120 NERVOUS SYSTEM. 



common salt, which is a powerful excitant for the nervous 

 irritability, the nerves that had been exposed to the alkaline 

 solution produced more powerful and prompt contractions 

 than those exposed to the acid. Now the electrolytic action 

 of a constant current tends to the accumulation of hydrogen 

 and an alkali near the cathode, and oxygen and an acid near 

 the anode ; and by this, Matteucci explains the increase of 

 excitability in catelectrotonus and the diminished excita- 

 bility in anelectrotonus. 1 As regards this question, we have 

 only to say, as in the case of general electrotonus, that the 

 conditions are susceptible of a partial explanation on purely 

 physical grounds ; but precisely how far the unexplained 

 physiological properties of the nerves are involved, it is im- 

 possible to say. 



Neutral Point. The anelectro tonic condition, on the 

 one hand, and the catelectrotonic condition at the other 

 pole of the battery, are marked in extra-polar portions of 

 the nerve, and are to be recognized as well in that portion 

 through which the current is passing; but between the 

 poles, is a point where these conditions .meet, as it were, 

 and where the excitability is unchanged. This has been 

 called the neutral point. When the galvanic current is of 

 moderate strength, this neutral point is about half-way be- 

 tween the poles. " When a weak current is used, the neu- 

 tral point approaches the positive pole, while in a strong 

 current, it approaches the negative pole. In other words, in 

 a weak current the negative pole rules over a wider territory 

 than the positive pole, whereas in a strong current the posi- 

 tive pole prevails." a 



Negative Variation. There remains one curious phe- 

 nomenon, discovered by Du Bois-Beymond, which depends 



1 MATTEUCCI, PMnomenes physico-chimiques des corps vivants. R?vue des 

 cours scientifigues, Paris, 1867-'68, tome v., p. 579. 

 8 RUTHERFORD, loc. cit., p. 92. 



