CHAP. IL] 



THE CONTRACTILE TISSUES. 



81 



may explain this result by saying that the polarizing current has caused 

 the appearance in the nerve outside the electrodes of a new current, 

 the 'electrotonic' current, having the same direction as itself, which 

 adds to, or takes away from, the natural nerve-current or " current of 

 rest " according as it is flowing in the same or in an opposite direction. 



.7 



H 



FIG. 15. DIAGRAM ILLUSTRATING ELECTROTOXIC CURRENTS. 



P the polarizing battery, with k a key, p the anode, and p' 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 + . The current at the other end of the piece of nerve, from h to h', 

 through the galvanometer 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. 



The strength of the electrotonic current is dependent on the strength 

 of the polarizing current, and on the length of the intrapolar region 

 which is exposed to the polarizing current. When a strong polarizing 

 current is used, the electromotive force of the electrotonic current may 

 be much greater than that of the natural nerve-current. 



The strergth of the electrotonic current varies with the irritability, 

 or vital condition of the nerve, being greater with the more irritable 

 nerve; and a dead nerve will not manifest electrotonic currents. More- 

 over, the propagation of the current is stopped by a ligature, or by 

 crushing the nerve. 



F. 6 



