BY DR. MICHAEL FOSTER. 391 



modify the increase of impulse due to katelectrotonus. When 

 the exciting pair is on the other side of the polarizing pair, 

 and the region of the exciting pair thrown into katelectroto- 

 nns, for instance, the increased impulse due to katelectrotonns 

 after passing through the region of katelectrotonns has to 

 make its way through a region of anelectrotoinis before it can 

 reach the muscle it has to struggle in this region against an- 

 tagonistic influences, and whether it reaches the muscle as an 

 impulse greater than, or less than, or simply equal to, that 

 which occurs in a nerve not electrotonized, will depend on the 

 relative amounts of the katelectrotonic increase of irritability 

 and the anelectrotonic decrease of conductivity. 



This will be found to depend largely on the intensity of the 

 polarizing current. 



If the current be weak, the katelectrotonic increase over the 

 normal impulse (of the non-electrotonized nerve), though less- 

 ened by having to pass through an anelectrotonic region, will 

 be evident as a larger contraction in the muscle. 



If the polarizing current be strong, the contraction caused 

 by the impulse originated in the katelectrotonic region will 

 not, only not be greater than the normal but will even be less, 

 or may be absent altogether with a very strong (three or four 

 Grove cells) polarizing current, owing to the impulse being 

 completely blocked in the anelectrotonic region. 



Mutatis mutandis, the same results are witnessed when the 

 effect of an anelectrotonic decrease has to pass through a kate- 

 lectrotonic region on its way to the muscle. 



Obs. XIV. By placing the polarizing electrodes sufficiently 

 far apart from each other, the exciting pair may be inserted 

 into the intrapolar region, and the following results ob- 

 tained : 



In the intrapolar region, as in the extrapolar, there is an in- 

 crease of irritability in the neighborhood of the negative, and 

 a decrease in the neighborhood of the positive pole. 



The increase and decrease respectively are greatest close to 

 the poles, and diminish towards a neutral point situate between 

 the poles. 



With a weak current, this neutral point lies rather nearer 

 to the negative pole than the positive. By increasing the 

 strength of the current it is driven nearer and nearer to the 

 positive pole. 



IV. Other Variations in Irritability. The farther 

 from the muscle the part of the nerve excited, the greater Ifie 

 contraction. 



Obs. XV. Arrange a nerve-muscle preparation with two pair 

 of electrodes, one close to the muscle, the other near to the cut 

 end of the nerve. 



Connect both electrodes with a double key (Chap. XIX., 



