IX ELECTRICAL EXCITATION OF NERVE 141 



The former is universally the case on the side of the kathode, 

 the latter on that of the anode. In a typical series of experi- 

 ments (the results and theoretical conclusions from which form 

 the contents of the classical work on " Electrotonus " so frequently 

 alluded to), Pfliiger has exhaustively investigated all the facts 

 which relate to this subject. 



If a constant current is led, by means of unpolarisable elec- 

 trodes, into the middle portion of a nerve that is still united at 

 one end with the muscle, the resulting alterations of excitability 

 are easily demonstrated in the tract of nerve that lies between 

 muscle and polarising current. As " test - stimulus " we may 

 employ either an easily graduated electrical, or a chemical, or 

 mechanical excitation, the height of the muscular contraction 

 being of course the gauge of excitability. If increase of 

 response is to be demonstrated, the twitch discharged by the 

 test-stimulus before the polarising current is made must obviously 

 be submaximal. When the latter is ascending, and the stimulus is 

 applied at a point of the nerve not too remote from the anode, in 

 the direction of the muscle, a more or less definite depression of 

 excitability inevitably appears, which increases in magnitude with 

 increasing strength of the polarising current. Under these condi- 

 tions a current that previously discharged a maximal twitch may 

 become totally ineffective, and in the same way a vigorous tetanus 

 produced by electrical or chemical excitation (concentrated salt- 

 solution) may be momentarily interrupted if a strong ascending 

 current is closed above the part excited. If different points of 

 the " myopolar " portion of the nerve (between muscle and polar- 

 ising current) are excited as equally as possible, it can easily be 

 determined that, on the one hand, the depression of excitability 

 spreads with increased intensity of current over an increasing 

 portion of the myopolar region, while, on the other, the degree of 

 alteration from the anode diminishes rapidly. With a descending 

 polarising current, the relations of excitability within the myopolar 

 region are precisely opposite in character. The response is now 

 augmented under all circumstances, in a greater degree in propor- 

 tion as the test-stimulus is nearer the kathode, and the polarising 

 current (other conditions being equal) stronger. Tetanising 

 stimuli, which previously elicited little or no trace of excitation, 

 evoke a vigorous tetanus, when a descending current of sufficient 

 strength is made above the point at which the nerve is excited. 



