CHAP. IL] 



THE CONTRACTILE TISSUES. 



131 



to a piece of nerve by means of two non-polarizable electrodes p, p', 

 the "currents of rest" obtainable from various points of the nerve 

 will be different during the passage of the polarizing current from 

 those which were manifest before or after the current was applied; and, 

 moreover, the changes in the nerve-currents produced by the polarizing 

 current will not be the same in the neighbourhood of the anode (p) 

 as those in the neighbourhood of the kathode (p'). Thus let G and H be 

 two galvanometers so connected with the two ends of the nerve as to 

 afford good and clear evidence of the " currents of rest." Before 

 the polarizing current is thrown into the nerve, the needle of H will 

 occupy a position indicating the passage of a current of a certain 

 intensity from h to h' through the galvanometer (from the positive 

 longitudinal surface to the negative cut end of the nerve), the circuit 

 being completed by a current in the nerve from h' to h, i.e. the current 



h 



FIG. 23. DIAGRAM ILLUSTRATING ELECTROTONIC 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 {/', 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 /< to /;', 

 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. 



92 



