PRODUCED BY A CONSTANT CURRENT. 17 



pended so that it does not rest on the nerve. If, however, the 

 arms of the reverser are placed in the cups of mercury 2, the current 

 is compelled, before reaching- the electro-magnet, to pass through 

 the metronome (M), which by its oscillations opens and closes 

 the circuit to the electro-magnet. Each time the circuit is broken 

 the lever falls on the nerve and stimulates it. When the circuit 

 is closed by the swing of the metronome in the opposite direction, 

 the lever rises again from the nerve. The direction of the current 

 is indicated in the figure by the arrows. 



The polarising current I obtained from a battery of Meidinger's 

 Elements (B^. First it was carried to a du Bois-Reymond's 

 rheochord (Rh\ where it divided, then to a Pohl's reverser, from there 

 to a du Bois-Reymond's key (K), and then to the nerve. The current 

 was closed and broken by means of the key, its strength being 

 graduated in the usual way by means of the rheochord. 



The current was, of course, led through the nerve in the ordinary 

 way by unpolarisable electrodes. To make them properly suit my 

 apparatus, I gave them a different form from what they usually 

 have ; and I take occasion here to describe them, as they seem to 

 me more practical and handier for stimulation experiments than 

 most of the others in use. 



Each consists simply of a box of ebonite 2\ centims high, the 

 base of which is I centim square. Underneath it is provided with 

 two steel prongs screwed in, by means of which it can be fixed on 

 the cork plate of my lever-apparatus, outside the bed for the nerve. 

 The inside of the trough is filled with paraffin to prevent any 

 contact between the clay and the steel prongs. The clay, having 

 been previously soaked with a 0-6 / solution of common salt, 

 is shaped with the hands into long rolls, which are then 

 placed in the trough, with their ends bent forwards. These 

 ends can afterwards have such a form given them as may best 

 suit each particular experiment. In the clay is stuck a glass 

 tube, which is filled with a concentrated solution of sulphate of 

 zinc ; and into this is introduced a well-amalgamated slip of zinc, 

 which is connected by means of a conducting wire with the wires 

 of the battery (Fig. i BJ. 



In certain experiments, where it was of primary consequence 

 to bring the mechanical exciter quite close to the electrodes, I 

 carried the current to the nerve through a strip of filter-paper 

 soaked in a 0-6 % solution of common salt, which was attached 

 to the unpolarisable electrodes just described. 



