POLAR EFFECTS OF ELECTRICAL CURRENT 203 



on a conducting tissue, then there is no such distinctive 

 isolation and the excitatory effect initiated at either of the 

 electrodes may be transmitted to the terminal motile organ, 

 there to induce contractile response. We have an example 

 of this in the nerve-and-muscle preparation of a frog, where 

 under feeble current excitatory contraction of the muscle 

 is seen to take place at make of both ascending and descend- 

 ing currents in the nerve. The terms * ascending ' and 

 ' descending ' may be found somewhat confusing, especially 

 in plant-experiments. The meaning will be clear if we 

 remember that the current is said to be descending when it 

 flows towards the responding motile organ. In the case of 

 descending current in a nerve-and-muscle preparation, the 

 kathode is proximal as regards the responding muscle ; 

 with the ascending current, the kathode is distal. 



As already stated, excitation takes place in nerve-and- 

 muscle preparation at make of both ascending and descend- 

 ing currents. This is explained as due to make of the 

 proximal kathode in the case of descending, and make of the 

 distal kathode in the case of ascending, currents. 



It is true that in the latter case the excitation has to 

 traverse the anodic area before it can reach the motile organ. 

 The anode induces, in general, a depression of conductivity 

 which may even block the passage of excitation. In the 

 present case, however, this blocking action is ineffective on 

 account of the feebleness of the current. 



The fact that it is the proximal kathode of the descending, 

 and the distal kathode of the ascending, current which form 

 respectively the seats of excitation, may be proved by 

 taking time-measurements of the interval between * make ' 

 of the current and the response of the muscle. With the 

 descending current the interval should be shorter, since in 

 such a case the kathode is proximal. This has been found 

 to be the case. 



Characteristics in every way similar are found in experi- 

 ments with the petiole and pulvinus of Mimosa. In this 

 case the petiole contains the conducting tissue and the 



