324 ELECTRO-PHYSIOLOGY CHAP. 



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

 of fluid reduces the hydrostatic pressure in BG, 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 ab 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 Pfliiger'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 



