120 THE NATURE OF THE 



to a currentless piece of nerve through unpolarisable electrodes, 

 must fall to about 0-6 of its whole strength in order to produce an 

 imperfect break-contraction. A descending current, on the other 

 hand, must fall to at least 0-3 of its original strength to produce 

 a similar excitation. Often, however, after frequent repetition of 

 the second experiment no contraction at all occurs, even though 

 the descending current is reduced to zero. 



The following experiment will serve as an example : 



EXPERIMENT. Nerve-muscle preparation of a grass-frog of medium size in the 

 moist chamber of the myograph. Unpolarisable electrodes about 7 mm - apart, the 

 distal one about I cm. from the muscle, at an indifference-point. Very weak ascending 

 and descending currents produce equally strong contractions. With key A open key B 

 is closed, and key A is afterwards closed regularly after an interval of a second, 

 followed by opening of B, while A remains closed. The first weak imperfect break- 

 contraction with gradually increasing falls of current-strength occurs with DD" = 

 250 cm., and increases in size regularly until DD" nearly = o. T.he current was 

 ascending, one Daniell being used. A current with DD" = 2f l o cm. corresponds 

 to 0-62 of the original current. It is of importance that in each experiment, 

 whether a contraction is produced or not, the key A should remain closed for a 

 short time, in order to get rid of the polarisation-currents. If this is overlooked the 

 results become uncertain and inconstant, so that an imperfect break-contraction first 

 appears at a different point if the current-strengths are altered in the opposite direc- 

 tion, i. e. if the current is first made to fall to zero, and the succeeding falls are made 

 less and less ; in other words, if the distance DD" is gradually increased from its 

 minimum. 



The following fact is also worth mentioning. With plain metallic electrodes of 

 copper, which give a strong polarisation- current, if key B remain open and key A be 

 repeatedly closed, several successive contractions, diminishing in size, may often occur. 

 These are caused by the polarisation-current in the nerve, which thus does not dis- 

 appear very rapidly, and passes at each closure of A without entirely exhausting 

 itself. It is therefore of great importance to keep the key A closed for a few seconds 

 after each experiment ; the results then become perfectly constant. 



With an ascending current, on the "other hand, the first imperfect break-contraction 

 occurs with_DD" = loocm., or sometimes not till DD" = 400111., that is to say, with a 

 reduction of current to about 0-22 or o-l of its original strength. On further repeti- 

 tion of the same experiment, or with other nerves, no imperfect break-contraction may 

 occur, even on reduction to zero. 



Let us try to explain these results on Hermann's theory that 

 the stimulation in the case of imperfect break-contractions always 

 occurs at the anode of the exciting current, and that there is no 

 reversal of the direction of the exciting current. In the case of 

 an ascending current the stimulation would occur at the anode 

 next the muscle, and would have to traverse a more or less con- 

 siderable extrapolar tract of anelectrotonic nerve in order to 

 reach the muscle. It would thus reach the muscle diminished in 

 strength according to the prevalent view, which may be considered 

 to be firmly established by the experiments of Pfliiger and by 

 Tigerstedt's recent researches. With a descending current the 



