82 PHYSIOLOGICAL PHYSICS. [Chap. vm. 



called ANELECTROTONUS, at the negative, KATELECTRO- 

 TONUS. Further, the condition is not limited to the 

 poles, but extends for some distance on either side of 

 them. There is, accordingly, an area in the neigh- 

 bourhood of the positive pole that is in the an- 

 electrotonic state, and, similarly, a katelectrotonic 

 area in the neighbourhood of the negative pole. On 

 the under side of Fig. 43 are represented electrodes 

 from a secondary coil, for stimulating at one time 

 next the muscle, at another time away from it. 



When the nerve is stimulated between the electro- 

 tonising electrodes and the muscle, the stimulation 

 is said to be MYOPOLAR, near to the muscle. When 

 the stimulus is applied beyond the electrotonising 

 electrodes, it is said to be CENTRO-POLAR, near the 

 centre from which the nerve proceeds. The dotted 

 lines in Fig. 43 represent the stimulating electrodes 

 in the centro-polar region. 



Thus, to show the effects of electrotonus on the 

 excitability of a nerve, the following things are neces- 

 sary : (1) a constant current for throwing the nerve 

 into an electrotonic state, (2) an apparatus for 

 varying the strength of the current at pleasure, the 

 rheocord, (3) a means of sending the constant 

 current at one moment up, at another down, the 

 nerve, i.e. a commutator, (4) a current for stimulating 

 the electrotonised nerve, an induction current, (5) 

 an arrangement for stimulating near or far from the 

 muscle at pleasure, another commutator. 



Fig. 44 is a diagram of the arrangements and 

 exact connections. 



At the upper right-hand corner of the figure is 

 the muscle telegraph, with the muscle preparation M 

 fixed in the forceps. The nerve of M is laid over the 

 electrodes El. These electrodes are shown in Fig. 45. 

 They are formed of platinum wires stretched across a 

 little box of ebonite. The wires are at least four 



