire of the electrodes, 

 would pass up to wire 3, by it back to cup 3 of DC, 

 and back to the battery by d', k, and d of the rheo- 

 cord ; that is, the current would travel up the nerve. 

 If, on the other hand, the commutator c be inclined in 

 the direction of the dotted arrow, then the wire to d of 

 the rheocord, as shown by , is + 3 the current goes 

 through k to d', out by 3 to the electrodes, and, in order 

 to gain wire 4 and get back to the battery, it must go 

 down the nerve. By the commutator c, with its cross 

 in, the electrotonising current is sent up or down the 

 nerve. Observe next that the wires from the key k' go 

 to the cups a 'b' of the right half of DC. As already 

 noted, if the bridge of DC be down towards 3 and 4 

 (that is, towards 1 and 2 also, since the two sides of 

 DC are connected together) the current from the Grove 

 cells (electrotonising current) will go by the cups 3 and 

 4 to the similarly numbered wires of the electrodes, 



