104 



THE CONTRACTILE TISSUES. 



kathode, but as the polarizing current increases in intensity, draws nearer 

 and nearer the kathode (see Fig. 33). 



The amount of increase and decrease is dependent : (1 ) On the strength 

 of the current, the stronger current up to a certain limit producing the 

 greater effect. (2) On the irritability of the nerve, the more irritable, 

 better conditioned nerve being the more affected by a current of the same 

 intensity. 



In the experiments just described the increase or decrease of irritability 

 is taken to mean that the same stimulus starts in the one case a larger or 

 more powerful, and in the other case a smaller or less energetic impulse ; 

 but we have reason to think that the mere propagation or condition of im- 

 pulses started elsewhere is also affected by the electrotonic condition. At all 

 events anelectrotonus appears to offer an obstacle to the passage of a nervous 

 impulse. 



$ 74. Electrotonic currents. During the passage of a constant current through 

 IL nerve, variations in the electric currents belonging to the nerve itself may be 



G H 



Diagram illustrating Electrotonic Currents : P the polarizing battery, with k a key, p the 

 anode, andp' the kathode. At the left end of the piece of nerve the natural current flows through 

 the galvanometer G from g to g', in the direction of the arrows ; its direction, therefore, is the 

 same as that of the polarizing current ; consequently it appears increased, as indicated by the 

 sign -K The current at the other end of the piece of nerve, from h to h' through the galvan- 

 ometer H, flows in a contrary direction to the polarizing current; it consequently appears to be 

 diminished, as indicated by the sign . 



N. B. For simplicity's sake, the polarizing current is here supposed to be thrown in at the 

 middle of a piece of nerve, and the galvanometer placed at the two ends. Of course it will be 

 understood that the former may be thrown in anywhere, and the latter connected with any two 

 pairs of points which will give currents. 



observed ; and these variations have certain relations to the variations of the irri- 

 tability of the nerve. Thus if a constant current supplied by the battery P 

 (Fig. 34) be applied to a piece of nerve by means of two non-polarizable electrodes 



