466 



NEE VE-IMP ULSES. 



called electrotonus. This has been concisely denned as "a change 

 of condition in nerves traversed by an electric current." It also 

 occurs in muscles. 



FIG. 267. Arrangement for measuring the velocity of the nerve-impulse: A, 

 travelling plate of spring myograph: M, muscle lying on a myograph plate; N, 

 nerve lying on two pairs of electrodes, E and E' ; C, Pohl's commutator without 

 cross-wires; K, knock-over key of spring myograph (only the binding screws 

 shown); K', simple key in primary circuit; B, battery; P, primary coil; 8, 

 secondary coil (Stewart). 



That an electrical change is produced in a nerve by passing a 

 constant current through it, the polarizing current, may be demon- 

 strated by connecting the nerve with a galvanometer. The 



FIG. 268. Electrotonic alterations of irritability caused by weak, medium, and 

 strong battery currents : A and E indicate the points of application of the elec- 

 trodes to the nerve, A being the anode, B the kathode. The horizontal line repre- 

 sents the nerve at normal irritability ; the curved lines illustrate how the irrita- 

 bility is altered at different parts of the nerve with currents of different strengths. 

 Curve y l shows the effect of a weak current, the part below the line indicating de- 

 creased, and that above the line increased irritability ; at x l the curve crosses the 

 line, this being the indifferent point at which the katelectrotouic effects are com- 

 pensated for by anelectrotonic effects; y 1 gives the effect of a stronger current, and 

 y 3 , of a still stronger current. As the strength of the current is increased the effect 

 becomes greater and extends farther into the extrapolar regions. In the intrapolar 

 region the indifferent point is seen to advance with increasing strengths of current 

 from the anode toward the kathode. 



current near the kathode is the katelectrotonic current, while that 

 near the anode is the anelectrotonic current. These currents occur 

 only as the result of passing a constant current through a nerve, 



