Chap, viii.] ELECTROTONIC EXPERIMENTS. 85 



and the current from the induction coil will go by the 

 cups 1 and 2 to these wires of the electrodes. In 

 other words, the electrotonising wires are 3 and 4, 

 next to the muscle, and the stimulating electrodes are 

 1 and 2, away from the muscle. The stimulus, there- 

 fore, is in the centro-polar region. But let DC be 

 reversed, so as to dip towards 1' 2' and 3' 4', then the 

 electrotonising current by the wires d' c' can no 

 longer get to the cups 3 and 4, the contact being 

 broken, but must go down to the opposite cups 1 ' and 

 2 ', where it catches the wires that carry it over to the 

 other half of DC to the cups 1 and 2. The wires 

 1 and 2, therefore, become electrotonising. In the 

 same way, the induction currents are led down to 

 3 ' and 4 ', and from them to 3 and 4, and so the wires 

 3 and 4 become the stimulating electrodes. The wires 

 have, therefore, by reversing DC, been reversed, and 

 the stimulation would now be applied by wires 3 and 

 4, between the muscle and the electrotonising wires, 

 in the myopolar region therefore. Thus by reversing 

 the commutator c, the constant current is sent up or 

 down the nerve, and by reversing the double commu- 

 tator DC, the stimulation is made centro-polar or 

 myopolar. The strength of the constant current is 

 regulated by the rheocord, and thus the desired con- 

 ditions are obtained. 



To perform tlie experiments it is necessary 

 to remember the rule that the excitability of a nerve 

 in the electrotonic state is increased in the neighbour- 

 hood of the negative, and diminished in the neighbour- 

 hood of the positive, pole. There are four cases, which 

 are represented in the diagram (Fig. 46). In the first 

 two cases the stimulation is in the myopolar region, 

 in the second two, in the centro-polar region ; and in 

 each set there is one case of downward and another 

 of upward current. In the first case, the stimulation 

 is next the muscle, that is, in the myopolar region, 



