ix ELECTRICAL EXCITATION OF NERVE 137 



In fresh, vigorous, motor -frog's nerves the closure of weak 

 currents therefore effects a twitch with both ascending and 

 descending direction of current, while the opening of the current 

 remains without effect in both cases (first stage of the law of 

 contraction). The opening contraction then makes its appearance 

 gradually with growing intensity of current, so that the second 

 stage is characterised throughout by twitches from the muscle, 

 which accompany the closure and opening of both ascending and 

 descending currents. The opposite effect of a contrary direction 

 of current first appears when the intensity has exceeded a certain 

 limit, the rule being then universal that only the closure of 

 descending and opening of ascending currents elicit any contraction, 

 while closure of ascending and opening of descending currents 

 invariably fail in effect. These consequences are so regular that 

 they can be employed as a means of determining the direction of 

 current physiologically, upon a rheoscopic preparation. In order 

 to demonstrate the law of contraction it is essential whenever 

 possible to test upon the same preparation the effect of different 

 strengths of current in one direction only, without changing the 

 position of the nerve upon the electrodes, the most convenient 

 method being to excite two nerve-muscle preparations from the 

 same frog simultaneously, by laying the two nerves in opposite 

 directions across the same unpolarisable electrodes. It is then 

 possible to observe at the same time all the changes in the reaction 

 of the muscle which follow upon increased intensity of current, as 

 when, in the third stage, one preparation twitches only on closing 

 the current, the other on opening it. 



Pfliiger was the first to give any satisfactory explanation of the 

 facts (at first sight very striking) which underlie the law of con- 

 traction. There is first of all the marked contrast between make 

 and break effects in the third stage. It is obvious that the mere 

 alteration in direction of the electrical current is not per se a 

 sufficient explanation, and if, as can hardly be doubted, the nerve 

 is also excited at closure of the ascending and opening of the 

 descending current, the failure of the excitation can only be 

 explained by the fact that it is in some way hindered from 

 expressing itself in the muscle. In other words, there must at 

 some part of the tract of nerve traversed be an alteration which 

 blocks the excitatory process on its way to the muscle, at closure 

 in the one case, on breaking the circuit in the other. 



