390 STIMULATION OF NERVES. 



been due not to vital changes in the nerve, but to escape of 

 current or simple electrical changes. 



Obs. IX. Having arranged a nerve-muscle preparation with 

 the polarizing, but without the exciting pair of electrodes, let 

 the nerve between the electrodes and the muscle hang down 

 in a loop. 



Let the extreme end of the loop dip into a drop of concen- 

 trated solution of common salt. As soon as the irregular 

 tetanic contractions resulting from the action of saline fluid 

 on the nerve make their appearance, pass an ascending current 

 through the electrodes. The tetanic spasms will be much less- 

 ened, or cease altogether. 



Tiiss a descending current through the electrodes, the spasms 

 will be increased. 



The general irritability, therefore, of the nerve is affected 

 in electrotonus, not simply its susceptibility to electrical modi- 

 fications. 



O/;.s. X. By introducing a rheochord between the battery 

 and the polarizing electrodes, and by varying the number of 

 cells used, the student will ascertain that the amount of in- 

 crease of irritability in katelectrotonus and decrease in anclec- 

 trotonus depends on the strength of the polarizing current, 

 being greater with the stronger. 



Obs. XI. By placing the polarizing electrodes at a variable 

 distance from each other, it will be found that, with the same 

 strength of current, the effect is greater the longer the piece of 

 nerve between the polarizing electrodes. 



Obs. XII. Bj ? shifting the exciting electrodes nearer to and 

 farther from the polarizing electrodes, it will be found that 

 the effects of both anelectrotonus and katelectrotonus are 

 greatest in the immediate neighborhood of the polarizing 

 pair, and diminish the farther the exciting pair is from the 

 polarizing. 



In all the above observations, the stimulus, whether electric 

 or chemical or other, is brought to bear on the nerve between 

 the polarizing pair and the muscle. 



06s. XIII. They may- be repeated with the polarizing pair 

 placed between the exciting pair (or chemical stimulus) and 

 the muscle. An ascending current will now throw the region 

 of the exciting pair into katelectrotonus, a descending into 

 anelectrotonus. 



The general results will be the same, but they will not come 

 out with the same distinctness, for the following reason: 

 When the exciting pair is placed nearer to the muscle than the 

 polarizing pair, the nerve between the exciting pair and the 

 muscle is simply in a state of katelectrotonus, the intensity of 

 which diminishes towards the muscle onwards. There is 

 nothing between the exciting electrodes and the muscle to 



