chap. xi.] THE MUSCLE CURRENT. 127 



over electrodes for conveying the constant current of 

 electricity. These electrodes should be of the non- 

 polarisable type, of the tube form shown in Fig. 59. 

 They are connected with a single Daniell's element, 

 through the medium of a commutator arranged for 



O 



reversing the direction of the current. The object of 

 the nerve being long is to have the electrodes which 

 convey the constant current, called the exciting 

 electrodes, as far away as possible from the galvano- 

 meter electrodes, which are called the deriving elec- 

 trodes. The key in connection with the galvanometer 

 is opened, and the natural nerve current is obtained. 

 The constant current is then passed through the piece 

 of nerve laid over the exciting electrodes, and a 

 variation is at once produced in the deflection of the 

 galvanometer, indicating some change in the electro- 

 motive force of the nerve. On reversing, by means of 

 the commutator, the direction of the constant current, 

 it is found that, w/ien the constant current floios in the 

 same direction as the nerve current, the deflection of 

 the needle is increased, and when the constant current 

 flows in the opposite direction the needle deflection is 

 diminished. The increase is called the positive phase, 

 and the decrease the negative phase. 



To prove that the change is not due to some of 

 the battery current passing downwards, and getting 

 into the galvanometer, a ligature is tightly applied 

 between the portion of the nerve on the exciting and 

 the portion on the deriving electrodes. This ligature 

 destroys the nervous conductivity, but does not destroy 

 the conductivity for the galvanic current. But it is 

 found that then electrotonus has no effect, so that it 

 was not the diffusion of the galvanic current that 

 produced the change. To succeed with this experiment 

 care must be taken that insulation is complete, and no 

 moisture must be allowed to be present to act as a 

 conductor. 



