324 ELECTRO-PHYSIOLOGY CHAP. 



is not replaced, the latter can only be instantaneous ; for the outflow 



of fluid reduces the hydrostatic pressure in BC, the spring once 



more pushes the piston D over the opening ; there could only be 



a momentary closure twitch. But if the potential energy given off 



is replaced, the discharge will also continue, like the outflow of 



fluid, provided always that fluid is poured into the vertical limb 



of the cylinder in the same quantity that flows out, i.e. there is 



closure tetanus. The opposite relations obtain on opening the 



current. At the moment of opening, the previous rise of inhibitory 



energy in the region of anelectrotonus returns to the normal ; 



potential energy then, of course, predominates, displacing the 



inhibition in its own direction, i.e. in the line of the arrow cd. 



The retreating inhibitions, however, come to rest as little as a 



pendulum, on again reaching the equilibrium from which they were 



driven by the anelectrotonus. They go slightly beyond it, so that 



the mouth g is opened for a moment for the discharge of the fluid ; 



this produces the opening twitch. It is obvious that no potential 



energy can be set in the katelectrotonised region, when, at the 



moment of opening the current, the reinforced elasticity of the spring 



displaces the inhibitions in the direction of the arrow ab, i.e. there 



can be no excitation. The hypothesis based on the phenomena 



of the law of contraction, to the effect that the closure of a given 



current excites more strongly than its opening, follows as the 



naturally corollary from Pfliiger's theory of discharge ; for, if at 



the closure of the current the inhibitions at the anode are as much 



displaced in the direction db as they are at the kathode in the 



direction cd, then, at opening, the inhibitions in the anelectro- 



tonised region are not displaced as far from the normal, in the 



direction cd, as the inhibitions in the region of katelectrotonus at 



closure, i.e. there cannot be the same discharge of potential energy. 



If a rapid succession of brief electrical currents is sent into the 



nerve, the apparently continuous tetanic excitation is due to the 



continuous alternating discharge of energy at anode and kathode, 



which persists as long as metabolism continues adequate between 



every shock to replace the energy dissipated in the last shock. 



The mechanics of the law of contraction are thus simply explained 



on Pfluger's hypothesis. 



We have next to see whether it can account for the after-effects 

 of the constant current, the so-called modifications of the nerve as 

 above described. We have seen that a state of heightened 



